Understanding Bearer Capabilities and their implications becomes significant when diagnosing problems, particularly when using diagnostic traces.

An example of the problems that can occur involves fax calls not being received correctly because there are two possible types of Bearer Capability for a fax call.

• 3.1kHz analogue call
  This often happens where the call originated inside the PSTN. The ISDN has no way of knowing exactly what type of equipment (telephone, fax, modem, etc.) placed the call, so it uses this "catch-all" bearer capability.
• Fax Group 3 call
  This is often used where the originates with a fax machine that is directly connected to an ISDN.

For the call to be successful, the chosen Bearer Capabilities must be supported by all equipment throughout the path taken by this call across the network. If any single piece of equipment does not support the requested Bearer Capabilities, the call will not be connected. In general, there is no support for negotiating Bearer Capabilities.

We therefore need to ask the questions:

• Do all the ISDN networks traversed by the call support the requested Bearer Capabilities?
• Does the PBX equipment at each end support these Bearer Capabilities?
• Has it been enabled?
• Has it been enabled for the port in question?