+ZNKXTKZTKZ]UXQY 3G^OS[SZXGTYSOYYOUTVGZN The maximum transmission path is made of five segments connected by four repeaters. The total number of segments can be made up of a maximum of three coax segments containing station nodes and two link segments, having no intermediate nodes. This is summarized as the 5-4-3-2 rule. These link segments are 10BaseFL fiber links as specified in IEEE 802.3. Table 3.3 5-4-3-2 rule It is important to verify that the above transmission rules are met by all paths between any two nodes on the network. Figure 3.9 Maximum transmission path Note that the maximum sized network of four repeaters supported by IEEE 802.3 can be susceptible to timing problems. The maximum configuration is limited by propagation delay. Note that 10Base2 segments should not be used to link 10Base5 segments. 3G^OS[STKZ]UXQYO`K 10Base5 = 2800 m node to node (5 × 500 m segments + 4 repeater cables + 2 AUI) 10Base2 = 925 m node to node (5 × 185 m segments) 10BaseT = 100 m node to hub 8KVKGZKXX[RKY Repeaters are connected to transceivers that count as one node on the segments. Special transceivers are used to connect repeaters and these do not implement the signal quality error test (SQE). Fiber optic repeaters are available giving up to 3000 m links at 10 Mbps. Check the vendor’s specifications for adherence with IEEE 802.3 repeater performance and compliance with the fiber optic inter repeater link (FOIRL) standard. 6XGIZOIGR:)6/6GTJ+ZNKXTKZ4KZ]UXQOTM )GHRKY_YZKSMXU[TJOTM Grounding has safety and noise connotations. IEEE 802.3 states that the shield conductor of each coaxial cable shall make electrical contact with an effective earth reference at one point only. The single point earth reference for an Ethernet system is usually located at one of the terminators. Most terminators for Ethernet have a screw terminal to which a ground lug can be attached using a braided cable preferably to ensure good earthing. Ensure that all other splices taps or terminators are jacketed so that no contact can be made with any metal objects. Insulating boots or sleeves should be used on all in-line coaxial connectors to avoid unintended earth contacts. 4 Fast and gigabit Ethernet systems Objectives When you have completed study of this chapter you should be able to: • List the basic methods used to achieve high transmission speeds on UTP cables • Describe the operation of the 100Base-TX system • List the different physical media options for 100Base-T systems • Explain the basic differences between a Class I and Class II repeater • Explain the packet bursting technique used by gigabit Ethernet • List the different media options used by gigabit Ethernet 4.1 Achieving higher speed Although Ethernet with over 200 million installed nodes world-wide is the most popular method of linking computers on a network, its 10 Mbps speed is too slow for very data intensive or real-time applications. From a philosophical point of view there are several ways to increase speed on a network. The easiest, conceptually, is to increase the bandwidth and allow faster changes of the data signal. This requires a high bandwidth medium and generates a considerable amount of high frequency electrical noise on copper cables, which is difficult to suppress. The second approach is to move away from the serial transmission of data on one circuit to a parallel method of transmitting over multiple circuits at each instant. A third approach is to use data compression techniques to enable more than one bit to be transferred for each electrical transition. A fourth approach used with 1000 Mbps gigabit Ethernet is to operate circuits full-duplex, enabling simultaneous transmission in both directions. All of the three approaches are used to achieve 100 Mbps fast Ethernet and 1000 Mbps gigabit Ethernet transmission on both fiber optic and copper cables using the current high-speed LAN technologies. 60 Practical TCP/IP and Ethernet Networking 4.1.1 Cabling limitations Typically most LAN systems use either coaxial cable, shielded (STP) or unshielded twisted pair (UTP) or fiber optic cables. The capacitance of the coaxial cable imposes a serious limit to the distance over which the higher frequencies that can be handled. Consequently 100 Mbps systems do not use coaxial cables. The unshielded twisted pair is obviously popular because of ease of installation and low cost. This is the basis of the 10Base-T Ethernet standard. The category 3 cable enables us to achieve only 10 Mbps while category 5 cables can attain 100 Mbps data rates, whilst the four pairs in the standard cable enable several parallel data streams to be handled. As we have seen fiber optic cables have enormous bandwidths and excellent noise immunity so are the obvious choice for high-speed LAN systems. 