Connecting Your Wide Area Network (WAN) Devices
For any WAN connection type that you may choose to use, you must establish a connection between the service provider and your router. How you do so will vary based on the type of connection. In general, you will use serial ports or ISDN ports or implement a Channel Service Unit/Data Service Unit (CSU/DSU).
One of the main methods of connecting the router to your service provider's network is via a serial port. Depending on your service provider connection, you may be provided with a CSU/DSU. The CSU/DSU serves the same purpose a modem (modulator/demodulator) in that it prepares your digital data to be sent over a telephone company network that may not be carrying a digital signal.
These devices connect to the service provider connection, such as a T1 link, which is then connected to your router. Figure 5-2 illustrates this final link.
This breaks the connection into two types of devices: Data Terminal Equipment (DTE) which end user devices, and Data Communications Equipment (DCE) which are telephone company devices.
In the computer world, a computer represents the DTE and a modem represents the DCE, but when dealing with your service provider, your router is the DTE, and the service provider's communication devices are the DCE. So, the cable that connects to your router connects to the DTE, whereas the end of the cable that connects to the service provider's equipment connects to the DCE
If you want to simulate a router-to-router serial connection for testing before deploying your network equipment, you can use a special serial cable (back-to-back serial cable), which is defined with two specific ends, for DTE and DCE.
Over the years, to reduce cost or complexity, it has become common for people to integrate the CSU/DSU components into their routers. Rather than having another bulky device sitting in your communications room, you can have the functionality of the CSU/DSU integrated into the circuitry of your router.
The drawback is that if you decide to change the type of connection you have, you may need to change your router. Consequently, Cisco came up with the WAN Interface Card (WIC) port, which allows you to change the type of service provider connection you use at a minimum cost, allowing you to reuse your existing router.
With the prevalence of ISDN connections, Cisco has created and sold routers in a fixed configuration — rather than use WIC ports — that are preconfigured for ISDN environments. Your connectivity options are limited, but the price of these routers can represent an attractive price-point if you do not anticipate changing the connection types you use with your service provider.