Client/server
5.1.13 This page will describe a client/server environment.
In a client/server arrangement, network services are located on a dedicated computer called a server. The server responds to the requests of clients. The server is a central computer that is continuously available to respond to requests from clients for file, print, application, and other services. Most network operating systems adopt the form of a client/server relationship. Typically, desktop computers function as clients and one or more computers with additional processing power, memory, and specialized software function as servers.
Servers are designed to handle requests from many clients simultaneously. Before a client can access the server resources, the client must be identified and be authorized to use the resource. Each client is assigned an account name and password that is verified by an authentication service. The authentication service guards access to the network. With the centralization of user accounts, security, and access control, server-based networks simplify the administration of large networks.
The concentration of network resources such as files, printers, and applications on servers also makes it easier to back-up and maintain the data. Resources can be located on specialized, dedicated servers for easier access. Most client/server systems also include ways to enhance the network with new services that extend the usefulness of the network.
The centralized functions in a client/server network has substantial advantages and some disadvantages. Although a centralized server enhances security, ease of access, and control, it introduces a single point of failure into the network. Without an operational server, the network cannot function at all. Servers require a trained, expert staff member to administer and maintain. Server systems also require additional hardware and specialized software that add to the cost.
This page concludes this lesson. The next lesson will discuss cabling WANs. The first page focuses on the WAN physical layer.
Tuesday, December 29, 2009
Host connectivity / Peer-to-peer
Host connectivity
5.1.11 This page will explain how NICs provide network connectivity.
The function of a NIC is to connect a host device to the network medium. A NIC is a printed circuit board that fits into the expansion slot on the motherboard or peripheral device of a computer. The NIC is also referred to as a network adapter. On laptop or notebook computers a NIC is the size of a credit card.
NICs are considered Layer 2 devices because each NIC carries a unique code called a MAC address. This address is used to control data communication for the host on the network. More will be learned about the MAC address later. NICs control host access to the medium.
In some cases the type of connector on the NIC does not match the type of media that needs to be connected to it. A good example is a Cisco 2500 router. This router has an AUI connector. That AUI connector needs to connect to a UTP Category 5 Ethernet cable. A transceiver is used to do this. A transceiver converts one type of signal or connector to another. For example, a transceiver can connect a 15-pin AUI interface to an RJ-45 jack. It is considered a Layer 1 device because it only works with bits and not with any address information or higher-level protocols.
NICs have no standardized symbol. It is implied that, when networking devices are attached to network media, there is a NIC or NIC-like device present. A dot on a topology map represents either a NIC interface or port, which acts like a NIC.
The next page discusses peer-to-peer networks.
Peer-to-peer
5.1.12 This page covers peer-to-peer networks.
When LAN and WAN technologies are used, many computers are interconnected to provide services to their users. To accomplish this, networked computers take on different roles or functions in relation to each other. Some types of applications require computers to function as equal partners. Other types of applications distribute their work so that one computer functions to serve a number of others in an unequal relationship.
Two computers generally use request and response protocols to communicate with each other. One computer issues a request for a service, and a second computer receives and responds to that request. The requestor acts like a client and the responder acts like a server.
In a peer-to-peer network, networked computers act as equal partners, or peers. As peers, each computer can take on the client function or the server function. Computer A may request for a file from Computer B, which then sends the file to Computer A. Computer A acts like the client and Computer B acts like the server. At a later time, Computers A and B can reverse roles.
In a peer-to-peer network, individual users control their own resources. The users may decide to share certain files with other users. The users may also require passwords before they allow others to access their resources. Since individual users make these decisions, there is no central point of control or administration in the network. In addition, individual users must back up their own systems to be able to recover from data loss in case of failures. When a computer acts as a server, the user of that machine may experience reduced performance as the machine serves the requests made by other systems.
Peer-to-peer networks are relatively easy to install and operate. No additional equipment is necessary beyond a suitable operating system installed on each computer. Since users control their own resources, no dedicated administrators are needed.
As networks grow, peer-to-peer relationships become increasingly difficult to coordinate. A peer-to-peer network works well with ten or fewer computers. Since peer-to-peer networks do not scale well, their efficiency decreases rapidly as the number of computers on the network increases. Also, individual users control access to the resources on their computers, which means security may be difficult to maintain. The client/server model of networking can be used to overcome the limitations of the peer-to-peer network.
The next page discusses a client/server network.
5.1.11 This page will explain how NICs provide network connectivity.
The function of a NIC is to connect a host device to the network medium. A NIC is a printed circuit board that fits into the expansion slot on the motherboard or peripheral device of a computer. The NIC is also referred to as a network adapter. On laptop or notebook computers a NIC is the size of a credit card.
NICs are considered Layer 2 devices because each NIC carries a unique code called a MAC address. This address is used to control data communication for the host on the network. More will be learned about the MAC address later. NICs control host access to the medium.
In some cases the type of connector on the NIC does not match the type of media that needs to be connected to it. A good example is a Cisco 2500 router. This router has an AUI connector. That AUI connector needs to connect to a UTP Category 5 Ethernet cable. A transceiver is used to do this. A transceiver converts one type of signal or connector to another. For example, a transceiver can connect a 15-pin AUI interface to an RJ-45 jack. It is considered a Layer 1 device because it only works with bits and not with any address information or higher-level protocols.
NICs have no standardized symbol. It is implied that, when networking devices are attached to network media, there is a NIC or NIC-like device present. A dot on a topology map represents either a NIC interface or port, which acts like a NIC.
The next page discusses peer-to-peer networks.
Peer-to-peer
5.1.12 This page covers peer-to-peer networks.
When LAN and WAN technologies are used, many computers are interconnected to provide services to their users. To accomplish this, networked computers take on different roles or functions in relation to each other. Some types of applications require computers to function as equal partners. Other types of applications distribute their work so that one computer functions to serve a number of others in an unequal relationship.
Two computers generally use request and response protocols to communicate with each other. One computer issues a request for a service, and a second computer receives and responds to that request. The requestor acts like a client and the responder acts like a server.
In a peer-to-peer network, networked computers act as equal partners, or peers. As peers, each computer can take on the client function or the server function. Computer A may request for a file from Computer B, which then sends the file to Computer A. Computer A acts like the client and Computer B acts like the server. At a later time, Computers A and B can reverse roles.
In a peer-to-peer network, individual users control their own resources. The users may decide to share certain files with other users. The users may also require passwords before they allow others to access their resources. Since individual users make these decisions, there is no central point of control or administration in the network. In addition, individual users must back up their own systems to be able to recover from data loss in case of failures. When a computer acts as a server, the user of that machine may experience reduced performance as the machine serves the requests made by other systems.
Peer-to-peer networks are relatively easy to install and operate. No additional equipment is necessary beyond a suitable operating system installed on each computer. Since users control their own resources, no dedicated administrators are needed.
As networks grow, peer-to-peer relationships become increasingly difficult to coordinate. A peer-to-peer network works well with ten or fewer computers. Since peer-to-peer networks do not scale well, their efficiency decreases rapidly as the number of computers on the network increases. Also, individual users control access to the resources on their computers, which means security may be difficult to maintain. The client/server model of networking can be used to overcome the limitations of the peer-to-peer network.
The next page discusses a client/server network.
Bridges / Switches
Bridges
5.1.9 This page will explain the function of bridges in a LAN.
There are times when it is necessary to break up a large LAN into smaller and more easily managed segments. This decreases the amount of traffic on a single LAN and can extend the geographical area past what a single LAN can support. The devices that are used to connect network segments together include bridges, switches, routers, and gateways. Switches and bridges operate at the data link layer of the OSI model. The function of the bridge is to make intelligent decisions about whether or not to pass signals on to the next segment of a network.
When a bridge receives a frame on the network, the destination MAC address is looked up in the bridge table to determine whether to filter, flood, or copy the frame onto another segment. This decision process occurs as follows:
• If the destination device is on the same segment as the frame, the bridge will not send the frame onto other segments. This process is known as filtering.
• If the destination device is on a different segment, the bridge forwards the frame to the appropriate segment.
• If the destination address is unknown to the bridge, the bridge forwards the frame to all segments except the one on which it was received. This process is known as flooding. If placed strategically, a bridge can greatly improve network performance.
The next page will describe switches.
Switches
5.1.10 This page will explain the function of switches.
A switch is sometimes described as a multiport bridge. A typical bridge may have only two ports that link two network segments. A switch can have multiple ports based on the number of network segments that need to be linked. Like bridges, switches learn information about the data packets that are received from computers on the network. Switches use this information to build tables to determine the destination of data that is sent between computers on the network.
Although there are some similarities between the two, a switch is a more sophisticated device than a bridge. A bridge determines whether the frame should be forwarded to the other network segment based on the destination MAC address. A switch has many ports with many network segments connected to them. A switch chooses the port to which the destination device or workstation is connected. Ethernet switches are popular connectivity solutions because they improve network speed, bandwidth, and performance.
Switching is a technology that alleviates congestion in Ethernet LANs. Switches reduce traffic and increase bandwidth. Switches can easily replace hubs because switches work with the cable infrastructures that are already in place. This improves performance with minimal changes to a network.
All switching equipment perform two basic operations. The first operation is called switching data frames. This is the process by which a frame is received on an input medium and then transmitted to an output medium. The second is the maintenance of switching operations where switches build and maintain switching tables and search for loops.
Switches operate at much higher speeds than bridges and can support new functionality, such as virtual LANs.
An Ethernet switch has many benefits. One benefit is that it allows many users to communicate at the same time through the use of virtual circuits and dedicated network segments in a virtually collision-free environment. This maximizes the bandwidth available on the shared medium. Another benefit is that a switched LAN environment is very cost effective since the hardware and cables in place can be reused.
The Lab activity will help students understand the price of a LAN switch.
5.1.9 This page will explain the function of bridges in a LAN.
There are times when it is necessary to break up a large LAN into smaller and more easily managed segments. This decreases the amount of traffic on a single LAN and can extend the geographical area past what a single LAN can support. The devices that are used to connect network segments together include bridges, switches, routers, and gateways. Switches and bridges operate at the data link layer of the OSI model. The function of the bridge is to make intelligent decisions about whether or not to pass signals on to the next segment of a network.
When a bridge receives a frame on the network, the destination MAC address is looked up in the bridge table to determine whether to filter, flood, or copy the frame onto another segment. This decision process occurs as follows:
• If the destination device is on the same segment as the frame, the bridge will not send the frame onto other segments. This process is known as filtering.
• If the destination device is on a different segment, the bridge forwards the frame to the appropriate segment.
• If the destination address is unknown to the bridge, the bridge forwards the frame to all segments except the one on which it was received. This process is known as flooding. If placed strategically, a bridge can greatly improve network performance.
The next page will describe switches.
Switches
5.1.10 This page will explain the function of switches.
A switch is sometimes described as a multiport bridge. A typical bridge may have only two ports that link two network segments. A switch can have multiple ports based on the number of network segments that need to be linked. Like bridges, switches learn information about the data packets that are received from computers on the network. Switches use this information to build tables to determine the destination of data that is sent between computers on the network.
Although there are some similarities between the two, a switch is a more sophisticated device than a bridge. A bridge determines whether the frame should be forwarded to the other network segment based on the destination MAC address. A switch has many ports with many network segments connected to them. A switch chooses the port to which the destination device or workstation is connected. Ethernet switches are popular connectivity solutions because they improve network speed, bandwidth, and performance.
Switching is a technology that alleviates congestion in Ethernet LANs. Switches reduce traffic and increase bandwidth. Switches can easily replace hubs because switches work with the cable infrastructures that are already in place. This improves performance with minimal changes to a network.
All switching equipment perform two basic operations. The first operation is called switching data frames. This is the process by which a frame is received on an input medium and then transmitted to an output medium. The second is the maintenance of switching operations where switches build and maintain switching tables and search for loops.
Switches operate at much higher speeds than bridges and can support new functionality, such as virtual LANs.
An Ethernet switch has many benefits. One benefit is that it allows many users to communicate at the same time through the use of virtual circuits and dedicated network segments in a virtually collision-free environment. This maximizes the bandwidth available on the shared medium. Another benefit is that a switched LAN environment is very cost effective since the hardware and cables in place can be reused.
The Lab activity will help students understand the price of a LAN switch.
Repeaters / Hub / Wireless
Repeaters
5.1.6 This page will discuss how a repeater is used on a network.
The term repeater comes from the early days of long distance communication. A repeater was a person on one hill who would repeat the signal that was just received from the person on the previous hill. The process would repeat until the message arrived at its destination. Telegraph, telephone, microwave, and optical communications use repeaters to strengthen signals sent over long distances.
A repeater receives a signal, regenerates it, and passes it on. It can regenerate and retime network signals at the bit level to allow them to travel a longer distance on the media. Ethernet and IEEE 802.3 implement a rule, known as the 5-4-3 rule, for the number of repeaters and segments on shared access Ethernet backbones in a tree topology. The 5-4-3 rule divides the network into two types of physical segments: populated (user) segments, and unpopulated (link) segments. User segments have users' systems connected to them. Link segments are used to connect the network repeaters together. The rule mandates that between any two nodes on the network, there can only be a maximum of five segments, connected through four repeaters, or concentrators, and only three of the five segments may contain user connections.
The Ethernet protocol requires that a signal sent out over the LAN reach every part of the network within a specified length of time. The 5-4-3 rule ensures this. Each repeater that a signal goes through adds a small amount of time to the process, so the rule is designed to minimize transmission times of the signals. Too much latency on the LAN increases the number of late collisions and makes the LAN less efficient.
The next page will discuss hubs.
Hub
5.1.7 This page will describe the three types of hubs.
Hubs are actually multiport repeaters. The difference between hubs and repeaters is usually the number of ports that each device provides. A typical repeater usually has two ports. A hub generally has from 4 to 24 ports. Hubs are most commonly used in Ethernet 10BASE-T or 100BASE-T networks.
The use of a hub changes the network from a linear bus with each device plugged directly into the wire to a star topology. Data that arrives over the cables to a hub port is electrically repeated on all the other ports connected to the network segment.
