Published April 2002

ETHERNET TECHNOLOGY

Part 2: The 100- and 1000-Mbps Ethernet
by James Antonakos

Start • 100BASET4 • Fast Link Pulses • Gigabit Ethernet • Wireless Ethernet • Sources and PDF

100BASET4

In the 100BaseT4 technology, 8B6T coding replaces 8-bit data values with six ternary codes, which may have the values –, +, or 0. Table 2 shows a small sample of the 256 code patterns used in 8B6T encoding. The patterns are chosen to provide good DC characteristics, error detection, and reduced high-frequency effects. Special patterns can also be used as markers or control codes.

A multilevel signaling scheme is used, which allows more than one bit of data to be encoded into a signal transition. This is why a 12.5-MHz frequency carries a 33.3-Mbps stream. Think about it this way: each cycle of the 12.5-MHz carrier contains two levels. This gives 25 million level changes per second on a single UTP pair. The signals on each of the three UTP pairs change a total of 75 million times each second. Dividing 75 million levels per second by six levels per 8B6T symbol gives 12.5 million symbols per second. Each symbol is equivalent to a unique 8-bit pattern, so multiplying 12.5 million symbols per second by 8 bits per symbol gives 100-million bps, the required data rate. Figure 2 shows a sample 8B6T encoded waveform. Note that the 12.5-MHz signaling frequency is within the 16-MHz limit for the Cat 3 cable.

Figure 2—An example of how 8 bits of data are encoded into a sequence of six ternary codes can be seen here.

100BASETX

Table 3 shows the 4B5B encoding for all 16 4-bit data patterns. Notice that there is always a mixture of ones and zeros in each 5-bit pattern. This is done to prevent long strings of ones or zeros from being encoded, which contributes to loss of synchronization on the signal.

Figure 3 shows how a three-level signal called multiple level transition (MLT-3) is used to represent the 4B5B bitstream. Each 4-bit data value is replaced by its 5-bit 4B5B counterpart. Thus, the 100-Mbps datastream becomes a 125-Mbps 4B5B encoded datastream. Using MLT-3 allows the 125-Mbps 4B5B datastream to be carried using a signal rate of 31.25 MHz (31.25 MHz × 4 bits per cycle = 125 Mbps). Because the signaling frequency of 31.25 MHz is greater than the 16-MHz limit of the Cat 3 cable, a better cable, Cat 5, is required. Cat 5 cable has a frequency limit of 100 MHz.

100BASEFX

In this technology, the 4B5B encoded data is transmitted using non-return-to-zero, invert-on-one (NRZI) signaling. A 4B5B data rate of 125 Mbps is obtained using a 62.5-MHz carrier. Figure 4 illustrates a sample encoding and waveform. NRZI is well suited for fiber because of its bilevel nature.

Figure 4—Eight bits of data are 4B5B encoded and transmitted using NRZI signaling.

100BASET2

Sending 100 Mbps over only two pairs of UTP requires yet another encoding and signaling scheme. In the 100BaseT2 technology, two five-level pulse amplitude modulation (PAM) signals are sent over the UTP pairs, with a signaling rate of 12.5 MHz. Each cycle of the signal provides two PAM5x5 level changes, so there are 25 million level changes per UTP pair. Each pair of PAM signals (called A and B) encode a different 4-bit pattern (along with other, special patterns for Idle mode) using combinations of these levels: +2, +1, 0, –1, –2. So, 25-million PAM5x5 pairs × 4 bits per pair = 100 Mbps.

Figures 5a–c show the symbol constellations found in PAM5x5 encoding, as well as a sample pair of waveforms. Note that when it is not transmitting data, the 100BaseT2 link transmits an idle signal to maintain synchronization. During Idle mode, the signals on A and B alternate between +1 and –1, and +2, 0, and –2.

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