Sec.1.3 Layering 17 through the network according to the current information available at the nodes visited. Virtual circuit routing is generally used in practice,although there are many interesting intermediate positions between virtual circuit routing and dynamic routing.The general issue of routing is treated in Chapter 5. 1.3 LAYERING Layering,or layered architecture,is a form of hierarchical modularity that is central to data network design.The concept of modularity (although perhaps not the name) is as old as engineering.In what follows,the word module is used to refer either to a device or to a process within some computer system.What is important is that the module performs a given function in support of the overall function of the system.Such a function is often called the service provided by the module.The designers of a module will be intensely aware of the internal details and operation of that module.Someone who uses that module as a component in a larger system,however,will treat the module as a "black box."That is,the user will be uninterested in the internal workings of the module and will be concerned only with the inputs,the outputs,and,most important, the functional relation of outputs to inputs (i.e.,the service provided).Thus,a black box is a module viewed in terms of its input-output description.It can be used with other black boxes to construct a more complex module,which again will be viewed at higher levels as a bigger black box. This approach to design leads naturally to a hierarchy of modules in which a module appears as a black box at one layer of the hierarchy,but appears as a system of lower-layer black boxes at the next lower layer of the hierarchy (see Fig.1.6).At the overall system level (i.e.,at the highest layer of the hierarchy),one sees a small collection of top-layer modules,each viewed as black boxes providing some clear-cut service.At the next layer down,each top-layer module is viewed as a subsystem of lower-layer black boxes,and so forth,down to the lowest layer of the hierarchy.As shown in Fig.1.6,each layer might contain not only black boxes made up of lower-layer modules but also simple modules that do not require division into yet simpler modules. As an example of this hierarchical viewpoint,a computer system could be viewed as a set of processor modules,a set of memory modules,and a bus module.A processor module could,in turn,be viewed as a control unit,an arithmetic unit,an instruction fetching unit,and an input-output unit.Similarly,the arithmetic unit could be broken into adders.accumulators,and so on. In most cases,a user of a black box does not need to know the detailed response of outputs to inputs.For example,precisely when an output changes in response to an input is not important as long as the output has changed by the time it is to be used.Thus,modules (i.e.,black boxes)can be specified in terms of tolerances rather than exact descriptions.This leads to standardized modules,which leads,in turn.to the possibility of using many identical,previously designed (i.e.,off-the-shelf)modules in the same system.In addition,such standardized modules can easily be replaced with new,functionally equivalent modules that are cheaper or more reliable.Sec. 1.3 Layering 17 through the network according to the current infonnation available at the nodes visited. Virtual circuit routing is generally used in practice, although there are many interesting intennediate positions between virtual circuit routing and dynamic routing. The general issue of routing is treated in Chapter 5. 1.3 LAYERING Layering, or layered architecture, is a fonn of hierarchical modularity that is central to data network design. The concept of modularity (although perhaps not the name) is as old as engineering. In what follows, the word module is used to refer either to a device or to a process within some computer system. What is important is that the module perfonns a given function in support of the overall function of the system. Such a function is often called the service provided by the module. The designers of a module will be intensely aware of the internal details and operation of that module. Someone who uses that module as a component in a larger system, however, will treat the module as a "black box." That is, the user will be uninterested in the internal workings of the module and will be concerned only with the inputs, the outputs, and, most important, the functional relation of outputs to inputs (i.e., the service provided). Thus, a black box is a module viewed in tenns of its input-output description. It can be used with other black boxes to construct a more complex module, which again will be viewed at higher levels as a bigger black box. This approach to design leads naturally to a hierarchy of modules in which a module appears as a black box at one layer of the hierarchy, but appears as a system of lower-layer black boxes at the next lower layer of the hierarchy (see Fig. 1.6). At the overall system level (i.e., at the highest layer of the hierarchy), one sees a small collection of top-layer modules, each viewed as black boxes providing some clear-cut service. At the next layer down, each top-layer module is viewed as a subsystem of lower-layer black boxes, and so forth, down to the lowest layer of the hierarchy. As shown in Fig. 1.6, each layer might contain not only black boxes made up of lower-layer modules but also simple modules that do not require division into yet simpler modules. As an example of this hierarchical viewpoint, a computer system could be viewed as a set of processor modules, a set of memory modules, and a bus module. A processor module could, in tum, be viewed as a control unit, an arithmetic unit, an instruction fetching unit, and an input-output unit. Similarly, the arithmetic unit could be broken into adders, accumulators, and so on. In most cases, a user of a black box does not need to know the detailed response of outputs to inputs. For example, precisely when an output changes in response to an input is not important as long as the output has changed by the time it is to be used. Thus, modules (i.e., black boxes) can be specified in tenns of tolerances rather than exact descriptions. This leads to standardized modules, which leads, in tum, to the possibility of using many identical, previously designed (i.e., off-the-shelf) modules in the same system. In addition, such standardized modules can easily be replaced with new, functionally equivalent modules that are cheaper or more reliable