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Chapter 6. Ethernet > 100BaseT2

6.5. 100BaseT2

100BaseT2 is the latest member of the 100BaseT 100-Mbps Ethernet group. The 100BaseT2 IEEE activity was established in March 1995, and the standard was approved in March 1997. 100BaseT2 is the only Ethernet standard that supports a 100-Mbps transmission rate over two pairs of Category 3 twisted pair cabling. If the cable has more than two twisted pairs, it also permits the additional pairs to carry other services such as digital phone, 10BaseT, or more 100BaseT2 connections. Extensive digital signal processing techniques, such as fractionally spaced adaptive channel equalization, echo cancellation, and NEXT noise cancellation, are utilized to enable a very high transmission throughput over this voice-grade Category 3 twisted pair cable.

100BaseT2 employs a “dual duplex baseband transmission” scheme to transmit data over each wire pair in each direction simultaneously. In a 100BaseTX and 100BaseT4 tradition of utilizing zero voltage as a possible signal level, 100BaseT2 transmits quinary (five-level) data symbols that can have values of –2, –1, 0, +1, or +2 on each twisted pair. Each quinary symbol carries only two bits. In comparison with a four-level signaling system of the same level separation value, the use of quinary symbols has an energy penalty of 5/6 in return for some coding redundancy. In fact, each pair of two bits is first mapped to only four signal levels: +1, 0, –1, and –2. The +2 signal level is produced through the sign reverse operation under the control of a binary pseudo-random sequence. Therefore, the probability of occurrence for +2 and –2 is only half that for +1, 0, and –1 signal levels. With two pairs each carrying five signal levels, a joint constellation of 5 by 5 is created for carrying four information bits during each symbol time interval. The 100-Mbps transmission throughput is achieved with a symbol rate of 25 MHz.


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