Information Technology Reference
In-Depth Information
The master-slave determination procedure allows terminals in a call to
determine what terminal is the master and what is the slave. The terminal
status may be redetermined at any time during a call; however, a terminal
may only initiate the master-slave determination process if no procedure
that depends upon its result is locally active.
3.2.2.2 Capability Exchange Signaling Entity
The capability exchange procedures are intended to ensure that the only
multimedia signals to be transmitted are those that can be received and
treated appropriately by the receiving terminal. This requires that the
capabilities of each terminal to receive and decode be known to the other
terminal. It is not necessary that a terminal understands or stores all incom-
ing capabilities; those that are not understood, or can not be used shall be
ignored, and no fault shall be considered to have occurred. When a capabil-
ity is received that contains extensions not understood by the terminal, the
capability shall be accepted as if it did not contain the extensions.
The total capability of a terminal to receive and decode various signals is
made known to the other terminal by transmission of its capability set.
Receiving capabilities describe the terminal's ability to receive and process
incoming information streams. Transmitters shall limit the content of their
transmitted information to which the receiver has indicated it is capable of
receiving. The absence of a receive capability indicates that the terminal can-
not receive (is a transmitter only).
Transmit capabilities describe the terminal's ability to transmit information
streams. Transmit capabilities serve to offer receivers a choice of possible
modes of operation, so that the receiver may request the mode that it prefers
to receive. The absence of a transmit capability indicates that the terminal is
not offering a choice of preferred modes to the receiver (but it may still trans-
mit anything within the capability of the receiver).
These capability sets provide for more than one stream of a given medium
type to be sent simultaneously. For example, a terminal may declare its ability
to receive (or send) two independent H.263 video streams and two indepen-
dent G.731.1 audio streams at the same time. Capability messages have been
defined to allow a terminal to indicate that it does not have fixed capabilities,
but that they depend on other modes that are being used simultaneously. For
example, it is possible to indicate that higher resolution video can be decoded
when a simpler audio algorithm is used; or that either two low resolution
video sequences can be decoded or a single high resolution one. It is also pos-
sible to indicate trade-offs between the capability to transmit and the capabil-
ity to receive.
Nonstandard capabilities and control messages may be issued using the
NonstandardParameter structure. Note that while the meaning of nonstan-
dard messages is defined by individual organizations, equipment built by any
manufacturer may signal any nonstandard message, if the meaning is known.
