Bearer Capabilities
This is a mechanism by which a user can inform the ISDN of the protocol that he is using in a B channel.
Strictly speaking, the Bearer Capability is one of three mechanisms that are used to communicate this information. However, the other two mechanisms (called High Layer Compatibility and Low Layer Compatibility) are always used in conjunction with Bearer Capabilities, so they are often known collectively as bearer capabilities. The High Layer Compatibility and Low Layer Compatibility are used to provide additional details about the B channel protocol. On this page, Bearer Capabilities (capitalised) refers to the mechanism, bearer capabilities (lower case) refer to the protocol in use.
When a user requests a connection through an ISDN, he is obliged to specify the Bearer Capabilities. If he does not, his request is rejected. If he is using a protocol that the ISDN does not understand he will specify that the data is transparent (or unstructured). High Layer Compatibility and Low Layer Compatibility are both optional.
The bearer capabilities are communicated to the destination, so that he may act upon them. For example, an analogue call could be presented to a modem, but a transparent data call would be presented to an ISDN adapter.
The ISDN might also use the bearer capabilities for its own purpose. For example, in North America, some network operators make different charges for voice and for data calls. Some networks are able to optimise their internal performance by using the bearer capabilities.