CCNA :) Be a Good Network Administrator

10BASE5 7.1.2 This page will discuss the original 1980 Ethernet product, which is 10BASE5. 10BASE5 transmitted 10 Mbps over a single think coaxial cable bus. 10BASE5 is important because it was the first medium used for Ethernet. 10BASE5 was part of the original 802.3 standard. The primary benefit of 10BASE5 was length. 10BASE5 may be found in legacy installations. It is not recommended for new installations. 10BASE5 systems are inexpensive and require no configuration. Two disadvantages are that basic components like NICs are very difficult to find and it is sensitive to signal reflections on the cable. 10BASE5 systems also represent a single point of failure. 10BASE5 uses Manchester encoding. It has a solid central conductor. Each segment of thick coax may be up to 500 m (1640.4 ft) in length. The cable is large, heavy, and difficult to install. However, the distance limitations were favorable and this prolonged its use in certain applications. When the medium is a single coa...

10-Mbps Ethernet 7.1.1 This page will discuss 10-Mbps Ethernet technologies. 10BASE5, 10BASE2, and 10BASE-T Ethernet are considered Legacy Ethernet. The four common features of Legacy Ethernet are timing parameters, the frame format, transmission processes, and a basic design rule. Figure displays the parameters for 10-Mbps Ethernet operation. 10-Mbps Ethernet and slower versions are asynchronous. Each receiving station uses eight octets of timing information to synchronize its receive circuit to the incoming data. 10BASE5, 10BASE2, and 10BASE-T all share the same timing parameters. For example, 1 bit time at 10 Mbps = 100 nanoseconds (ns) = 0.1 microseconds = 1 10-millionth of a second. This means that on a 10-Mbps Ethernet network, 1 bit at the MAC sublayer requires 100 ns to transmit. For all speeds of Ethernet transmission 1000 Mbps or slower, transmission can be no slower than the slot time. Slot time is just longer than the time it theoretically can take to go from one e...

Overview Ethernet has been the most successful LAN technology mainly because of how easy it is to implement. Ethernet has also been successful because it is a flexible technology that has evolved as needs and media capabilities have changed. This module will provide details about the most important types of Ethernet. The goal is to help students understand what is common to all forms of Ethernet. Changes in Ethernet have resulted in major improvements over the 10-Mbps Ethernet of the early 1980s. The 10-Mbps Ethernet standard remained virtually unchanged until 1995 when IEEE announced a standard for a 100-Mbps Fast Ethernet. In recent years, an even more rapid growth in media speed has moved the transition from Fast Ethernet to Gigabit Ethernet. The standards for Gigabit Ethernet emerged in only three years. A faster Ethernet version called 10-Gigabit Ethernet is now widely available and faster versions will be developed. MAC addresses, CSMA/CD, and the frame format have not been...

Summary This page summarizes the topics discussed in this module. Ethernet is not one networking technology, but a family of LAN technologies that includes Legacy, Fast Ethernet, and Gigabit Ethernet. When Ethernet needs to be expanded to add a new medium or capability, the IEEE issues a new supplement to the 802.3 standard. The new supplements are given a one or two letter designation such as 802.3u. Ethernet relies on baseband signaling, which uses the entire bandwidth of the transmission medium. Ethernet operates at two layers of the OSI model, the lower half of the data link layer, known as the MAC sublayer and the physical layer. Ethernet at Layer 1 involves interfacing with media, signals, bit streams that travel on the media, components that put signals on media, and various physical topologies. Layer 1 bits need structure so OSI Layer 2 frames are used. The MAC sublayer of Layer 2 determines the type of frame appropriate for the physical media. The one thing common to all...

Link establishment and full and half duplex 6.2.10 This page will explain how links are established through Auto-Negotiation and introduce the two duplex modes. Link partners are allowed to skip offering configurations of which they are capable. This allows the network administrator to force ports to a selected speed and duplex setting, without disabling Auto-Negotiation. Auto-Negotiation is optional for most Ethernet implementations. Gigabit Ethernet requires its implementation, though the user may disable it. Auto-Negotiation was originally defined for UTP implementations of Ethernet and has been extended to work with other fiber optic implementations. When an Auto-Negotiating station first attempts to link it is supposed to enable 100BASE-TX to attempt to immediately establish a link. If 100BASE-TX signaling is present, and the station supports 100BASE-TX, it will attempt to establish a link without negotiating. If either signaling produces a link or FLP bursts are received...

