Digital Signal Encoding Techniques for Gigabit Ethernet
The encoding scheme used for all of the Gigabit Ethernet options except twisted pair is 8B/10B. This scheme is also used in Fibre Channel. With 8B/10B, each 8 bits of data is converted into 10 bits for transmission. The 8B/10B scheme was developed and patented by IBM for use in its 200 megabaud ESCON interconnect system.
The developers of this code list the following advantages:
It can be implemented with relatively simple and reliable transceivers at low cost.
It is well balanced, with minimal deviation from the occurrence of an equal number of 1 and 0 bits across any sequence.
It provides good transition density for easier clock recovery.
It provides useful error-detection capability.
The 8B/10B code is an example of the more general mBnB code, in which m binary source bits are mapped into n binary bits for transmission. Redundancy is built into the code to provide the desired transmission features by making n > m. For 8B/10B, a mapping is defined that maps each of the possible 8-bit source blocks into a 10-bit code block. There is also a function called disparity control. In essence, this function keeps track of the excess of zeros over ones or ones over zeros. An excess in either direction is referred to as a disparity. If there is a disparity, and if the current code block would add to that disparity, 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.
The encoding mechanism also includes a control-line input, K, which indicates whether the lines A through H are data or control bits. In the latter case, a special nondata 10-bit block is generated. A total of 12 of these nondata blocks are defined as valid in the standard. These are used for synchronization and other control purposes.
For twisted-pair Gigabit Ethernet, the encoding scheme used is PAM-5, over four twisted-pair links. Therefore, each link must provide a data rate of 250 Mbps. PAM-5 provides better bandwidth utilization than simple binary signaling by using five different signaling levels. Each signal element can represent two bits of information (using four signaling levels), plus a fifth signal level is used in a forward error-correction scheme.