CCNA 1 and 2 Companion Guide, Revised (Cisco Networking Academy Program) part 24 doc

Signals and Noise on Networking Media 199 Figure 4-15 Power Sum Near-End Crosstalk (PSNEXT) 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 important tests. Cable Testing Standards The TIA/EIA-568-B standard specifies ten tests that a copper cable must pass if it is to 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 primary test parameters that must be verified for a cable link to meet TIA/EIA-568-B standards are as follows: ■ 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 has a particular purpose, as shown in Figure 4-16. A network interface card (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 chpt_04.fm Page 199 Tuesday, May 27, 2003 9:01 AM 200 Chapter 4: Cable Testing and Cabling LANs and WANs that no open or short circuits exist in 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. Figure 4-16 Ethernet Standards for RJ-45 Connectors The wire map test also verifies that all eight wires are connected to the correct pins on both ends of the cable. The wire map test can detect several different wiring faults. The reversed-pair fault occurs when a wire pair is correctly installed on one connector, but reversed on the other connector. If the orange striped wire is on pin 1 and the orange wire is on pin 2 at one end, but the orange striped wire is on pin 2 and the orange wire is on pin 1 at the other end, then the cable has a reversed-pair fault, as demonstrated in Figure 4-17. Figure 4-17 Cable Wire Map Problems Pair 2 Pinouts 1 = G/W 2 = Green 3 = O/W 4 = Blue 5 = Blue/W 6 = Orange 7 = Brown/W 8 = Brown Pinouts 1 = O/W 2 = Orange 3 = G/W 4 = Blue 5 = Blue/W 6 = Green 7 = Brown/W 8 = Brown Pair 3 Pair 1 Pair 4 12345678 T568A Pair 3 Pair 2 Pair 1 Pair 4 12345678 T568B chpt_04.fm Page 200 Tuesday, May 27, 2003 9:01 AM Signals and Noise on Networking Media 201 A split-pair wiring fault occurs when two wires from different wire pairs are connected to the wrong pins on both ends of the cable. Look carefully at the pin numbers in Fig- ure 4-17 to detect the wiring fault. A split pair creates two transmit or receive pairs each with two wires that are not twisted together. Transposed-pair wiring faults occur when a wire pair is connected to completely different pins at both ends. Contrast this with a reversed-pair where the same pair of pins is used at both ends. Transposed pairs also occur when two different colors codes on punch-down blocks (representing T568A and T568B) are used at different locations on the same link. Lab Activity Fluke 620 Cable Tester: Wire Map In this lab, you learn the wire mapping features of the Fluke 620 LAN CableMeter. Lab Activity Fluke 620 Cable Tester: Faults In this lab, you learn the Cable Fault Test—Pass/Fail features of the Fluke 620 LAN CableMeter. Lab Activity Fluke 620 Cable Tester: Length In this lab, you learn the cable length feature of the Fluke 620 LAN CableMeter. Lab Activity Fluke LinkRunner: LAN Tests In this lab, you use the Fluke LinkRunner to determine whether a cable drop is active and identify its speed, duplex capabilities, and service type. You also verify network layer connectivity with ping. Lab Activity Fluke LinkRunner: Cable and NIC Tests In this lab, you use the Fluke LinkRunner to verify cable length and integrity and determine where a cable terminates. You also verify PC NIC functionality. chpt_04.fm Page 201 Tuesday, May 27, 2003 9:01 AM 202 Chapter 4: Cable Testing and Cabling LANs and WANs Other Test Parameters The combination of the effects of signal attenuation and impedance discontinuities on a communications link is called insertion loss. Insertion loss increases as data trans- mission speeds and frequencies increase. Insertion loss is measured in decibels. The TIA/EIA-568-B 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: ■ NEXT ■ ELFEXT ■ PSELFEXT ■ Return loss 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. 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 mea- surement 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 dis- continuities 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 strike the receiver at different intervals causing signal jitter. Time-Based Parameters 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 the 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. chpt_04.fm Page 202 Tuesday, May 27, 2003 9:01 AM Other Test Parameters 203 Propagation delay measurements are the basis of the cable length measurement. TIA/ EIA-568-B-1 specifies that the physical length of the link is 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 the physical length of the cable jacket. Because 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, which can calculate the approximate distance to the wiring fault. This calculation can be helpful in locating a faulty connection point, such as a wall jack, along a cable run. 