GB/T 15845.4-2003 Technical requirements for audio-visual user terminals Narrow-band audio-visual systems and terminal equipment
Some standard content:
ICS 33.160.60
National Standard of the People's Republic of China
GB/T 15845,4-—2003/ITU-T H.320:1999 replaces GB/T 15845. 4—1995
Technical requirements for audiovisual user terminals
Narrow-band audiovisual systems and terminal equipment
Performance reguirements of audiovisual terminals-Narrow-band audio visual system and terminal eguipment(ITU-T H. 320:1999,Systems and terminal equipment for audiovisual services-
Narrow-band visual telephone systems and terminal eguipment,IDT)2003-01-17 Issued by
General Administration of Quality Supervision, Inspection and Quarantine of the People's Republic of China
2003-08-01Implementation
GB/T 15845 is divided into the following five parts:
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GB/T 15845. 4—2003/ITU-T H, 320: 19991.GB/T15845.11995 "Technical requirements for audio-visual user terminals: Frame structure of 641920kbit/s channels in audio-visual user terminal services"
2.GB/T15845.2-1995 "Technical requirements for audio-visual user terminals: Control and indication signals for frame synchronization in audio-visual systems" 3. GB/T15845.3-1995 "Technical requirements for audio-visual user terminals: Methods for establishing communication between audio-visual user terminals using data channels within 2Mbit/s"
4.GB/T15845.4-2003 "Technical requirements for audio-visual user terminals: Narrowband audio-visual systems and terminal equipment" 5.GB/T15845.5-1995 "Technical requirements for audio-visual user terminals: P×64kbit/s t/s Video Codecs for Audiovisual Services" The above parts are equivalent to ITU-T H.221, H.230, H.242, H.320 and H.261 recommendations respectively. This part is the fourth part of GB/T15845. This part is equivalent to ITU-T H.320 recommendation (1999 version). In terms of the writing format, it adds a chapter of "normative reference documents" according to GB/T1.1-2000. ITU-T H.320 recommendation is the framework standard of conference video system. The recommendation involves frame structure, audio coding, frequency coding, intercommunication with other types of terminals and other aspects. This part replaces GB/T15845.4-1995 "Technical requirements for audiovisual terminals - narrowband audiovisual telephone systems and terminal equipment" GB/T 15845. 4-1995 is formulated on the basis of the 1992 version of H.320 recommendation. It is the fourth part of GB/T 15845-1995.
This instrument has revised GB/T15845.4-1995, and the remaining parts will be revised after the corresponding international recommended versions are relatively stable.
Compared with GB/T15845.4-1995, this part mainly adds the following contents: - The ITU-T recommendations are directly adopted in the normative reference documents; - The content of Table 1 has changed significantly compared with the 1995 version, for example, six notes on the audio-visual telephone mode have been added to the table; - The content of Table 2 has changed significantly compared with the 1995 version, for example, the table stipulates the modes that the terminal must support and the modes that can be supported:
Added "4.3.2.2 Optional requirements for single-channel devices running on the channel aggregator recommended by H.244". - In each stage of the normal process of call establishment in 4.4.1, the content has been increased to varying degrees compared with the 1995 version - 4.4.5 Added the content of audio information transmission in Chapter 4.5.1; added some content:
--- Added "4.5.3 Restricted Network"--- Added "4.5.3 Restricted Network"--- Added "5 Terminal Requirements"--- Added audio arrangement and measurement or principles; added some content in "5.3 Delay Compensation of Audio Path", such as the requirements and methods for delay compensation insertion,
--- Added "5.4 Control and Indication"--- Added the content of optional control and indication signals--- Added "6.3 Intercommunication with other audiovisual terminals"--- Added "Intercommunication with H.324 terminals, intercommunication with H.322 terminals, intercommunication with H.323 terminals", added an informative appendix A\H. Optional enhanced video algorithm for use in 320 systems". GB/T 15845.4--2003/1ITU-T H.320:1999 The appendix to this part is an informative appendix.