Hubs come in three basic types:
• Passive – A passive hub serves as a physical connection point only. It does not manipulate or view the traffic that crosses it. It does not boost or clean the signal. A passive hub is used only to share the physical media. A passive hub does not need electrical power.
• Active – An active hub must be plugged into an electrical outlet because it needs power to amplify a signal before it is sent to the other ports.
• Intelligent – Intelligent hubs are sometimes called smart hubs. They function like active hubs with microprocessor chips and diagnostic capabilities. Intelligent hubs are more expensive than active hubs. They are also more useful in troubleshooting situations.
Devices attached to a hub receive all traffic that travels through the hub. If many devices are attached to the hub, collisions are more likely to occur. A collision occurs when two or more workstations send data over the network wire at the same time. All data is corrupted when this occurs. All devices that are connected to the same network segment are members of the same collision domain.
The next page discusses wireless networks.
Wireless
5.1.8 This page will explain how a wireless network can be created with much less cabling than other networks.
Wireless signals are electromagnetic waves that travel through the air. Wireless networks use radio frequency (RF), laser, infrared (IR), satellite, or microwaves to carry signals between computers without a permanent cable connection. The only permanent cabling can be to the access points for the network. Workstations within the range of the wireless network can be moved easily without the need to connect and reconnect network cables.
A common application of wireless data communication is for mobile use. Some examples of mobile use include commuters, airplanes, satellites, remote space probes, space shuttles, and space stations.
At the core of wireless communication are devices called transmitters and receivers. The transmitter converts source data to electromagnetic waves that are sent to the receiver. The receiver then converts these electromagnetic waves back into data for the destination. For two-way communication, each device requires a transmitter and a receiver. Many networking device manufacturers build the transmitter and receiver into a single unit called a transceiver or wireless network card. All devices in a WLAN must have the correct wireless network card installed.
The two most common wireless technologies used for networking are IR and RF. IR technology has its weaknesses. Workstations and digital devices must be in the line of sight of the transmitter to work correctly. An infrared-based network can be used when all the digital devices that require network connectivity are in one room. IR networking technology can be installed quickly. However, the data signals can be weakened or obstructed by people who walk across the room or by moisture in the air. New IR technologies will be able to work out of sight.
RF technology allows devices to be in different rooms or buildings. The limited range of radio signals restricts the use of this kind of network. RF technology can be on single or multiple frequencies. A single radio frequency is subject to outside interference and geographic obstructions. It is also easily monitored by others, which makes the transmissions of data insecure. Spread spectrum uses multiple frequencies to increase the immunity to noise and to make it difficult for outsiders to intercept data transmissions.
Two approaches that are used to implement spread spectrum for WLAN transmissions are Frequency Hopping Spread Spectrum (FHSS) and Direct Sequence Spread Spectrum (DSSS). The technical details of how these technologies work are beyond the scope of this course.
A large LAN can be broken into smaller segments. The next page will explain how bridges are used to accomplish this.
5.1.6 This page will discuss how a repeater is used on a network.
The term repeater comes from the early days of long distance communication. A repeater was a person on one hill who would repeat the signal that was just received from the person on the previous hill. The process would repeat until the message arrived at its destination. Telegraph, telephone, microwave, and optical communications use repeaters to strengthen signals sent over long distances.
A repeater receives a signal, regenerates it, and passes it on. It can regenerate and retime network signals at the bit level to allow them to travel a longer distance on the media. Ethernet and IEEE 802.3 implement a rule, known as the 5-4-3 rule, for the number of repeaters and segments on shared access Ethernet backbones in a tree topology. The 5-4-3 rule divides the network into two types of physical segments: populated (user) segments, and unpopulated (link) segments. User segments have users' systems connected to them. Link segments are used to connect the network repeaters together. The rule mandates that between any two nodes on the network, there can only be a maximum of five segments, connected through four repeaters, or concentrators, and only three of the five segments may contain user connections.
The Ethernet protocol requires that a signal sent out over the LAN reach every part of the network within a specified length of time. The 5-4-3 rule ensures this. Each repeater that a signal goes through adds a small amount of time to the process, so the rule is designed to minimize transmission times of the signals. Too much latency on the LAN increases the number of late collisions and makes the LAN less efficient.
The next page will discuss hubs.
Hub
5.1.7 This page will describe the three types of hubs.
Hubs are actually multiport repeaters. The difference between hubs and repeaters is usually the number of ports that each device provides. A typical repeater usually has two ports. A hub generally has from 4 to 24 ports. Hubs are most commonly used in Ethernet 10BASE-T or 100BASE-T networks.
The use of a hub changes the network from a linear bus with each device plugged directly into the wire to a star topology. Data that arrives over the cables to a hub port is electrically repeated on all the other ports connected to the network segment.
Hubs come in three basic types:
• Passive – A passive hub serves as a physical connection point only. It does not manipulate or view the traffic that crosses it. It does not boost or clean the signal. A passive hub is used only to share the physical media. A passive hub does not need electrical power.
• Active – An active hub must be plugged into an electrical outlet because it needs power to amplify a signal before it is sent to the other ports.
• Intelligent – Intelligent hubs are sometimes called smart hubs. They function like active hubs with microprocessor chips and diagnostic capabilities. Intelligent hubs are more expensive than active hubs. They are also more useful in troubleshooting situations.
Devices attached to a hub receive all traffic that travels through the hub. If many devices are attached to the hub, collisions are more likely to occur. A collision occurs when two or more workstations send data over the network wire at the same time. All data is corrupted when this occurs. All devices that are connected to the same network segment are members of the same collision domain.
The next page discusses wireless networks.
Wireless
5.1.8 This page will explain how a wireless network can be created with much less cabling than other networks.
Wireless signals are electromagnetic waves that travel through the air. Wireless networks use radio frequency (RF), laser, infrared (IR), satellite, or microwaves to carry signals between computers without a permanent cable connection. The only permanent cabling can be to the access points for the network. Workstations within the range of the wireless network can be moved easily without the need to connect and reconnect network cables.
A common application of wireless data communication is for mobile use. Some examples of mobile use include commuters, airplanes, satellites, remote space probes, space shuttles, and space stations.
At the core of wireless communication are devices called transmitters and receivers. The transmitter converts source data to electromagnetic waves that are sent to the receiver. The receiver then converts these electromagnetic waves back into data for the destination. For two-way communication, each device requires a transmitter and a receiver. Many networking device manufacturers build the transmitter and receiver into a single unit called a transceiver or wireless network card. All devices in a WLAN must have the correct wireless network card installed.
The two most common wireless technologies used for networking are IR and RF. IR technology has its weaknesses. Workstations and digital devices must be in the line of sight of the transmitter to work correctly. An infrared-based network can be used when all the digital devices that require network connectivity are in one room. IR networking technology can be installed quickly. However, the data signals can be weakened or obstructed by people who walk across the room or by moisture in the air. New IR technologies will be able to work out of sight.
RF technology allows devices to be in different rooms or buildings. The limited range of radio signals restricts the use of this kind of network. RF technology can be on single or multiple frequencies. A single radio frequency is subject to outside interference and geographic obstructions. It is also easily monitored by others, which makes the transmissions of data insecure. Spread spectrum uses multiple frequencies to increase the immunity to noise and to make it difficult for outsiders to intercept data transmissions.
Two approaches that are used to implement spread spectrum for WLAN transmissions are Frequency Hopping Spread Spectrum (FHSS) and Direct Sequence Spread Spectrum (DSSS). The technical details of how these technologies work are beyond the scope of this course.
A large LAN can be broken into smaller segments. The next page will explain how bridges are used to accomplish this.
Ethernet media and connector requirements / Connection media
Ethernet media and connector requirements
5.1.3 This page provides important considerations for an Ethernet implementation. These include the media and connector requirements and the level of network performance.
The cables and connector specifications used to support Ethernet implementations are derived from the EIA/TIA standards. The categories of cabling defined for Ethernet are derived from the EIA/TIA-568 SP-2840 Commercial Building Telecommunications Wiring Standards.
Figure compares the cable and connector specifications for the most popular Ethernet implementations. It is important to note the difference in the media used for 10-Mbps Ethernet versus 100-Mbps Ethernet. Networks with a combination of 10- and 100-Mbps traffic use Category 5 UTP to support Fast Ethernet.
The next page will discuss the different connection types.
Connection media
5.1.4 This page describes the different connection types used by each physical layer implementation, as shown in Figure . The RJ-45 connector and jack are the most common. RJ-45 connectors are discussed in more detail in the next section.
The connector on a NIC may not match the media to which it needs to connect. As shown in Figure , an interface may exist for the 15-pin attachment unit interface (AUI) connector. The AUI connector allows different media to connect when used with the appropriate transceiver. A transceiver is an adapter that converts one type of connection to another. A transceiver will usually convert an AUI to an RJ-45, a coax, or a fiber optic connector. On 10BASE5 Ethernet, or Thicknet, a short cable is used to connect the AUI with a transceiver on the main cable.
The next page will discuss UTP cables.
UTP implementation
5.1.5 This page provides detailed information for a UTP implementation.
EIA/TIA specifies an RJ-45 connector for UTP cable. The letters RJ stand for registered jack and the number 45 refers to a specific wiring sequence. The RJ-45 transparent end connector shows eight colored wires. Four of the wires, T1 through T4, carry the voltage and are called tip. The other four wires, R1 through R4, are grounded and are called ring. Tip and ring are terms that originated in the early days of the telephone. Today, these terms refer to the positive and the negative wire in a pair. The wires in the first pair in a cable or a connector are designated as T1 and R1. The second pair is T2 and R2, the third is T3 and R3, and the fourth is T4 and R4.
The RJ-45 connector is the male component, which is crimped on the end of the cable. When a male connector is viewed from the front, the pin locations are numbered from 8 on the left to 1 on the right as seen in Figure .
The jack is the female component in a network device, wall outlet, or patch panel as seen in Figure . Figure shows the punch-down connections at the back of the jack where the Ethernet UTP cable connects.
For electricity to run between the connector and the jack, the order of the wires must follow T568A or T568B color code found in the EIA/TIA-568-B.1 standard, as shown in Figure . To determine the EIA/TIA category of cable that should be used to connect a device, refer to the documentation for that device or look for a label on the device near the jack. If there are no labels or documentation available, use Category 5E or greater as higher categories can be used in place of lower ones. Then determine whether to use a straight-through cable or a crossover cable.
If the two RJ-45 connectors of a cable are held side by side in the same orientation, the colored wires will be seen in each. If the order of the colored wires is the same at each end, then the cable is a straight-through, as seen in Figure .
In a crossover cable, the RJ-45 connectors on both ends show that some of the wires are connected to different pins on each side of the cable. Figure shows that pins 1 and 2 on one connector connect to pins 3 and 6 on the other.
Figure shows the guidelines that are used to determine the type of cable that is required to connect Cisco devices.
Use straight-through cables for the following connections:
• Switch to router
• Switch to PC or server
• Hub to PC or server
Use crossover cables for the following connections:
• Switch to switch
• Switch to hub
• Hub to hub
• Router to router
• PC to PC
• Router to PC
The category of UTP cable required is based on the type of Ethernet that is chosen.
The next page explains how repeaters work.
5.1.3 This page provides important considerations for an Ethernet implementation. These include the media and connector requirements and the level of network performance.
The cables and connector specifications used to support Ethernet implementations are derived from the EIA/TIA standards. The categories of cabling defined for Ethernet are derived from the EIA/TIA-568 SP-2840 Commercial Building Telecommunications Wiring Standards.
Figure compares the cable and connector specifications for the most popular Ethernet implementations. It is important to note the difference in the media used for 10-Mbps Ethernet versus 100-Mbps Ethernet. Networks with a combination of 10- and 100-Mbps traffic use Category 5 UTP to support Fast Ethernet.
The next page will discuss the different connection types.
Connection media
5.1.4 This page describes the different connection types used by each physical layer implementation, as shown in Figure . The RJ-45 connector and jack are the most common. RJ-45 connectors are discussed in more detail in the next section.
The connector on a NIC may not match the media to which it needs to connect. As shown in Figure , an interface may exist for the 15-pin attachment unit interface (AUI) connector. The AUI connector allows different media to connect when used with the appropriate transceiver. A transceiver is an adapter that converts one type of connection to another. A transceiver will usually convert an AUI to an RJ-45, a coax, or a fiber optic connector. On 10BASE5 Ethernet, or Thicknet, a short cable is used to connect the AUI with a transceiver on the main cable.
The next page will discuss UTP cables.
UTP implementation
5.1.5 This page provides detailed information for a UTP implementation.
EIA/TIA specifies an RJ-45 connector for UTP cable. The letters RJ stand for registered jack and the number 45 refers to a specific wiring sequence. The RJ-45 transparent end connector shows eight colored wires. Four of the wires, T1 through T4, carry the voltage and are called tip. The other four wires, R1 through R4, are grounded and are called ring. Tip and ring are terms that originated in the early days of the telephone. Today, these terms refer to the positive and the negative wire in a pair. The wires in the first pair in a cable or a connector are designated as T1 and R1. The second pair is T2 and R2, the third is T3 and R3, and the fourth is T4 and R4.
The RJ-45 connector is the male component, which is crimped on the end of the cable. When a male connector is viewed from the front, the pin locations are numbered from 8 on the left to 1 on the right as seen in Figure .
The jack is the female component in a network device, wall outlet, or patch panel as seen in Figure . Figure shows the punch-down connections at the back of the jack where the Ethernet UTP cable connects.
For electricity to run between the connector and the jack, the order of the wires must follow T568A or T568B color code found in the EIA/TIA-568-B.1 standard, as shown in Figure . To determine the EIA/TIA category of cable that should be used to connect a device, refer to the documentation for that device or look for a label on the device near the jack. If there are no labels or documentation available, use Category 5E or greater as higher categories can be used in place of lower ones. Then determine whether to use a straight-through cable or a crossover cable.
