Terminology for Troubleshooting After the Physical Layer

This section focuses on the other aspects of troubleshooting ISDN, when the problems aren't found at the Physical layer. To better understand this (and the rest of this chapter), a quick terminology lesson is necessary to familiarize you with the terms used to describe the ISDN components shown in Figure 12.2. After that, we will discuss the techniques and commands for troubleshooting ISDN configurations.

Figure 12.2. The ISDN Physical layer components.

The following lists the terms you need to know:

• Line termination (LT) point ” Handles line termination of the local loop and switching functions. An NT1 device is located at the local service provider's site.

• Exchange termination (ET) point ” Handles line termination of the local loop and switching functions.

• Network termination (NT1) ” The network termination point. This is often the demarcation point.

• Terminal equipment type 1 (TE1) ” This is a device that uses a four-wire, twisted-pair digital interface. Most ISDN devices found in today's networks are this type.

• Terminal equipment type 2 (TE2) ” This is a device that does not contain an ISDN interface and that requires a terminal adapter (TA).

• R reference point (R) ” Identifies the reference point between non-ISDN equipment and a TA. This point in the network allows a non-ISDN device to appear to the network as an ISDN device.

• S reference point (S) ” The connection point between the user-end equipment and an NT1 or an NT2. The user -end equipment can be the ISDN router, a TE1, or a TA.

• T reference point (T) ” Defines the reference point between an NT1 device and an NT2 device.

• S/T interface ” A combination of both the S and T interfaces. This interface is governed by the ITU I.430 standard, which defines the connection as a four-wire connection and the ISO 8877 physical connector. The S/T interface is an RJ 45, 8-pin cable using pins 3 and 6 to receive data and pins 4 and 5 to transmit data. A straight-through pin configuration connects the terminal end point (TE) to the NT.

• U reference point (U) ” The connection point between an NT1 and the LE. It is normally serviced on a single twisted-pair cable to reduce data delivery costs and simplify installations.

• V reference point (V) ” The termination point within the local loop.

Both the LT and the ET typically are just referred to as the local exchange (LE).