Wide Area Networks

Chapter 11Wide AreaNetworksUnderstand point-to-point WAN technologies: T-lines andSONET networks.Understand X.25 technology and why it is losing popularity.Understand Frame Relay technology and how it was designedto replace X.25.Understand ATM technology and its importance in today’smarket.After reading this chapter, the reader should be able to:OBJECTIVESUnderstand ATM LANs.POINT-TO-POINTWANs11.1Figure 11-1T-1 lineA T-1 line has a data rate of1.544 MbpsNote:A T-3 line has a data rate of44.736 MbpsNote:Technical Focus: Fractional T-LinesMany subscribers may not need the entire capacity of a T- line. To accommodate these customers, telephone companies offer fractional T-line services, which allow several subscribers to share one line by multiplexing theirtransmissions.Figure 11-2SONETSWITCHEDWANs11.2Technical Focus: TunnelingTo use an X.25 network, an IP packet uses a technique called tunneling. Because X.25 does not allow IP packets to use theirown network layer protocol, the IP packets are encapsulated in the network layer of the X.25 protocol. This can be compared to a car entering a tunnel. To an observer, the car disappears at one side of the tunnel and reappears at the other side. Likewise, an IP packet disappears at the entry point of an X.25 network and reappears at the exit point. Figure 11-3Frame Relay networkFigure 11-4Frame Relay layersTechnical Focus: DLCIsA Frame Relay network uses data link connection identifiers (DLCIs) which act as addresses. Each DLCI defines a channel between two adjacent devices (DTEs or DCEs). A path between a device at one end and another device at the other end is made of several DLCIs as shown in the following figure:A cell network uses the cell as the basic unit of data exchange. A cell is defined as a small fixed-sized block of information.Note:Figure 11-5ATM multiplexingFigure 11-6Architecture of an ATM networkTechnical Focus: VPIs and VCIsIn an ATM network, connection between two end points is accomplished through transmission paths, virtual paths, and virtual circuits as shown in the following figure: The outer cylinder shows a transmission path. The inner cylinders show the virtual path identifiers (VPIs). The lines show virtual circuit identifiers (VCIs). Figure 11-7An ATM cellFigure 11-8ATM layersThe IP protocol usesthe AAL5 layer.Note:ATMLANs11.3Figure 11-9ATM LAN architecturesTechnical Focus: LANEConnectionless vs. Connection-orientedPhysical Address vs. Virtual Connection IdentifierMulticasting and Broadcasting DeliveryInteroperabilityAt the surface level, the use of ATM technology in LANs seems plausible. However, on close inspection, we see that many issues need to be resolved, as summarized below:An approach called local area network emulation (LANE) solves the above-mentioned problems and allows stations in a mixed architecture to communicate with each other.