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With 8B6T, the data to be transmitted is handled in 8-bit blocks. Each block of 8 bits is mapped into a code group of 6 ternary symbols. The stream of code groups is then transmitted in round-robin fashion across the three output channels (see Exhibit 1-5-6).


Exhibit 1-5-6.  6B6T Transmission Scheme

Exhibit 1-5-7 shows a portion of the 8B6T code table; the full table maps all possible 8-bit patterns into a unique code group of 6 ternary symbols. The mapping was chosen with two requirements in mind: synchronization and DC balance. For synchronization, the codes were chosen to maximize the average number of transitions per code group. The second requirement is to maintain DC balance, so that the average voltage on the line is zero. For this purpose, all of the selected code groups either have an equal number of positive and negative symbols or an excess of one positive symbol. To maintain balance, a DC balancing algorithm is used. In essence, this algorithm monitors the cumulative weight of the of all code groups transmitted on a single pair. Each code group has a weight of 0 or 1. To maintain balance, the algorithm may negate a transmitted code group by changing all plus symbols to minus symbols and all minus symbols to plus symbols so that the cumulative weight at the conclusion of each code group is always either 0 or 1.


Exhibit 1-5-7.  4B5B Code

5B6B Code

As with 100BaseT4, a key objective of the 100VG-AnyLAN effort is to be able to achieve 100M bps over short distances using ordinary voice-grade (i.e., Category 3) cabling. To meet the objective, 100VG-AnyLAN specifies a novel encoding scheme that involves using four pairs to transmit data in a half-duplex mode. Thus to achieve a data rate of 100M bps, a data rate of only 25M bps is needed on each channel. An encoding scheme known as 5B6B is used.

The 5B6B encoding is used in the 100VG-AnyLAN specification. The bit stream to be transmitted is divided into 5-bit chunks (quintets) and each successive chunk is transmitted over a different channel in round-robin fashion. To achieve a data rate of 100M bps, a data rate of only 25M bps is needed on each channel.

The 5B6B scheme, which is used to ensure adequate transitions on each line for synchronization, is based on the same strategy as the 4B5B scheme. In this case, each group of 5 input bits is mapped into a set of 6 output bits. Thus for an effective data rate of 25M bps, a signaling rate of 30M baud is required.

With the 5B6B scheme, there are 32 possible 5-bit inputs. The ideal situation would be to assign each 5-bit input a 6-bit code that has an equal number of ones and zeros. This would maintain a DC balance of zero. However, there are only twenty 6-bit code words that have three ones and zeros. These codes are assigned to 20 of the input patterns. For the remaining 12 input patterns, two code words are assigned, one with four zeros and two ones (mode 2) and one with two zeros and four ones (mode 4).

Successive instances of any of these 24 unbalanced code words must alternate between mode 2 and mode 4 output to maintain balance. If, during reception, a station or repeater receives two of the same type of unbalanced words in a row (with any number of intervening balanced words), the receiver knows a transmission error has occurred and will ask for a retransmission of the data.

Exhibit 1-5-8 shows the complete 5B6B encoding scheme. There is a unique output code word for 12 of the input patterns. For the rest, the transmitter keeps track of whether the last unbalanced transmitted word was mode 2 or mode 4 and transmits the appropriate output code word to maintain balance.


Exhibit 1-5-8.  8B6T Transmission Scheme

8B10B Code

Fibre Channel is a new switched-LAN specification designed for use over optical fiber at data rates up to 800M bps. The encoding scheme used for Fibre Channel is 8B10B, in which each 8 bits of data is converted into 10 bits for transmission. This scheme has a similar philosophy to the 4B5B scheme used for FDDI. The 8B10B scheme is more powerful than 4B5B in terms of transmission characteristics and error detection capability. The advantages of this code are that it:

  Can be implemented with relatively simple and reliable transceivers at low cost.
  Is well-balanced, with minimal deviation from the occurrence of an equal number of 1 and 0 bits across any sequence.
  Provides good transition density for easier clock recovery.
  Provides useful error detection capability.

With 8B10B, each group of 8 input bits is mapped into a 10-bit code block. There is also a function called disparity control, which keeps track of the excess of 0s over 1s or 1s over 0s. If there is an excess in either direction, this is referred to as a disparity. If there is a disparity, and if the current code block would add to that disparity, then the disparity control block complements the 10-bit code block. This has the effect of either eliminating the disparity or at least moving it in the opposite direction of the current disparity.

SUMMARY

The evolution of LANs to higher speeds, sometimes over lower-quality transmission media, has sparked a corresponding evolution in digital signal encoding techniques. The NRZ codes commonly used in computing equipment and for magnetic recordin g are not efficient enough for high-speed LAN use. Even Manchester and Differential Manchester, developed for Ethernet and Token Ring systems, fail to meet evolving needs. In recent years, a variety of schemes that combine bit mapping and signal encoding have appeared to meet the need.


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