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multi level transmission (MLT)

MLT coding, Multi Level Transmission, is a ternary method that uses three-valued signal levels (+V, 0 V, -V) for coding. It is called MLT3 coding because, unlike the well-known two-level methods Non Return to Zero (NRZ), Non Return to Zero Inverted (NRZ-I) and others, the level can take three level values.

The advantage of this Multi Level Coding (MLC) is the reduction of the base frequency, which results in lower cabling requirements. In addition, radiation is reduced and bit error rates are minimized. However, ternary methods require the receiver to distinguish between three level values.

MLT-3 coding

MLT-3 coding

The MLT method is used in the form of MLT-3 for FDDI over TP cables. Here, 4 bits are transmitted in one transmission oscillation. This means that the actual transmission frequency is reduced to a quarter of the actual value. The transmitted signal experiences a level change only at a logical "1". If the level is positive at the first logical "1", it becomes zero at the following logical "1" and negative at the one after that. Since FDDI uses 100 Mbit/s for user data transmission and 25 Mbit/s for control data, this results in a total transmission rate of 125 Mbit/s, or a transmission frequency of 31.25 MHz on the cable.

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Englisch: multi level transmission - MLT
Updated at: 11.10.2013
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