Although the ISDN doesn't always need to know what protocol is in use in the B channel, there are circumstances where this information is useful.

For instance, if you make an telephone call over ISDN, it is useful to tell the network that this is a voice call so that it can connect your call with an analogue telephone in the PSTN. If the ISDN network thinks that your B channel contains a protocol unknown to it, then it can only connect your call directly to another ISDN line.

You have the opportunity to tell the ISDN about the protocol that you're using in the B channel when you request the call. This information is added to the signalling information sent on the D channel when the call is requested. This call information is often called Bearer Capabilities.

The receiver of the call can also see the bearer capabilities when a call is offered to him by the network.

Generally, there is no possibility to negotiate bearer capabilities. You must decide what bearer capabilities you want to use before placing the call. Bearer capabilities are fixed for the lifetime of the call and cannot be dynamically changed. If you attempt to place a call without specifying the bearer capabilities, the request will be rejected by the ISDN.