18 Introduction and Layered Network Architecture Chap.1 High level module Lower level Black box black box Simple Black bo× module Lower level black box Simple module Black box Figure 1.6 Hierarchy of nested black boxes.Each black box (except that at the lowest level)contains black boxes at a lower level,plus perhaps other modules. All of these advantages of modularity (i.e.,simplicity of design:understandability; and standard,interchangeable,widely available modules)provide the motivation for a layered architecture in data networks.A layered architecture can be regarded as a hierarchy of nested modules or black boxes,as described above.Each given layer in the hierarchy regards the next lower layer as one or more black boxes which provide a specified service to the given higher layer. What is unusual about the layered architecture for data networks is that the black boxes at the various layers are in fact distributed black boxes.The bottom layer of the hierarchy consists of the physical communication links,and at each higher layer,each black box consists of a lower-layer black box communication system plus a set of simple modules,one at each end of the lower-layer communication system.The simple modules associated with a black box at a given layer are called peer processes or peer modules (see Fig.1.7). In the simplest case,a black box consists of two peer processes,one at each of two nodes,and a lower-layer black box communication system connecting the two peer processes.One process communicates with its peer at the other node by placing a message18 Introduction and Layered Network Architecture High level module Chap. 1 Lower level black box D Simple module Black box Lower level black box Black box Black box Figure 1.6 Hierarchy of nested black boxes. Each black box (except that at the lowest level) contains black boxes at a lower level, plus perhaps other modules. All of these advantages of modularity (i.e., simplicity of design; understandability; and standard, interchangeable, widely available modules) provide the motivation for a layered architecture in data networks. A layered architecture can be regarded as a hierarchy of nested modules or black boxes, as described above. Each given layer in the hierarchy regards the next lower layer as one or more black boxes which provide a specified service to the given higher layer. What is unusual about the layered architecture for data networks is that the black boxes at the various layers are in fact distributed black boxes. The bottom layer of the hierarchy consists of the physical communication links, and at each higher layer, each black box consists of a lower-layer black box communication system plus a set of simple modules, one at each end of the lower-layer communication system. The simple modules associated with a black box at a given layer are called peer processes or peer modules (see Fig. 1.7). In the simplest case, a black box consists of two peer processes, one at each of two nodes, and a lower-layer black box communication system connecting the two peer processes. One process communicates with its peer at the other node by placing a message