CCNA :) Be a Good Network Administrator

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 u...

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 ...

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...

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 connecte...

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...

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 5...

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...

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 th...

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...

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 ope...

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 impedanc...

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 conn...

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 digita...

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. Narrow...

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 descr...

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 descri...

Sine waves and square waves (Core) 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 desc...

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 deteriora...

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 elec...

Signals and noise on a WLAN 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 faci...

How wireless LANs communicate 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 Ac...