Switching Equipment - AT&T MERLIN LEGEND Release 3.1 System Manager's Manual

About Telecommunications
The services provided by channels can greatly enhance a company's
effectiveness and efficiency. If the MERLIN LEGEND Communications System
has a DS1 facility as one of its incoming trunks, the one facility provides 24
"lines" that are called channels or B-channels , depending on the type of service
the facility is programmed for (T1 or PRI). If programmed for T1 service, the
channels can be configured to emulate different types of trunks (loop-start,
ground-start, DID, and tie) according to business needs, and can provide a
variety of special services. If programmed for PRI service, additional special
services are available and each B-channel can be programmed to dynamically
provide different services over the same B-channel.

Switching Equipment

The primary function of switching equipment is to interconnect the transmission
facilities at various key locations and route the telephone signals through the
telephone network. The telephone network is composed of a number of these
key centralized locations called central offices (COs). At the COs, the electrical
signal carried on one telephone circuit is connected, or switched, to another
circuit, forming a communications path; that is, the caller's line is connected to
the called party's line so they can hold a conversation.
In the early telephone network, switching was performed manually by human
operators. Today, electronic computer-controlled switching systems are fast,
flexible, and economical, providing reliable, efficient, and cost-effective service.
There are now also private switches that, rather than being located at the
telephone company's CO, are located on a company's premises. These
systems, called private branch exchanges (PBXs), were developed because
most of a business' calls are between telephones on-site within the company.
The MERLIN LEGEND Communications System is a such a switch, located on
the customer's premises, that provides advanced services and features and yet
also provides many of the special functions originally performed by human
operators.
The Evolution of Switches
The method, type, capabilities, and capacities of switches have evolved as
geographic areas expanded and technological advances became available.
Three basic types of technology have been used in switching:
Manually-Operated Switching. Human operators made the connections
of circuits by placing plug-ended wires into jacks on a switchboard.
Manual switching was slow, labor-intensive and, therefore, costly but
afforded some special functionality: calls could be forwarded, messages
taken, and calls interrupted.
System Manager's Guide
B–5