OSI Layer 1 for Pointto Point WANs

The OSI physical layer, or Layer 1, defines the details of how to move data from one device to another. In fact, many people think of OSI Layer 1 as "sending bits." Higher layers encapsulate the data, as described in Chapter 2, "The TCP/IP and OSI Networking Models." No matter what the other OSI layers do, eventually the sender of the data needs to actually transmit the bits to another device. The OSI physical layer defines the standards and protocols used to create the physical network and to send the bits across that network.

A point-to-point WAN link acts like a trunk between two Ethernet switches in many ways. For perspective, look at Figure 4-1, which shows a LAN with two buildings and two switches in each building.

As a brief review, remember that Ethernet uses a twisted pair of wires to transmit and another twisted pair to receive, to reduce electromagnetic interference. You typically use straight-through Ethernet cables between end user devices and the switches. For the trunk links between the switches, you use crossover cables because each switch transmits on the same pair, so the crossover cable connects one device's transmit pair to the other device's receive pair. The lower part of the figure reminds you of the basic idea behind a crossover cable.

Figure 4-1 Example LAN, Two Buildings

Figure 4-1 Example LAN, Two Buildings

Cross-over Cables

Cross-over Cable Conceptual View

Now imagine that the buildings are 1000 miles apart instead of right next to each other. You are immediately faced with two problems:

■ Ethernet does not support any type of cabling that allows an individual trunk to run for 1000 miles.

■ Even if Ethernet supported a 1000-mile trunk, you do not have the rights of way needed to bury a cable over the 1000 miles of real estate between buildings.

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 possibly among buildings in a campus using optical cabling approved for Ethernet. WAN connections typically run longer distances than Ethernet, across town or between cities. Often, only one or a few companies even have the rights to run cables under the ground between the sites. So, the people who created WAN standards needed to use different physical specifications than Ethernet to send data 1000 km or more (WAN).

NOTE Besides LANs and WANs, the term metropolitan-area network (MAN) is sometimes used for networks that extend between buildings and through rights-of-ways. The term typically implies a network that does not reach as far as a WAN, generally in a sinle metropolitan area. 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.

To create such long links, or circuits, the actual physical cabling is owned, installed, and managed by a company that has the right of way to run cables under streets. Because a company that needs to send data over the WAN circuit does not actually own the cable or line, it is called a leased line. Companies that can provide leased WAN lines typically started life as the local telephone company, or telco. In many countries, the telco is still a government-regulated or government-controlled monopoly; these companies are sometimes called public telephone and telegraph (PTT) companies. Today many people use the generic term service provider to refer to a company that provides any form of WAN connectivity, including Internet services.

Point-to-point WAN links provide basic connectivity between two points. To get a point-to-point WAN link, you would work with a service provider to install a circuit. What the phone company or service provider gives you is similar to what you would have if you made a phone call between two sites but you never hung up. The two devices on either end of the WAN circuit could send and receive bits between each other any time they want, without needing to dial a phone number. And because the connection is always available, a point-to-point WAN connection sometimes is called a leased circuit or leased line because you have the exclusive right to use that circuit, as long as you keep paying for it.

Now back to the comparison of the LAN between two nearby buildings versus the two buildings that are 1000 miles apart. The physical details are different, but the same general functions need to be accomplished, as shown in Figure 4-2.

Figure 4-2 Conceptual View of Point-to-Point Leased Line

Figure 4-2 Conceptual View of Point-to-Point Leased Line

1000 Miles

Building 2

Switc

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* R2 Switc

Keep in mind that Figure 4-2 provides a conceptual view of a point-to-point WAN link. In concept, the telco installs a physical cable, with a transmit and a receive twisted pair, between the buildings. The cable has been connected to each router, and each router, in turn, has been connected to the LAN switches. As a result of this new physical WAN link and the logic used by the routers connected to it, data now can be transferred between the two sites. In practice, the telco does not actually run a cable between the two buildings. In the next section, you will learn more about the physical details of the WAN link.

NOTE Ethernet switches have many different types of interfaces, but all the interfaces are some form of Ethernet. Routers provide the capability to connect many different types of OSI Layer 1 and 2 technologies. So, when you see a LAN connected to some other site using a WAN connection, you will see a router connected to each, as in Figure 4-2.

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