Chapter 2: Network Services and Layered Architectures

OVERVIEW

Many networks provide transportation services. The postal system, whose services include the transfer of letter and parcels, is a familiar example. The postal system's services are differentiated by quality: there is registered mail, overnight delivery, surface mail, third-class mail. These services are built from basic transportation services, such as truck or rail or air transport. The postal system uses these basic services to create the more sophisticated services that its customers purchase. If you mail a letter, the system selects a route over which to send it; puts the letter with others going on the same route in a larger container; ships the container using air transport, say; transfers the container to a post office near the destination using truck transport; and finally brings the letter to the destination using yet another service, namely hand delivery by the letter carrier.

Characteristics of the basic services and system performance are summarized and measured by a few parameters: the volume of letters that can be handled per day or per hour, the speed of delivery, the fraction of letters that are lost. To a considerable extent these characteristics are determined by the capabilities of the postal system "hardware": the number of trucks, train cars, and airplanes the postal system has; their speeds; the routes they can use; and so on. These hardware-determined capabilities are then managed and controlled by the postal system's "intelligence" embodied in hardware or software systems or postal system employees to produce the more sophisticated services offered to customers. Naturally,...