Chapter 8
Designing for the WAN

CISCO INTERNETWORK DESIGN EXAM OBJECTIVES COVERED IN THIS CHAPTER:

  üList common concerns that customers have about WAN designs.

  üExamine statements made by a customer and distinguish issues that affect the choice of WAN designs.

  üDesign core WAN connectivity to maximize availability and optimize utilization of resources.

  üDesign a full- or partial-mesh Frame Relay nonbroadcast multiaccess (NBMA) core for full or partial connectivity.

  üChoose a scalable topology for NBMA Frame Relay.

  üUse subinterface Frame Relay configurations to design robust core WANs.

Network designers frequently need to connect geographically distant locations with relatively high-speed links. Unfortunately, costs generally increase as the available bandwidth increases, and thus the designer is compelled to find the best solution in terms of cost, performance, scalability, and availability.

There are a number of ways to connect networks across large geographical areas. In the earliest networks, this required the use of expensive leased lines or slow dial-up connections—both of which were limited in terms of bandwidth compared to modern, cheaper solutions. Today’s offerings, which are substantially cheaper on a per-megabyte basis, include Frame Relay, ATM (Asynchronous Transfer Mode), and SMDS (Switched Multi-megabit Data Service). Each of these technologies relies on the reliability of modern fiber-optic and copper networks and scales to support at least DS-3 (45Mbps) bandwidth—ATM is currently available in OC-48 and OC-192 (optical carrier) offerings, yielding up to 10Gbps of bandwidth.

This chapter does not focus so much on the increasing performance of modern WAN technologies, such as DWDM (dense wavelength division multiplexing), which multiplies the number of signals that can traverse a fiber, or the issues surrounding OC-192 and OC-48 networks. Rather, each of these technologies (Frame Relay, ATM, and SMDS) is presented in detail, and the differences between frame-based and cell-based transports are discussed. Additionally, this chapter focuses on the general concepts of wide area networking technologies. Beyond nontechnical concerns such as cost, this chapter reviews more technological factors, including scalability, reliability, and latency.

While SMDS is included in the CID exam objectives, its availability has waned in recent years. Standard ATM services have effectively replaced such installations, while Frame Relay has always held a substantial market share. SMDS did not fail due to technology—in fact, it was a very good protocol. Rather, it required additional expertise and expensive equipment compared to the alternatives. Many providers never offered the technology.

Wide Area Network Technologies

The design goals—and technologies—of a wide area network (WAN) are slightly different than those for local area network (LAN) installations. For example, it is fairly simple to add an extra connection in a LAN, while in a WAN this may take 90 days or more. Also, in a LAN, most designers are concerned with port density and broadcast control, while in a WAN, bandwidth and cost are frequently the foremost concern. Further, the interactions with outside vendors required in a WAN can alter significantly the issues involved in the design.

There are two categories of WAN design technology in use today: dedicated services and switched services. Dedicated services include the traditional leased T1 and T3 services. They are called dedicated services because only one connectionpoint, which follows a pre-determined path, exists within the circuit. This connection may be transported over shared media within the provider’s network; however, the full amount of bandwidth will always be allocated (dedicated) for the specific connection.

Readers may notice a lack of emphasis on dedicated services in this chapter. This is primarily due to the text’s focus on the Cisco exam objectives and the actual test. However, it is also presumed that most CCDP candidates are familiar with the basic concepts of these connections from their experience or the CCDA, CCNA, and CCNP materials. If the concepts of time division multiplexing, inverse multiplexing, and the serial protocols (HDLC, PPP) are unfamiliar, please make sure that you review this material before continuing your certification efforts. While the test does not ask questions outside the constraints of the objectives, it presumes a certain foundation.

Switched services include circuit, packet, and cell-switched connections; ISDN, telephone service (POTS), X.25, Frame Relay, ATM, and SMDS. Switched services typically incorporate charges for distance and bandwidth used, but this is dependent on the specific tariff in use. Most telecommunications services are charged based on a tariff— a set, regulated price structure that includes parameters for installation and administration processes.

There are two benefits of switched technologies. First, in the case of dynamic circuits, the designer can establish a connection to any other eligible recipient. For example, both POTS and ISDN connections can be established with a simple access number—the connection into the network is sufficient, and there is no requirement to define each possible link ahead of time. Second, switched services typically share bandwidth better within the cloud. As this chapter’s discussion turns to committed information rates (CIR—Frame Relay) and SCR (Sustained Cell Rate—ATM), you will see that the network can logically adapt to the requirements of the users and allow bursts of traffic within the constraints of total capacity.