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Chapter 6. Ethernet > 100BaseT4

6.3. 100BaseT4

All versions of Ethernet with a transmission throughput of 100 Mbps belong to the group of 100BaseT. They include 100BaseT4 on four pairs of a Category 3 or better twisted pair cable, 100BaseTX on two pairs of a Category 5 twisted pair cable, 100BaseFX on two multimode fibers, and 100BaseT2 on two pairs of a Category 3 or better twisted pair cable. All twisted pair–based 100BaseT Ethernet will be discussed in the remainder of this chapter. All these twisted pair–based Ethernet versions use the same RJ-45 connector. To recognize each Ethernet transceiver's physical layer capabilities and to obtain the best performance match between multimode transceivers, an autonegotiation process, which will be discussed at the end of this chapter, is normally used at the beginning of each transmission session. On the other hand, a Media Independent Interface between different physical layers and the common MAC layer is also defined as a part of the Ethernet standard.

All four twisted pairs of a Category 3 cable are occupied by 100BaseT4. Among these four pairs, one is always used for transmitting, another one is always used for receiving, and the remaining two are used for either activity, depending on the data traffic direction. The 100-Mbps transmission throughput is equally divided among three transmitting pairs. Each byte (8 bits) group of binary information bits on one twisted pair is further converted into a group of six ternary symbols of three possible values: –1, 0, and +1. The symbol baud rate on each twisted pair is therefore 100/3 x 6/8 = 25 MHz. The always transmitting pair is connected to pin #1 and pin #2 and the always receiving pair is connected to pin #3 and pin #6 on an RJ-45 connector for a 100BaseT4 NIC as shown in Figure 6.24. These transmitting and receiving pair pin positions are consistent with those of 10BaseT; therefore, the autonegotiation procedure can be carried out without signaling conflict. Two 100BaseT4 NICs can also be connected directly with a crossover cable as defined in Figure 6.25.


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