MAS.632 Conversational Computer Systems - MIT OpenCourseWare

248 VOICE (COMMUNICATION WITH COMPUTERS
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Figure 11.11. A server allows centralization of some resources with a network
interface to each workstation.
switch
An alternative arrangement is for the server to communicate with the switch
directly through a custom interface. Some experimental systems, such as MICE,
use small PBXs designed to be controlled by a host computer. Larger PBXs are
less flexible but may still provide a simple computer interface. For example, some
switches support a message set called SMDI (Simple Message Desk Interface),
which carries selected signaling information from the switch over a serial line.
Interfaces such as SMDI are often used by voice mail systems in the following
fashion: When a call gets transferred to voice mail, the switch identifies the originally
dialed number and a trunk line switches onto that which the call has been
transferred so that the voice mail system can record a message into the proper
mailbox. The signaling interface is bidirectional so the voice mail system can control
the telephone's message waiting light via the switch. Such a PBX interface
can extend call control to a single workstation, which communicates with the
PBX and acts as a server to other workstations. SCAI (Switch-Computer Application
Interface) is a proposed standard for external switch control. It is currently
under study by the T1S1 committee of ANSI (the American National Standards
Institute).