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WAN
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Wide area networks (WANs) are used to connect LANs together. Typically, WANs are used when the LANs that must be connected are separated by a large distance. Whereas a corporation provides its own infrastructure for a LAN, WANs are leased from carrier networks, such as telephone companies. Four basic types of connections, or circuits, are used in WAN services: circuit-switched, cell-switched, packet-switched, and dedicated connections.A wide array ofWAN services are availble, including analog dialup, ATM,
dedicated circuits, cable, DSL (digital subscriber line) Frame Relay, ISDN, Switched Multi-megabit Data Services (SMDS), and X.25. Here, analog dialup and ISDN are examples of circuit switched services, ATM and SMDS are examples of cell-switched services, and Frame Relay and X.25 are examples of packet-switched services.
Circuit-switched services provide a temporary connection across a phone circuit. These are typically used for backup of primary circuits and for temporary boots of bandwidth. A dedicated circuit is a permanent connection between two sites where the bandwidth is dedicated.
These circuits are common where you have a variety of services, such as voice, video, and data, that must traverse the connection and you are concerned about delay issues with the traffic and guaranteed bandwidth.
Cell-switched services can provide the same features that dedicated circuits offer. Their advantage over dedicated circuits is that a single device can connect to multiple devices on the same interface. The downside of these services is that they are not available at all locations, they are difficult to set up and troubleshoot, and the equipment is expensive when compared to using dedicated circuits.
Packet-switched services are similar to cell-switched services. Whereas cell-switched services switch fixed-length packets, called cells, packet-switched services switch variable-length packets. This feature makes them better suited for data services, but they can nonetheless provide some of the Quality of Service (QoS) features that cell-switched services provide.
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EXAM WATCH:
Circuit-switched connections, like analog and ISDN are typically used for temporary or
backup connections. Dedicated circuits, like leased lines, are used to provide guaranteed bandwidth for applications across short distances. Cell-switched services and cell-switches services are used when you only want to use a single connection to the WAN, but provide a
partially or full-meshed network. Cellswitched services, like ATM and SMDS, can provide a granular level of quality of service (QoS) for an application and are typically used to provide voice and video connections. Packet-switched services, like Frame Relay and X.25, provide a more cost-effective solution than cell-switched services, but not with the same level of
QoS support.
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Two newerWAN services that are very popular in the U.S. are cable and DSL. DSL provides speeds up to 2 Mbps and costs much less than a typicalWAN circuit from the carrier. It supports both voice and video and doesn’t require a dialup connection (it’s always enabled). Cable access uses coaxial copper connections—the same medium used to provide television broadcast services. It supports higher data rates than DSL, but
like DSL, it provides a full-time connection. However, it has one major drawback: it is a shared service and functions in a logical bus topology (discussed in Chapter 2) much like Ethernet—the more customers in an area that connect via cable, the less bandwidth each customer has.
Examples of networking devices used in WAN connections include cable and DSL modems, carrier switches, CSU/DSUs, firewalls, modems, NT1s, and routers.
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