Although all of these businesses need WANs, they might not need the same kinds of WANs.Depending on the traffic load, budget, geographical breadth, and commercially available technology, each might implement a different transmission method. For every business need, a fewappropriate WAN connection types might exist. However, many WAN technologies can coexist on the same network.WANs and LANs have several fundamental properties in common. Both are designed toenable communication between clients and hosts for resource sharing. In general, both usethe same protocols from Layers 3 and higher of the OSI model. And both networks typicallycarry digitized data via packet-switched connections.However, LANs and WANs often differ at Layers 1 and 2 of the OSI model in access methods, topologies, and, sometimes, media. They also differ in the extent to which the organization that uses the network is responsible for the network. LANs that depend on wire-boundtransmission use a building’s internal cabling, such as twisted pair, which runs from workareas to the walls, through plenum areas, and to a telecommunications closet. Such wiringis private; it belongs to the building owner. In contrast, WANs typically send data over publicly available communications networks, which are owned by local and long-distance telecommunications carriers. Such carriers, which are privately owned corporations, are alsoknown asNSPs (network service providers). Some large NSPs based in the United States
include AT&T, Verizon, and Sprint. Customers lease connections from these carriers, with
payments based on the amount of bandwidth they need. For best throughput and quality,
organizations lease dedicated lines, or continuously available communications channels,
from a telecommunications provider, such as a local telephone company or ISP. Dedicated
lines come in a variety of types that are distinguished by their capacity and transmission
characteristics.
The individual geographic locations connected by a WAN are known as WAN sites. AWAN
link is a connection between one WAN site (or point) and another site (or point). Most WAN
links are point-to-point, connecting one site to only one other site. That is, the link does not
connect one site to several other sites, in the way that LAN routers connect multiple segments
or workstations. Nevertheless, one location may be connected to more than one location by
multiple WAN links. Figure 7-1 illustrates the difference between WAN and LAN
connectivity.
The following section describes different topologies used on WANs.
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