Avaya S8700 Maintenance Manual page 40

Maintenance Architecture
Data cannot be carried over analog facilities unless inside the system it is
represented as a PCM-encoded analog signal. To do this for data
originating at a digital terminal, the signal enters the system at a digital port
and exits the system at a digital port. The signal then reenters the system
through a modem-pool connection (data-module to modem to analog-port)
and exits the system again at an analog port.
Although DS1 is commonly called a trunk speed, here it names the protocol
used at layer 1 for digital trunks. Some trunks use different signaling
methods but use DS1 protocol at layer 1 (for example, PRI and
24th-channel signaling trunks).
1-16 Issue 1 May 2002 555-233-143