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Time Division Multiplexing


Definition
1. Time Division multiplexing, or TDM, is the process used by digital trunks Such as T-1 or E-1 and digital voice buses such as MVIP, PEB or SC bus to transmit audio and signaling information. The data is transmitted as a series of frames. Each frame is divided into 24 (T-1) or 32 (E-1) different time-slots, which are 8 bits long. A complete frame is then 24 x 8 or 32 x 8 bits long, together with a one or more extra bits for synchronization. The 24 or 32 channels are combined at one end of the trunk and split apart at the other end. This process happens so quickly that each conversation is seamless.

In telephony applications, each "slot" is usually an 8-bit sample of the amplitude of the sound in the conversation, encoded using PCM (Pulse Code Modulation). There are 8,000 samples transmitted per second. The number represented by the bit sample is a direct measure of the loudness (amplitude) of the sound, converted on a non-linear scale. Unfortunately, E-1 and T-1 use different scales to do the conversion: the scale usually used by T-1 is called the Mu-law scale, E-1 generally uses A-law.

2. TDM

The division of a transmission facility into multiple channels by allotting the facility to different channels, one at a time at regular intervals.

Related Terms:

Bandwidth