Ethernet auto-negotiation 6.2.9 This page explains auto-negotiation and how it is accomplished. As Ethernet grew from 10 to 100 and 1000 Mbps, one requirement was to make each technology interoperable, even to the point that 10, 100, and 1000 interfaces could be directly connected. A process called Auto-Negotiation of speeds at half or full duplex was developed. Specifically, at the time that Fast Ethernet was introduced, the standard included a method of automatically configuring a given interface to match the speed and capabilities of the link partner. This process defines how two link partners may automatically negotiate a configuration offering the best common performance level. It has the additional advantage of only involving the lowest part of the physical layer. 10BASE-T required each station to transmit a link pulse about every 16 milliseconds, whenever the station was not engaged in transmitting a message. Auto-Negotiation adopted this signal and renamed it a Normal Li...

FCS and beyond 6.2.8 This page will focus on additional errors that occur on an Ethernet network. A received frame that has a bad Frame Check Sequence, also referred to as a checksum or CRC error, differs from the original transmission by at least one bit. In an FCS error frame the header information is probably correct, but the checksum calculated by the receiving station does not match the checksum appended to the end of the frame by the sending station. The frame is then discarded. High numbers of FCS errors from a single station usually indicates a faulty NIC and/or faulty or corrupted software drivers, or a bad cable connecting that station to the network. If FCS errors are associated with many stations, they are generally traceable to bad cabling, a faulty version of the NIC driver, a faulty hub port, or induced noise in the cable system. A message that does not end on an octet boundary is known as an alignment error. Instead of the correct number of binary bits forming ...

Ethernet errors 6.2.7 This page will define common Ethernet errors. Knowledge of typical errors is invaluable for understanding both the operation and troubleshooting of Ethernet networks. The following are the sources of Ethernet error: • Collision or runt – Simultaneous transmission occurring before slot time has elapsed • Late collision – Simultaneous transmission occurring after slot time has elapsed • Jabber, long frame and range errors – Excessively or illegally long transmission • Short frame, collision fragment or runt – Illegally short transmission • FCS error – Corrupted transmission • Alignment error – Insufficient or excessive number of bits transmitted • Range error – Actual and reported number of octets in frame do not match • Ghost or jabber – Unusually long Preamble or Jam event While local and remote collisions are considered to be a normal part of Ethernet operation, late collisions are considered to be an error. The presence of errors on a network a...

Types of collisions 6.2.6 This page covers the different types of collisions and their characteristics. Collisions typically take place when two or more Ethernet stations transmit simultaneously within a collision domain. A single collision is a collision that was detected while trying to transmit a frame, but on the next attempt the frame was transmitted successfully. Multiple collisions indicate that the same frame collided repeatedly before being successfully transmitted. The results of collisions, collision fragments, are partial or corrupted frames that are less than 64 octets and have an invalid FCS. Three types of collisions are: • Local • Remote • Late To create a local collision on coax cable (10BASE2 and 10BASE5), the signal travels down the cable until it encounters a signal from the other station. The waveforms then overlap, canceling some parts of the signal out and reinforcing or doubling other parts. The doubling of the signal pushes the voltage level of the s...

Error handling 6.2.5 This page will describe collisions and how they are handled on a network. The most common error condition on Ethernet networks are collisions. Collisions are the mechanism for resolving contention for network access. A few collisions provide a smooth, simple, low overhead way for network nodes to arbitrate contention for the network resource. When network contention becomes too great, collisions can become a significant impediment to useful network operation. Collisions result in network bandwidth loss that is equal to the initial transmission and the collision jam signal. This is consumption delay and affects all network nodes possibly causing significant reduction in network throughput. The considerable majority of collisions occur very early in the frame, often before the SFD. Collisions occurring before the SFD are usually not reported to the higher layers, as if the collision did not occur. As soon as a collision is detected, the sending stations tran...