If the two RJ-45 connectors of a cable are held side by side in the same orientation, the colored wires will be seen in each. If the order of the colored wires is the same at each end, then the cable is a straight-through, as seen in Figure .
In a crossover cable, the RJ-45 connectors on both ends show that some of the wires are connected to different pins on each side of the cable. Figure shows that pins 1 and 2 on one connector connect to pins 3 and 6 on the other.
Figure shows the guidelines that are used to determine the type of cable that is required to connect Cisco devices.
Use straight-through cables for the following connections:
• Switch to router
• Switch to PC or server
• Hub to PC or server
Use crossover cables for the following connections:
• Switch to switch
• Switch to hub
• Hub to hub
• Router to router
• PC to PC
• Router to PC
The category of UTP cable required is based on the type of Ethernet that is chosen.
The next page explains how repeaters work.
Cabling LANs / LAN physical layer / Ethernet in the campus
LAN physical layer
5.1.1 This page describes the LAN physical layer.
Various symbols are used to represent media types. Token Ring is represented by a circle. FDDI is represented by two concentric circles and the Ethernet symbol is represented by a straight line. Serial connections are represented by a lightning bolt.
Each computer network can be built with many different media types. The function of media is to carry a flow of information through a LAN. Wireless LANs use the atmosphere, or space, as the medium. Other networking media confine network signals to a wire, cable, or fiber. Networking media are considered Layer 1, or physical layer, components of LANs.
Each type of media has advantages and disadvantages. These are based on the following factors:
• Cable length
• Cost
• Ease of installation
• Susceptibility to interference
Coaxial cable, optical fiber, and space can carry network signals. This module will focus on Category 5 UTP, which includes the Category 5e family of cables.
Many topologies support LANs, as well as many different physical media. Figure shows a subset of physical layer implementations that can be deployed to support Ethernet.
The next page explains how Ethernet is implemented in a campus environment.
Ethernet in the campus
5.1.2 This page will discuss Ethernet.
Ethernet is the most widely used LAN technology. Ethernet was first implemented by the Digital, Intel, and Xerox group (DIX). DIX created and implemented the first Ethernet LAN specification, which was used as the basis for the Institute of Electrical and Electronics Engineers (IEEE) 802.3 specification, released in 1980. IEEE extended 802.3 to three new committees known as 802.3u for Fast Ethernet, 802.3z for Gigabit Ethernet over fiber, and 802.3ab for Gigabit Ethernet over UTP.
A network may require an upgrade to one of the faster Ethernet topologies. Most Ethernet networks support speeds of 10 Mbps and 100 Mbps.
The new generation of multimedia, imaging, and database products can easily overwhelm a network that operates at traditional Ethernet speeds of 10 and 100 Mbps. Network administrators may choose to provide Gigabit Ethernet from the backbone to the end user. Installation costs for new cables and adapters can make this prohibitive.
There are several ways that Ethernet technologies can be used in a campus network:
• An Ethernet speed of 10 Mbps can be used at the user level to provide good performance. Clients or servers that require more bandwidth can use 100-Mbps Ethernet.
• Fast Ethernet is used as the link between user and network devices. It can support the combination of all traffic from each Ethernet segment.
• Fast Ethernet can be used to connect enterprise servers. This will enhance client-server performance across the campus network and help prevent bottlenecks.
• Fast Ethernet or Gigabit Ethernet should be implemented between backbone devices, based on affordability.
The media and connector requirements for an Ethernet implementation are discussed on the next page.
5.1.1 This page describes the LAN physical layer.
Various symbols are used to represent media types. Token Ring is represented by a circle. FDDI is represented by two concentric circles and the Ethernet symbol is represented by a straight line. Serial connections are represented by a lightning bolt.
Each computer network can be built with many different media types. The function of media is to carry a flow of information through a LAN. Wireless LANs use the atmosphere, or space, as the medium. Other networking media confine network signals to a wire, cable, or fiber. Networking media are considered Layer 1, or physical layer, components of LANs.
Each type of media has advantages and disadvantages. These are based on the following factors:
• Cable length
• Cost
• Ease of installation
• Susceptibility to interference
Coaxial cable, optical fiber, and space can carry network signals. This module will focus on Category 5 UTP, which includes the Category 5e family of cables.
Many topologies support LANs, as well as many different physical media. Figure shows a subset of physical layer implementations that can be deployed to support Ethernet.
The next page explains how Ethernet is implemented in a campus environment.
Ethernet in the campus
5.1.2 This page will discuss Ethernet.
Ethernet is the most widely used LAN technology. Ethernet was first implemented by the Digital, Intel, and Xerox group (DIX). DIX created and implemented the first Ethernet LAN specification, which was used as the basis for the Institute of Electrical and Electronics Engineers (IEEE) 802.3 specification, released in 1980. IEEE extended 802.3 to three new committees known as 802.3u for Fast Ethernet, 802.3z for Gigabit Ethernet over fiber, and 802.3ab for Gigabit Ethernet over UTP.
A network may require an upgrade to one of the faster Ethernet topologies. Most Ethernet networks support speeds of 10 Mbps and 100 Mbps.
The new generation of multimedia, imaging, and database products can easily overwhelm a network that operates at traditional Ethernet speeds of 10 and 100 Mbps. Network administrators may choose to provide Gigabit Ethernet from the backbone to the end user. Installation costs for new cables and adapters can make this prohibitive.
There are several ways that Ethernet technologies can be used in a campus network:
• An Ethernet speed of 10 Mbps can be used at the user level to provide good performance. Clients or servers that require more bandwidth can use 100-Mbps Ethernet.
• Fast Ethernet is used as the link between user and network devices. It can support the combination of all traffic from each Ethernet segment.
• Fast Ethernet can be used to connect enterprise servers. This will enhance client-server performance across the campus network and help prevent bottlenecks.
• Fast Ethernet or Gigabit Ethernet should be implemented between backbone devices, based on affordability.
The media and connector requirements for an Ethernet implementation are discussed on the next page.
Module 5 : Cabling LANs and WANs Overview
Cabling LANs and WANs
Overview
Even though each LAN is unique, there are many design aspects that are common to all LANs. For example, most LANs follow the same standards and use the same components. This module presents information on elements of Ethernet LANs and common LAN devices.
There are several types of WAN connections. They range from dial-up to broadband access and differ in bandwidth, cost, and required equipment. This module presents information on the various types of WAN connections.
This module covers some of the objectives for the CCNA 640-801, INTRO 640-821, and ICND 640-811 exams.
Students who complete this module should be able to perform the following tasks:
• Identify characteristics of Ethernet networks
• Identify straight-through, crossover, and rollover cables
• Describe the function, advantages, and disadvantages of repeaters, hubs, bridges, switches, and wireless network components
• Describe the function of peer-to-peer networks
• Describe the function, advantages, and disadvantages of client-server networks
• Describe and differentiate between serial, ISDN, DSL, and cable modem WAN connections
• Identify router serial ports, cables, and connectors
• Identify and describe the placement of equipment used in various WAN configurations
Overview
Even though each LAN is unique, there are many design aspects that are common to all LANs. For example, most LANs follow the same standards and use the same components. This module presents information on elements of Ethernet LANs and common LAN devices.
There are several types of WAN connections. They range from dial-up to broadband access and differ in bandwidth, cost, and required equipment. This module presents information on the various types of WAN connections.
This module covers some of the objectives for the CCNA 640-801, INTRO 640-821, and ICND 640-811 exams.
Students who complete this module should be able to perform the following tasks:
• Identify characteristics of Ethernet networks
• Identify straight-through, crossover, and rollover cables
• Describe the function, advantages, and disadvantages of repeaters, hubs, bridges, switches, and wireless network components
• Describe the function of peer-to-peer networks
• Describe the function, advantages, and disadvantages of client-server networks
• Describe and differentiate between serial, ISDN, DSL, and cable modem WAN connections
• Identify router serial ports, cables, and connectors
• Identify and describe the placement of equipment used in various WAN configurations
Thursday, December 24, 2009
Summary of Module 4
Summary
Data symbolizing characters, words, pictures, video, or music can be represented electrically by voltage patterns on wires and in electronic devices. The data represented by these voltage patterns can be converted to light waves or radio waves, and then back to voltage patterns. Waves are energy traveling from one place to another, and are created by disturbances. All waves have similar attributes such as amplitude, period, and frequency. Sine waves are periodic, continuously varying functions. Analog signals look like sine waves. Square waves are periodic functions whose values remain constant for a period of time and then change abruptly. Digital signals look like square waves.
Exponents are used to represent very large or very small numbers. The base of a number raised to a positive exponent is equal to the base multiplied by itself exponent times. For example, 103 = 10x10x10 = 1000. Logarithms are similar to exponents. A logarithm to the base of 10 of a number equals the exponent to which 10 would have to be raised in order to equal the number. For example, log10 1000 = 3 because 103 = 1000.
Decibels are measurements of a gain or loss in the power of a signal. Negative values represent losses and positive values represent gains. Time and frequency analysis can both be used to graph the voltage or power of a signal.
Undesirable signals in a communications system are called noise. Noise originates from other cables, radio frequency interference (RFI), and electromagnetic interference (EMI). Noise may affect all signal frequencies or a subset of frequencies.
Analog bandwidth is the frequency range that is associated with certain analog transmission, such as television or FM radio. Digital bandwidth measures how much information can flow from one place to another in a given amount of time. Its units are in various multiples of bits per second.
On copper cable, data signals are represented by voltage levels that correspond to binary ones and zeros. In order for the LAN to operate properly, the receiving device must be able to accurately interpret the bit signal. Proper cable installation according to standards increases LAN reliability and performance.
Signal degradation is due to various factors such as attenuation, impedance mismatch, noise, and several types of crosstalk. Attenuation is the decrease in signal amplitude over the length of a link. Impedance is a measurement of resistance to the electrical signal. Cables and the connectors used on them must have similar impedance values or some of the data signal may be reflected back from a connector. This is referred to as impedance mismatch or impedance discontinuity. Noise is any electrical energy on the transmission cable that makes it difficult for a receiver to interpret the data sent from the transmitter. Crosstalk involves the transmission of signals from one wire to a nearby wire. There are three distinct types of crosstalk: Near-end Crosstalk (NEXT), Far-end Crosstalk (FEXT), Power Sum Near-end Crosstalk (PSNEXT).
STP and UTP cable are designed to take advantage of the effects of crosstalk in order to minimize noise. Additionally, STP contains an outer conductive shield and inner foil shields that make it less susceptible to noise. UTP contains no shielding and is more susceptible to external noise but is the most frequently used because it is inexpensive and easier to install.
Fiber-optic cable is used to transmit data signals by increasing and decreasing the intensity of light to represent binary ones and zeros. The strength of a light signal does not diminish like the strength of an electrical signal does over an identical run length. Optical signals are not affected by electrical noise, and optical fiber does not need to be grounded. Therefore, optical fiber is often used between buildings and between floors within a building.
The TIA/EIA-568-B standard specifies ten tests that a copper cable must pass if it will be used for modern, high-speed Ethernet LANs. Optical fiber must also be tested according to networking standards. Category 6 cable must meet more rigorous frequency testing standards than Category 5 cable.
Data symbolizing characters, words, pictures, video, or music can be represented electrically by voltage patterns on wires and in electronic devices. The data represented by these voltage patterns can be converted to light waves or radio waves, and then back to voltage patterns. Waves are energy traveling from one place to another, and are created by disturbances. All waves have similar attributes such as amplitude, period, and frequency. Sine waves are periodic, continuously varying functions. Analog signals look like sine waves. Square waves are periodic functions whose values remain constant for a period of time and then change abruptly. Digital signals look like square waves.
Exponents are used to represent very large or very small numbers. The base of a number raised to a positive exponent is equal to the base multiplied by itself exponent times. For example, 103 = 10x10x10 = 1000. Logarithms are similar to exponents. A logarithm to the base of 10 of a number equals the exponent to which 10 would have to be raised in order to equal the number. For example, log10 1000 = 3 because 103 = 1000.
Decibels are measurements of a gain or loss in the power of a signal. Negative values represent losses and positive values represent gains. Time and frequency analysis can both be used to graph the voltage or power of a signal.
Undesirable signals in a communications system are called noise. Noise originates from other cables, radio frequency interference (RFI), and electromagnetic interference (EMI). Noise may affect all signal frequencies or a subset of frequencies.
Analog bandwidth is the frequency range that is associated with certain analog transmission, such as television or FM radio. Digital bandwidth measures how much information can flow from one place to another in a given amount of time. Its units are in various multiples of bits per second.
On copper cable, data signals are represented by voltage levels that correspond to binary ones and zeros. In order for the LAN to operate properly, the receiving device must be able to accurately interpret the bit signal. Proper cable installation according to standards increases LAN reliability and performance.
Signal degradation is due to various factors such as attenuation, impedance mismatch, noise, and several types of crosstalk. Attenuation is the decrease in signal amplitude over the length of a link. Impedance is a measurement of resistance to the electrical signal. Cables and the connectors used on them must have similar impedance values or some of the data signal may be reflected back from a connector. This is referred to as impedance mismatch or impedance discontinuity. Noise is any electrical energy on the transmission cable that makes it difficult for a receiver to interpret the data sent from the transmitter. Crosstalk involves the transmission of signals from one wire to a nearby wire. There are three distinct types of crosstalk: Near-end Crosstalk (NEXT), Far-end Crosstalk (FEXT), Power Sum Near-end Crosstalk (PSNEXT).
STP and UTP cable are designed to take advantage of the effects of crosstalk in order to minimize noise. Additionally, STP contains an outer conductive shield and inner foil shields that make it less susceptible to noise. UTP contains no shielding and is more susceptible to external noise but is the most frequently used because it is inexpensive and easier to install.
Fiber-optic cable is used to transmit data signals by increasing and decreasing the intensity of light to represent binary ones and zeros. The strength of a light signal does not diminish like the strength of an electrical signal does over an identical run length. Optical signals are not affected by electrical noise, and optical fiber does not need to be grounded. Therefore, optical fiber is often used between buildings and between floors within a building.
