Switching to VoIP

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Time Division Multiplexing

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signal is transmitted at a data rate of 64 kbps, which is 8 bits times 8,000 samples per

second. Lined up one after the other, 8,000 times per second, each 8-bit word forms

a stream of binary data that travels the TDM bus in a single time slice. The common

name for a 64 kbps voice channel is DS0, and the signaling and encoding technique

is called PCM, or pulse code modulation.

DS0 channels are the building blocks of ISDN and T1 voice circuits, too. Twenty-

four of them together comprise a T1. This is why T1s have a maximum bandwidth of

1.5 Mbps, which is the same as 24 times 64 kbps.

Channel Banks

Channel banks are devices that multiplex (combine) separate digital voice signals into

a single T1 circuit and demux (split) it back into its individual signals again. In a nut-

shell, a channel bank combines up to 24 separate phone lines into a single digital

link.

Channel banks are not DSU/CSUs, which are data transmission devices, though

some DSU/CSU devices can perform channel bank functions like connection of one

or more single analog phone lines to a T1. For example, certain AdTran DSU/CSUs

Figure 4-7. An analog sample of part of a waveform signal

Figure 4-8. A digitized sample of the same part of the waveform signal in Figure 4-7

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