Interframe spacing and backoff 6.2.4 This page explains how spacing is used in an Ethernet network for data transmission. The minimum spacing between two non-colliding frames is also called the interframe spacing. This is measured from the last bit of the FCS field of the first frame to the first bit of the preamble of the second frame. After a frame has been sent, all stations on a 10-Mbps Ethernet are required to wait a minimum of 96 bit-times (9.6 microseconds) before any station may legally transmit the next frame. On faster versions of Ethernet the spacing remains the same, 96 bit-times, but the time required for that interval grows correspondingly shorter. This interval is referred to as the spacing gap. The gap is intended to allow slow stations time to process the previous frame and prepare for the next frame. A repeater is expected to regenerate the full 64 bits of timing information, which is the preamble and SFD, at the start of any frame. This is despite the potent...

Ethernet timing 6.2.3 This page explains the importance of slot times in an Ethernet network. The basic rules and specifications for proper operation of Ethernet are not particularly complicated, though some of the faster physical layer implementations are becoming so. Despite the basic simplicity, when a problem occurs in Ethernet it is often quite difficult to isolate the source. Because of the common bus architecture of Ethernet, also described as a distributed single point of failure, the scope of the problem usually encompasses all devices within the collision domain. In situations where repeaters are used, this can include devices up to four segments away. Any station on an Ethernet network wishing to transmit a message first “listens” to ensure that no other station is currently transmitting. If the cable is quiet, the station will begin transmitting immediately. The electrical signal takes time to travel down the cable (delay), and each subsequent repeater introduces a s...

MAC 6.2.1 This page will define MAC and provide examples of deterministic and non-deterministic MAC protocols. MAC refers to protocols that determine which computer in a shared-media environment, or collision domain, is allowed to transmit data. MAC and LLC comprise the IEEE version of the OSI Layer 2. MAC and LLC are sublayers of Layer 2. The two broad categories of MAC are deterministic and non-deterministic. Examples of deterministic protocols include Token Ring and FDDI. In a Token Ring network, hosts are arranged in a ring and a special data token travels around the ring to each host in sequence. When a host wants to transmit, it seizes the token, transmits the data for a limited time, and then forwards the token to the next host in the ring. Token Ring is a collisionless environment since only one host can transmit at a time. Non-deterministic MAC protocols use a first-come, first-served approach. CSMA/CD is a simple system. The NIC listens for the absence of a signal on ...

Ethernet frame structure 6.1.6 This page will describe the frame structure of Ethernet networks. At the data link layer the frame structure is nearly identical for all speeds of Ethernet from 10 Mbps to 10,000 Mbps. However, at the physical layer almost all versions of Ethernet are very different. Each speed has a distinct set of architecture design rules. In the version of Ethernet that was developed by DIX prior to the adoption of the IEEE 802.3 version of Ethernet, the Preamble and Start-of-Frame (SOF) Delimiter were combined into a single field. The binary pattern was identical. The field labeled Length/Type was only listed as Length in the early IEEE versions and only as Type in the DIX version. These two uses of the field were officially combined in a later IEEE version since both uses were common. The Ethernet II Type field is incorporated into the current 802.3 frame definition. When a node receives a frame it must examine the Length/Type field to determine which highe...

Layer 2 framing 6.1.5 This page will explain how frames are created at Layer 2 of the OSI model. Encoded bit streams, or data, on physical media represent a tremendous technological accomplishment, but they, alone, are not enough to make communication happen. Framing provides essential information that could not be obtained from coded bit streams alone. This information includes the following: • Which computers are in communication with each other • When communication between individual computers begins and when it ends • Which errors occurred while the computers communicated • Which computer will communicate next Framing is the Layer 2 encapsulation process. A frame is the Layer 2 protocol data unit. A voltage versus time graph could be used to visualize bits. However, it may be too difficult to graph address and control information for larger units of data. Another type of diagram that could be used is the frame format diagram, which is based on voltage versus time gr...

Naming 6.1.4 This page will discuss the MAC addresses used by Ethernet networks. An address system is required to uniquely identify computers and interfaces to allow for local delivery of frames on the Ethernet. Ethernet uses MAC addresses that are 48 bits in length and expressed as 12 hexadecimal digits. The first six hexadecimal digits, which are administered by the IEEE, identify the manufacturer or vendor. This portion of the MAC address is known as the Organizational Unique Identifier (OUI). The remaining six hexadecimal digits represent the interface serial number or another value administered by the manufacturer. MAC addresses are sometimes referred to as burned-in MAC addresses (BIAs) because they are burned into ROM and are copied into RAM when the NIC initializes. At the data link layer MAC headers and trailers are added to upper layer data. The header and trailer contain control information intended for the data link layer in the destination system. The data from upper...

