A WAN(Wide Area Network) spans a large geographical area, often a country or continent. We will begin our discussion with wired WANs, using the example of a company with branch offices in different cities.
The WAN in Fig. 1 is a network that connects offices in Perth, Melbourne, and Brisbane. Each of these offices contains computers intended for running user (i.e., application) programs. We will follow traditional usage and call these machineshosts. The rest of the network that connects these hosts is then called thecommunication subnet, or justsubnetfor short. The job of the subnet is to carry messages from host to host, just as the telephone system carries words (really just sounds) from speaker to listener.
In most WANs, the subnet consists of two distinct components: transmission lines and switching elements.Transmission linesmove bits between machines. They can be made of copper wire, optical fiber, or even radio links. Most companies do not have transmission lines lying about, so instead they lease the lines from a telecommunications company.Switching elements, or justswitches, are specialized computers that connect two or more transmission lines. When data
arrive on an incoming line, the switching element must choose an outgoing line on which to forward them. These switching computers have been called by various names in the past; the namerouteris now most commonly used. Unfortunately, some people pronounce it ‘‘rooter’’ while others have it rhyme with ‘‘doubter.’’ Determining the correct pronunciation will be left as an exercise for the reader. (Note: the perceived correct answer may depend on where you live.)
A short comment about the term ‘‘subnet’’ is in order here. Originally, itsonlymeaning was the collection of routers and communication lines that moved packets from the source host to the destination host. Readers should be aware that it has acquired a second, more recent meaning in conjunction with network addressing.
The WAN as we have described it looks similar to a large wired LAN, but there are some important differences that go beyond long wires. Usually in a WAN, the hosts and subnet are owned and operated by different people. In our example, the employees might be responsible for their own computers, while the company’s IT department is in charge of the rest of the network. We will see clearer boundaries in the coming examples, in which the network provider or telephone company operates the subnet. Separation of the pure communication aspects of the network (the subnet) from the application aspects (the hosts) greatly simplifies the overall network design.
A second difference is that the routers will usually connect different kinds of networking technology. The networks inside the offices may be switched Ethernet, for example, while the long-distance transmission lines may be SONET links. Some device needs to join them. The astute reader will notice that this goes beyond our definition of a network. This means that many WANs will in fact beinternetworks, or composite networks that are made up of more than one network. We will have more to say about internetworks in the next section.
A final difference is in what is connected to the subnet. This could be individual computers, as was the case for connecting to LANs, or it could be entire LANs. This is how larger networks are built from smaller ones. As far as the subnet is concerned, it does the same job. We are now in a position to look at two other varieties of WANs. First, rather than lease dedicated transmission lines, a company might connect its offices to the Internet This allows connections to be made between the offices as virtual links that use the underlying capacity of the Internet. This arrangement, shown in Fig. 2, is called aVPN(Virtual Private Network). Compared to the dedicated arrangement, a VPN has the usual advantage of virtualization, which is that it provides flexible reuse of a resource (Internet connectivity). Consider how easy it is to add a fourth office to see this. A VPN also has the usual disadvantage of virtualization, which is a lack of control over the underlying resources. With a dedicated line, the capacity is clear. With a VPN your mileage may vary with your Internet service.
The second variation is that the subnet may be run by a different company. The subnet operator is known as anetwork service providerand the offices are its customers. This structure is shown in Fig. 3. The subnet operator will connect to other customers too, as long as they can pay and it can provide service.
Since it would be a disappointing network service if the customers could only send packets to each other, the subnet operator will also connect to other networks that are part of the Internet. Such a subnet operator is called anISP(InternetService Provider) and the subnet is anISP network. Its customers who connect to the ISP receive Internet service.
We can use the ISP network to preview some key issues that we will study in later chapters. In most WANs, the network contains many transmission lines, each connecting a pair of routers. If two routers that do not share a transmission line wish to communicate, they must do this indirectly, via other routers. There may be many paths in the network that connect these two routers. How the network makes the decision as to which path to use is called therouting algorithm.Many such algorithms exist. How each router makes the decision as to where to send a packet next is called theforwarding algorithm. Many of them exist too.
Other kinds of WANs make heavy use of wireless technologies. In satellite systems, each computer on the ground has an antenna through which it can send data to and receive data from to a satellite in orbit. All computers can hear the outputfromthe satellite, and in some cases they can also hear the upward transmissions of their fellow computerstothe satellite as well. Satellite networks are inherently broadcast and are most useful when the broadcast property is important.
The cellular telephone network is another example of a WAN that uses wireless technology. This system has already gone through three generations and a fourth one is on the horizon. The first generation was analog and for voice only. The second generation was digital and for voice only. The third generation is digital and is for both voice and data. Each cellular base station covers a distance much larger than a wireless LAN, with a range measured in kilometers rather than tens of meters. The base stations are connected to each other by a backbone network that is usually wired. The data rates of cellular networks are often on the order of 1 Mbps, much smaller than a wireless LAN that can range up to on the order of 100 Mbps.