AT&T 7200 series manual Alternating Mark Inversion, Zero Code Suppression ZCS Line Coding

MULTIPLEXED COMMUNICATION 1-19

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Line Coding

Line coding is the pattern that data assumes as it is propagated over a communications channel. Governing line coding is a set of parameters that must be defined for all digital transmissions. These transmission parameters specify the voltage level and patterns in which 1s and 0s can appear on the line.

The parameters chosen for a given transmission stream must meet the requirements set by the hardware through which the data is to be transmitted. Most notable among these requirements are two established by the AT&T network. The first of these requirements states that the voltage on the line should be a net DC 0. Alternating mark inversion (AMI) was adopted to fulfill this requirement. The second requirement, known as the ones density requirement, states that in every stream of 15 consecutive bits, a one must appear. Zero code suppression (ZCS) and binary 8-zero substitution (B8ZS) line coding were adopted to meet this requirement.

Alternating Mark Inversion

All transmissions generated by DS1 ports are encoded in AMI. AMI was designed to fill the line repeater’s need for dependable and recurring voltage changes on the line. With AMI, every 1 in the transmission stream changes the line state alternately to either +3 volts or −3 volts. That is, every other 1 is represented as +3 volts, with the following 1 being represented as −3 volts. A zero is always represented as 0 volts. (See figure 1-5.)

v(t)
+3
								t
-3
0	1	0	1	1	0	1	1	1

Figure 1-5.Alternating Mark Inversion

Zero Code Suppression (ZCS) Line Coding

ZCS ensures that each transmitted byte contains at least one 1. DS1 ports that are administered for ZCS line coding arbitrarily insert a 1 in the second least-significant bit of each all-zeros byte.

The arbitrary insertion of 1s in the transmission stream presents a problem for data because of the obvious corruption that ZCS line coding produces. To avoid data corruption, at the originating data module the data is encoded with inverted HDLC or inverted LAPD protocol. (See appendix D for protocol explanations.) Using HDLC or LAPD with signal inversion (all 1s become 0s; all 0s become 1s) ensures that no byte will