This part was proposed by the Ministry of Information Industry of the People's Republic of China and is under the jurisdiction of the Telecommunication Research Institute of the Ministry of Information Industry. Drafting unit of this part: Telecommunication Transmission Research Institute of the Ministry of Information Industry. Main drafters of this part: Shi Denian, Liu Xiaohong. This part was first issued in December 1995 and this is the first revision. 1 Scope
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GB/T 15845. 4—2003/IT-T H. 320: 1999 Technical requirements for audio-visual user terminals
Narrowband audio-visual systems and terminal equipment
This part of GB/T15845 specifies the technical requirements for narrowband audio-visual systems and terminal equipment. The requirements for audio-visual services for videophones are specified in Recommendation F.720, the requirements for audio-visual services for conference video are specified in Recommendation F.702, and the technical requirements for audio coding, video coding and other common parts of audio-visual services are specified in other recommendations in the H.200/F.720 series of recommendations. This part applies to narrowband audio-visual systems and terminal equipment, the channel rate of which does not exceed 1920 kbit/s. 2 Normative references
The clauses in the following documents become the clauses of this part through reference in this part of GB/T15845. For any dated referenced document, all subsequent amendments (excluding errata) or revisions are not applicable to this part. However, parties to an agreement based on this part are encouraged to study whether the latest versions of these documents can be used. For any undated referenced document, the latest version applies to this section.
ITU-T G.167 Acoustic echo controller
7 kHz audio coding at 64 kbit/s G.722
16 kbit/s speech coding using low-delay code excited linear prediction Multilayer procedures in audiovisual systems
Structure of 64 to 1920 khit/s channels in audiovisual user terminal services Control and indication signals for frame synchronisation in audiovisual systems Multipoint control equipment for audiovisual systems using digital channels up to 2 Mbit/s Method for establishing communication between audiovisual terminals using digital channels up to 2 Mbit/s Method for establishing communication between two or more audiovisual terminals using digital channels up to 2 Mbit/s Synchronous convergence of multiple 64 or 56 kbit?s channels Video codec for P×64 kbit/s audiovisual services ITU-T
1.400 ISDN user-network interface
3 Terms and definitions
The following terms and definitions apply to this standard: 3. 1
Batch rate allocation value number (BAs) bit-rate allocation signa 8-bit data in the H.221 recommendation frame structure, transmitting such as command, control and indication signals, capabilities, etc. 3.2
Control and indication signal (C&I) control and indication End-to-end signaling between two terminals including "control" that causes receiver status changes and "indication" that provides information such as system function, see H.230 recommendation
GB/T15845.4—2003/ITU-TH,320,19993.3
Data port data part
Input/output port for transmitting user data in a service channel or subchannel in accordance with H.221 recommendation. 3.4
Human-machine interface human-machine interface The interface between the user and the end/system, including the physical part (electric-to-acoustic converter, electric-to-optical converter, keyboard, etc.) and the logical part related to the functional operation status.
In-band signaling in-band signaling
BAS signaling through the H. 221 frame structure. 3.6
Lip sync lip syachronization
It is an operation that makes people feel that the speaking action is synchronized with the spoken sound. 3.7
Multipoint control equipment multipointcontrolanit (MCU) A device installed in a connection point or a terminal of the network. The device receives information from several channels from an access port, processes the audio and video information according to certain criteria and distributes them to the connected channels. 3.8
Narrow-band narrow-band
Channel with a bit rate in the range of 64 to 1920 khit/s. This channel capacity can be provided in ISDN as a single B/H./H1/Hi channel or multiple B/H. channels.
Out-band signaling
Signaling transmitted over channels other than B/H./Hu/H1: channels (defined by the I-400 series of recommendations). 3.10
Visual telephone serviceA set of audiovisual services, including the videotelephone service defined in Recommendation F.721 and the videoconferencing service defined in Recommendation F.730 under construction.
4 System description
4.1 Block diagram and element identification
·A general audiovisual telephone system block diagram is shown in Figure 1. It consists of terminal equipment, network, multipoint control unit (MCU) and other system operation entities.
