Tips and Training >> Definitions >> ISDN


The term ISDN stands for Integrated Services Digital Network.

ISDN was developed as an early form of a networking protocol that was intended to carry switched multimedia messages. Enabling users to make multimedia connections by "calling" from one device to another was a breakthrough enabled by ISDN. Prior to ISDN, users arranged digital connections between devices such as video conferencing systems, by contacting their telephone provider to request a reconfiguration of their network which sometimes took hours or even days.

ISDN uses out of band signaling to allow for full 64Kbps transmission on switched data channels. Switched data channels prior to the advent of ISDN required signals for call supervision such as ringing indicators to be carried "in band" and this reduced the communications bandwidth for user data channels to 56Kbps each.

ISDN B Channels

User data channels are called "bearer" channels and because ISDN uses out of band signaling, each ISDN bearer or "B" channel provides 64Kbps of bandwidth. Calls can be made at 56Kbps but this is a reduced rate feature of ISDN. In this way, ISDN is backwards compatible with older style communications channels.

ISDN telephone lines are available in low and high capacity. A low capacity ISDN telephone line can be delivered to a user just as a regular telephone line, however it can provide up to two B channels at 64Kbps each. There are actually three channels on a low capacity ISDN line, 2B channels and 1D Channel (sometimes referred to as 2B+D). The third channel is called the "D" channel to distinguish it from the "B" or data bearing channels. Common names for this channel are: Data, Delta or simply the D channel. The third, or D channel, is primarily used for signaling. An example of signaling is sending an alert signal to begin ringing for a telephone.

Low capacity ISDN telephone lines are called Basic Rate ISDN lines or BRI lines. High capacity ISDN lines include 23 or 24 B channels. The high capacity ISDN lines are delivered to users on a T-1 line and here again, each individual "B" channel provides 64Kbps of user data bandwidth.

What is an ISDN D Channel?

An ISDN D channel is primarily a signaling channel. It carries information about the types of calls, requested services (such as 56Kbps speech or 64Kbps data) and status of calls on B channels. D channels follow very specific and well accepted Q.921 and Q.931 signaling which provide significant help in problem isolation and network diagnostics.

One D channel is provisioned on each Basic Rate ISDN line and it is required for the B channels to function. An NT1 device physically connects to the ISDN BRI line and it manages the signaling between multiple endpoints that are possible to connect to a BRI Line. Often, users only connect one device to an ISDN line and in this case, an NT1 is still required.

Primary Rate ISDN, as the high capacity lines are known, does not require an NT1. Generally, a channel signaling unit or CSU terminates a T-1 and then the T-1 connects to an ISDN switch or video endpoint. Primary Rate ISDN lines or PRI lines as they are called can provide 23 or 24 B channels to users. Each channel must be associated with at least one signaling D channel and so if only one PRI is used in an application, the D channel is generally assigned to the 24th time slot channel of the T-1.

If multiple T-1/PRI's are used in an application, a single D channel can be assigned to several PRI's. In this case, some PRI lines can be provisioned for all 24 channels of the T-1 to be used for the "B" Bearer Data channels (up to 64Kbps each). This is called NFAS signaling.

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