Troubleshooting Connections at the Physical Layer

One of the first areas to consider when troubleshooting ISDN is the Physical layer, particularly with new installations. The wiring is one of the most important aspects to consider when no connection can be made. When using ISDN for videoconferencing equipment or connecting to PBX equipment in the local network, you can use Category 3, 4, 5, or even 6 cabling.

An RJ-45, RJ-11, or RJ-14 can be used for ISDN terminations, however, it is recommended that you use an RJ-45.

This chapter, however, will focus on asynchronous Basic Rate Interface (BRI) connections, which use standard copper -pair wiring and which are the standard connection type in the United States.

ISDN BRI was originally designed to provide digital services over existing pairs of copper telephone wire so that the already existing analog phone lines wouldn't need to be replaced . A BRI can be used for videoconferencing, voice services, or data. An additional control channel, called the D channel, is used by BRI as a replacement for legacy X.25 networks.

BRI interface connections are different from primary rate interface (PRI) connections. PRI uses a T-1 or DS-1, which is the equivalent of 24 individual 64K channels. A voice connection basic rate is referred to as a DS-0, or a single 64Kbps B channel of the T-1 connection.

Each ISDN BRI is a 192Kbps circuit that is divided into three individual channels in a connection. The B channels are used as the primary data channels, providing 64Kbps of bandwidth in each direction. The D channel provides 16Kbps of bandwidth for control signaling. The remaining bandwidth of 48Kbps is used for overhead.

A frame that traverses the BRI interfaces is 48 bits, and each BRI circuit can send 4,000 frames per second. A service called Always On can use the D channel to send data, which reflects a nondemand mode for the channel. The Always On service is usually used to replace X.25 in point-of-sale (POS) circuits that use 9.6Kbps of bandwidth for the application.

For the exam you should remember the total circuit bandwidth for a BRI, the bandwidth of the B and D channels, as well as the size of frames that can traverse the link.

The Local Loop and Its Connections

It is good to keep in mind that a local loop exists on the remote side that must be functioning properly. The local loop is the circuit between your side of the demarcation point and the central office (CO), also known as the carrier operations. The demarcation point is the point at which your ISDN connection connects to the wall in your facility. This is also the point at which the telephone company begins and ends its responsibility for the connection. The local service provider's responsibility is to verify that a connection exists at the wall, and from there it is the local administrator's responsibility to get the connection functioning. Of course, it is also the responsibility of the local service provider to provide you with the proper SPIDs and connection type information.

The local loop at this point in the network is referred to as the RT. The RT allows the connection to overcome distance limitations so that a clear signal can reach the CO. The local loop connects your local ISDN router and the ISDN switch belonging to the local service provider. The local loop is necessary because all digital signals have distance limitations between two devices.

Now that you know about the local loop and the connections, you are ready to learn how to troubleshoot beyond the Physical Layer connectivity. The next section discusses that troubleshooting process.