The size of an exchange area (also called a serving area) obviously will depend largely on subscriber (or potential subscriber) density and distribution. Subscriber traffic is another factor to be considered. If statistics on subscriber traffic intensity are not available, use Table 2.8 based on ITU-T data.
Exchange sizes are often in units of 10,000 lines. Although the number of subscribers initially connected should be considerably smaller than when an exchange is installed, 10,000 is the number of subscribers that may be connected when an exchange reaches "exhaust," where it is filled and no more subscribers can be connected.
Ten thousand is not a magic number, but it is a convenient one. It lends itself to crossbar (switch) unit size and is a mean unit for subscriber densities in suburban areas and mid-sized towns in fairly well developed countries. More important, though, is its significance in telephone numbering (the assignment of telephone numbers). Consider a seven-digit number. Now break that down into a three-number group and a four-number group. The first three digits—that is, the first three dialed—identify the local exchange. The last four identify the individual subscriber and is called the subscriber number. Note the breakdown in the following sample:
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