Numbering, on basis of switching
A telephone subscriber looking into a telecommunication network sees a repeatedly branching tree of links. At each branch point there are multiple choices. Assume that a calling subscriber wishes to contact one particular distant subscriber. To reach that distant subscriber, a connection is built up utilizing one choice at each branch point. Of course, some choices lead to the desired end point, and others lead away from it. Alternative paths are also presented. A call is directed through this maze, which we call a telephone network, by a telephone number. It is this number that activates the switch or switches at the maze branch point(s).
Actually, a telephone number performs two important functions: (1) It routes the call, (2) it activates the necessary equipment for proper call charging. Each telephone subscriber is assigned a distinct number, which is cross referenced in the telephone directory with the subscribers name and address; in the local serving exchange (switch), this number is associated with a distinct subscriber line.
If a subscriber wishes to make a telephone call, she lifts her receiver off hook (i.e., takes the handset out of its cradle) and awaits a dial tone that indicates readiness of her serving switch to receive instructions. These instructions are the number that the subscriber dials (or the buttons that she punches) giving the switch certain information necessary to (a) route the call to the distant subscriber with whom she wishes to communicate and (b) set up the call charging equipment.
A subscriber number is the number to be dialled or called to reach a subscriber in the same local (serving) area. Remember that our definition of a local serving area is the area served by a single switch (exchange). The thinking that follows ties that switch capacity in total lines to the number of digits in the telephone number.
If we had a switch with a capacity of 100 lines,
It could serve up to 100 subscribers and we could assign telephone numbers 00 through 99.
If we had a switch with a capacity of 1000 lines,
It could serve up to 1000 subscribers and we could assign telephone numbers 000 through 999.
If we had a switch with a capacity of 10,000 lines,
It could serve up to 1000 subscribers and we could assign telephone number 0000 through 9999.
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