TRANSMISSION AND SWITCHING: CORNERSTONES OF A NETWORK
A simplified functional diagram of an SPC exchange.
The call store is often referred to as the "scratch-pad" memory. This is temporary
storage of incoming call information ready for use, on command from the central proces-
sor. It also contains availability and status information of lines, trunks, and service circuits
under internal switch-circuit conditions. Circuit status information is brought to the mem-
ory by a method of scanning. All speech circuits are scanned for a busy/idle condition.
The program store provides basic instructions to the controller (central processor). In
many installations, translation information is held in this store (memory), such as DN to
EN translation and trunk signaling information.
A simplified functional diagram of a basic SPC system is shown in Figure 4.12.
Concentrators and Remote Switching
In Chapter 5 we discuss the design of a subscriber loop. There we will find that there are
very definite length limitations on subscriber loops. As we delve further into subscriber
loop design, methods of extending loops still further are described. One way to extend
such loops is with a remote concentrator or switch.
The simplest form of extending a switch is to use a concentrator some distance from the
switch (exchange). Concentrators or line concentrators consolidate subscriber loops, are
remotely operated, and are a part of the concentration and expansion portion of a switch
placed at a remote location. The concentrator may be based on electromechanical facilities
or solid-state cross-points for the concentration matrix. For instance, a 10:1 concentrator
might serve 100 subscriber loops and deliver 10 trunks to the "mother" exchange. A
concentrator does no switching whatsoever. All switching is carried out at the controlling
or "mother" exchange. A typical line concentrator is illustrated in Figure 4.13, where 100
subscriber loops are consolidated to 20 trunks plus 2 trunks for control from the nearby
"mother" exchange. Of course, the ratio of loops to trunks is a key issue, and it is based
on calling habits and whether the subscribers are predominantly business or residential.
A remote switch, sometimes called a satellite, or satellite exchange, originates and
terminates calls from the parent exchange. It differs from a concentrator in that local calls
(i.e., calls originating and terminating inside the same satellite serving area) are served by
the remote switch and do not have to traverse the parent exchange as remote concentrator
calls do. A block of telephone numbers is assigned to the satellite serving area and is
usually part of the basic number block assigned to the parent exchange. Because of the
numbering arrangement, a satellite exchange can discriminate between local calls and
calls to be handled by the parent exchange. A satellite exchange can be regarded as a
component of the parent exchange that has been dislocated and moved to a distant site.
The use of remote switching is very common in rural areas, and the distance a remote
switch is from the parent exchange can be as much as 100 miles (160 km). Satellite
exchanges range in size from 300 to 2000 lines. Concentrators are cost effective for 300
or less subscribers. However, AT&T's SLC-96 can serve 1000 subscribers or more.