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 might not be intended for this type of data transmission, test- ing for delay skew helps ensure that the link supports future upgrades to high-speed networks. All cable links in a LAN must pass all of the tests covered in the preceding text as spec- ified in the TIA/EIA-568-B standard to ensure that they function reliably at high speeds and frequencies. Perform cable tests when the cable is installed, and afterward on a regular basis to ensure that LAN cabling meets industry standards. Use high-quality cable test instruments correctly to ensure that the tests are accurate. Carefully document test results. Testing Fiber-Optic Cables A fiber link consists of two separate glass fibers functioning as separate and indepen- dent 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 no crosstalk problems exist on fiber-optic cable. The jacket, external EMI or noise has no affect on fiber cabling. Attenuation does occur on fiber links, but to a lesser extent than on copper cabling. chpt_04.fm Page 203 Tuesday, May 27, 2003 9:01 AM 204 Chapter 4: Cable Testing and Cabling LANs and WANs Fiber links are subject to the optical equivalent of UTP impedance discontinuities. When light encounters an optical discontinuity, some of the light signal is reflected back in the opposite direction with only a fraction of the original light signal continuing down the fiber towards the receiver. This results in a reduced amount of light energy arriving at the receiver making signal recognition more difficult. Just as with UTP cable, improp- erly 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 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 with- out dropping below the requirements of the receiver must be calculated. This calcula- tion is referred to as the optical link loss budget. A fiber test instrument checks whether the optical link loss budget has been exceeded. If the fiber fails the test, the cable test instrument indicates where the optical discontinuities occur along the length of the cable link. Usually, the problem is one or more improperly attached connectors. The cable test instrument indicates the location of the faulty connections that must be replaced. When the faults are corrected, the cable must be retested. A New Cabling Standard On June 20, 2002, the Category 6 (CAT 6) addition to the TIA-568 standard was pub- lished. The official title of the standard is ANSI/TIA/EIA-568-B.2-1. This new standard specifies the original set of ten tests for Ethernet cabling and the passing scores for each of these tests. Cables certified as CAT 6 cable must pass all ten tests. Although the CAT 6 tests are essentially the same as the CAT 5 standard specifies, CAT 6 cable must pass the tests with higher scores to be certified. CAT 6 cable must be capa- ble 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 CAT 5, CAT 5e, and CAT 6 cable certi- fications of both permanent links and channel links. chpt_04.fm Page 204 Tuesday, May 27, 2003 9:01 AM Cabling the LANs 205 Cabling the LANs The cabling aspect of the LAN exists at Layer 1 of the OSI reference model. To under- stand the types of cabling used to cable networking devices, you need to understand the LAN physical layer implementation of Ethernet, which is a LAN technology specified at the data link layer. It is important that you be able to identify the usages of different types of cable and to differentiate among the types of connectors that can be used to connect Ethernet. This section addresses the LAN physical layer implementation and the main principle of implementing Ethernet in a campus LAN. This chapter also discusses the different types of connectors specified for Ethernet use as well as the UTP wiring standards. LAN Physical Layer Ethernet is the most widely used LAN technology. Ethernet was first implemented by a group called DIX (Digital, Intel, and Xerox). DIX created and implemented the first Ethernet LAN specification, which was used as the basis for the Institute of Electrical and Electronics Engineer (IEEE) 802.3 specification released in 1980. Later, the IEEE extended the 802.3 committee to three new committees known as 802.3u (Fast Ether- net), 802.3z (Gigabit Ethernet over Fiber), and 802.3ab (Gigabit Ethernet over UTP). The cabling aspect of the LAN exists at Layer 1 of the Open System Interconnection (OSI) reference model. Many topologies support LANs, as well as different physical media. Figure 4-18 shows a subset of physical layer implementations that you can deploy to support Ethernet. Figure 4-18 LAN Physical Layer Implementation chpt_04.fm Page 205 Tuesday, May 27, 2003 9:01 AM 206 Chapter 4: Cable Testing and Cabling LANs and WANs The symbols for media vary. For example, the symbol for a serial line looks an elongated letter z or a lighting bolt; the Ethernet symbol is typically a straight line with perpen- dicular lines projecting from it; the Token Ring network symbol is a circle with hosts attached to it; and for FDDI, the symbol is two concentric circles with attached devices, as shown in Figure 4-19. Figure 4-19 LAN Physical Layer Media Symbols The basic functions of media are to carry a flow of information, in the form of bits and bytes, through a LAN. Other than wireless LANs (that use the atmosphere, or space, as the medium), networking media confines network signals to a wire, cable, or fiber. Networking media are considered Layer 1 components of LANs. Computer networks can be built with many different media types. Each media has advantages and disadvantages. What is an advantage for one media (CAT 5 cost) might be a disadvantage for another (fiber-optic cost). The primary advantage and disadvantage comparison categories are as follows: ■ Cable length ■ Cost ■ Ease of installation Coaxial cable, optical fiber, and even free space can carry network signals. However, the