As shown in Figure 1, the configuration of the terminal equipment is composed of several functional units. "Video 1/0 equipment" includes cameras, monitors and video processing units that provide functions such as screen segmentation solutions. "Audio 1/0 equipment includes microphones, speakers and audio processing units that provide functions such as voice cancellation (see Recommendation G.167). "Telematics equipment" includes audio-visual auxiliary equipment, such as electronic blackboards, text conversion equipment and still image transceivers to enhance basic audio-visual telephone communications. The "system control unit" performs the following functions: access the network through "end-to-network signaling", and implement end-to-end control through "end-to-end signaling" to establish a common operating mode and notify the terminal of appropriate operations. The "video codec" performs encoding and decoding of video signals to reduce redundant magic, and the "audio codec" does the same for audio signals. The "delay of the audio channel" is used to compensate for the delay of the video codec to maintain "lip synchronization". The "multiplexing/demultiplexing" unit multiplexes the video, audio, data and control signals to be transmitted into a single bit stream, and decomposes the received bit stream signal into the multimedia signals contained. * Network interface\ Performs necessary adaptation between the network and the terminal in accordance with the requirements of the user-network interface defined in the .400 series of recommendations.
Video gateway equipment
Background/core equipment
Video decoding
H. 200/AV 250-Series Recs.
Audio response
T·H.200/AV 270-Series Recs Remote information processing equipment
Recommendations H.242, H.230, H.221
-Terminal signaling
System control
MCU multipoint control unit
Terminal-network signaling
1.400-Series Recommendations
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GB/T 15845.4--2003/ITU-T H.320:1999 Recommendation H.221
Multiplexing/ Demultiplexing
Figure 1 Video Telephone System
I.400-series equipment
1 Interfaces
Note: For leased line networks, the network interfaces within the range of 64 kbit/s-2048 kbit/s are defined in Recommendation G.703. An alternative interface is defined in Recommendation X.21. For TxEI channels, the time slot allocation of the G.703 interface is specified in Section 5 of Recommendation G.704. It should be emphasized that the interaction with the ISI)N network requires synchronous operation of the leased line network. 4.2 Signals
The signals of the audio-visual telephone system are divided into video, audio, data and control signals: a) The audio signal is a continuous bit stream and requires real-time transmission; Note: In order to reduce the average bit rate of the audio signal, a voice activation mechanism can be introduced (in this case, the audio signal is no longer continuous). b) Video signals are also continuous bit streams: the bit rate allocated to video signals should be as high as possible to optimize the image quality within the available channel capacity;
) Data signals include still images, faxes and documents, or other media, such as text conversion. Data signals only occur occasionally as required and can temporarily replace all or part of the audio-visual signal content. It should be noted that data signals are only associated with enhanced options of the basic audio-visual telephone system, so the opening of the channel carrying data signals is after negotiation between terminals; d) Control signals are defined-some system control signals. The channel for terminal-to-network control signals is provided by the D channel, according to the mechanism defined in Recommendation H.221, and the channel for terminal-to-terminal control signals is provided by the BAS, or by the service channel when necessary.
4.3 Bit rate options and infrastructure
4.3.1 Control modes of audio-visual telephones
The communication modes of audio-visual telephones are defined in Table! according to their channel configuration and coding. The establishment of special operation communication modes is carried out according to the procedures specified in Recommendation H.242.
4.3.2 Terminal types for audio-visual telephony
Terminal types are distinguished according to the type of channels available for terminal communication. Terminals are classified into the following types: m×B (type X with parameters a~), n×II, (type Y with parameters 1 to 5, see note), Hn/Hre (type Z with parameters α to β) or a combination thereof. Table 2 gives the terminal types for audio-visual telephony and the communication modes that each type of terminal is required to support. Note: Type Y terminals must have SM-comp or 6B-H, compatible modes defined in Recommendation H.221 for increasing network interoperability (see 4.3.2.2).