4.2 100Base-T (100Base-TX, -T4, -FX, -T2) This is the preferred approach to 100 Mbps transmission, which uses the existing Ethernet MAC layer with various enhanced physical media dependent (PMD) layers to improve the speed. These are described in the IEEE 802.3u and 802.3y standards as follow. IEEE 802.3u defines three different versions based on the physical media: • 100Base-TX which uses two pairs of category 5 UTP or STP • 100Base-T4 which uses four pairs of wires of category 3, 4 or 5 UTP • 100Base-FX which uses multimode or single-mode fiber optic cable IEEE 802.3y: • 100Base-T2 which uses two pairs of wires of category 3, 4 or 5 UTP Figure 4.1 Summary of 100Base-T standards This approach is possible because the original 802.3 specifications defined the MAC layer independently of the various physical PMD layers it supports. As you will recall, the MAC layer defines the format of the Ethernet frame and defines the operation of the CSMA/CD access mechanism. The time dependent parameters are defined in the 802.3 specifications in terms of bit-time intervals and so is speed independent. The 10 Mbps Ethernet interframe gap is actually defined as an absolute time interval of 9.60 microseconds, equivalent to 96 bit times; while the 100 Mbps system reduces this by ten times to 960 nanoseconds. One of the limitations of the 100Base-T systems is the size of the collision domain, which is 250 m. This is the maximum sized network in which collisions can be detected; being one tenth of the size of the maximum 10 Mbps network. This limits the distance Fast and gigabit Ethernet systems 61 between our workstation and hub to 100 m, the same as for 10Base-T, but usually only one hub is allowed in a collision domain. This means that networks larger than 200 m must be logically connected together by store and forward type devices such as bridges, routers or switches. However, this is not a bad thing, since it segregates the traffic within each collision domain, reducing the number of collisions on the network. The use of bridges and routers for traffic segregation, in this manner, is often done on industrial CSMA/CD networks. The dominant 100Base-T system is 100Base-TX, which accounts for about 95% of all fast Ethernet shipments. The 100Base-T4 systems were developed to use four pairs of category 3 cable; however few users had the spare pairs available and T4 systems are not capable of full-duplex operation, so this system has not been widely used. The 100Base- T2 system has not been marketed at this stage, however its underlying technology using digital signal processing (DSP) techniques is used for the 1000Base-T systems on two category 5 pairs. With category 3 cable diminishing in importance, it is not expected that the 100Base-T2 systems will become significant. 4.2.1 IEEE 802.3u 100Base-T standards arrangement The IEEE 802.3u standard fits into the OSI model as shown in Figure 4.2. You will note that the unchanged IEEE 802.3 MAC layer sits beneath the LLC as the lower half of the data link layer of the OSI model. Its physical layer is divided into the following two sub layers and their associated interfaces: • PHY – physical medium independent layer • MII – medium independent interface • PMD – physical medium dependent layer • MDI – medium dependent interface A convergence sub layer is added for the 100Base-TX and -FX systems, which use the ANSI X3T9.5 PMD layer which was developed for the reliable transmission of 100 Mbps over the twisted pair version of FDDI. The FDDI PMD layer operates as a continuous full-duplex 125 Mbps transmission system, so a convergence layer is needed to translate this into the 100 Mbps half-duplex data bursts expected by the IEEE 802.3 MAC layer. Figure 4.2 100Base-T standards architecture . Ethernet and 1000 Mbps gigabit Ethernet transmission on both fiber optic and copper cables using the current high-speed LAN technologies. 60 Practical TCP/IP and Ethernet Networking 4.1.1 Cabling. network. The easiest, conceptually, is to increase the bandwidth and allow faster changes of the data signal. This requires a high bandwidth medium and generates a considerable amount of high frequency. I and Class II repeater • Explain the packet bursting technique used by gigabit Ethernet • List the different media options used by gigabit Ethernet 4.1 Achieving higher speed Although Ethernet