principal medium that is studied is called Category 5 unshielded twisted-pair cable (CAT 5 UTP). Ethernet in the Campus Given the variety of Ethernet speeds that you can deploy in the campus, you need to determine when, if, and where to upgrade to one or more of the Fast Ethernet imple- mentations. With the correct hardware and cabling infrastructure, 10- or 100-Mbps chpt_04.fm Page 206 Tuesday, May 27, 2003 9:01 AM Cabling the LANs 207 Ethernet can be run anywhere in the network. As noted in Table 4-2, 10-Mbps Ether- net typically is implemented at the end-user level to connect to desktops, and faster technologies are used to interconnect to servers and network devices, such as routers and switches. In today’s installations, although customers are providing Gigabit Ethernet from the backbone to the end user, costs for cabling and switch ports can make this prohibitive. Before making this decision, you must determine network requirements. For example, a network running at traditional Ethernet speeds of 10 Mbps can be easily overwhelmed with the new generation of multimedia, imaging, and database products. In general, you can use Ethernet technologies in a campus LAN in several different ways: ■ An Ethernet speed of Fast Ethernet can be used at the user level to provide good performance. Also, Fast Ethernet or Gigabit Ethernet can be used for clients or servers that consume high bandwidth. Table 4-2 Ethernet Connectivity Recommendations Ethernet 10BASE-T Position Fast Ethernet Position Gigabit Ethernet Position End-User Level (End-user device to workgroup device) Provides connec- tivity between the end-user device and the user-level switch. Gives high-perfor- mance PC work- stations 100-Mbps access to the server. Not typically used at this level. Workgroup Level (Workgroup device to backbone) Not typically used at this level. Provides connectiv- ity between the end user and work- groups. Provides connectivity from the workgroup to backbone. Provides connectivity from the server block to the backbone. Provides high- performance con- nectivity from the workgroup to back- bone. Provides high-performance connectivity to the enterprise server block. Backbone Level Not typically used at this level. Provides connectiv- ity for low- to medium-volume applications. Provides high- speed backbone and network device connectivity. chpt_04.fm Page 207 Tuesday, May 27, 2003 9:01 AM 208 Chapter 4: Cable Testing and Cabling LANs and WANs ■ Fast Ethernet is often used as the link between the user-level and network devices, supporting the aggregate traffic from each Ethernet segment on the access link. ■ Many client/server networks suffer from too many clients trying to access the same server, creating a bottleneck where the server attaches to the LAN. To enhance client/server performance across the campus LAN and avoid bottlenecks at the server, you can use Fast Ethernet or Gigabit Ethernet links to connect enterprise servers. Fast Ethernet or Gigabit Ethernet creates an effective solution for avoid- ing slow networks. ■ You also can use Fast Ethernet links to provide the connection between the workgroup level and the backbone. Because the campus LAN model supports dual links between each workgroup router and backbone switch, you can load balance the aggregate traffic from multiple-access switches across the links. ■ You can use Fast Ethernet (or Gigabit Ethernet) between switches and the back- bone. Implement the fastest medium affordable between backbone switches. Ethernet Media and Connector Requirement In addition to network need, and before selecting an Ethernet implementation, you must consider the media and connector requirements for each implementation. The cables and connector specifications used to support Ethernet implementations are derived from the Electronic Industries Association and (newer) Telecommunications Industry Association (EIA/TIA) standards body. The categories of cabling defined for the Ethernet are derived from the EIA/TIA-568 (SP-2840) Commercial Building Tele- communications Wiring Standards. The EIA/TIA specifies an RJ-45 connector for UTP cable. The letters RJ stand for registered jack, and the number 45 refers to the physical connector that has eight conductors. Table 4-3 compares the cable and connector specifications for the most popular Ether- net implementations. The important difference to note is the medium used for 10-Mbps Ethernet versus 100-Mbps and 1000-Mbps Ethernet. In today’s networks, in which you see a mix of 10- and 1000-Mbps requirements, you must be aware of the need to change over to UTP CAT 5 to support Fast Ethernet. Figure 4-20 illustrates some of different connection types used by the physical layer implementation. chpt_04.fm Page 208 Tuesday, May 27, 2003 9:01 AM . Brown Pair 3 Pair 1 Pair 4 12 345678 T568A Pair 3 Pair 2 Pair 1 Pair 4 12 345678 T568B chpt_04.fm Page 20 0 Tuesday, May 27 , 20 03 9: 01 AM Signals and Noise on Networking Media 2 01 A split-pair wiring. integrity and determine where a cable terminates. You also verify PC NIC functionality. chpt_04.fm Page 2 01 Tuesday, May 27 , 20 03 9: 01 AM 20 2 Chapter 4: Cable Testing and Cabling LANs and WANs Other. Cabling Standard On June 20 , 20 02, the Category 6 (CAT 6) addition to the TIA-568 standard was pub- lished. The official title of the standard is ANSI/TIA/EIA-568-B .2 -1. This new standard specifies