Ccna routing and Switching Complete Study Guide

Defining WAN Terms

Before you run out and order a WAN service type from a provider, you really need to 

understand the following terms that service providers typically use. Take a look at these in 

Figure 21.4:

f i g u r e   21. 4     WAN terms

Provider

toll network

CO

Corporate

Local loop

CSU/DSU

Demarc

CPE

Customer premises equipment (CPE)    Customer premises equipment (CPE) is equipment 

that’s typically owned by the subscriber and located on the subscriber’s premises.

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Chapter 21 

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  Wide Area Networks

CSU/DSU    A channel service unit/data service unit (CSU/DSU) is a device that is used to 

connect data terminal equipment (DTE) to a digital circuit, such as a T1/T3 line. A device 

is considered DTE if it is either a source or destination for digital data—for example, PCs, 

servers, and routers. In Figure 21.4, the router is considered DTE because it is passing data 

to the CSU/DSU, which will forward the data to the service provider. Although the CSU/

DSU connects to the service provider infrastructure using a telephone or coaxial cable, such 

as a T1 or E1 line, it connects to the router with a serial cable. The most important aspect 

to remember for the CCNA objectives is that the CSU/DSU provides clocking of the line to 

the router. You really need to understand this completely, which is why I’ll cover it in depth 

later in the cabling the serial WAN interface configuration section!