The TIA/EIA-568-B standard specifies ten tests that a copper cable must pass if it will be used for modern, high-speed Ethernet LANs. Optical fiber must also be tested according to networking standards. Category 6 cable must meet more rigorous frequency testing standards than Category 5 cable.
A new standard
A new standard (Optional)
4.2.9 This page discusses the new test standards for Category 6 cable.
On June 20, 2002, the Category 6 addition to the TIA-568 standard was published. The official title of the standard is ANSI/TIA/EIA-568-B.2-1. This new standard specifies the original set of performance parameters that need to be tested for Ethernet cabling as well as the passing scores for each of these tests. Cables certified as Category 6 cable must pass all ten tests.
Although the Category 6 tests are essentially the same as those specified by the Category 5 standard, Category 6 cable must pass the tests with higher scores to be certified. Category 6 cable must be capable of carrying frequencies up to 250 MHz and must have lower levels of crosstalk and return loss.
A quality cable tester similar to the Fluke DSP-4000 series or Fluke OMNIScanner2 can perform all the test measurements required for Category 5, Category 5e, and Category 6 cable certifications of both permanent links and channel links. Figure shows the Fluke DSP-4100 Cable Analyzer with a DSP-LIA013 Channel/Traffic Adapter for Category 5e.
The Lab Activities will teach students how to use a cable tester.
This page concludes this lesson. The next page will summarize the main points from the module.
4.2.9 This page discusses the new test standards for Category 6 cable.
On June 20, 2002, the Category 6 addition to the TIA-568 standard was published. The official title of the standard is ANSI/TIA/EIA-568-B.2-1. This new standard specifies the original set of performance parameters that need to be tested for Ethernet cabling as well as the passing scores for each of these tests. Cables certified as Category 6 cable must pass all ten tests.
Although the Category 6 tests are essentially the same as those specified by the Category 5 standard, Category 6 cable must pass the tests with higher scores to be certified. Category 6 cable must be capable of carrying frequencies up to 250 MHz and must have lower levels of crosstalk and return loss.
A quality cable tester similar to the Fluke DSP-4000 series or Fluke OMNIScanner2 can perform all the test measurements required for Category 5, Category 5e, and Category 6 cable certifications of both permanent links and channel links. Figure shows the Fluke DSP-4100 Cable Analyzer with a DSP-LIA013 Channel/Traffic Adapter for Category 5e.
The Lab Activities will teach students how to use a cable tester.
This page concludes this lesson. The next page will summarize the main points from the module.
Cable testing standards / Test parameters / Time base parameters / Testing Optical Fiber
Cable testing standards (Core)
4.2.5 This page will describe the TIA/EIA-568-B standard. This standard specifies ten tests that a copper cable must pass if it will be used for modern, high-speed Ethernet LANs.
All cable links should be tested to the maximum rating that applies for the category of cable being installed.
The ten primary test parameters that must be verified for a cable link to meet TIA/EIA standards are:
• Wire map
• Insertion loss
• Near-end crosstalk (NEXT)
• Power sum near-end crosstalk (PSNEXT)
• Equal-level far-end crosstalk (ELFEXT)
• Power sum equal-level far-end crosstalk (PSELFEXT)
• Return loss
• Propagation delay
• Cable length
• Delay skew
The Ethernet standard specifies that each of the pins on an RJ-45 connector have a particular purpose. A NIC transmits signals on pins 1 and 2, and it receives signals on pins 3 and 6. The wires in UTP cable must be connected to the proper pins at each end of a cable. The wire map test insures that no open or short circuits exist on the cable. An open circuit occurs if the wire does not attach properly at the connector. A short circuit occurs if two wires are connected to each other.
The wire map test also verifies that all eight wires are connected to the correct pins on both ends of the cable. There are several different wiring faults that the wire map test can detect. The reversed-pair fault occurs when a wire pair is correctly installed on one connector, but reversed on the other connector. If the white/orange wire is terminated on pin 1 and the orange wire is terminated on pin 2 at one end of a cable, but reversed at the other end, then the cable has a reversed-pair fault. This example is shown in the graphic.
A split-pair wiring fault occurs when one wire from one pair is switched with one wire from a different pair at both ends. Look carefully at the pin numbers in the graphic to detect the wiring fault. A split pair creates two transmit or receive pairs each with two wires that are not twisted together. This mixing hampers the cross-cancellation process and makes the cable more susceptible to crosstalk and interference. Contrast this with a reversed-pair, where the same pair of pins is used at both ends.
Other test parameters (Optional)
4.2.6 This page will explain how cables are tested for crosstalk and attenuation.
The combination of the effects of signal attenuation and impedance discontinuities on a communications link is called insertion loss. Insertion loss is measured in decibels at the far end of the cable. The TIA/EIA standard requires that a cable and its connectors pass an insertion loss test before the cable can be used as a communications link in a LAN.
Crosstalk is measured in four separate tests. A cable tester measures NEXT by applying a test signal to one cable pair and measuring the amplitude of the crosstalk signals received by the other cable pairs. The NEXT value, expressed in decibels, is computed as the difference in amplitude between the test signal and the crosstalk signal measured at the same end of the cable. Remember, because the number of decibels that the tester displays is a negative number, the larger the number, the lower the NEXT on the wire pair. As previously mentioned, the PSNEXT test is actually a calculation based on combined NEXT effects.
The equal-level far-end crosstalk (ELFEXT) test measures FEXT. Pair-to-pair ELFEXT is expressed in dB as the difference between the measured FEXT and the insertion loss of the wire pair whose signal is disturbed by the FEXT. ELFEXT is an important measurement in Ethernet networks using 1000BASE-T technologies. Power sum equal-level far-end crosstalk (PSELFEXT) is the combined effect of ELFEXT from all wire pairs.
Return loss is a measure in decibels of reflections that are caused by the impedance discontinuities at all locations along the link. Recall that the main impact of return loss is not on loss of signal strength. The significant problem is that signal echoes caused by the reflections from the impedance discontinuities will strike the receiver at different intervals causing signal jitter.
Time-based parameters (Optional)
4.2.7 This page will discuss propegation delay and how it is measured.
Propagation delay is a simple measurement of how long it takes for a signal to travel along the cable being tested. The delay in a wire pair depends on its length, twist rate, and electrical properties. Delays are measured in hundredths of nanoseconds. One nanosecond is one-billionth of a second, or 0.000000001 second. The TIA/EIA-568-B standard sets a limit for propagation delay for the various categories of UTP.
Propagation delay measurements are the basis of the cable length measurement. TIA/EIA-568-B.1 specifies that the physical length of the link shall be calculated using the wire pair with the shortest electrical delay. Testers measure the length of the wire based on the electrical delay as measured by a Time Domain Reflectometry (TDR) test, not by the physical length of the cable jacket. Since the wires inside the cable are twisted, signals actually travel farther than the physical length of the cable. When a cable tester makes a TDR measurement, it sends a pulse signal down a wire pair and measures the amount of time required for the pulse to return on the same wire pair.
The TDR test is used not only to determine length, but also to identify the distance to wiring faults such as shorts and opens. When the pulse encounters an open, short, or poor connection, all or part of the pulse energy is reflected back to the tester. This can be used to calculate the approximate distance to the wiring fault. The approximate distance can be helpful in locating a faulty connection point along a cable run, such as a wall jack.
The propagation delays of different wire pairs in a single cable can differ slightly because of differences in the number of twists and electrical properties of each wire pair. The delay difference between pairs is called delay skew. Delay skew is a critical parameter for high-speed networks in which data is simultaneously transmitted over multiple wire pairs, such as 1000BASE-T Ethernet. If the delay skew between the pairs is too great, the bits arrive at different times and the data cannot be properly reassembled. Even though a cable link may not be intended for this type of data transmission, testing for delay skew helps ensure that the link will support future upgrades to high-speed networks.
All cable links in a LAN must pass all of the tests previously mentioned as specified in the TIA/EIA-568-B standard in order to be considered standards compliant. A certification meter must be used to ensure that all of the tests are passed in order to be considered standards compliant. These tests ensure that the cable links will function reliably at high speeds and frequencies. Cable tests should be performed when the cable is installed and afterward on a regular basis to ensure that LAN cabling meets industry standards. High quality cable test instruments should be correctly used to ensure that the tests are accurate. Test results should also be carefully documented.
Testing optical fiber (Optional)
4.2.8 This page will explain how optical fiber is tested.
A fiber link consists of two separate glass fibers functioning as independent data pathways. One fiber carries transmitted signals in one direction, while the second carries signals in the opposite direction. Each glass fiber is surrounded by a sheath that light cannot pass through, so there are no crosstalk problems on fiber optic cable. External electromagnetic interference or noise has no affect on fiber cabling. Attenuation does occur on fiber links, but to a lesser extent than on copper cabling.
Fiber links are subject to the optical equivalent of UTP impedance discontinuities. When light encounters an optical discontinuity, like an impurity in the glass or a micro-fracture, some of the light signal is reflected back in the opposite direction. This means only a fraction of the original light signal will continue down the fiber towards the receiver. This results in a reduced amount of light energy arriving at the receiver, making signal recognition difficult. Just as with UTP cable, improperly installed connectors are the main cause of light reflection and signal strength loss in optical fiber.
Because noise is not an issue when transmitting on optical fiber, the main concern with a fiber link is the strength of the light signal that arrives at the receiver. If attenuation weakens the light signal at the receiver, then data errors will result. Testing fiber optic cable primarily involves shining a light down the fiber and measuring whether a sufficient amount of light reaches the receiver.
On a fiber optic link, the acceptable amount of signal power loss that can occur without dropping below the requirements of the receiver must be calculated. This calculation is referred to as the optical link loss budget. A fiber test instrument, known as a light source and power meter, checks whether the optical link loss budget has been exceeded. If the fiber fails the test, another cable test instrument can be used to indicate where the optical discontinuities occur along the length of the cable link. An optical TDR known as an OTDR is capable of locating these discontinuities. Usually, the problem is one or more improperly attached connectors. The OTDR will indicate the location of the faulty connections that must be replaced. When the faults are corrected, the cable must be retested.
The standards for testing are updated regularly. The next page will introduce a new standard.
4.2.5 This page will describe the TIA/EIA-568-B standard. This standard specifies ten tests that a copper cable must pass if it will be used for modern, high-speed Ethernet LANs.
All cable links should be tested to the maximum rating that applies for the category of cable being installed.
The ten primary test parameters that must be verified for a cable link to meet TIA/EIA standards are:
• Wire map
• Insertion loss
• Near-end crosstalk (NEXT)
• Power sum near-end crosstalk (PSNEXT)
• Equal-level far-end crosstalk (ELFEXT)
• Power sum equal-level far-end crosstalk (PSELFEXT)
• Return loss
• Propagation delay
• Cable length
• Delay skew
The Ethernet standard specifies that each of the pins on an RJ-45 connector have a particular purpose. A NIC transmits signals on pins 1 and 2, and it receives signals on pins 3 and 6. The wires in UTP cable must be connected to the proper pins at each end of a cable. The wire map test insures that no open or short circuits exist on the cable. An open circuit occurs if the wire does not attach properly at the connector. A short circuit occurs if two wires are connected to each other.
The wire map test also verifies that all eight wires are connected to the correct pins on both ends of the cable. There are several different wiring faults that the wire map test can detect. The reversed-pair fault occurs when a wire pair is correctly installed on one connector, but reversed on the other connector. If the white/orange wire is terminated on pin 1 and the orange wire is terminated on pin 2 at one end of a cable, but reversed at the other end, then the cable has a reversed-pair fault. This example is shown in the graphic.
A split-pair wiring fault occurs when one wire from one pair is switched with one wire from a different pair at both ends. Look carefully at the pin numbers in the graphic to detect the wiring fault. A split pair creates two transmit or receive pairs each with two wires that are not twisted together. This mixing hampers the cross-cancellation process and makes the cable more susceptible to crosstalk and interference. Contrast this with a reversed-pair, where the same pair of pins is used at both ends.
Other test parameters (Optional)
4.2.6 This page will explain how cables are tested for crosstalk and attenuation.
The combination of the effects of signal attenuation and impedance discontinuities on a communications link is called insertion loss. Insertion loss is measured in decibels at the far end of the cable. The TIA/EIA standard requires that a cable and its connectors pass an insertion loss test before the cable can be used as a communications link in a LAN.
Crosstalk is measured in four separate tests. A cable tester measures NEXT by applying a test signal to one cable pair and measuring the amplitude of the crosstalk signals received by the other cable pairs. The NEXT value, expressed in decibels, is computed as the difference in amplitude between the test signal and the crosstalk signal measured at the same end of the cable. Remember, because the number of decibels that the tester displays is a negative number, the larger the number, the lower the NEXT on the wire pair. As previously mentioned, the PSNEXT test is actually a calculation based on combined NEXT effects.
The equal-level far-end crosstalk (ELFEXT) test measures FEXT. Pair-to-pair ELFEXT is expressed in dB as the difference between the measured FEXT and the insertion loss of the wire pair whose signal is disturbed by the FEXT. ELFEXT is an important measurement in Ethernet networks using 1000BASE-T technologies. Power sum equal-level far-end crosstalk (PSELFEXT) is the combined effect of ELFEXT from all wire pairs.
Return loss is a measure in decibels of reflections that are caused by the impedance discontinuities at all locations along the link. Recall that the main impact of return loss is not on loss of signal strength. The significant problem is that signal echoes caused by the reflections from the impedance discontinuities will strike the receiver at different intervals causing signal jitter.
Time-based parameters (Optional)
4.2.7 This page will discuss propegation delay and how it is measured.
Propagation delay is a simple measurement of how long it takes for a signal to travel along the cable being tested. The delay in a wire pair depends on its length, twist rate, and electrical properties. Delays are measured in hundredths of nanoseconds. One nanosecond is one-billionth of a second, or 0.000000001 second. The TIA/EIA-568-B standard sets a limit for propagation delay for the various categories of UTP.
