Electronic Interface Dictionary Terms

MAC: Abbreviation for medium access control.

Manchester code: A code in which (a) data and clock signals are combined to form a single self synchronizing data stream, (b) each encoded bit contains a transition at the midpoint of a bit period, (c) the direction of transition determines whether the bit is a 0 or a 1, and (d) the first half is the true bit true bit value. Contrast with non-return-to zero.

Mean Time Between Failures (MTBF): An indicator of expected system reliability calculated on a statistical basis from the known failure rates of various components of the system. More on MTBF

Microprocessor: A central processing unit implemented on a single chip. [uP Manufacturers]

Modem: Acronym for modulator/demodulator. In general, a device that both modulates and demodulates signals. In computer communications, a device used for converting digital, signals into, and recovering them from, quasi-analog signals suitable for transmission over analog communications channels. [PC Modem Manufacturers]

Modified AMI code: A T-carrier AMI 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 Mb/s) does not use bipolar violations. The European T1 line code (2.048 Mb/s) may use bipolar violations when 8 or more consecutive zeros are present. This line code is called bipolar with eight-zero substitution (B8ZS). (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 Mb/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 Mb/s), bipolar violations are inserted if 3 or more consecutive zeros occur. This line code is called bipolar with three-zero substitution (B3ZS).