Cisco IOS Software supports a number of WAN protocols. In this chapter, we will introduce you to High-Level Data Link Control (HDLC), Point-to-Point Protocol (PPP), and Frame Relay on serial interfaces. We will also learn how to configure these WAN protocols on Cisco routers. We also give you a brief introductions to virtual private networks (VPNs) as an alternate to traditional WAN solutions.
The OSI Layer 1 (physical layer) and Layer 2 (data link layer) work together to deliver data across a wide variety of network types. Local-Area Network (LAN) standards and protocols define how to network devices that are relatively close together, hence the term local-area in the acronym LAN. Wide-Area Network (WAN) standards and protocols define how to network devices that are relatively far apart, hence the term wide-area in the acronym WAN. LANs and WANs both implement the same OSI Layer 1 and Layer 2 functions but with different mechanisms.
The big distinction between LANs and WANs relates to how far apart the devices can be and still be capable of sending and receiving data. LANs tend to reside in a single building or at most among nearby buildings in a campus using optical cabling approved for Ethernet. WAN connections typically run much longer distances than Ethernet LANs: across town, between cities, or even between continents. Usually only one or a few companies even have the rights to run cables under the ground between the sites. For example, a company may have two offices just across a road such that the distance between the two buildings is well within the maximum distance supported by Ethernet technologies. However, the two companies still cannot simply run a cable under the ground between the two offices due to right-of-way restrictions. When Ethernet LANs are used to connect buildings, it normally is inside a campus like a university or office complex.
Besides LANs and WANs, the term Metropolitan-Area Network (MAN) is sometimes used for networks that extend between buildings and through rights-of-way. The term MAN typlically implies a network that does not reach as far as a WAN, and generally spans a single metropolitan area. However, you should keep in mind that the distinctions between LANs, MANs, and WANs are blurry. There is no set distance that means a link is a LAN, MAN, or WAN link. For example, the 1000BASE-ZX Ethernet standard with extended wavelength, single-mode (SM) fiber cabling can achieve distances upto 100 km!
A company that needs to send data over longer distances does not actually own the line or cable; it rather leases it from the company that actually own it and that’s why it is called a leased line. The company that owns, manages, and installs such long links, or circuits has the right-of-way to run cables under streets, highways, rivers etc. The generic term service provider is used to refer to a company that provides leased lines for WAN connectivity.
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