I must Create a System, or be enslav'd by another Man's; I will not Reason and Compare: my business is to Create. —William Blake
Problem: Building a Network
Suppose you want to build a computer network, one that has the potential to grow to global proportions and to support applications as diverse as teleconferencing, video on demand, electronic commerce, distributed computing, and digital libraries. What available technologies would serve as the underlying building blocks, and what kind of software architecture would you design to integrate these building blocks into an effective communication service? Answering this question is the overriding goal of this book—to describe the available building materials and then to show how they can be used to construct a network from the ground up.
Before we can understand how to design a computer network, we should first agree on exactly what a computer network is. At one time, the term network meant the set of serial lines used to attach dumb terminals to mainframe computers. Other important networks include the voice telephone network and the cable TV network used to disseminate video signals. The main things these networks have in common are that they are specialized to handle one particular kind of data (keystrokes, voice, or video) and they typically connect to special-purpose devices (terminals, hand receivers, and television sets).
What distinguishes a computer network from these other types of networks? Probably the most important characteristic of a computer network is its generality. Computer networks are built primarily from general-purpose programmable hardware, and they are not optimized for a particular application like making phone calls or delivering television signals. Instead, they are able to carry many different types of data, and they support a wide, and ever growing, range of applications. Today's computer networks have pretty much taken over the functions previously performed by single-use networks. This chapter looks at some typical applications of computer networks and discusses the requirements that a network designer who wishes to support such applications must be aware of.
Once we understand the requirements, how do we proceed? Fortunately, we will not be building the first network. Others, most notably the community of researchers responsible for the Internet, have gone before us. We will use the wealth of experience generated from the Internet to guide our design. This experience is embodied in a network architecture that identifies the available hardware and software components and shows how they can be arranged to form a complete network system.
In addition to understanding how networks are built, it is increasingly important to understand how they are operated or managed and how network applications are developed. Almost all of us now have computer networks in our homes, offices, and in some cases in our cars, so operating networks is no longer a matter only for a few specialists. And with the proliferation of smartphones, many more of this generation are developing networked applications than in the past. So we need to consider networks from these multiple perspectives: builders, operators, application developers.
To start us on the road toward understanding how to build, operate, and program a network, this chapter does four things. First, it explores the requirements that different applications and different communities of people place on the network. Second, it introduces the idea of a network architecture, which lays the foundation for the rest of the book. Third, it introduces some of the key elements in the implementation of computer networks. Finally, it identifies the key metrics that are used to evaluate the performance of computer networks.