Propagation delay measurements are the basis of the cable length measurement. TIA/EIA-568-B.1 specifies that the physical length of the link shall be calculated using the wire pair with the shortest electrical delay. Testers measure the length of the wire based on the electrical delay as measured by a Time Domain Reflectometry (TDR) test, not by the physical length of the cable jacket. Since the wires inside the cable are twisted, signals actually travel farther than the physical length of the cable. When a cable tester makes a TDR measurement, it sends a pulse signal down a wire pair and measures the amount of time required for the pulse to return on the same wire pair.
The TDR test is used not only to determine length, but also to identify the distance to wiring faults such as shorts and opens. When the pulse encounters an open, short, or poor connection, all or part of the pulse energy is reflected back to the tester. This can be used to calculate the approximate distance to the wiring fault. The approximate distance can be helpful in locating a faulty connection point along a cable run, such as a wall jack.
The propagation delays of different wire pairs in a single cable can differ slightly because of differences in the number of twists and electrical properties of each wire pair. The delay difference between pairs is called delay skew. Delay skew is a critical parameter for high-speed networks in which data is simultaneously transmitted over multiple wire pairs, such as 1000BASE-T Ethernet. If the delay skew between the pairs is too great, the bits arrive at different times and the data cannot be properly reassembled. Even though a cable link may not be intended for this type of data transmission, testing for delay skew helps ensure that the link will support future upgrades to high-speed networks.
All cable links in a LAN must pass all of the tests previously mentioned as specified in the TIA/EIA-568-B standard in order to be considered standards compliant. A certification meter must be used to ensure that all of the tests are passed in order to be considered standards compliant. These tests ensure that the cable links will function reliably at high speeds and frequencies. Cable tests should be performed when the cable is installed and afterward on a regular basis to ensure that LAN cabling meets industry standards. High quality cable test instruments should be correctly used to ensure that the tests are accurate. Test results should also be carefully documented.
Testing optical fiber (Optional)
4.2.8 This page will explain how optical fiber is tested.
A fiber link consists of two separate glass fibers functioning as independent data pathways. One fiber carries transmitted signals in one direction, while the second carries signals in the opposite direction. Each glass fiber is surrounded by a sheath that light cannot pass through, so there are no crosstalk problems on fiber optic cable. External electromagnetic interference or noise has no affect on fiber cabling. Attenuation does occur on fiber links, but to a lesser extent than on copper cabling.
Fiber links are subject to the optical equivalent of UTP impedance discontinuities. When light encounters an optical discontinuity, like an impurity in the glass or a micro-fracture, some of the light signal is reflected back in the opposite direction. This means only a fraction of the original light signal will continue down the fiber towards the receiver. This results in a reduced amount of light energy arriving at the receiver, making signal recognition difficult. Just as with UTP cable, improperly installed connectors are the main cause of light reflection and signal strength loss in optical fiber.
Because noise is not an issue when transmitting on optical fiber, the main concern with a fiber link is the strength of the light signal that arrives at the receiver. If attenuation weakens the light signal at the receiver, then data errors will result. Testing fiber optic cable primarily involves shining a light down the fiber and measuring whether a sufficient amount of light reaches the receiver.
On a fiber optic link, the acceptable amount of signal power loss that can occur without dropping below the requirements of the receiver must be calculated. This calculation is referred to as the optical link loss budget. A fiber test instrument, known as a light source and power meter, checks whether the optical link loss budget has been exceeded. If the fiber fails the test, another cable test instrument can be used to indicate where the optical discontinuities occur along the length of the cable link. An optical TDR known as an OTDR is capable of locating these discontinuities. Usually, the problem is one or more improperly attached connectors. The OTDR will indicate the location of the faulty connections that must be replaced. When the faults are corrected, the cable must be retested.
The standards for testing are updated regularly. The next page will introduce a new standard.
Attenuation / Sources of noise / Types of crosstalk (Core)
Attenuation and insertion loss on copper media (Core)
4.2.2 This page explains insertion loss caused by signal attenuation and impedance discontinuities.
Attenuation is the decrease in signal amplitude over the length of a link. Long cable lengths and high signal frequencies contribute to greater signal attenuation. For this reason, attenuation on a cable is measured by a cable tester with the highest frequencies that the cable is rated to support. Attenuation is expressed in dBs with negative numbers. Smaller negative dB values are an indication of better link performance.
There are several factors that contribute to attenuation. The resistance of the copper cable converts some of the electrical energy of the signal to heat. Signal energy is also lost when it leaks through the insulation of the cable and by impedance caused by defective connectors.
Impedance is a measurement of the resistance of the cable to alternating current (AC) and is measured in ohms. The normal impedance of a Category 5 cable is 100 ohms. If a connector is improperly installed on Category 5, it will have a different impedance value than the cable. This is called an impedance discontinuity or an impedance mismatch.
Impedance discontinuities cause attenuation because a portion of a transmitted signal is reflected back, like an echo, and does not reach the receiver. This effect is compounded if multiple discontinuities cause additional portions of the signal to be reflected back to the transmitter. When the reflected signal strikes the first discontinuity, some of the signal rebounds in the original direction, which creates multiple echo effects. The echoes strike the receiver at different intervals. This makes it difficult for the receiver to detect data values. This is called jitter and results in data errors.
The combination of the effects of signal attenuation and impedance discontinuities on a communications link is called insertion loss. Proper network operation depends on constant characteristic impedance in all cables and connectors, with no impedance discontinuities in the entire cable system.
The next page will discuss sources of noise on copper cable.
Sources of noise on copper media (Core)
4.2.3 This page will describe the sources of noise on copper cables.
Noise is any electrical energy on the transmission cable that makes it difficult for a receiver to interpret the data sent from the transmitter. TIA/EIA-568-B certification now requires cables to be tested for a variety of types of noise.
Crosstalk involves the transmission of signals from one wire to a nearby wire. When voltages change on a wire, electromagnetic energy is generated. This energy radiates outward from the wire like a radio signal from a transmitter. Adjacent wires in the cable act like antennas and receive the transmitted energy, which interferes with data on those wires. Crosstalk can also be caused by signals on separate, nearby cables. When crosstalk is caused by a signal on another cable, it is called alien crosstalk. Crosstalk is more destructive at higher transmission frequencies.
Cable testing instruments measure crosstalk by applying a test signal to one wire pair. The cable tester then measures the amplitude of the unwanted crosstalk signals on the other wire pairs in the cable.
Twisted-pair cable is designed to take advantage of the effects of crosstalk in order to minimize noise. In twisted-pair cable, a pair of wires is used to transmit one signal. The wire pair is twisted so that each wire experiences similar crosstalk. Because a noise signal on one wire will appear identically on the other wire, this noise be easily detected and filtered at the receiver.
Twisted wire pairs in a cable are also more resistant to crosstalk or noise signals from adjacent wire pairs. Higher categories of UTP require more twists on each wire pair in the cable to minimize crosstalk at high transmission frequencies. When connectors are attached to the ends of UTP cable, the wire pairs should be untwisted as little as possible to ensure reliable LAN communications.
The next page will explain the three types of crosstalk
Types of crosstalk (Core)
4.2.4 This page defines the three types of crosstalk:
• Near-end Crosstalk (NEXT)
• Far-end Crosstalk (FEXT)
• Power Sum Near-end Crosstalk (PSNEXT)
Near-end crosstalk (NEXT) is computed as the ratio of voltage amplitude between the test signal and the crosstalk signal when measured from the same end of the link. This difference is expressed in a negative value of decibels (dB). Low negative numbers indicate more noise, just as low negative temperatures indicate more heat. By tradition, cable testers do not show the minus sign indicating the negative NEXT values. A NEXT reading of 30 dB (which actually indicates -30 dB) indicates less NEXT noise and a cleaner signal than does a NEXT reading of 10 dB.
NEXT needs to be measured from each pair to each other pair in a UTP link, and from both ends of the link. To shorten test times, some cable test instruments allow the user to test the NEXT performance of a link by using larger frequency step sizes than specified by the TIA/EIA standard. The resulting measurements may not comply with TIA/EIA-568-B, and may overlook link faults. To verify proper link performance, NEXT should be measured from both ends of the link with a high-quality test instrument. This is also a requirement for complete compliance with high-speed cable specifications.
Due to attenuation, crosstalk occurring further away from the transmitter creates less noise on a cable than NEXT. This is called far-end crosstalk, or FEXT. The noise caused by FEXT still travels back to the source, but it is attenuated as it returns. Thus, FEXT is not as significant a problem as NEXT.
Power Sum NEXT (PSNEXT) measures the cumulative effect of NEXT from all wire pairs in the cable. PSNEXT is computed for each wire pair based on the NEXT effects of the other three pairs. The combined effect of crosstalk from multiple simultaneous transmission sources can be very detrimental to the signal. TIA/EIA-568-B certification now requires this PSNEXT test.
Some Ethernet standards such as 10BASE-T and 100BASE-TX receive data from only one wire pair in each direction. However, for newer technologies such as 1000BASE-T that receive data simultaneously from multiple pairs in the same direction, power sum measurements are very important tests.
The next page will discuss cable testing standards.
4.2.2 This page explains insertion loss caused by signal attenuation and impedance discontinuities.
Attenuation is the decrease in signal amplitude over the length of a link. Long cable lengths and high signal frequencies contribute to greater signal attenuation. For this reason, attenuation on a cable is measured by a cable tester with the highest frequencies that the cable is rated to support. Attenuation is expressed in dBs with negative numbers. Smaller negative dB values are an indication of better link performance.
There are several factors that contribute to attenuation. The resistance of the copper cable converts some of the electrical energy of the signal to heat. Signal energy is also lost when it leaks through the insulation of the cable and by impedance caused by defective connectors.
Impedance is a measurement of the resistance of the cable to alternating current (AC) and is measured in ohms. The normal impedance of a Category 5 cable is 100 ohms. If a connector is improperly installed on Category 5, it will have a different impedance value than the cable. This is called an impedance discontinuity or an impedance mismatch.
Impedance discontinuities cause attenuation because a portion of a transmitted signal is reflected back, like an echo, and does not reach the receiver. This effect is compounded if multiple discontinuities cause additional portions of the signal to be reflected back to the transmitter. When the reflected signal strikes the first discontinuity, some of the signal rebounds in the original direction, which creates multiple echo effects. The echoes strike the receiver at different intervals. This makes it difficult for the receiver to detect data values. This is called jitter and results in data errors.
The combination of the effects of signal attenuation and impedance discontinuities on a communications link is called insertion loss. Proper network operation depends on constant characteristic impedance in all cables and connectors, with no impedance discontinuities in the entire cable system.
The next page will discuss sources of noise on copper cable.
Sources of noise on copper media (Core)
4.2.3 This page will describe the sources of noise on copper cables.
Noise is any electrical energy on the transmission cable that makes it difficult for a receiver to interpret the data sent from the transmitter. TIA/EIA-568-B certification now requires cables to be tested for a variety of types of noise.
Crosstalk involves the transmission of signals from one wire to a nearby wire. When voltages change on a wire, electromagnetic energy is generated. This energy radiates outward from the wire like a radio signal from a transmitter. Adjacent wires in the cable act like antennas and receive the transmitted energy, which interferes with data on those wires. Crosstalk can also be caused by signals on separate, nearby cables. When crosstalk is caused by a signal on another cable, it is called alien crosstalk. Crosstalk is more destructive at higher transmission frequencies.
Cable testing instruments measure crosstalk by applying a test signal to one wire pair. The cable tester then measures the amplitude of the unwanted crosstalk signals on the other wire pairs in the cable.
Twisted-pair cable is designed to take advantage of the effects of crosstalk in order to minimize noise. In twisted-pair cable, a pair of wires is used to transmit one signal. The wire pair is twisted so that each wire experiences similar crosstalk. Because a noise signal on one wire will appear identically on the other wire, this noise be easily detected and filtered at the receiver.
Twisted wire pairs in a cable are also more resistant to crosstalk or noise signals from adjacent wire pairs. Higher categories of UTP require more twists on each wire pair in the cable to minimize crosstalk at high transmission frequencies. When connectors are attached to the ends of UTP cable, the wire pairs should be untwisted as little as possible to ensure reliable LAN communications.
The next page will explain the three types of crosstalk
Types of crosstalk (Core)
4.2.4 This page defines the three types of crosstalk:
• Near-end Crosstalk (NEXT)
• Far-end Crosstalk (FEXT)
• Power Sum Near-end Crosstalk (PSNEXT)
Near-end crosstalk (NEXT) is computed as the ratio of voltage amplitude between the test signal and the crosstalk signal when measured from the same end of the link. This difference is expressed in a negative value of decibels (dB). Low negative numbers indicate more noise, just as low negative temperatures indicate more heat. By tradition, cable testers do not show the minus sign indicating the negative NEXT values. A NEXT reading of 30 dB (which actually indicates -30 dB) indicates less NEXT noise and a cleaner signal than does a NEXT reading of 10 dB.
NEXT needs to be measured from each pair to each other pair in a UTP link, and from both ends of the link. To shorten test times, some cable test instruments allow the user to test the NEXT performance of a link by using larger frequency step sizes than specified by the TIA/EIA standard. The resulting measurements may not comply with TIA/EIA-568-B, and may overlook link faults. To verify proper link performance, NEXT should be measured from both ends of the link with a high-quality test instrument. This is also a requirement for complete compliance with high-speed cable specifications.
Due to attenuation, crosstalk occurring further away from the transmitter creates less noise on a cable than NEXT. This is called far-end crosstalk, or FEXT. The noise caused by FEXT still travels back to the source, but it is attenuated as it returns. Thus, FEXT is not as significant a problem as NEXT.
Power Sum NEXT (PSNEXT) measures the cumulative effect of NEXT from all wire pairs in the cable. PSNEXT is computed for each wire pair based on the NEXT effects of the other three pairs. The combined effect of crosstalk from multiple simultaneous transmission sources can be very detrimental to the signal. TIA/EIA-568-B certification now requires this PSNEXT test.
Some Ethernet standards such as 10BASE-T and 100BASE-TX receive data from only one wire pair in each direction. However, for newer technologies such as 1000BASE-T that receive data simultaneously from multiple pairs in the same direction, power sum measurements are very important tests.
