Part of an ISDN configuration is knowing the switch type that the router is connected to. Many manufacturers created their own distinct ways of signaling, and if a router is configured for a 5-ESS, it won't be able to talk with an NI-1 switch. There are different switch types for PRI versus BRI as well.
When configuring a PRI connection, remember that an E-1 uses timeslot 15 for signaling and a T1 uses timeslot 23.
A dialer-map statement tells the router what phone number to use to get to a given network address. The statement includes the network address; the phone number; and, if using CHAP, the username or device name that has been configured on the remote router.
A TE1 is a native ISDN device, whereas a TE2 requires an adapter for ISDN operation. An analog phone would be a TE2, and an ISDN phone would be a TE1.
The U reference point uses two wires, but the S/T reference point uses four wires.
The NT1 connects the U to the S/T. The NT1 can be built into some devices or can be a separate piece of equipment from others. A TA is used to connect TE2 devices to the ISDN network.
T1 and E1 PRI circuits support up to 1.544Mbps and 2.048Mbps of bandwidth, respectively, whereas a BRI connection supports two 64Kbps B-channels and a 64Kbps D-channel. It's important to know what the local service provider is willing to sell you and what you need to achieve connectivity.
T1 and E1 circuits seem as if they would be compatible, but they aren't. Even though you can configure both with alternate mark inversion (AMI) and both use 64Kbps channels, T1s need either SuperFrame or Extended SuperFrame. An E1 needs cyclic redundancy check (CRC)-4 or No CRC-4 Framing. These types are not compatible.
Use show debug isdn q921 to view constantly updating Layer 2 ISDN information and debug isdn q931 or debug isdn event to view Layer 3 ISDN information.
