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1.6 Encryption and extension-BAS capabilities
lie capabilities are defined in Recommendation H.221.
I Transmission
1.1 Transmission modes
Ludio modes of operation are defíned in Annex A/H.221 audio commands.
!or analogue telephone termináis, it may be assumed that the speech sígnal is converted to PCM to G.711 at a digital
etwork interface. These termináis are viewed as workíng in mode OU when connected to wideband speech termináis.
'he vídeo transmission is govemed by the video-on and video-off commands. When switched on, the video signal
ccupies all of the capacily, both in the initial channel and in any additional channels, whích is not specifícally
Ilocated to other signáis by other commands. Thus different video bit rates will result from audio, transfer-rate, ECS
nd data commands, the resultant video bit rate being: [transfer rate, less audio rate, less data rate if present, less
ncryption control channel if present, less FAS and BAS in all the channels/tíme-slots where they are present}.
'ransfer-rate modes are defined in Recommendation H.221, and specify the total capacity of íhe communication
ffectíve in the following the BAS command sub-multiframe.
lata modes are defined in Recommendation H.221, and specify only the bit rate and bit positíons used for a user data
ígnal. The protocol used for data applications is defined by the termináis, but see also 9.
.2 Establishment of compatible modes of operation
,t the beginníng of the communication phase of a cali, all termináis start to work in mode OF (outgoing signal
•amed). Termináis other than those limited to G.71 1 capability will then begin an initialization procedure.
his procedure (further described in 6) consists of;
— the transmission of Information conceming the capabilities of the respective termináis for receiving and
decoding audio, video, transfer rate, data rates and other capabilities;
— the determination of a suitable transmission mode, consistent with the known capabilities of both
termináis. An example is given ín IV.1, in which the transmission mode is the same in both dírections,
but the H.242 procedures are equally applicable to systems in which asymmetric bidirectional
communication is optimal (examples are surveillance — see IV.2 — and retrieval servicesj;
— switching to thís mode; and establishing additional channels if relevant.
he termináis connected to a cali may change during the cali. This may require re-initialization in order to identify the
rminal type and to re-establish the desired mode of operation. In panicular, this feature is used in mode O forcing,
hich is necessary Ín the case of a cali transfer (see 8).
Frame structure
ti e frame structure described in Recommendation H.221 is used for mode initialization and dynamic mode switching
ee the following subclauses) and more generally to define the multiplex of the various bit streams (audio, vídeo, data,
icryption control signal, frame structure) wíthin the frame.
ecommendation H.221 defines a bit rate allocation signal (BAS) whích is used ínter alia to allocate sub-channels and
indícate the coding algorithm(s).
A.S codes are classified by the valué of the first three bits which represent the BAS attribute; each attribute may
e.refore have, un to 32 defíned valúes.