Network Plus 2005 In Depth
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Now that you understand the basic transmission media, network models, and networking hardware associated with LANs (local area networks), you need to expand that knowledge to encompass WANs (wide area networks). As you have learned, a WAN is a network that con-
nects two or more geographically distinct LANs. You might assume that WANs are the same as LANs, only bigger. Although a WAN is based on the same principles as a LAN, including reliance on the OSI Model, its distance requirements affect its entire infrastructure. As a result, WANs differ from LANs in nearly every respect.
To understand the difference between a LAN and WAN, think of the hallways and stairs of your house as LAN pathways. These interior passages allow you to go from room to room. To reach destinations outside your house, however, you need to use sidewalks and streets. These public thoroughfares are analogous to WAN pathways—except that WAN pathways are not necessarily public.
This chapter discusses the technical differences between LANs and WANs and describes in detail WAN transmission media and methods. It also notes the potential pitfalls in establishing and maintaining WANs. In addition, it introduces you to remote connectivity for LANs— a technology that, in some cases, can be used to extend a LAN into a WAN. Remote connectivity and WANs are significant concerns for organizations attempting to meet the needs of telecommuting workers, global business partners, and Internet-based commerce. To pass the Network+ certification exam, you must be familiar with the variety of WAN and remote connectivity options. You also need to understand the hardware and software requirements for dial-up networking.
WAN Essentials
A WAN is a network that traverses some distance and usually connects LANs, whether across the city or across the nation. You are probably familiar with at least one WAN—the Internet, which is the largest WAN in existence today. However, the Internet is not a typical WAN. Most WANs arise from the simple need to connect one building to another. As an organization grows, the WAN might grow to connect more and more sites, located across the city or around the world. Only an organization’s information technology budget and aspirations limit the dimensions of its WAN.
Why might an organization need a WAN? Any business or government institution with sites scattered over a wide geographical area needs a way to exchange data between those sites. Each of the following scenarios demonstrates a need for a WAN:
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A bank with offices around the state needs to connect those offices to gather transaction and account information into a central database.
Regional sales representatives for a national pharmaceutical company need to submit their sales figures to a file server at the company’s headquarters and receive e-mail from the company’s mail server.
An automobile manufacturer in Detroit contracts out its plastic parts manufacturing to a Delaware-based company. Through WAN links, the auto manufacturer can videoconference with the plastics manufacturer, exchange specification data, and even examine the parts for quality online.
A clothing manufacturer sells its products over the Internet to customers throughout the world.
Although all of these businesses need WANs, they may not need the same kinds of WANs. Depending on the traffic load, budget, geographical breadth, and commercially available technology, each might implement a different transmission method. For every business need, only a few (or possibly only one) appropriate WAN connection types may exist. However, many WAN technologies can coexist on the same network.
WANs and LANs are similar in some fundamental ways. They both are designed to enable communication between clients and hosts for resource sharing. In general, both use the same protocols from Layers 3 and higher of the OSI Model. And both networks typically carry digitized data via packet-switched connections.
However, LANs and WANs often differ at Layers 1 and 2 of the OSI Model, in access methods, topologies, and sometimes, media. They also differ in the extent to which the organization that uses the network is responsible for the network. LANs use a building’s internal cabling, such as twisted-pair, that runs from work area to the wall, through plenum areas and to a telecommunications closet. Such wiring is private; it belongs to the building owner. In contrast, WANs typically send data over publicly available communications networks, which are owned by local and long-distance telecommunications carriers. Such carriers, which are privately owned corporations, are also known as NSPs (network service providers). Some popular NSPs include AT&T, PSInet, Sprintlink, and UUNET (MCI Worldcom). Customers lease connections from these carriers, paying them to use a specified amount of bandwidth on their networks. For better throughput, an organization might lease a dedicated line, or a continuously available communications channel, from a telecommunications provider, such as a local telephone company or ISP. Dedicated lines come in a variety of types that are distinguished by their capacity and transmission characteristics.
The individual geographic locations connected by a WAN are known as WAN sites. A WAN link is a connection between one WAN site (or point) and another site (or point). A WAN link is typically described as point-to-point—because it connects one site to only one other site. That is, the link does not connect one site to several other sites, in the way that LAN hubs or switches connect multiple segments or workstations. Nevertheless, one location may be connected to more than one location by multiple WAN links. Figure 7-1 illustrates the difference between WAN and LAN connectivity.
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FIGURE 7-1 Differences in LAN and WAN connectivity
The following section describes different topologies used on WANs.
WAN Topologies
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WAN topologies resemble LAN topologies, but their details differ because of the distance |
1.1they must cover, the larger number of users they serve, and the heavy traffic they often handle. For example, WAN topologies connect sites via dedicated and, usually, high-speed links. As a consequence, WANs use different connectivity devices. For example, to connect two buildings via high-speed T1 carrier lines, each location must use a special type of terminating device, a multiplexer, plus a router. And because WAN connections require routers or other Layer 3 devices to connect locations, their links are not capable of carrying nonroutable protocols, such as NetBEUI. The following sections describe common WAN topologies and special considerations for using each.
