a. The medium might be wire-pair cable, fiber-optic cable, microwave radio, and, stretching the imagination, satellite communications. In the conventional telephone plant, coaxial cable has fallen out of favor as a transmission medium for this application. Of course, in the long-distance plant, satellite communication is fairly widely employed, particularly for international service. Our reference above was for local service. 1.3.2 Telephone Numbering and Routing Every subscriber in the world is identified by a number, which is geographically tied to a physical location.4 4 This is the telephone number. The telephone number, as we used it 4This will change. At least in North America, we expect to have telephone number portability. Thus, whenever one moves to a new location, she/he takes their telephone number with them. Will we see a day when telephone numbers are issued at birth, much like social security numbers? INTRODUCTORY CONCEPTS above, is seven digits long. For example: 234-5678 The last four digits identify the subscriber line; the first three digits (i.e., 234) identify the serving switch (or exchange). For a moment, let's consider theoretical numbering capacity. The subscriber number, consisting of the last four digits, has a theoretical numbering capacity of 10,000. The first telephone number issued could be 0000; the second number, if it were assigned in sequence, would be 0001, the third would be 0002, and SO on. At the point where the numbers ran out, the last number issued would be 9999. The first three digits of the example above contain the exchange code (or central office code). These three digits identify the exchange or switch. The theoretical maximum capacity is 1000. If again we assign numbers in sequence, the first exchange would have 001, the next 002, then 003, and finally 999. However, particularly in the case of the exchange code, there are blocked numbers. Numbers starting with 0 may not be desirable because in North America 0 is used to dial the operator. The numbering system for Nor