Switching to VoIP

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Chapter 4: Circuit-Switched Telephony

Time Division Multiplexing

Traditional PBX systems use a digital bus to carry sound information between inter-

faces where phones and/or trunks are connected. The signals flowing across this bus

are digitized audio that travel in an aggregate form—that is, one bus can carry many

separate signals within a single bit stream. The transmission technique is called time

division multiplexing (TDM). To understand how all modern telephony solutions

work, including VoIP, a basic understanding of TDM is important.

Multiplexing means combining many signals onto a single transport mechanism,

such as a T1 or a PBX bus. This bus can be a connection between two points—like a

point-to-point T1 circuit, or it can be a large group of digital phones, like a PBX’s

bus.

Time division is the method of combining, and later dividing, the signals, with the

purpose of yielding greater efficiency over the data link, be it a T1 circuit or a PBX

backplane. Each signal is given a time slice, a small piece of the total bandwidth of

the bus. At a very high rate, a TDM bus transmits a fixed sequence of time slices that

are equal in duration. Each time slice contains a digitally sampled representation of

the original analog waveform signal.

Each endpoint pulls a particular time ...