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.
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