Modified AMI code
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Modified AMI code is a T-carrier AMI (alternate mark inversion) line code in which bipolar violations may be deliberately inserted to maintain system synchronization.
The clock rate of an incoming T-carrier signal is extracted from its bipolar line code. T-carrier was originally developed for voice applications. When voice signals are digitized for transmission via T-carrier, there is no problem in maintaining system synchronization, because of the nature of the digitized signals. However, when used for the transmission of digital data, the conventional AMI line code may fail to have sufficient marks, i.e., "1's," to permit recovery of the incoming clock, and synchronization is lost. This happens when there are too many consecutive zeros in the user data being transported. To prevent loss of synchronization when a long string of zeros is present in the user data, deliberate bipolar violations are inserted into the line code, to create a sufficient number of marks to maintain synchronization. The receive terminal equipment recognizes the bipolar violations and removes from the user data the marks attributable to the bipolar violations.
The exact pattern of bipolar violations that is transmitted in any given case depends on the line rate and the polarity of the last valid mark in the user data prior to the unacceptably long string of zeros.
The number of consecutive zeros that can be tolerated in user data depends on the data rate, i.e., the level of the line code in the T-carrier hierarchy. The North American T1 line code (1.544 Mbit/s) did not originally use bipolar violations, but it is now common to use bipolar violations when 8 or more consecutive zeros are present. This line code is called bipolar with eight-zero substitution (B8ZS). B8ZS is also used in the European E1 rate (2.048 Mbit/s). In all levels of the European T-carrier hierarchy, the patterns of bipolar violations that are used differ from those used in the North American hierarchy. At the North American T2 rate (6.312 Mbit/s), bipolar violations are inserted if 6 or more consecutive zeros occur. This line code is called bipolar with six-zero substitution (B6ZS). At the North American T3 rate (44.736 Mbit/s), bipolar violations are inserted if 3 or more consecutive zeros occur. This line code is called "bipolar with three-zero substitution" (B3ZS).
This article contains material from the Federal Standard 1037C, which, as a work of the United States Government, is in the public domain.