Bus
A WAN in which each site is directly connected to no more than two other sites in a serial fashion is known as a bus topology WAN. A bus topology WAN is similar to a bus topology LAN in that each site depends on every other site in the network to transmit and receive its traffic. However, bus topology LANs use computers with shared access to one cable, whereas the WAN bus topology uses different locations, each one connected to another one through point-to-point links.
A bus topology WAN is often the best option for organizations with only a few sites and the capability to use dedicated circuits. Some examples of dedicated circuits include T1, DSL, and
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1.1Figure 7-2 depicts a bus topology WAN using T1 and DSL connections.
Bus WAN topologies are suitable for only small WANs. Because all sites between the sending and receiving location must participate in carrying traffic, this model does not scale well. The addition of more sites can cause performance to suffer. Also, a single failure on a bus topology WAN can take down communications between all sites.
FIGURE 7-2 A bus topology WAN
Ring
In a ring topology WAN, each site is connected to two other sites so that the entire WAN forms a ring pattern, as shown in Figure 7-3. This architecture is similar to the simple ring topology used on a LAN, except that a WAN ring topology connects locations rather than local nodes and in most WANs, a ring topology uses two parallel paths for data. This means that unlike a ring topology LAN, a ring topology WAN cannot be taken down by the loss of one site; instead, if one site fails, data can be rerouted around the WAN in a different direction. On the other hand, expanding ring-configured WANs can be difficult, and it is more expensive than expanding a bus topology WAN. For these reasons, WANs that use the ring topology are only practical for connecting fewer than four or five locations.
Star
The star topology WAN mimics the arrangement of a star topology LAN. A single site acts as the central connection point for several other points, as shown in Figure 7-4. This arrangement provides separate routes for data between any two sites. That means that if a single connection fails, only one location loses WAN access. For example, if the T1 link between the
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FIGURE 7-3 A ring topology WAN
FIGURE 7-4 A star topology WAN
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Oak Street and Main Street locations fails, the Watertown and Columbus locations can still |
1.1communicate with the Main Street location because they use different routes. In a bus or ring topology, however, a single connection failure would halt all traffic between all sites. Another advantage of a star WAN is that when all of its dedicated circuits are functioning, a star WAN provides shorter data paths between any two sites.
Extending a star WAN is relatively simple and less costly than extending a bus or ring topology WAN. For example, if the organization that uses the star WAN pictured in Figure 7-4 wanted to add a Maple Street, Madison, location to its topology, it could simply lease a new dedicated circuit from the Main Street office to its Maple Street office. None of the other offices would be affected by the change. If the organization were using a bus or ring WAN topology, however, two separate dedicated connections would be required to incorporate the new location into the network.
As with star LAN topologies, the greatest drawback of a star WAN is that a failure at the central connection point can bring down the entire WAN. In Figure 7-4, for example, if the Main Street office suffered a catastrophic fire, the entire WAN would fail. Similarly, if the central connection point is overloaded with traffic, performance on the entire WAN will be adversely affected.
Mesh
A mesh topology WAN incorporates many directly interconnected sites. Because every site is interconnected, data can travel directly from its origin to its destination. If one connection suffers a problem, routers can redirect data easily and quickly. Mesh WANs are the most faulttolerant type of WAN because they provide multiple routes for data to follow between any two points. For example, if the Madison office in Figure 7-5 suffered a catastrophic fire, the Dubuque office could still send and transmit data to and from the Detroit office by going directly to the Detroit office. If both the Madison and Detroit offices failed, the Dubuque and Indianapolis offices could still communicate.
The type of mesh topology in which every WAN site is directly connected to every other site is called a full mesh WAN. One drawback to a full mesh WAN is the cost. If more than a few sites are involved, connecting every site to every other requires leasing a large number of dedicated circuits. As WANs grow larger, the expense multiplies. To reduce costs, a network administrator might choose to implement a partial mesh WAN, in which only critical WAN sites are directly interconnected and secondary sites are connected through star or ring topologies, as shown in Figure 7-5. Partial mesh WANs are more common in today’s business world than full mesh WANs because they are more economical.
Tiered
In a tiered topology WAN, sites connected in star or ring formations are interconnected at different levels, with the interconnection points being organized into layers to form hierarchical groupings. Figure 7-6 depicts a tiered WAN. In this example, the Madison, Detroit, and New
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FIGURE 7-5 Full mesh and partial mesh WANs
FIGURE 7-6 A tiered topology WAN
York offices form the upper tier, and the Dubuque, Indianapolis, Toronto, Toledo, Washington, and Boston offices form the lower tier. If the Detroit office suffers a failure, the Toronto and Toledo offices cannot communicate with any other nodes on the WAN, nor can the Washington, Boston, and New York locations exchange data with the other six locations. Yet the Washington, Boston, and New York locations can still exchange data with each other, as can the Indianapolis, Dubuque, and Madison locations.
