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^^ by Bert Latamore

Networks Wire You to the World

Strung out between the terminals and microcomputers of America and the computer banks of The Source, Compuserve and several hundred other timesharing services worldwide, are thousands of miles of leased telephone lines.

Originally designed for use by larger computers, this international system of linked common-carrier networks and the vast majority of the databanks they serve are compatible with TRS-80s and Tandy's Videotex terminals.

While most services are specialized, their large number ensures that they have something to offer everyone, whether it's timely legal advice, medical information or a handy way to communicate with someone overseas.

The networks cut the cost of reaching the data banks. Some of them can handle certain compatibility problems between different computers and protect data from transmission problems ranging from line noise to physical breaks in the cables.

Most users, however, are only aware of the service they are subscribing to.

Earty Experiments

Common carrier data transmission networks like Tymnet and Telenet grew out of experiments in the late 1960s and 1970s. Telenet, Vienna, VA, was one of the first experiments which went public after several years of successful private operation.

Lawrence G. Roberts, one of the networking pioneers involved in Telenet from the start, writes in his article "The Evolution of Packet Switching" (IEEE Proceedings, June 1978) that the networks are based on a very different approach to communications than that used for most electronic communication methods.

Telephones, Telex and most radio transmissions, for instance, are pre-allocation methods. That is, a transmission channel is allocated exclusively to each user, either on a permanent basis as with commercial radio frequencies or on a per-use basis as with the telephone. Even if the assigned user is not communicating at a given moment, no one else may use any part of the channel allocated to him.

An alternative to this method is dynamic allocation which incorporates a time-sharing concept. This is the system used by telegraph and postal systems.

Rather than giving each user a direct link to the person he wishes to communicate with, these systems allocate channels only after a message is received. The messages ate moved from point to point (for example, post office to post office) within the system until they reach their destination. The system does not provide a direct, complete link between message sender and receiver.

While both kinds were tried for computer networking, a form of dynamic allocation called packet switching proved to be more effective. This method was developed simultaneously in the mid-1960's in Britain by Donald Davies of the National Physical Laboratories, who named the system, and in the US by Lawrence G. Roberts at the Advanced Research Projects Agency (ARPA), Cambridge, MA.

Both these networks consisted of communications cables connecting minicomputer packet switches. To use the system you call the nearest packet switch, tell it who you want to contact and give it the message you want to send. It chops your message up into discreet packets, each a standardized size. Davies' network, for instance, used 128-byte packets.

The network determines the optimal path for the message, and the computer feeds the packets out to the next switch in that particular path. This method allows each packet to be checked to make sure it has not been garbled and determines the route for the packet to the next switch.

While Davies wrote the original design for the network, European communications people were slow to recognize its value, and Davies was only able to set up a

52 » 80 Micrdboffiputing, September 1981

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