The next page will discuss cable testing standards.
Signals over copper and fiber optic cables (Core)
4.2.1 This page discusses signals over copper and fiber optic cables.
On copper cable, data signals are represented by voltage levels that represent binary ones and zeros. The voltage levels are measured based on a reference level of 0 volts at both the transmitter and the receiver. This reference level is called the signal ground. It is important for devices that transmit and receive data to have the same 0-volt reference point. When they do, they are said to be properly grounded.
For a LAN to operate properly, the devices that receive data must be able to accurately interpret the binary ones and zeros transmitted as voltage levels. Since current Ethernet technology supports data rates of billions of bps, each bit must be recognized and the duration of each bit is very small. This means that as much of the original signal strength as possible must be retained, as the signal moves through the cable and passes through the connectors. In anticipation of faster Ethernet protocols, new cable installations should be made with the best cable, connectors, and interconnect devices such as punch-down blocks and patch panels.
The two basic types of copper cable are shielded and unshielded. In shielded cable, shielding material protects the data signal from external sources of noise and from noise generated by electrical signals within the cable.
Coaxial cable is a type of shielded cable. It consists of a solid copper conductor surrounded by insulating material and a braided conductive shield. In LAN applications, the braided shielding is electrically grounded to protect the inner conductor from external electrical noise. The shield also keeps the transmitted signal confined to the cable, which reduces signal loss. This helps make coaxial cable less noisy than other types of copper cabling, but also makes it more expensive. The need to ground the shielding and the bulky size of coaxial cable make it more difficult to install than other copper cabling.
Two types of twisted-pair cable are shielded twisted-pair (STP) and unshielded twisted pair (UTP).
STP cable contains an outer conductive shield that is electrically grounded to insulate the signals from external electrical noise. STP also uses inner foil shields to protect each wire pair from noise generated by the other pairs. STP cable is sometimes called screened twisted pair (ScTP) in error. ScTP generally refers to Category 5 or Category 5e twisted pair cabling, while STP refers to an IBM specific cable containing only two pairs of conductors. ScTP cable is more expensive, more difficult to install, and less frequently used than UTP. UTP contains no shielding and is more susceptible to external noise but is the most frequently used because it is inexpensive and easier to install.
Fiber-optic cable increases and decreases the intensity of light to represent binary ones and zeros in data transmissions. The strength of a light signal does not diminish as much as the strength of an electrical signal does over an identical run length. Optical signals are not affected by electrical noise and optical fiber does not need to be grounded unless the jacket contains a metal or a metalized strength member. Therefore, optical fiber is often used between buildings and between floors within a building. As costs decrease and speeds increase, optical fiber may become a more commonly used LAN media.
The next page explains the concept of insertion loss.
Bandwidth
4.1.8 This page will describe bandwidth, which is an extremely important concept in networks.
Two types of bandwidth that are important for the study of LANs are analog and digital.
Analog bandwidth typically refers to the frequency range of an analog electronic system. Analog bandwidth could be used to describe the range of frequencies transmitted by a radio station or an electronic amplifier. The unit of measurement for analog bandwidth is hertz (Hz), the same as the unit of frequency.
Digital bandwidth measures how much information can flow from one place to another in a given amount of time. The fundamental unit of measurement for digital bandwidth is bps. Since LANs are capable of speeds of thousands or millions of bits per second, measurement is expressed in kbps or Mbps. Physical media, current technologies, and the laws of physics limit bandwidth.
During cable testing, analog bandwidth is used to determine the digital bandwidth of a copper cable. The digital waveforms are made up of many sinewaves (analog waves). Analog frequencies are transmitted from one end and received on the opposite end. The two signals are then compared, and the amount of attenuation of the signal is calculated. In general, media that will support higher analog bandwidths without high degrees of attenuation will also support higher digital bandwidths.
This page concludes this lesson. The next lesson will discuss signals and noise. The first page describes copper and fiber optic cables.
Noise in time and frequency (Optional)
4.1.7 Noise is an important concept in networks such as LANs. Noise usually refers to sounds. However, noise related to communications refers to undesirable signals. Noise can originate from natural or technological sources and is added to the data signals in communications systems.
All communications systems have some amount of noise. Even though noise cannot be eliminated, its effects can be minimized if the sources of the noise are understood. There are many possible sources of noise:
• Nearby cables that carry data signals
• RFI from other signals that are transmitted nearby
• EMI from nearby sources such as motors and lights
• Laser noise at the transmitter or receiver of an optical signal
Noise that affects all transmission frequencies equally is called white noise. Noise that only affects small ranges of frequencies is called narrowband interference. White noise on a radio receiver would interfere with all radio stations. Narrowband interference would affect only a few stations whose frequencies are close together. When detected on a LAN, white noise could affect all data transmissions, but narrowband interference might disrupt only certain signals.
The Interactive Media Activity will allow students to generate white noise and narrowband noise.
The next page will describe analog bandwidth and digital bandwidth.
Thursday, December 17, 2009
Decibels (Optional) / Time and frequency of signals (Optional) / Analog and digital signals (Core)
Decibels (Optional)
4.1.4 The study of logarithms is beyond the scope of this course. However, the terminology is often used to calculate decibels and measure signals on copper, optical, and wireless media. The decibel is related to the exponents and logarithms described in prior sections. There are two formulas that are used to calculate decibels:
dB = 10 log10 (Pfinal / Pref)
dB = 20 log10 (Vfinal / Vref)
In these formulas, dB represents the loss or gain of the power of a wave. Decibels can be negative values which would represent a loss in power as the wave travels or a positive value to represent a gain in power if the signal is amplified.
The log10 variable implies that the number in parentheses will be transformed with the base 10 logarithm rule.
Pfinal is the delivered power measured in watts.
Pref is the original power measured in watts.
Vfinal is the delivered voltage measured in volts.
Vref is the original voltage measured in volts.
The first formula describes decibels in terms of power (P), and the second in terms of voltage (V). The power formula is often used to measure light waves on optical fiber and radio waves in the air. The voltage formula is used to measure electromagnetic waves on copper cables. These formulas have several things in common.
In the formula dB = 10 log10 (Pfinal / Pref), enter values for dB and Pref to discover the delivered power. This formula could be used to see how much power is left in a radio wave after it travels through different materials and stages of electronic systems such as radios. Try the following examples with the Interactive Media Activities:
• If the source power of the original laser, or Pref is seven microwatts (1 x 10-6 Watts), and the total loss of a fiber link is 13 dB, how much power is delivered?
• If the total loss of a fiber link is 84 dB and the source power of the original laser, or Pref is 1 milliwatt, how much power is delivered?
• If 2 microvolts, or 2 x 10-6 volts, are measured at the end of a cable and the source voltage was 1 volt, what is the gain or loss in decibels? Is this value positive or negative? Does the value represent a gain or a loss in voltage?
The next page will explain how an oscilloscope is used to analyze and view signals.
Time and frequency of signals (Optional)
4.1.5 One of the most important facts of the information age is that characters, words, pictures, video, or music can be represented electrically by voltage patterns on wires and in electronic devices. The data represented by these voltage patterns can be converted to light waves or radio waves, and then back to voltage waves. Consider the example of an analog telephone. The sound waves of the caller’s voice enter a microphone in the telephone. The microphone converts the patterns of sound energy into voltage patterns of electrical energy that represent the voice.
If the voltage is graphed over time, the patterns that represent the voice will be displayed. An oscilloscope is an important electronic device used to view electrical signals such as voltage waves and pulses. The x-axis on the display represents time and the y-axis represents voltage or current. There are usually two y-axis inputs, so two waves can be observed and measured at the same time.
The analysis of signals with an oscilloscope is called time-domain analysis. The x-axis or domain of the mathematical function represents time. Engineers also use frequency-domain analysis to study signals. In frequency-domain analysis, the x-axis represents frequency. An electronic device called a spectrum analyzer creates graphs for frequency-domain analysis.
Electromagnetic signals use different frequencies for transmission so that different signals do not interfere with each other. Frequency modulation (FM) radio signals use frequencies that are different from television or satellite signals. When listeners change the station on a radio, they change the frequency that the radio receives.
The next page examines the variations of network signals.
Analog and digital signals (Core)
4.1.6 This page will explain how analog signals vary with time and with frequency.
First, consider a single-frequency electrical sine wave, whose frequency can be detected by the human ear. If this signal is transmitted to a speaker, a tone can be heard.
Next, imagine the combination of several sine waves. This will create a wave that is more complex than a pure sine wave. This wave will include several tones. A graph of the tones will show several lines that correspond to the frequency of each tone.
Finally, imagine a complex signal, like a voice or a musical instrument. If many different tones are present, the graph will show a continuous spectrum of individual tones.
The Interactive Media Activity draws sine waves and complex waves based on amplitude, frequency, and the phase.
The next page will discuss noise.
4.1.4 The study of logarithms is beyond the scope of this course. However, the terminology is often used to calculate decibels and measure signals on copper, optical, and wireless media. The decibel is related to the exponents and logarithms described in prior sections. There are two formulas that are used to calculate decibels:
dB = 10 log10 (Pfinal / Pref)
dB = 20 log10 (Vfinal / Vref)
In these formulas, dB represents the loss or gain of the power of a wave. Decibels can be negative values which would represent a loss in power as the wave travels or a positive value to represent a gain in power if the signal is amplified.
The log10 variable implies that the number in parentheses will be transformed with the base 10 logarithm rule.
Pfinal is the delivered power measured in watts.
Pref is the original power measured in watts.
Vfinal is the delivered voltage measured in volts.
Vref is the original voltage measured in volts.
The first formula describes decibels in terms of power (P), and the second in terms of voltage (V). The power formula is often used to measure light waves on optical fiber and radio waves in the air. The voltage formula is used to measure electromagnetic waves on copper cables. These formulas have several things in common.
In the formula dB = 10 log10 (Pfinal / Pref), enter values for dB and Pref to discover the delivered power. This formula could be used to see how much power is left in a radio wave after it travels through different materials and stages of electronic systems such as radios. Try the following examples with the Interactive Media Activities:
• If the source power of the original laser, or Pref is seven microwatts (1 x 10-6 Watts), and the total loss of a fiber link is 13 dB, how much power is delivered?
• If the total loss of a fiber link is 84 dB and the source power of the original laser, or Pref is 1 milliwatt, how much power is delivered?
• If 2 microvolts, or 2 x 10-6 volts, are measured at the end of a cable and the source voltage was 1 volt, what is the gain or loss in decibels? Is this value positive or negative? Does the value represent a gain or a loss in voltage?
The next page will explain how an oscilloscope is used to analyze and view signals.
4.1.5 One of the most important facts of the information age is that characters, words, pictures, video, or music can be represented electrically by voltage patterns on wires and in electronic devices. The data represented by these voltage patterns can be converted to light waves or radio waves, and then back to voltage waves. Consider the example of an analog telephone. The sound waves of the caller’s voice enter a microphone in the telephone. The microphone converts the patterns of sound energy into voltage patterns of electrical energy that represent the voice.
If the voltage is graphed over time, the patterns that represent the voice will be displayed. An oscilloscope is an important electronic device used to view electrical signals such as voltage waves and pulses. The x-axis on the display represents time and the y-axis represents voltage or current. There are usually two y-axis inputs, so two waves can be observed and measured at the same time.
The analysis of signals with an oscilloscope is called time-domain analysis. The x-axis or domain of the mathematical function represents time. Engineers also use frequency-domain analysis to study signals. In frequency-domain analysis, the x-axis represents frequency. An electronic device called a spectrum analyzer creates graphs for frequency-domain analysis.
Electromagnetic signals use different frequencies for transmission so that different signals do not interfere with each other. Frequency modulation (FM) radio signals use frequencies that are different from television or satellite signals. When listeners change the station on a radio, they change the frequency that the radio receives.
The next page examines the variations of network signals.
4.1.6 This page will explain how analog signals vary with time and with frequency.
First, consider a single-frequency electrical sine wave, whose frequency can be detected by the human ear. If this signal is transmitted to a speaker, a tone can be heard.
Next, imagine the combination of several sine waves. This will create a wave that is more complex than a pure sine wave. This wave will include several tones. A graph of the tones will show several lines that correspond to the frequency of each tone.
Finally, imagine a complex signal, like a voice or a musical instrument. If many different tones are present, the graph will show a continuous spectrum of individual tones.
The Interactive Media Activity draws sine waves and complex waves based on amplitude, frequency, and the phase.
The next page will discuss noise.
Decibels (Optional)
Decibels (Optional)
4.1.4 The study of logarithms is beyond the scope of this course. However, the terminology is often used to calculate decibels and measure signals on copper, optical, and wireless media. The decibel is related to the exponents and logarithms described in prior sections. There are two formulas that are used to calculate decibels:
dB = 10 log10 (Pfinal / Pref)
dB = 20 log10 (Vfinal / Vref)
In these formulas, dB represents the loss or gain of the power of a wave. Decibels can be negative values which would represent a loss in power as the wave travels or a positive value to represent a gain in power if the signal is amplified.
The log10 variable implies that the number in parentheses will be transformed with the base 10 logarithm rule.
Pfinal is the delivered power measured in watts.
Pref is the original power measured in watts.
Vfinal is the delivered voltage measured in volts.
Vref is the original voltage measured in volts.
The first formula describes decibels in terms of power (P), and the second in terms of voltage (V). The power formula is often used to measure light waves on optical fiber and radio waves in the air. The voltage formula is used to measure electromagnetic waves on copper cables. These formulas have several things in common.
In the formula dB = 10 log10 (Pfinal / Pref), enter values for dB and Pref to discover the delivered power. This formula could be used to see how much power is left in a radio wave after it travels through different materials and stages of electronic systems such as radios. Try the following examples with the Interactive Media Activities:
• If the source power of the original laser, or Pref is seven microwatts (1 x 10-6 Watts), and the total loss of a fiber link is 13 dB, how much power is delivered?
