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Two Varieties of Networks: Local and Wide Area

The topology of a computer network is an important feature of its composition. Another is the geographical composition: the network’s coverage. That is to say, how far does it extend? The span of a network—its physical girth—often dictates how it goes about sending, receiving, and otherwise managing data.

LANs

A local area network (LAN) is so-named because the nodes are in close proximity to each other, usually within a building or inside a home. In the past, the procedures (protocols) employed to manage a LAN depended on the nodes being close to each other—within a kilometer or so. The older Ethernet bus topology is an example of this distance-limited idea. Another way to describe a LAN is that it is usually a private network. It is owned, operated, and used by a company or an individual, to the exclusion of other companies and individuals.

Also, in the old days (a couple decades ago), a LAN was noted for its “high-speed” capacity. The original Ethernet LAN sent and received data at 10 million bits per second (bps)—a phenomenal transfer rate in those days. Today, this capacity and beyond is enjoyed by both LANs and wide area networks (WANs), discussed next.

The term “speed” to describe (1) a high-capacity system (2) offering fast response times is just fine. Speed in this context describes the rate at which data is transmitted or the measurement of how long it takes for a function to be performed.

WANs

As their name implies, wide area networks (WANs) are geographically scattered. They are usually connected to local networks with a router. This machine relays the packets between computers, which often reside on LANs, as seen in Figure 1.2. Access to a WAN is obtained with a dial-up telephone line or with a broadband link, such as a Digital Subscriber Line (DSL), a cable TV link, or a satellite link. The dial-up option, although widely used, is quite limited in its capacity, perhaps operating at only 56,000bps. In contrast, broadband links transport data in the megabit per second (Mbps) range. Once you’ve used broadband, you likely won’t be happy with dial-up. Downloading a web page on a dial-up line might take several minutes, in contrast to a broadband link, which takes a few seconds.

Figure 1.2

Figure 1.2 LANs and WANs

The term “broadband” can be confusing. Strictly speaking, it refers to the frequency spectrum with a broad band of frequencies, but it also describes a network or communications link that sends and receives data at a high bit rate, such as 4,000,000bps. If eight bits are used to comprise a character, such as the letter A, this broadband link can accommodate 1/2 million alphabetic characters per second (4,000,000/8 = 500,000). It’s easy to understand why broadband is so popular.

WANs are often public networks. That is, they’re available to anyone who wants to use them and pay for their use. The telephone system is a WAN public network facility. So is the Internet. Some WANs are private networks, owned and operated by companies or other enterprises. An example of a private WAN is a bank’s ATM network. Typically, the bank leases communications links from a communications carrier, such as AT&T, and then installs its own ATM machines and routers, configuring them for its own unique requirements. As a bank customer, we can use the ATM network, but we can’t connect our computers to it. In this regard, it’s a private network.

Examples of Network Topologies, LANs, and WANs

Insofar as possible, we’ve avoided using buzz words to describe network topologies. It might be helpful to associate specific names with these topologies, but you can skip this section if you prefer. Here’s a list of common computer networks and their associated topologies; all will be examined in subsequent hours:

  • Star networks—Switched Ethernet (LAN); Asynchronous Transfer Mode (ATM) (LAN or WAN); Frame Relay (WAN); the Internet (WAN); Synchronous Optical Network (SONET) (WAN)
  • Ring networks—Token Ring (LAN); IBM Token Ring (LAN); Fiber Distributed Data Interface (FDDI) (LAN); Synchronous Optical Network (SONET) (WAN)
  • Bus networks—Ethernet Bus (LAN); Token Bus (LAN)
  • Cellular networks—The cell phone networks (WAN); Bluetooth (LAN); Wi-Fi (LAN)
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