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When reviewing the WAN technologies and designs presented in this chapter, it is important to consider the following issues: reliability, latency, cost, and traffic flows and traffic types. Most network designers focus ultimately on cost as the most important design consideration; however, reliability may require additional expense. Latency, various traffic flows, and traffic types are supported with most modern technologies and thus lose some importance in modern designs. Of course, this text ignores some of the older and more limited protocols in WAN design, such as BiSYNC and digital data system (DDS) circuitstwo areas in which these issues deserve more prominence.
Network Design in the Real World: SONET
Private SONET rings (Synchronous Optical Networks), wireless, and certain point-to-point technologies are outside the scope of Ciscos exam objectives. However, these solutions are frequently selected for an array of reasons, including facilities, security, and cost. The most scalable installations, at the lowest cost, frequently use Frame Relay and, to an increasing degree, ATM. Wireless technologies are well suited to temporary installations and areas where wire-based services are unavailable, although this alternative has gained favor as a means to reduce dependency on the carriers. SONET offers high reliability and is a fundamental transport technology in the carrier world. Packet over SONET (PoS) and Dynamic Packet Transport (DPT) can both operate over these rings.
Unlike LAN connections, WAN links tend to be a bit unstable and often are unreliable. This may be due to fiber cuts, equipment failure, or misconfiguration by the service provider. Unfortunately, it is difficult to add reliability to WAN installations simply by selecting a different technology. For example, Frame Relay is just as susceptible to a fiber cut as ATM; in fact, ATM transports most Frame-Relay installations in the providers core network.
Since reliability is a physical-layer concern, augmented by the higher layers, designers typically have to think of the physical layer first. For example, fiber cuts can be circumvented by wireless technologies, yet these are sometimes degraded by snow, rain, or fog. To augment reliability from a physical context, the designer needs to consider the available options, many of which are beyond the scope of this text. (Consult with your vendors for the most current information regarding WAN options.)
However, it is possible to add a degree of reliability to the network with the selection of a WAN technology. This chapter addresses some forms of redundancy for network designers to consider. Frame Relay and ATM, with their ability to service multiple connections from a single port, typically provide more reliability than point-to-point connectionsshould one virtual link fail, the other should still be available (presuming the lack of a port or local loop failure).
Latency, the delay introduced by network equipment, has become a minor concern in most designs as protocols have migrated toward delay tolerance in the data arena. However, with voice and video integration on data networks, even todays wire-speed offerings may require the attention once afforded time-sensitive protocols on slower links; this would include SNAP (Sub-Network Access Protocol), used in mainframe connectivity. Modern network designs can address these issues with queuing, low-latency hardware, cell-based technologies like ATM, and prioritization. One of the benefits afforded by ATM is a consistent latency within the network.
The latency category frequently incorporates throughput and delay factors. Compared to LANs, most wide area links are very slow, and performance suffers as a result. Designers should work with application developers and server administrators to tune the network to address this limitation. Possible solutions include compression and prioritization (queuing), yet these functions can degrade performance more than the link if not deployed correctly. Designers should also make use of static routes or quiet routing protocols and employ other techniques, such as IPX watchdog spoofing (discussed in Chapter 6), to control overhead traffic. Under the best circumstances, designers should focus on moving limited amounts of data between servers on very slow WAN links whenever possible.
Network Design in the Real World: WAN Technologies and Latency
Historically, WAN access was provided by telecommunications service providers on circuits originally provisioned for voice services. The system of T1 and E1 channels was mapped directly to the number of voice channel time slots afforded (24 for T1, 30 for E1). The technologies presented in this chapter are based upon these solutions.
Recently, however, advances in laser technology and the availability of fiber optics has provided designers with new solutions, including the option to use GigabitEthernet in some wide area solutions. While limited to approximately 55 miles, this connectivity works well in a metropolitan installation. Microwave and wireless laser solutions are also available to designers who wish to reduce the cost and installation time of traditional remote access.
In the context of latency, all of these options provide a more consistent transport infrastructure. Rather than converting from Ethernet to Frame Relay and back to Ethernet, the designer can install fairly long connections and maintain Ethernet throughout. This lack of conversion can substantially reduce the complexity of the installation and the latency.
WAN networking costs typically exceed those for a LAN. There are a number of reasons for this; the most significant factor is the recurring costs that exist in WAN networks. Unlike the LAN, where the company owns the connections between routers, the WAN infrastructure is owned by the telecommunications provider. As a result, the provider leases its fiber or copper cables. This differs from LAN installations, where the company purchases and installs its own cable. The initial cost of establishing a WAN may be greater, but the lack of recurring costs quickly reduces the amortized impact.
The technologies used to reduce WAN costsFrame Relay, ATM, and SDMSare presented throughout this chapter. Yet in short, Frame Relay provides the greatest savings per megabit. ATM is quickly providing savings in WAN costs, but this is based more on the integration of voice and data than on lower tariffs.
|Though not discussed in this book, both MAN-based Ethernet and DSL, a shorter-range technology, appear to further reduce WAN costs.|
Point-to-point circuits generally represent the highest cost in the WAN. This is because, unlike Frame Relay, the bandwidth is dedicated, which can oversubscribe and group users based on actual usage. Oversubscribing is the intentional configuration of more theoretical bandwidth on a circuit than it could accommodate. This is similar to providing 10 phones for 100 people if, on average, only seven concurrent phone calls occur, there is sufficient capacity, even though the system is oversubscribed overall. The risk of 20 callers is very real, but the savings of not providing 100 lines is substantial.
Compared to local area networks, some WANs provide limited protocol support. This may be for simplification, but in most cases this results from the desire to conserve the bandwidth that typically is needed to support additional protocols. The designer can consider encapsulation and other methodologies, including conversion and isolation, to remove or omit protocols from the WAN. Many designers are converting from AppleTalk and IPX to IP.
Previous chapters have addressed the concept of tunneling in the context of AppleTalk and other protocols. However, the general concepts and concerns regarding tunnels are universal. The most significant issue with tunnels is performance, though troubleshooting is also a major issue that can be complicated by encapsulation.
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