Consider this example of the subtleties of Bearer Capability requirements.

A user of an ISDN PBX (ISPBX) may have no problem making analogue calls, yet experience difficulties sending faxes. The Bearer Capability required to send a fax might be:

• Enabled for the port in use on the remote PBX
• Available as a facility on the local and remote PBX
• Supported by the local and remote providers
• Enabled for various ports on the local PBX



• But not enabled on the particular PBX port experiencing the problem.

The chain of enabled Bearer Capabilities must be complete for the service to be operable. In this instance, the PBX will refuse to recognise the outgoing fax calls from the local port.