Ethernet and the OSI model 6.1.3 This page will explain how Ethernet relates to the OSI model. Ethernet operates in two areas of the OSI model. These are the lower half of the data link layer, which is known as the MAC sublayer, and the physical layer. Data that moves from one Ethernet station to another often passes through a repeater. All stations in the same collision domain see traffic that passes through a repeater. A collision domain is a shared resource. Problems that originate in one part of a collision domain will usually impact the entire collision domain. A repeater forwards traffic to all other ports. A repeater never sends traffic out the same port from which it was received. Any signal detected by a repeater will be forwarded. If the signal is degraded through attenuation or noise, the repeater will attempt to reconstruct and regenerate the signal. To guarantee minimum bandwidth and operability, standards specify the maximum number of stations per segment, max...

IEEE Ethernet naming rules 6.1.2 This page focuses on the Ethernet naming rules developed by IEEE. Ethernet is not one networking technology, but a family of networking technologies that includes Legacy, Fast Ethernet, and Gigabit Ethernet. Ethernet speeds can be 10, 100, 1000, or 10,000 Mbps. The basic frame format and the IEEE sublayers of OSI Layers 1 and 2 remain consistent across all forms of Ethernet. When Ethernet needs to be expanded to add a new medium or capability, the IEEE issues a new supplement to the 802.3 standard. The new supplements are given a one or two letter designation such as 802.3u. An abbreviated description, called an identifier, is also assigned to the supplement. The abbreviated description consists of the following elements: • A number that indicates the number of Mbps transmitted • The word base to indicate that baseband signaling is used • One or more letters of the alphabet indicating the type of medium used. For example, F = fiber optical ...

Introduction to Ethernet 6.1.1 This page provides an introduction to Ethernet. Most of the traffic on the Internet originates and ends with Ethernet connections. Since it began in the 1970s, Ethernet has evolved to meet the increased demand for high-speed LANs. When optical fiber media was introduced, Ethernet adapted to take advantage of the superior bandwidth and low error rate that fiber offers. Now the same protocol that transported data at 3 Mbps in 1973 can carry data at 10 Gbps. The success of Ethernet is due to the following factors: • Simplicity and ease of maintenance • Ability to incorporate new technologies • Reliability • Low cost of installation and upgrade The introduction of Gigabit Ethernet has extended the original LAN technology to distances that make Ethernet a MAN and WAN standard. The original idea for Ethernet was to allow two or more hosts to use the same medium with no interference between the signals. This problem of multiple user access to a sha...

Overview of Module 6 Ethernet Fundamentals Ethernet is now the dominant LAN technology in the world. Ethernet is a family of LAN technologies that may be best understood with the OSI reference model. All LANs must deal with the basic issue of how individual stations, or nodes, are named. Ethernet specifications support different media, bandwidths, and other Layer 1 and 2 variations. However, the basic frame format and address scheme is the same for all varieties of Ethernet. Various MAC strategies have been invented to allow multiple stations to access physical media and network devices. It is important to understand how network devices gain access to the network media before students can comprehend and troubleshoot the entire network. 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: • Describe the basics of Ethernet technology • Explain nami...

Summary Ethernet is the most widely used LAN technology and can be implemented on a variety of media. Ethernet technologies provide a variety of network speeds, from 10 Mbps to Gigabit Ethernet, which can be applied to appropriate areas of a network. Media and connector requirements differ for various Ethernet implementations. The connector on a network interface card (NIC) must match the media. A bayonet nut connector (BNC) connector is required to connect to coaxial cable. A fiber connector is required to connect to fiber media. The registered jack (RJ-45) connector used with twisted-pair wire is the most common type of connector used in LAN implementations. Ethernet When twisted-pair wire is used to connect devices, the appropriate wire sequence, or pinout, must be determined as well. A crossover cable is used to connect two similar devices, such as two PCs. A straight-through cable is used to connect different devices, such as connections between a switch and a PC. A rollove...