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Variations on this topology abound. Indeed, flexibility makes the tiered approach quite practical. A network architect can determine the best placement of top-level routers based on traffic patterns or critical data paths. In addition, tiered systems allow for easy expansion and inclusion of redundant links to support growth. On the other hand, their enormous flexibility means that creation of tiered WANs requires careful consideration of geography, usage patterns, and growth potential.
Now that you understand the fundamental shapes that WANs may take, you are ready to learn about specific technologies and types. WAN technologies discussed in the following sections differ in terms of speed, reliability, cost, distance covered, and security. Also, some are defined by specifications at the Data Link layer, whereas others are defined by specifications at the Physical layer of the OSI Model. As you learn about each technology, pay attention to its characteristics and think about its possible applications. To qualify for Network+ certification, you must be familiar with the variety of WAN connection types and be able to identify the networking environments that each suits best.
PSTN
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PSTN, which stands for Public Switched Telephone Network, refers to the network of typ- |
2.15ical telephone lines and carrier equipment that service most homes. PSTN may also be called POTS (plain old telephone service). It was originally composed of analog lines and designed to handle voice-based traffic. The PSTN comprises the entire telephone system, from the lines that connect homes and businesses to the network centers that connect different regions of a country. Now, except for the lines connecting homes, nearly all of the PSTN uses digital transmission. Its traffic is carried by fiber-optic and copper twisted-pair cable, microwave, and satellite connections. The PSTN is often used by individuals connecting to a WAN (such as the Internet) via a dial-up connection. A dial-up connection is one in which a user connects, via a modem, to a distant network from a computer and stays connected for a finite period of time. Most of the time, the term dial-up refers to a connection that uses a PSTN line.
When computers connect via the PSTN, modems are necessary at both the source and destination, because not all of the PSTN is capable of handling digital transmission. A modem converts a computer’s digital pulses into analog signals before it issues them to the telephone line, then converts the analog signals back into digital pulses at the receiving computer’s end. Unlike other types of WAN connections, dial-up connections provide a fixed period of access to the network, just as the phone call you make to a friend has a fixed length, determined by when you initiate and terminate the call.
Between the two modems, a signal travels through a carrier’s network of switches and, possibly, long-distance connections. To understand this network, it’s useful to trace the path of a dial-up call. Imagine you dial into your ISP to surf the Web through a 56-Kbps modem. You first initiate a call through your computer’s dial-up software, which instructs your modem to dial the number for your ISP’s remote access server. Next, your modem attempts to establish a connection. It then converts the digital signal from your computer into an analog signal that travels over
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the phone line to the local telephone company’s network until it reaches the central office. A cen- |
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2.15tral office is the place where a telephone company terminates lines and switches calls between different locations. Between your house and a central office, the call might go through one or more of the telephone company’s remote switching facilities. The portion of the PSTN that connects your house to the nearest central office is known as the local loop, or the last mile, and is illustrated in Figure 7-7.
FIGURE 7-7 Local loop portion of the PSTN
At either a remote switching facility or at the central office, your signal is converted back to digital pulses. If your home and your ISP share the same central office, the signal is switched from your incoming connection to your ISP’s connection. In most cases, the ISP would have a dedicated connection to a central office. If so, your signal is issued over this dedicated connection multiplexed together with many other signals. But suppose you are dialing your ISP from a hotel in another city. The first part of the process is the same as if you were at home— you initiate a call and connect to the local telephone company’s central office, where your signal is converted to digital pulses. However, this time your signal cannot go straight to your ISP, because your ISP doesn’t have a connection in that carrier’s central office. Instead, the local telephone company forwards the signal to a regional central office. This regional office may have to forward the signal to a second regional office, if you are far from the ISP. The closest regional central office to your ISP directs the signal to your ISP’s local central office. Finally, the signal is sent to the ISP’s location. Figure 7-8 illustrates the path a signal takes in a longdistance dial-up connection.
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FIGURE 7-8 A long-distance dial-up connection
The advantages to using the PSTN are its ubiquity, ease of use, and low cost. A person can travel virtually anywhere in the world and have access to a phone line and, therefore, remote access to a network. Within the United States, the dial-up configuration for one location differs little from the dial-up configuration in another location. And nearly all mobile personal computers contain a modem, the only peripheral hardware a computer requires to establish this type of connection.
But, the PSTN comes with significant disadvantages. Most limiting is its low throughput. Currently, manufacturers of PSTN modems advertise a connection speed of 56 Kbps. However, the 56-Kbps maximum is only a theoretical threshold that assumes that the connection between the initiator and the receiver is pristine. Splitters, fax machines, or other devices that a signal