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CISCO INTERNETWORK DESIGN EXAM OBJECTIVES COVERED IN THIS CHAPTER:
While the technologies presented in this chapter are different from the WAN systems discussed in Chapter 8, readers should find some similarities between them. All WAN systems ultimately introduce factors that are not present in LAN designssometimes these factors are significant. Consider the fact that most WAN solutions reduce the amount of control availed to the administrator. This loss of control may be due to a partnership with a telecommunications provider or to end-user activity. Either factor can greatly complicate troubleshooting and support.
Another common factor in remote access and WAN solutions is performance. While it is possible to obtain OC-48 SONET (Synchronous Optical Network) rings (yielding over 2Gbps) for WAN connectivity, these solutions are also very costly (up to and exceeding $30,000 a month, depending on distance). Remote-access solutions typically utilize significantly slower connection methods, including X.25, ISDN, and standard telephone services (PSTN/POTS or Public Switched Telephone Network/plain old telephone service). Therefore, designers should work with users and application support staff to minimize the demands on the remote access solution, thereby providing the greatest performance for users.
This chapter will address X.25 and ISDN technologies in detail. It will also present the various ways remote users access the corporate network, including remote gateways, remote control, and remote nodes.
This chapter will include a section on xDSL technologies as well. While xDSL is not on the current CID examination, the quick growth of this transport technology will certainly play a role in future network designs.
The X.25 protocol was intended to address the connectivity demands of low-bandwidth, poor-quality connections. As a result, the protocol contains a significant amount of overhead related to error-checking that is typically unnecessary in modern networks. However, it is also a widely available protocol, so network designers may likely need to integrate legacy X.25 into more modern network designs. The protocol remains quite prevalent in some countries and in the telecommunications industry as well. Companies with networks outside the US and Japan should consider X.25 for lower cost, lower bandwidth connections, especially as a transport for IP traffic however, X.25 will transport most protocols.
The basic tenet of X.25 is that the protocol should be reliable. Therefore, the protocol is based on LAPB (Link Access Procedure, Balanced), which provides flow control and reliable transport at the data-link layer. One feature in X.25 is the use of channels, which are effectively logical virtual circuits. As indicated previously, compared to other protocols, including Frame Relay, X.25 has very low throughput and high latencya characteristic of packet-relay transports. While most X.25 implementations connect to a public network, a significant number of private systems exist. These are frequently found in telecommunications and financial environments, although ISDN, xDSL, and low-bandwidth Frame Relay are slowly eroding this market share.
In a Cisco-based network design, the X.25 protocol is used to create WAN links where the carrier provides the DCE (data circuit-terminating equipment) and the router takes on the role of the DTE (data terminal equipment). However, the router can be configured as the DCE when necessary. Connections are established by defining an X.121 address in the router. X.121 addresses are comprised of a four-digit Data Network Identification Code (DNIC) and a National Terminal Number (NTN), which may be up to 10 digits in length. It is important to note that most X.25 services are billed on a per-packet basis, so most designers use static routes and filters to limit the traffic on the network.
Most designers without X.25 experience typically have some Frame Relay expertise. This expertise is beneficial, as Frame Relay compares with X.25 from an overall topology perspective. The network core can be configured via X.25, but it is generally recommended that a full-mesh design be implemented. In addition, careful consideration should be given to over-subscription, as bandwidth is limited. Designers also need to consider X.25 under the same guidelines as any NBMA (nonbroadcast multiaccess) configuration, which was covered in the Frame Relay section of the previous chapter.
Cisco introduced subinterface support for X.25 in IOS version 10.0. This eliminated the NBMA factors of partial-mesh connectivity and split-horizon, so the designer can provide full connectivity with a partial-mesh configuration. As with other subinterfaces, each link is a different network.
The router can also provide the functions of an X.25 switch via its serial ports. This allows connectivity between two packet assembler/disassembler (PAD) devices. Unfortunately, X.25 and LAPB are the only protocols supported on the link, which precludes other encapsulations. Both PVC (permanent virtual circuit) and SVC (switched virtual circuit) links are supported.
Integrated Services Digital Network (ISDN) technology was developed in large part from the conversion to digital networks from analog switches by the telephone companies in North America, which at the time was AT&T for the United States. This conversion, which started in the 1960s, resulted in the following features:
ISDN was originally conceived as a means to move the digital network into the home, where a single ISDN connection would provide two standard phone lines and digital services for data. This migration from the analog phone would continue to use the existing copper wire plant while adding services that would ultimately increase revenues.
Unfortunately, users failed to accept ISDN in the numbers desired. This was especially true in the United States, where installation problems, service availability, and pricing all combined to hinder acceptance.
Standard ISDN service is popular for videoconferencing and as a residential connection to the Internet. Cable modems and xDSL technologies will probably replace this market in the 21st century, however.
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