• If the total loss of a fiber link is 84 dB and the source power of the original laser, or Pref is 1 milliwatt, how much power is delivered?
• If 2 microvolts, or 2 x 10-6 volts, are measured at the end of a cable and the source voltage was 1 volt, what is the gain or loss in decibels? Is this value positive or negative? Does the value represent a gain or a loss in voltage?
The next page will explain how an oscilloscope is used to analyze and view signals.
4.1.4 The study of logarithms is beyond the scope of this course. However, the terminology is often used to calculate decibels and measure signals on copper, optical, and wireless media. The decibel is related to the exponents and logarithms described in prior sections. There are two formulas that are used to calculate decibels:
dB = 10 log10 (Pfinal / Pref)
dB = 20 log10 (Vfinal / Vref)
In these formulas, dB represents the loss or gain of the power of a wave. Decibels can be negative values which would represent a loss in power as the wave travels or a positive value to represent a gain in power if the signal is amplified.
The log10 variable implies that the number in parentheses will be transformed with the base 10 logarithm rule.
Pfinal is the delivered power measured in watts.
Pref is the original power measured in watts.
Vfinal is the delivered voltage measured in volts.
Vref is the original voltage measured in volts.
The first formula describes decibels in terms of power (P), and the second in terms of voltage (V). The power formula is often used to measure light waves on optical fiber and radio waves in the air. The voltage formula is used to measure electromagnetic waves on copper cables. These formulas have several things in common.
In the formula dB = 10 log10 (Pfinal / Pref), enter values for dB and Pref to discover the delivered power. This formula could be used to see how much power is left in a radio wave after it travels through different materials and stages of electronic systems such as radios. Try the following examples with the Interactive Media Activities:
• If the source power of the original laser, or Pref is seven microwatts (1 x 10-6 Watts), and the total loss of a fiber link is 13 dB, how much power is delivered?
• If the total loss of a fiber link is 84 dB and the source power of the original laser, or Pref is 1 milliwatt, how much power is delivered?
• If 2 microvolts, or 2 x 10-6 volts, are measured at the end of a cable and the source voltage was 1 volt, what is the gain or loss in decibels? Is this value positive or negative? Does the value represent a gain or a loss in voltage?
The next page will explain how an oscilloscope is used to analyze and view signals.
Sine and Square waves (Core) / Exponents and logarithms (Optional)
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4.1.2 Sine waves, or sinusoids, are graphs of mathematical functions. Sine waves are periodic, which means that they repeat the same pattern at regular intervals. Sine waves vary continuously, which means that no adjacent points on the graph have the same value.
Sine waves are graphical representations of many natural occurrences that change regularly over time. Some examples of these occurrences are the distance from the earth to the sun, the distance from the ground while riding a Ferris wheel, and the time of day that the sun rises. Since sine waves vary continuously, they are examples of analog waves.
Square waves, like sine waves, are periodic. However, square wave graphs do not continuously vary with time. The wave maintains one value and then suddenly changes to a different value. After a short amount of time it changes back to the original value. Square waves represent digital signals, or pulses. Like all waves, square waves can be described in terms of amplitude, period, and frequency.
The next page reviews exponents and logarithms.
Exponents and logarithms (Optional)
4.1.3 In networking, there are three important number systems:
• Base 2 – binary
• Base 10 – decimal
• Base 16 – hexadecimal
Recall that the base of a number system refers to the number of different symbols that can occupy one position. For example, binary numbers have only two placeholders, which are zero and one. Decimal numbers have ten different placeholders, the numbers 0 to 9. Hexadecimal numbers have 16 different placeholders, the numbers 0 to 9 and the letters A to F.
Remember that 10 x 10 can be written as 102. 102 means ten squared or ten raised to the second power. 10 is the base of the number and 2 is the exponent of the number. 10 x 10 x 10 can be written as 103. 103 means ten cubed or ten raised to the third power. The base is ten and the exponent is three. Use the Interactive Media Activity to calculate exponents. Enter a value for x to calculate y or a value for y to calculate x.
The base of a number system also refers to the value of each digit. The least significant digit has a value of base0, or one. The next digit has a value of base1. This is equal to 2 for binary numbers, 10 for decimal numbers, and 16 for hexadecimal numbers.
Numbers with exponents are used to easily represent very large or very small numbers. It is much easier and less error-prone to represent one billion numerically as 109 than as 1000000000. Many cable-testing calculations involve numbers that are very large and require exponents. Use the Interactive Media Activity to learn more about exponents.
One way to work with the very large and very small numbers is to transform the numbers based on the mathematical rule known as a logarithm. Logarithm is abbreviated as "log". Any number may be used as a base for a system of logarithms. However, base 10 has many advantages not obtainable in ordinary calculations with other bases. Base 10 is used almost exclusively for ordinary calculations. Logarithms with 10 as a base are called common logarithms. It is not possible to obtain the logarithm of a negative number.
To take the log of a number use a calculator or the Interactive Media Activity. For example, the log of (109) = 9. It is possible to take the logarithm of numbers that are not powers of ten. It is not possible to determine the logarithm of a negative number. The study of logarithms is beyond the scope of this course. However, the terminology is often used to calculate decibels and measure signal intensity on copper, optical, and wireless media.
The next page will explain how to calculate decibels.
Wednesday, December 16, 2009
Module 4: Cable Testing (Overview) / Frequency-Based Cable Testing (Core)- Waves
Cable Testing
Overview
Networking media is the backbone of a network. Networking media is literally and physically the backbone of a network. Inferior quality of network cabling results in network failures and unreliable performance. Copper, optical fiber, and wireless networking media all require testing to ensure that they meet strict specification guidelines. These tests involve certain electrical and mathematical concepts and terms such as signal, wave, frequency, and noise. These terms will help students understand networks, cables, and cable testing.
The first lesson in this module will provide some basic definitions to help students understand the cable testing concepts presented in the second lesson.
The second lesson of this module describes issues related to cable testing for physical layer connectivity in LANs. In order for the LAN to function properly, the physical layer medium should meet the industry standard specifications.
Attenuation, which is signal deterioration, and noise, which is signal interference, can cause problems in networks because the data sent may be interpreted incorrectly or not recognized at all after it has been received. Proper termination of cable connectors and proper cable installation are important. If standards are followed during installations, repairs, and changes, attenuation and noise levels should be minimized.
After a cable has been installed, a cable certification meter can verify that the installation meets TIA/EIA specifications. This module also describes some important tests that are performed.
This module covers some of the objectives for the CCNA 640-801, INTRO 640-821, and ICND 640-811 exams.
Students who complete this module should be able to perform the following tasks:
• Differentiate between sine waves and square waves
• Define and calculate exponents and logarithms
• Define and calculate decibels
• Define basic terminology related to time, frequency, and noise
• Differentiate between digital bandwidth and analog bandwidth
• Compare and contrast noise levels on various types of cabling
• Define and describe the affects of attenuation and impedance mismatch
• Define crosstalk, near-end crosstalk, far-end crosstalk, and power sum near-end crosstalk
• Describe how twisted pairs help reduce noise
• Describe the ten copper cable tests defined in TIA/EIA-568-B
• Describe the difference between Category 5 and Category 6 cable
4.1 Frequency-Based Cable Testing (Core)
Waves
4.1.1 This lesson provides definitions that relate to frequency-based cable testing. This page defines waves.
A wave is energy that travels from one place to another. There are many types of waves, but all can be described with similar vocabulary.
It is helpful to think of waves as disturbances. A bucket of water that is completely still does not have waves since there are no disturbances. Conversely, the ocean always has some sort of detectable waves due to disturbances such as wind and tide.
Ocean waves can be described in terms of their height, or amplitude, which could be measured in meters. They can also be described in terms of how frequently the waves reach the shore, which relates to period and frequency. The period of the waves is the amount of time between each wave, measured in seconds. The frequency is the number of waves that reach the shore each second, measured in hertz (Hz). 1 Hz is equal to 1 wave per second, or 1 cycle per second. To experiment with these concepts, adjust the amplitude and frequency in Figure .
Networking professionals are specifically interested in voltage waves on copper media, light waves in optical fiber, and alternating electric and magnetic fields called electromagnetic waves. The amplitude of an electrical signal still represents height, but it is measured in volts (V) instead of meters (m). The period is the amount of time that it takes to complete 1 cycle. This is measured in seconds. The frequency is the number of complete cycles per second. This is measured in Hz.
If a disturbance is deliberately caused, and involves a fixed, predictable duration, it is called a pulse. Pulses are an important part of electrical signals because they are the basis of digital transmission. The pattern of the pulses represents the value of the data being transmitted.
The next page will introduce sine waves and square waves.
Overview
Networking media is the backbone of a network. Networking media is literally and physically the backbone of a network. Inferior quality of network cabling results in network failures and unreliable performance. Copper, optical fiber, and wireless networking media all require testing to ensure that they meet strict specification guidelines. These tests involve certain electrical and mathematical concepts and terms such as signal, wave, frequency, and noise. These terms will help students understand networks, cables, and cable testing.
The first lesson in this module will provide some basic definitions to help students understand the cable testing concepts presented in the second lesson.
The second lesson of this module describes issues related to cable testing for physical layer connectivity in LANs. In order for the LAN to function properly, the physical layer medium should meet the industry standard specifications.
Attenuation, which is signal deterioration, and noise, which is signal interference, can cause problems in networks because the data sent may be interpreted incorrectly or not recognized at all after it has been received. Proper termination of cable connectors and proper cable installation are important. If standards are followed during installations, repairs, and changes, attenuation and noise levels should be minimized.
After a cable has been installed, a cable certification meter can verify that the installation meets TIA/EIA specifications. This module also describes some important tests that are performed.
This module covers some of the objectives for the CCNA 640-801, INTRO 640-821, and ICND 640-811 exams.
Students who complete this module should be able to perform the following tasks:
• Differentiate between sine waves and square waves
• Define and calculate exponents and logarithms
• Define and calculate decibels
• Define basic terminology related to time, frequency, and noise
• Differentiate between digital bandwidth and analog bandwidth
• Compare and contrast noise levels on various types of cabling
• Define and describe the affects of attenuation and impedance mismatch
• Define crosstalk, near-end crosstalk, far-end crosstalk, and power sum near-end crosstalk
• Describe how twisted pairs help reduce noise
• Describe the ten copper cable tests defined in TIA/EIA-568-B
• Describe the difference between Category 5 and Category 6 cable
Waves
4.1.1 This lesson provides definitions that relate to frequency-based cable testing. This page defines waves.
A wave is energy that travels from one place to another. There are many types of waves, but all can be described with similar vocabulary.
It is helpful to think of waves as disturbances. A bucket of water that is completely still does not have waves since there are no disturbances. Conversely, the ocean always has some sort of detectable waves due to disturbances such as wind and tide.
Ocean waves can be described in terms of their height, or amplitude, which could be measured in meters. They can also be described in terms of how frequently the waves reach the shore, which relates to period and frequency. The period of the waves is the amount of time between each wave, measured in seconds. The frequency is the number of waves that reach the shore each second, measured in hertz (Hz). 1 Hz is equal to 1 wave per second, or 1 cycle per second. To experiment with these concepts, adjust the amplitude and frequency in Figure .
Networking professionals are specifically interested in voltage waves on copper media, light waves in optical fiber, and alternating electric and magnetic fields called electromagnetic waves. The amplitude of an electrical signal still represents height, but it is measured in volts (V) instead of meters (m). The period is the amount of time that it takes to complete 1 cycle. This is measured in seconds. The frequency is the number of complete cycles per second. This is measured in Hz.
If a disturbance is deliberately caused, and involves a fixed, predictable duration, it is called a pulse. Pulses are an important part of electrical signals because they are the basis of digital transmission. The pattern of the pulses represents the value of the data being transmitted.
The next page will introduce sine waves and square waves.
Friday, December 11, 2009
Module 3 Summary
Summary
This page summarizes the topics discussed in this module.
Copper cable carries information using electrical current. The electrical specifications of a cable determines the kind of signal a particular cable can transmit, the speed at which the signal is transmitted and the distance the signal will travel.
An understanding of the following electrical concepts is helpful when working with computer networks:
• Voltage – the pressure that moves electrons through a circuit from one place to another
• Resistance – opposition to the flow of electrons and why a signal becomes degraded as it travels along the conduit
• Current – flow of charges created when electrons move
• Circuits – a closed loop through which an electrical current flows
Circuits must be composed of conducting materials, and must have sources of voltage. Voltage causes current to flow, while resistance and impedance oppose it. A multimeter is used to measure voltage, current, resistance, and other electrical quantities expressed in numeric form.
Coaxial cable, unshielded twisted pair (UTP) and shielded twisted pair (STP) are types of copper cables that can be used in a network to provide different capabilities. Twisted-pair cable can be configured for straight through, crossover, or rollover signaling. These terms refer to the individual wire connections, or pinouts, from one end to the other end of the cable. A straight-through cable is used to connect unlike devices such as a switch and a PC. A crossover cable is used to connect similar devices such as two switches. A rollover cable is used to connect a PC to the console port of a router. Different pinouts are required because the transmit and receive pins are in different locations on each of these devices.
Optical fiber is the most frequently used medium for the longer, high-bandwidth, point-to-point transmissions required on LAN backbones and on WANs. Light energy is used to transmit large amounts of data securely over relatively long distances The light signal carried by a fiber is produced by a transmitter that converts an electrical signal into a light signal. The receiver converts the light that arrives at the far end of the cable back to the original electrical signal.
Every fiber-optic cable used for networking consists of two glass fibers encased in separate sheaths. Just as copper twisted-pair uses separate wire pairs to transmit and receive, fiber-optic circuits use one fiber strand to transmit and one to receive.
The part of an optical fiber through which light rays travel is called the core of the fiber. Surrounding the core is the cladding. Its function is to reflect the signal back towards the core. Surrounding the cladding is a buffer material that helps shield the core and cladding from damage. A strength material surrounds the buffer, preventing the fiber cable from being stretched when installers pull it. The material used is often Kevlar. The final element is the outer jacket that surrounds the cable to protect the fiber against abrasion, solvents, and other contaminants.
