WAN Connections from the Customer Viewpoint

The concepts behind a point-to-point connection are simple. However, to fully understand what the service provider does to build his network to support your point-to-point line, you would need to spend lots of time studying and learning. However, most of what you need to know about WANs for the INTRO exam relates to how WAN connections are implemented between the telephone company and a customer site. Along the way, you will need to learn a little about the terminology used by the provider.

In Figure 4-2, you saw that a WAN leased line acts as if the telco gave you two twisted pairs of wires between the two sites on each end of the line. Well, it's not that simple. Of course, a lot more underlying technology must be used to create the circuit, and telcos use a lot of terminology that is different from LAN terminology. The telco seldom actually runs a 1000-mile cable for you between the two sites. Instead, it has built a large network already and even runs extra cables from the local central office (CO) to your building. (A CO is just a building where the telco locates the devices used to create its own network.) However the telco works out the details, what you receive is the equivalent of a four-wire leased circuit between two buildings.

Figure 4-3 introduces some of those key concepts and terms relating to WAN circuits.

Figure 4-3 Point-to-Point Leased Line: Components and Terminology

- Short Cables (Usually Less than 50 Feet)

Long Cables (Can Be Several Miles Long)

Figure 4-3 Point-to-Point Leased Line: Components and Terminology

- Short Cables (Usually Less than 50 Feet)

Long Cables (Can Be Several Miles Long)

Typically, routers connect to a device called an external channel service unit/digital service unit (CSU/DSU). The router connects to the CSU/DSU with a relatively short cable, typically less than 50 feet, because the CSU/DSUs typically get placed in a rack near the router. The much longer four-wire cable from the telco plugs into the CSU/DSU. That cable leaves the building, running through the hidden (typically buried) cables that you always see phone company workers fixing by the side of the road. The other end of that cable ends up in something called a central office (CO), which is simply a building where the phone company puts its equipment. The actual physical line terminates in a device generically called a WAN switch, of which there are many types.

The same general physical connectivity exists on each side of the point-to-point WAN link. In between the two COs, the service provider can build its network with several competing different types of technology, all of which is beyond the scope of either CCNA exam. However, the perspective in Figure 4-2 remains true—the two routers can send and receive data simultaneously across the point-to-point WAN link.

From a legal perspective, two different companies own the various components of the equipment and lines in Figure 4-3. For instance, the router cable and typically the CSU/DSU are owned by one company, and the wiring to the CO and the gear inside the CO are owned by the telco. So, the telco uses the term demarc, which is short for demarcation point, to refer to the point at which the telco's responsibility is on one side and the customer's responsibility is on the other. The demarc is not a separate device or cable, but instead a concept of where each company's responsibilities end.

In the United States, the demarc is typically where the telco physically terminates the set of two twisted pairs inside the customer building. Typically, the customer asks the telco to terminate the cable in a particular room, and most, if not all, the lines from the telco into that building terminate in the same room.

The term customer premises equipment (CPE) refers to devices that are at the customer site, from the telco's perspective. For instance, both the CSU/DSU and the router are CPE devices in this case.

The demarc does not always reside between the telco and all CPE. In some cases, the telco actually could own the CSU/DSU, and the demarc would be on the router side of the CSU/ DSU. In some cases today, the telco even owns and manages the router at the customer site, again moving the point that would be considered the demarc. Regardless of where the demarc sits from a legal perspective, the term CPE still refers to the equipment at the telco customer's location.

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