Information Technology Reference
In-Depth Information
This phase is out of the range of 3G324M protocol stack (H.324, H.245, and
H.223) but done by modem. A terminal should be able to work under both
an active mode, dialing a call-setup, and a passive mode, answering a call-
setup request. In our implementation, (Hayes At Command) AT Commands
are used to make call-setup with the remote terminal, the procedure is pre-
sented in the following codes, including the active mode and passive mode.
3.2.1.3.2 Initialization (H.245)
After digital communication has been established, a minimum of 16 HDLC
flags shall be transmitted in order to ensure synchronization. Following
this, system-to-system communication shall be initiated using the H.245
control channel. Since no multiplex table entries have yet been sent to
the receiver, initial control messages shall be sent using multiplex table
entry 0.
Terminal system capabilities are exchanged by transmission of the H.245
TerminalCapabilitySet message. This capability protocol data unit (PDU)
shall be the first message sent. The H.245 MasterSlaveDetermination mes-
sage shall also be sent at this time in which the terminals exchange random
numbers, according to the procedure in ITU-T Rec. H.245, to determine the
master and slave terminals. H.324 terminals shall be capable of operating
in both master and slave modes, and shall set terminalType to 128 and set
StatusDeterminationNumber to a random number in the range 0 to 224. Only
one random number shall be chosen by the terminal for each call, except in
the case of identical random numbers, as described in ITU-T Rec. H.245.
If the initial capability exchange or master/slave determination procedures
fail, these should be retried at least two additional times before the terminal
abandons the connection attempt and proceeds to phase D.
The range of terminalTypes from 0 to 127 is reserved for possible use by mul-
tipoint control units (MCUs) or other nonterminal devices that may need to be a
slave at all times, and the range 129 to 255 is reserved for possible use by MCUs
or other nonterminal devices that may need to be master at all the times.
After these procedures are completed, and the far-end capabilities have
been received, the procedures of H.245 may then be used to open logical
channels for various information streams. Multiplex table entries may be
sent before or after logical channels have been opened, but information shall
not be transmitted over a logical channel until the channel is opened, and an
appropriate H.223 multiplex table entry has been defined.
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Exchange of Video by Mutual Agreement
The indication videoIndicateReadyToActivate, “Video Indicate
Ready-To-Activate,” is defined in H.245. Its use is optional, but when
used the procedure shall be as follows.
Terminal X has been set so that video is not transmitted unless, and
until, the remote terminal has also indicated readiness to transmit
