Traditional WAN Technologies

Traditional WAN technologies include the following:

■ Leased lines: Point-to-point connections indefinitely reserved for transmissions, rather than used only when transmission is required. The carrier establishes the connection either by dedicating a physical wire or by delegating a channel using frequency division multiplexing or time-division multiplexing (TDM). Leased-line connections usually use synchronous transmission.

■ Circuit-switched networks: A type of network that, for the duration of the connection, obtains and dedicates a physical path for a single connection between two network endpoints. Ordinary voice phone service over the PSTN is circuit-switched; the telephone company reserves a specific physical path to the number being called for the call's duration. During that time, no one else can use the physical lines involved. Other circuit-switched examples include asynchronous serial transmission and ISDN.

■ Packet-switched and cell-switched networks: A carrier creates permanent virtual circuits (PVC) or switched virtual circuits (SVC) that deliver packets of data among customer sites. Users share common carrier resources and can use different paths through the WAN (for example, when congestion or delay is encountered). This allows the carrier to use its infrastructure more efficiently than it can with leased point-to-point links. Examples of packet-switched networks include X.25, Frame Relay, and Switched Multimegabit Data Service.

Leased lines and circuit-switched networks offer users dedicated bandwidth that other users cannot take. In contrast, packet-switched networks have traditionally offered more flexibility and used network bandwidth more efficiently than circuit-switched networks. Cell switching combines some aspects of circuit switching and packet switching to produce networks with low latency and high throughput.

Circuit-/Packet-/Cell-Switched Versus the Open Systems Interconnection Model

Circuit-switched technologies properly fit into Layer 1 of the Open Systems Interconnection (OSI) model—the physical layer. Layer 1 OSI protocols describe methods for binary encoding on physical transmission media. PSTN networks, however, use analog methods to encode data on a phone line. For a network device such as a router to interface with this analog network, a means of converting binary-encoded data to analog is required. This function is provided by a modulator/ demodulator (modem). ISDN networks, on the other hand, are digital (the "D" in ISDN stands for "digital"). There is no need to convert from digital to analog, so devices adapt to an ISDN network using not a modem, but a terminal adapter.

In contrast, packet- and cell-switched networks operate at the data link layer (Layer 2) of the OSI model. As such, they use protocols that define methods to control access to the physical layer, allowing many conversations to multiplex over the same physical transmission medium. This is achieved by framing the binary transmission at Layer 2 and providing addressing to identify the endpoints of the data link. Virtual circuits (either permanent or switched) provide logical paths between the endpoints in the same way that circuit-switched technologies create a physical path.

Packet-Switched Network Topologies

As shown in Figure 5-2, packet-switched networks use three basic topologies: star, full mesh, and partial mesh.

Star Topology

A star topology (also called a hub-and-spoke topology) features a single internetworking hub (for example, a central router) that provides access from remote networks into the core router. Communication between remote networks is possible only through the core router. The advantages of a star approach are simplified management and minimized tariff costs, which result from the low number of circuits. However, the disadvantages are significant, including the following:

■ The central router (the hub) is a single point of failure.

■ The central router limits overall performance for access to centralized resources because all traffic intended for the centralized resources or for the other regional routers goes through this single device.

■ The topology is not scalable. Fully Meshed Topology

In a fully meshed topology, each routing node on the periphery of a given packet-switching network has a direct path to every other node, providing any-to-any connectivity. The key rationale for creating a fully meshed environment is to provide a high level of redundancy; however, a fully meshed topology is not scalable to large packet-switched networks. Key issues include the following:

■ The large number of virtual circuits required—one for every connection between routers. The number of circuits required in a fully meshed topology is n(n-1 )/2, where n is the number of routers.

■ The problems associated with the requirement for large numbers of packet and broadcast replications.

■ The configuration complexity of routers that must handle the absence of multicast support in nonbroadcast environments.

Partially Meshed Topology

A partially meshed topology reduces, within a region, the number of routers that have direct connections to all other nodes within that region. Not all nodes are connected to all other nodes; for a nonmeshed node to communicate with another nonmeshed node, it must send traffic through one of the fully connected routers.

There are many forms of partially meshed topologies. In general, partially meshed approaches provide the best balance for regional topologies in terms of the number of virtual circuits, redundancy, and performance.

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