Bearer Capability is one of the Information Elements (fields) of the Q.931

Q.931

ITU-T Recommendation Q.931 is ISDN's connection control protocol, roughly comparable to TCP in the Internet Protocol stack. Q.931 doesn't provide flow control or perform retransmission, since the underlying layers are assumed to be reliable and the circuit-oriented nature of ISDN allocates...

 SETUP message. It is used by the calling party to specify the kind of B channel

B channel

B channel is a telecommunications term which refers to the ISDN channel in which the primary data or voice communication is carried. It has a bit rate of 64 kbit/s in full duplex....

 that is being requested.

For example, if a Bearer Capability of Speech is specified, then the network is free to select a set of facilities that are able to provide this service (including, in this case, analogue trunks and routes involving echo cancellers or DCME).

If, on the other hand, a Bearer Capability of Unrestricted Digital Information is specified, then the network is required to set up the call using only end-to-end digital facilities (without the use of robbed-bit signalling, echo cancellers, DCME nor A-law

A-law algorithm

An A-law algorithm is a standard companding algorithm, used in European digital communications systems to optimize, i.e., modify, the dynamic range of an analog signal for digitizing.It is similar to the μ-law algorithm used in North America and Japan....

 to mu-law

Mu-law algorithm

The µ-law algorithm is a companding algorithm, primarily used in the digital telecommunication systems of North America and Japan. Companding algorithms reduce the dynamic range of an audio signal...

conversion), or reject the call if no end-to-end digital circuit is available.

The Bearer Capability is augmented by the Higher Layer Compatibility (HLC) Information Element (another field in the Q.931 SETUP message), which informs the network of the use that the caller intends to make of the channel.