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Now that we're done with the stuff that college professors want you to know to understand networks, let's get back to the real world and look at how networks are put together. Networks link computers, but a link can take many forms, which a visit to any sausage shop will readily confirm.
With networks, computers link together in many ways. The physical configuration (something those professors call a topology), describes how computers are connected, like the sausages that squirt out of the grinder. Sausages link in a straight line, one after another. Networks are more versatile.
Moreover, by their nature, networks define hierarchies of computers. Some networks make some computers more important than others. The terms describing these arrangements are often bandied about as if they mean something. The descriptions that follow should help you keep up with the conversation.
In mathematics, topology is an elusive word. Some mathematicians see it as the study of geometry without shapes or distance. Some definitions call it the study of deformation. It is the properties of an object that survive when you change its size or shape by stretching or twisting it. A beach ball has the same topology with or without air. Concepts such as inside and outside survive the deformation, although they change physical places. It's a rich, rewarding, and fun part of mathematics.
Topology describes how the parts of an object relate to and connect with one another. The same word is applied to networks to describe how network nodes connect together. It describes the network without reference to distance or physical location. It shows the paths that signals must travel from one node to another.
Designers have developed several topologies for computer networks. Most can be reduced to one of four basic layouts: linear, ring, star, and tree. The names describe how the cables run throughout an installation.
A network with linear cabling has a single backbone, one main cable that runs from one end of the system to the other. Along the way, computers tap into this backbone to send and receive signals. The computers link to the backbone with a single cable through which they both send and receive. In effect, the network backbone functions as a data bus, and this configuration is often called a bus topology. It's as close as a network comes to the sausages streaming from the grinding machine. Figure 12.2 shows a network bus.
In the typical installation, a wire leads from the computer to the backbone, and a T-connector links the two. The network backbone has a definite beginning and end. In most cases, these ends are terminated with a resistor matching the characteristic impedance of the cable in the background. That is, a 61-ohm network cable will have a 61-ohm termination at either end. These terminations prevent signals from reflecting from the ends of the cable, thus helping ensure signal integrity.
The ring topology looks like a linear network that's biting its own tail. The backbone is a continuous loop, a ring, with no end. But the ring is not a single, continuous wire. Instead, it is made of short segments daisy-chained from one computer to the next, the last connected, in turn, to the first. Each computer thus has two connections. One wire connects a computer to the computer before it in the ring, and a second wire leads to the next computer in the ring. Signals must traverse through one computer to get to the next, and the signals typically are listened to and analyzed along the way. You can envision it as a snake eating its own tail (or the first sausage swallowing the last link). If neither image awakens your imagination, Figure 12.3 also shows a network ring.
Just as rays blast out from the core of a star, in the star topology, connecting cables emanate from a centralized location called a hub, and each cable links a single computer to the network. A popular image for the star topology is an old-fashioned wagon wheel—the network hub is the hub of the wheel, the cables are the spokes, and the computers are ignored in the analogy. Try visualizing them as clumps of mud clinging to the rim (which, depending on your particular network situation, may be an apt metaphor). Figure 12.4 shows a small network using the star topology.
Star-style networks have become popular because their topology matches that of other office wiring. In the typical office building, the most common wiring is used by telephones, and telephone wiring converges at the wiring closet, which is the Private Branch Exchange, or PBX (the telephone switching equipment for a business). Star-style topologies require only a single cable and connection for each device to link to the central location where all cables converge into the network hub.
When multiple hubs are connected together, the result is a tree. More like a family tree than some great oak, the tree spreads out and, at least potentially, connects many more nodes than might a single hub. Figure 12.5 shows a small network tree.
Computers have classes—or at least a class system—in some networking schemes. Whereas some networks treat all computers the same, others elevate particular computers to a special, more important role as servers. Although the network performs many of the same functions in either case, these two hierarchical systems enforce a few differences in how the network is used.
In the client/server system, the shared resources of the network—files, high-speed connections with the Internet, printers, and the email system—are centralized on one or more powerful computers with very large disk storage capacity called servers. Individual workstation computers used by workers at the company are called clients.
Exactly what is on the server (or servers) and what is local to each client depends on the choices of the network manager. The server may host as little as a single shared database. The server may also host the data file used by each client. In extreme cases, programs used by each client load from a disk in the server.
As the network places more reliance on the server, the load on both the network and server increase. Because nearly every computer operation becomes a network operation, normal day-to-day use swallows up network bandwidth, and a slowdown in the network or the server slows work on every computer.
The strong point of the centralized client/server network is ease of administration. The network and all its connected clients are easier to control. Centralizing all data files on the server makes data and programs easier to secure and back up. Also, it helps ensure that everyone on the network is using the same programs and the same data.
The client/server system is a royalist system, particularly if you view a nation's leader as a servant of the people rather than a profiteer. The opposite is the true democracy, in which every computer is equal. Computers share files and other resources (such as printers) among one another. They share equally, each as the peer of the others, so this scheme is called peer-to-peer networking.
Peer-to-peer means that there is no dedicated file server. All computers have their own, local storage, but each computer is (or can be) granted access to the disk drives and printers connected to the other computers. The peer-to-peer system is not centralized. In fact, it probably has no center, only a perimeter.
When you use the Internet to share files (say, downloading an MP3 file from someone else's collection), you're using peer-to-peer networking.
In a peer-to-peer network, no one computer needs to be particularly endowed with overwhelming mass storage or an incomprehensible network operating system. But all computers need not be equal. In fact, one peer may provide a network resource, such as a connection to a printer or the Internet. It may even have a large disk that's used to back up the files from other peers.
In other words, the line between client/server and peer-to-peer systems can be fuzzy, indeed. There is no electrical difference between the two systems. A peer-to-peer network and a client/server network may use exactly the same topology. In fact, the same network may take different characterizations, depending on how it is used.
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