Information Technology Reference
In-Depth Information
3.2.2.3 Logical Channel Signaling Entity
An acknowledged protocol is defined for the opening and closing of logical
channels that carry the audiovisual and data information. The aim of these
procedures is to ensure that a terminal is capable of receiving and decoding
the data that will be transmitted on a logical channel at the time the logical
channel is opened rather than at the time the first data is transmitted on it;
and to ensure that the receiving terminal is ready to receive and decode the
data that will be transmitted on the logical channel before that transmission
starts. The OpenLogicalChannel message includes a description of the data
to be transported, for example, H.263 at 6 Mbit/s. Logical channels should
only be opened when there is sufficient capability to receive data on all open
logical channels simultaneously.
A part of this protocol is concerned with the opening of bidirectional chan-
nels. To avoid conflicts that may arise when two terminals initiate similar
events simultaneously, one terminal is defined as the master terminal, and
the other as the slave terminal. A protocol is defined to establish the terminal
that is the master and the one that is the slave. However, systems that use this
recommendation may specify the procedure specified in this recommenda-
tion or another means of determining the master and the slave.
3.2.2.4 Close Logical Channel Request Signaling Entity
A logical channel is opened and closed from the transmitter side. A mecha-
nism is defined that allows a receiving terminal to request the closure of an
incoming logical channel. The transmit terminal may accept or reject the
logical channel closure request. A terminal may, for example, use these pro-
cedures to request the closure of an incoming logical channel, which, for
whatever reason, cannot be decoded. These procedures may also be used to
request the closure of a bidirectional logical channel by the terminal that did
not open the channel.
3.2.2.5 H.223 Multiplex Table Entry Modification Signaling Entity
The H.223 multiplex table associates each octet within an H.223 MUX mes-
sage with a particular logical channel number. The H.223 multiplex table
may have up to 15 entries. A mechanism is provided that allows the transmit
terminal to specify and inform the receiver of new H.223 multiplex table
entries. A receiving terminal may also request the retransmission of a mul-
tiplex table entry.
3.2.2.6 Audiovisual and Data Mode Request Signaling Entity
When the capability exchange protocol has been completed, both terminals
will be aware of each other's capability to transmit and receive as specified
in the capability descriptors that have been exchanged. It is not mandatory
