AT&T 2500 series, 7500 series manual Statistical Multiplexing, Multiplexing Over DS1 Facilities

1-18MULTIPLEXED COMMUNICATION

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Time-division multiplexing can be bit interleaved or byte interleaved. With bit interleaving, the time slot assigned to a particular channel is one bit long and the channel recurs every 15.5 microseconds over a DS1 link. With byte interleaving, the time slot is one byte long and recurs every 125 microseconds. (See figure 1-4.)For voice communication, each byte-interleaved time slot carries a byte representing a PCM-encoded sample. For data communication, it carries the next sequential byte of information.

Byte-interleaved multiplexers are not compatible with bit-interleaved multiplexers.

Statistical Multiplexing

Statistical multiplexers dynamically assign signals from incoming lines onto a single outgoing communications path. This outgoing path is time-division multiplexed, but the time slots are assigned on demand, making it possible for the number of incoming lines to be greater than the number of outgoing time slots.

MULTIPLEXING OVER DS1 FACILITIES

The hierarchy of digital transmission rates is given as "digital signal" (DS) numbers. DS numbers increase from 0 to 4 as follows:

DS		Number of
Number	Speed	DS0 Channels
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DS0	64 kbps	1
DS1	1.544 Mbps	24
International DS1	2.048 Mbps	30
DS1C	3.152 Mbps	48
DS2	6.312 Mbps	96
DS3	44.736 Mbps	672
DS4	274.176 Mbps	4032

DS0 is the lowest rate and consists of a single 64-kbps digital channel. The DS0 data rate is achieved by sampling the signal 8000 times a second and encoding each sample into an 8-bit byte. Above DS0, DS1 is the first level at which multiplexing occurs and the higher levels represent the multiplexing of two or more DS1 sources (plus additional overhead).

The System 75, System 85, and DEFINITY Generic 1 (G1) and Generic 2 (G2) communications systems multiplex up to the DS1 level. In fact, DS1 facilities, like T1 trunks, are the medium through which many time-division multiplexed signals are carried to and from the switch. Communication over these facilities is managed through the DS1 port on the System 75, System 85, and DEFINITY G1 and G2 switches. How line coding, framing, and signaling are performed over a particular T1 trunk depends upon how the DS1 port is administered, which, in turn, depends upon the types of equipment through which the signal travels after it exits the switch. (For further clarification of the DS1 protocol, see DEFINITY Communications System and System 75 and System 85 DS1/DMI/ISDN PRI Reference, 555-025-101.)