Setting up console connections 5.2.7 This page will explain how console connections are set up. To initially configure the Cisco device, a management connection must be directly connected to the device. For Cisco equipment this management attachment is called a console port. The console port allows monitoring and configuration of a Cisco hub, switch, or router. The cable used between a terminal and a console port is a rollover cable, with RJ-45 connectors. The rollover cable, also known as a console cable, has a different pinout than the straight-through or crossover RJ-45 cables used with Ethernet or the ISDN BRI. The pinout for a rollover is as follows: 1 to 8  2 to 7 3 to 6 4 to 5 5 to 4 6 to 3 7 to 2 8 to 1 To set up a connection between the terminal and the Cisco console port, perform two steps. First, connect the devices using a rollover cable from the router console port to the workstation serial port. An RJ-45-to-DB-9 or an RJ-45-to-DB-25 adapter may be req...

Routers and ISDN BRI connections 5.2.4 This page will help students understand ISDN BRI connections. With ISDN BRI, two types of interfaces may be used, BRI S/T and BRI U. Determine who is providing the Network Termination 1 (NT1) device in order to determine which interface type is needed. An NT1 is an intermediate device located between the router and the service provider ISDN switch. The NT1 is used to connect four-wire subscriber wiring to the conventional two-wire local loop. In North America, the customer typically provides the NT1, while in the rest of the world the service provider provides the NT1 device. It may be necessary to provide an external NT1 if the device is not already integrated into the router. Reviewing the labeling on the router interfaces is usually the easiest way to determine if the router has an integrated NT1. A BRI interface with an integrated NT1 is labeled BRI U. A BRI interface without an integrated NT1 is labeled BRI S/T. Because routers can h...

Routers and serial connections 5.2.3 This page will describe how routers and serial connections are used in a WAN. Routers are responsible for routing data packets from source to destination within the LAN, and for providing connectivity to the WAN. Within a LAN environment the router contains broadcasts, provides local address resolution services, such as ARP and RARP, and may segment the network using a subnetwork structure. In order to provide these services the router must be connected to the LAN and WAN. In addition to determining the cable type, it is necessary to determine whether DTE or DCE connectors are required. The DTE is the endpoint of the user’s device on the WAN link. The DCE is typically the point where responsibility for delivering data passes into the hands of the service provider. When connecting directly to a service provider, or to a device such as a CSU/DSU that will perform signal clocking, the router is a DTE and needs a DTE serial cable. This is typical...

Routers and serial connections 5.2.3

WAN serial connections 5.2.2 This page will discuss WAN serial connections. For long distance communication, WANs use serial transmission. This is a process by which bits of data are sent over a single channel. This process provides reliable long distance communication and the use of a specific electromagnetic or optical frequency range. Frequencies are measured in terms of cycles per second and expressed in Hz. Signals transmitted over voice grade telephone lines use 4 kHz. The size of the frequency range is referred to as bandwidth. In networking, bandwidth is a measure of the bits per second that are transmitted. For a Cisco router, physical connectivity at the customer site is provided by one of two types of serial connections. The first type is a 60-pin connector. The second is a more compact ‘smart serial’ connector. The provider connector will vary depending on the type of service equipment. If the connection is made directly to a service provider, or a device that pr...

WAN serial connections

WAN physical layer 5.2.1 This page describes the WAN physical layer. The physical layer implementations vary based on the distance of the equipment from each service, the speed, and the type of service. Serial connections are used to support WAN services such as dedicated leased lines that run PPP or Frame Relay. The speed of these connections ranges from 2400 bps to T1 service at 1.544 Mbps and E1 service at 2.048 Mbps. ISDN offers dial-on-demand connections or dial backup services. An ISDN Basic Rate Interface (BRI) is composed of two 64 kbps bearer channels (B channels) for data, and one delta channel (D channel) at 16 kbps used for signaling and other link-management tasks. PPP is typically used to carry data over the B channels. As the demand for residential broadband high-speed services has increased, DSL and cable modem connections have become more popular. Typical residential DSL service can achieve T1/E1 speeds over the telephone line. Cable services use the coaxial ca...