2.21: A Robot Reaching for Communication

Let us imagine that Rob gets stand-alonely. He creates a "telephone system" with a string and two tin cans so as to communicate with the neighbouring robot Belle. A third robot, Sam, asks to join the network and insists on its own tin terminal. Whereas you needed only one string between Rob and Belle, you now need 3 strings - one between Rob and Belle, one between Rob and Sam, and one between Belle and Sam. An invitation to a fourth robot, Milly, would require 6 strings - one between Rob and Belle, one between Rob and Sam, one between Rob and Milly, one between Belle and Sam, one between Belle and Milly, and one between Sam and Milly.

This is beginning to get out of hand - so let us reduce it to a formula. Since each robot must be linked to each other - the number of links in the number of robots times the number of others (that is the number of robots minus one). However, since the link from Rob to Belle is the same as the link from Belle to Rob, the result must be divided by two. Figure 2-1a shows the relationship between the number of robots and the number of links between them. As you can see, the number of links increases quickly as the number of robots increases slowly.

As mentioned in Chapter 1, the number of telephone handsets in the world was estimated at 400 million in 1980 - you can imagine then the number of links required between them. One solution to this huge problem is, of course, not to link them in pairs. Groups of handsets are linked to other groups. Figure 2-2b shows how the number of wires is reduced in a simple case of ten handsets organized in two groups of five. This reduces the number of strings from (10 x 9) — 2 or 45 to 11 (count them).The nodes in this new network are called switching devices and the thick wire between them is called a trunk line.

This hierarchical organization of the telephone system enables you to zero in very quickly on one of the 400 million handsets. Let us say you want to call from Canada to your mother in Scotland. You dial 011 which gets you Great Britain, then 44 which gets you Scotland, then 505 which gets you Renfrewshire, then 843 which gets you Lochwinnoch, and then 364 which gets you the handset in your mother's home. Hello, Mum! (Actually, it is not that tidy but the example does serve to demonstrate the principle of zeroing in on a specific item from a large number of moving into smaller and smaller categories to which it belongs).

Note that the same principle is applied in getting a letter to one of the millions of homes in the world. You write the categories in reverse order - Mrs. J. K. Brown, 9 Calder Drive, Lochwinnoch, Renfrewshire, Scotland, Great Britain - and the mail system works backwards through this set of categories to deliver it. Hello, Mum! The Zip Code is an attempt to allow machines to automate mail delivery just as they have automated telephone call delivery. You may be familiar with the same divide-and-conquer principle in the game of Twenty Questions.