The laws of reflection and refraction are used to design fiber media that guides the light waves through the fiber with minimum energy and signal loss. Once the rays have entered the core of the fiber, there are a limited number of optical paths that a light ray can follow through the fiber. These optical paths are called modes. If the diameter of the core of the fiber is large enough so that there are many paths that light can take through the fiber, the fiber is called multimode fiber. Single-mode fiber has a much smaller core that only allows light rays to travel along one mode inside the fiber. Because of its design, single-mode fiber is capable of higher rates of data transmission and greater cable run distances than multimode fiber.
Fiber is described as immune to noise because it is not affected by external noise or noise from other cables. Light confined in one fiber has no way of inducing light in another fiber. Attenuation of a light signal becomes a problem over long cables especially if sections of cable are connected at patch panels or spliced.
Both copper and fiber media require that devices remains stationary permitting moves only within the limits of the media. Wireless technology removes these restraints. Understanding the regulations and standards that apply to wireless technology will ensure that deployed networks will be interoperable and in compliance with IEEE 802.11 standards for WLANs.
A wireless network may consist of as few as two devices. The wireless equivalent of a peer-to-peer network where end-user devices connect directly is referred to as an ad-hoc wireless topology. To solve compatibility problems among devices, an infrastructure mode topology can be set up using an access point (AP) to act as a central hub for the WLAN. Wireless communication uses three types of frames: control, management, and data frames. To avoid collisions on the shared radio frequency media WLANs use Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA).
WLAN authentication is a Layer 2 process that authenticates the device, not the user. Association, performed after authentication, permits a client to use the services of the access point to transfer data.
This page summarizes the topics discussed in this module.
Copper cable carries information using electrical current. The electrical specifications of a cable determines the kind of signal a particular cable can transmit, the speed at which the signal is transmitted and the distance the signal will travel.
An understanding of the following electrical concepts is helpful when working with computer networks:
• Voltage – the pressure that moves electrons through a circuit from one place to another
• Resistance – opposition to the flow of electrons and why a signal becomes degraded as it travels along the conduit
• Current – flow of charges created when electrons move
• Circuits – a closed loop through which an electrical current flows
Circuits must be composed of conducting materials, and must have sources of voltage. Voltage causes current to flow, while resistance and impedance oppose it. A multimeter is used to measure voltage, current, resistance, and other electrical quantities expressed in numeric form.
Coaxial cable, unshielded twisted pair (UTP) and shielded twisted pair (STP) are types of copper cables that can be used in a network to provide different capabilities. Twisted-pair cable can be configured for straight through, crossover, or rollover signaling. These terms refer to the individual wire connections, or pinouts, from one end to the other end of the cable. A straight-through cable is used to connect unlike devices such as a switch and a PC. A crossover cable is used to connect similar devices such as two switches. A rollover cable is used to connect a PC to the console port of a router. Different pinouts are required because the transmit and receive pins are in different locations on each of these devices.
Optical fiber is the most frequently used medium for the longer, high-bandwidth, point-to-point transmissions required on LAN backbones and on WANs. Light energy is used to transmit large amounts of data securely over relatively long distances The light signal carried by a fiber is produced by a transmitter that converts an electrical signal into a light signal. The receiver converts the light that arrives at the far end of the cable back to the original electrical signal.
Every fiber-optic cable used for networking consists of two glass fibers encased in separate sheaths. Just as copper twisted-pair uses separate wire pairs to transmit and receive, fiber-optic circuits use one fiber strand to transmit and one to receive.
The part of an optical fiber through which light rays travel is called the core of the fiber. Surrounding the core is the cladding. Its function is to reflect the signal back towards the core. Surrounding the cladding is a buffer material that helps shield the core and cladding from damage. A strength material surrounds the buffer, preventing the fiber cable from being stretched when installers pull it. The material used is often Kevlar. The final element is the outer jacket that surrounds the cable to protect the fiber against abrasion, solvents, and other contaminants.
The laws of reflection and refraction are used to design fiber media that guides the light waves through the fiber with minimum energy and signal loss. Once the rays have entered the core of the fiber, there are a limited number of optical paths that a light ray can follow through the fiber. These optical paths are called modes. If the diameter of the core of the fiber is large enough so that there are many paths that light can take through the fiber, the fiber is called multimode fiber. Single-mode fiber has a much smaller core that only allows light rays to travel along one mode inside the fiber. Because of its design, single-mode fiber is capable of higher rates of data transmission and greater cable run distances than multimode fiber.
Fiber is described as immune to noise because it is not affected by external noise or noise from other cables. Light confined in one fiber has no way of inducing light in another fiber. Attenuation of a light signal becomes a problem over long cables especially if sections of cable are connected at patch panels or spliced.
Both copper and fiber media require that devices remains stationary permitting moves only within the limits of the media. Wireless technology removes these restraints. Understanding the regulations and standards that apply to wireless technology will ensure that deployed networks will be interoperable and in compliance with IEEE 802.11 standards for WLANs.
A wireless network may consist of as few as two devices. The wireless equivalent of a peer-to-peer network where end-user devices connect directly is referred to as an ad-hoc wireless topology. To solve compatibility problems among devices, an infrastructure mode topology can be set up using an access point (AP) to act as a central hub for the WLAN. Wireless communication uses three types of frames: control, management, and data frames. To avoid collisions on the shared radio frequency media WLANs use Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA).
WLAN authentication is a Layer 2 process that authenticates the device, not the user. Association, performed after authentication, permits a client to use the services of the access point to transfer data.
Signals and noise on a WLAN / Wireless security
3.3.6 This page discusses how signals and noise can affect a WLAN.
On a wired Ethernet network, it is usually a simple process to diagnose the cause of interference. When using RF technology many kinds of interference must be taken into consideration.
Narrowband is the opposite of spread spectrum technology. As the name implies narrowband does not affect the entire frequency spectrum of the wireless signal. One solution to a narrowband interference problem could be simply changing the channel that the AP is using. Actually diagnosing the cause of narrowband interference can be a costly and time-consuming experience. To identify the source requires a spectrum analyzer and even a low cost model is relatively expensive.
All band interference affects the entire spectrum range. Bluetooth™ technologies hops across the entire 2.4 GHz many times per second and can cause significant interference on an 802.11b network. It is not uncommon to see signs in facilities that use wireless networks requesting that all Bluetooth™ devices be shut down before entering. In homes and offices, a device that is often overlooked as causing interference is the standard microwave oven. Leakage from a microwave of as little as one watt into the RF spectrum can cause major network disruption. Wireless phones operating in the 2.4GHZ spectrum can also cause network disorder.
Generally the RF signal will not be affected by even the most extreme weather conditions. However, fog or very high moisture conditions can and do affect wireless networks. Lightning can also charge the atmosphere and alter the path of a transmitted signal.
The first and most obvious source of a signal problem is the transmitting station and antenna type. A higher output station will transmit the signal further and a parabolic dish antenna that concentrates the signal will increase the transmission range.
In a SOHO environment most access points will utilize twin omnidirectional antennae that transmit the signal in all directions thereby reducing the range of communication.
The next page describes WLANs security.
3.3.7 This page will explain how wireless security can be achieved.
Where wireless networks exist there is little security. This has been a problem from the earliest days of WLANs. Currently, many administrators are weak in implementing effective security practices.
A number of new security solutions and protocols, such as Virtual Private Networking (VPN) and Extensible Authentication Protocol (EAP) are emerging. With EAP, the access point does not provide authentication to the client, but passes the duties to a more sophisticated device, possibly a dedicated server, designed for that purpose. Using an integrated server VPN technology creates a tunnel on top of an existing protocol such as IP. This is a Layer 3 connection as opposed to the Layer 2 connection between the AP and the sending node.
• EAP-MD5 Challenge – Extensible Authentication Protocol is the earliest authentication type, which is very similar to CHAP password protection on a wired network.
• LEAP (Cisco) – Lightweight Extensible Authentication Protocol is the type primarily used on Cisco WLAN access points. LEAP provides security during credential exchange, encrypts using dynamic WEP keys, and supports mutual authentication.
• User authentication – Allows only authorized users to connect, send and receive data over the wireless network.
• Encryption – Provides encryption services further protecting the data from intruders.
• Data authentication – Ensures the integrity of the data, authenticating source and destination devices.
VPN technology effectively closes the wireless network since an unrestricted WLAN will automatically forward traffic between nodes that appear to be on the same wireless network. WLANs often extend outside the perimeter of the home or office in which they are installed and without security intruders may infiltrate the network with little effort. Conversely it takes minimal effort on the part of the network administrator to provide low-level security to the WLAN.
This page concludes the lesson. The next page will summarize the main points from the module.
How wireless LANs communicate / Authentication and association / The radio wave and microwave spectrums
3.3.3 This page explains the communication process of a WLAN.
After establishing connectivity to the WLAN, a node will pass frames in the same manner as on any other 802.x network. WLANs do not use a standard 802.3 frame. Therefore, using the term wireless Ethernet is misleading. There are three types of frames: control, management, and data. Only the data frame type is similar to 802.3 frames. The payload of wireless and 802.3 frames is 1500 bytes; however, an Ethernet frame may not exceed 1518 bytes whereas a wireless frame could be as large as 2346 bytes. Usually the WLAN frame size will be limited to 1518 bytes as it is most commonly connected to a wired Ethernet network.
Since radio frequency (RF) is a shared medium, collisions can occur just as they do on wired shared medium. The major difference is that there is no method by which the source node is able to detect that a collision occurred. For that reason WLANs use Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA). This is somewhat like Ethernet CSMA/CD.
When a source node sends a frame, the receiving node returns a positive acknowledgment (ACK). This can cause consumption of 50% of the available bandwidth. This overhead when combined with the collision avoidance protocol overhead reduces the actual data throughput to a maximum of 5.0 to 5.5 Mbps on an 802.11b wireless LAN rated at 11 Mbps.
Performance of the network will also be affected by signal strength and degradation in signal quality due to distance or interference. As the signal becomes weaker, Adaptive Rate Selection (ARS) may be invoked. The transmitting unit will drop the data rate from 11 Mbps to 5.5 Mbps, from 5.5 Mbps to 2 Mbps or 2 Mbps to 1 Mbps.
The next page explains authentication and association.
Authentication and association
3.3.4 This page describes WLAN authentication and association.
WLAN authentication occurs at Layer 2. It is the process of authenticating the device not the user. This is a critical point to remember when considering WLAN security, troubleshooting and overall management.
Authentication may be a null process, as in the case of a new AP and NIC with default configurations in place. The client will send an authentication request frame to the AP and the frame will be accepted or rejected by the AP. The client is notified of the response via an authentication response frame. The AP may also be configured to hand off the authentication task to an authentication server, which would perform a more thorough credentialing process.
Association, performed after authentication, is the state that permits a client to use the services of the AP to transfer data.
Authentication and Association types
• Unauthenticated and unassociated
• The node is disconnected from the network and not associated to an access point.
• Authenticated and unassociated
• The node has been authenticated on the network but has not yet associated with the access point.
• Authenticated and associated
• The node is connected to the network and able to transmit and receive data through the access point.
Methods of authentication
IEEE 802.11 lists two types of authentication processes.
The first authentication process is the open system. This is an open connectivity standard in which only the SSID must match. This may be used in a secure or non-secure environment although the ability of low level network ‘sniffers’ to discover the SSID of the WLAN is high.
The second process is the shared key. This process requires the use of Wireless Equivalency Protocol (WEP) encryption. WEP is a fairly simple algorithm using 64 and 128 bit keys. The AP is configured with an encrypted key and nodes attempting to access the network through the AP must have a matching key. Statically assigned WEP keys provide a higher level of security than the open system but are definitely not hack proof.
The problem of unauthorized entry into WLANs is being addressed by a number of new security solution technologies.
The next page explains radio waves and modulation.
3.3.5 This page describes radio waves and modulation.
Computers send data signals electronically. Radio transmitters convert these electrical signals to radio waves. Changing electric currents in the antenna of a transmitter generates the radio waves. These radio waves radiate out in straight lines from the antenna. However, radio waves attenuate as they move out from the transmitting antenna. In a WLAN, a radio signal measured at a distance of just 10 meters (30 feet) from the transmitting antenna would be only 1/100th of its original strength. Like light, radio waves can be absorbed by some materials and reflected by others. When passing from one material, like air, into another material, like a plaster wall, radio waves are refracted. Radio waves are also scattered and absorbed by water droplets in the air.
These qualities of radio waves are important to remember when a WLAN is being planned for a building or for a campus. The process of evaluating a location for the installation of a WLAN is called making a Site Survey.
Because radio signals weaken as they travel away from the transmitter, the receiver must also be equipped with an antenna. When radio waves hit the antenna of a receiver, weak electric currents are generated in that antenna. These electric currents, caused by the received radio waves, are equal to the currents that originally generated the radio waves in the antenna of the transmitter. The receiver amplifies the strength of these weak electrical signals.
In a transmitter, the electrical (data) signals from a computer or a LAN are not sent directly into the antenna of the transmitter. Rather, these data signals are used to alter a second, strong signal called the carrier signal.
The process of altering the carrier signal that will enter the antenna of the transmitter is called modulation. There are three basic ways in which a radio carrier signal can be modulated. For example, Amplitude Modulated (AM) radio stations modulate the height (amplitude) of the carrier signal. Frequency Modulated (FM) radio stations modulate the frequency of the carrier signal as determined by the electrical signal from the microphone. In WLANs, a third type of modulation called phase modulation is used to superimpose the data signal onto the carrier signal that is broadcast by the transmitter.
In this type of modulation, the data bits in the electrical signal change the phase of the carrier signal.
A receiver demodulates the carrier signal that arrives from its antenna. The receiver interprets the phase changes of the carrier signal and reconstructs from it the original electrical data signal.
The first Interactive Media Activity explains electromagnetic fields and polarization.
The second Interactive Media Activity shows the names, devices, frequencies, and wavelengths of the EM spectrum.
The next page describes problems caused by signals and noise.
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