A New Class of (2p)B(2p+1)B d.c. Balanced Line Codes

A block code of the form (2p)B(2p+1)B cannot provide both d.c. balance and redundant codewords for control signals. We describe a new set of additional rules that may be applied to groups of consecutive codewords in order to provide both d.c. balance and a large control space, and show how 256 10-bit control symbols may be achieved with a d.c. balanced 4B5B code. Introduction: nBmB block coding, whereby n bits of data are mapped to m > n bits, is often used in data communication links in order to provide some redundant signal space which may be used for control signals. The transmission overhead introduced by the block code should be kept minimal, and so codes of the type nB(n+1)B are popular. It is also desirable that the size of the input data field (n) is as small as possible and is a factor of 8. These two properties enable low complexity designs for byteoriented data. The 4B5B code used by FDDI [1] and 100BaseTX [2] is an example of an nB(n+1)B code that meets these criteria. A further desirable property of a block code is that of d.c. balance. Data is often transmitted through a.c. coupled links (e.g. copper twisted pairs or optical fibres), and any d.c. content in the coded data may cause signal distortion, commonly referred to as baseline wander. A d.c. balanced code reduces baseline wander by ensuring the disparity of coded data (i.e. the difference between the number of ones and the number of zeros) is zero. For an nB(n+1)B code with n even, i.e. a (2p)B(2p+1)B code, d.c. balance can only be achieved by using two alphabets, for example one containing all codewords having positive disparity and one containing all codewords having negative disparity (there are no codewords with zero disparity). The running disparity (rd) is constrained by choosing codewords from the appropriate alphabet: if rd > 0, select a codeword having negative disparity; if rd <= 0, select a codeword having positive disparity. Unfortunately, such a scheme results in all codewords being used in the two data alphabets leaving no redundant codewords for control signalling. In practice, codes such as the FDDI 4B5B must provide some control codewords and are therefore not d.c. balanced. When n is odd, zero disparity codewords may be shared between the two alphabets leaving some redundant codewords. An example of this is the 5B6B code used by IEEE 802.12 networks [3]. However, these codes are harder to implement with byte-oriented data.