Modified AMI code

Modified AMI codes are a digital telecommunications technique to maintain system synchronization. Alternate Mark Inversion (AMI) line codes are modified by deliberate insertion of bipolar violations. There are several types of modified AMI codes, used in various T-carrier and E-carrier systems.

The clock rate of an incoming T-carrier is extracted from its bipolar line code. Each signal transition provides an opportunity for the receiver to see the transmitter's clock. The AMI code guarantees that transitions are always present before and after each mark (1 bit), but are missing between adjacent spaces (0 bits). To prevent loss of synchronization when a long string of zeros is present in the payload, deliberate bipolar violations are inserted into the line code, to create a sufficient number of transitions to maintain synchronization; this is a form of run length limited coding. The receive terminal equipment recognizes the bipolar violations and removes from the user data the marks attributable to the bipolar violations.

T-carrier was originally developed for voice applications. When voice signals are digitized for transmission via T-carrier, the data stream always includes ample 1 bits to maintain synchronization. (To help this, the μ-law algorithm for digitizing voice signals encodes silence as a continuous stream of 1 bits.) However, when used for the transmission of digital data, the conventional AMI line code may fail to have sufficient marks 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.

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