as they are described in IOS. Some CSU/DSUs even include a feature that lets them run BERT tests. While this be useful, if you're running BERT tests, you should probably call telco. Most problems discovered by BERT tests cannot be fixed with router configuration. Troubleshooting T1s CSU/DSU CSU/DSU Router A Router B Remote Remote payload payload Figure 21-13. Integrated CSU/DSU loopback points Configuring T1s There are two steps involved in configuring T1s for use on a router. The first step is the configuration of the CSU/DSUs. The second step is the configuration of the router interface. When you're using integrated CSU/DSUs, the lines might seem blurred, but the concepts remain the same. Configuring the router interface is just like configuring any serial interface. CSU/DSU Configuration To get a T1 up and operational, you must do the following: Configure both sides with the same encoding that matches the circuit's provisioned encoding Encoding options are AMI and B8ZS. Data T1s should always use B8ZS encoding. To configure encoding on a CSU/DSU WIC, use the service-module t1 line code interface command: Router (config)#int s0/1 Router (config-if)# service-module t1 linecode b8zs Configure both sides with the same framing that matches the circuit's provisioned framing Framing options are D4/SF and ESF. Data T1s should always use ESF framing. To configure framing on a CSU/DSU WIC, use the service-module t1 framing inter- face command: Router (config)#int s0/1 Router (config-if)# service-module t1 framing esf Configure how many channels will be used for the link, which channels will be used, and what speed they will be If the T1 is being split or you have had fewer than 24 channels delivered to you, you must tell the CSU/DSU how many channels are in use. You do this for the Chapter 21: T1 CSU/DSU WIC with the service-module t1 timeslots interface command. Here, I've specified that channels 7-12 will be used at a speed of 64 Kbps: Router (config)#int s0/1 Router (config-if)# service-module t1 timeslots 7-12 speed