Information Technology Reference
In-Depth Information
100BASE-T allows compatibility with 10BASE-T in that the first two pairs (Pair 1 and Pair
2) are used in the same way as 10BASE-T connections. 100BASE-T then uses the other two
pairs (Pair 3 and Pair 4) with half-duplex links between the hub and the node. The connec-
tions are illustrated in Figure 26.
13.
26.9.2 Line code
100BASE-4T uses four separate Cat-3 twisted-pair wires. The maximum clock rate that can
be applied to Cat-3 cable is 30 Mbps. Thus, some mechanism must be devised which reduces
the line bit rate to under 30 Mbps but gives a symbol rate of 100 Mbps. This is achieved with
a three-level code (+, - and 0) and is known as 8B6T. This code converts eight binary digits
into six ternary symbols.
Node
Hub
Pair 1
Compatible with
10BASE
Pair 2
Pair 3
Tx
Rx
Rx
Tx
Pair 4
Tx
Rx
Rx
Tx
Figure 26.13
100BASE-4T connections
The first six codes are:
Data byte
Code
Data byte
Code
Data byte
Code
00000000
-+0 0-+
00000001
0-+ -+0
00000010
0-+ 0-+
00000011
00000100
00001001
0-+ +0-
-+0 +0-
+0- -+0
Thus, the bit sequence 00000000 will be coded as a negative voltage, a positive voltage, a
zero voltage, a zero voltage, a negative voltage and a positive voltage.
The maximum base frequency for a 100 Mbps signal will be produced when the input
bitstream is 010101010 …01010. As each bit lasts 10 ns then the period between consecutive
levels is 20 ns. Thus, the minimum frequency contained will be 50 MHz. This is greater than
the bandwidth of Cat-3 cable, so it would not pass through the cable.
Apart from reducing the frequencies with the digital signal, the 8B6T code has the advan-
tage of reducing the DC content of the signal. Most of the codes contain the same number of
positive and negative voltages. This is because only 256 of the possible 729 (3
6
) codes are
actually used. The codes are also chosen to have at least two transitions in every code word,
thus the clock information is embedded into the signal.
Unfortunately, it is not possible to have all codes with the same number of negative volt-