GB/T 15845, 4—2003/ITU-T H.320: 1999 Table 1
Communication modes for audio-visual telephony
Audio-visual telephony mode
Footprint processing (combined audio coding)
Channel rate?
khit/s
ISDN interface
Basic rate
Not applicable
Primary rate
3 G. 723. 1
(Note 5)
Building H. 261
Recommendation H.262
Recommendation H.263
(Go to 6)
Note 1: When the audio-visual telephone terminal is interworking with a broadband voice terminal, G.722 audio coding (mode a) may replace G.711 audio coding (mode a). Note 2: When two terminals are connected at this rate and are running on G.711 (mode a) or G.722 (mode a1) coding, and both terminals have video capability, the H.261, H.262 or H.263 video coding protocols may be used. Note that due to the low bit rate at this time, video performance may be limited.
Note 3: For multiple B/H, channels, all channels must be synchronized on the end in accordance with the requirements of Recommendation 2.7/H.221. When the bit rate exceeds 64εbit/s, the bonding of multiple B channels will be performed in accordance with the requirements of Recommendation H.244 or IS0/TEC:13871. Note 1: When multiple basic rate access is used, this mode is applicable to the ISDN basic rate interface mode. Note 5: For i.723.1 audio coding mode, channels can be allocated in accordance with the requirements of Recommendation H.221. Audio data and its packet format are defined in Recommendations G.723. 1 and H. 223. The use of bits not used for G.723. 1 audio coding is specified in Recommendation H.221. Note 5. H.261, H.262 and H.The levels of 263 are defined in Appendix A. Table 2
Videophone terminal type
Terminal type
(Note 1)
ar , bo
ar , ho + ca
an + hn +th+d.
&+ho+o, dh,ea
an tbn - h, da,eo -in
Modes to be supported
All other an
All other + br
All other ab, icawwW.bzxz.Net
All other aab..cd
All other ahaCasd,，en
All other a,，b,cn+d,Cnt.
Terminal type
(Note 2)
Ho.hesi,,jo
ga-he,io,j.,l
Table 2 (continued)
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GB/T 15845. 4—2003/ITU-T H. 320: 1999 Modes to be supported
gt-hy,i
g+hrsiil
All others
All others,h,
All others,h,,i,
All others,hn,i,·i
All othersga,hn,i,jn+l.
All othersk,
All others
Note 1: If the videophone terminal interoperates with the broadband voice terminal, the terminal may support G.722 coding (mode a) in addition to G.721 coding (mode a).
Note 2: This type of terminal shall comply with the requirements of 4.3.2.2.4.3.2. 1 Example
Type X with G.711, G.722 and G.728 audio codecs is a terminal capable of operating in antal.az, bo, bla)
and b2 modes over B or 2B channels.
Type XhY with G.711, G. 722 and G.728 audio codecs is a terminal capable of operating in a+ai, a2 b)
bubl, bz, g, g and g modes over B, 2B or H channels. Type X+Y,Z with G.711, G. 722 and G. 728 audio codecs is a terminal capable of operating in XrX, and X (X=a~k) modes over (1~6)B (1~4)H. or H11 channels. The type of the remote terminal is identified by the transmission rate capability exchange defined in the H.242 recommendation. 4.3.2.2 Optional requirements for single-channel devices operating on a channel aggregator in accordance with Recommendation I.244 If a single-channel device, such as a Type Y or Type B terminal, and a terminal with sub-primary group rate channel access capability operate on a channel aggregate (CAU) that complies with Model 2 in Recommendation I.244: These terminals should have the following corresponding characteristics: 1) The terminal should be able to complete the (SM-comp) command defined in Recommendation H.221... - be able to free up 8 bits of the first 16 bytes of any time slot (except time slot 1) in each frame and be able to receive input signals with the same structure. The terminal should be able to receive capability set 2 defined in Recommendation H.244 including repeated null) values. The terminal should be able to operate at rates that are integer multiples of 64 kbit/s up to the maximum rate (this item is optional 3)
When the channel is lost or unavailable, it can operate at the available channel capacity). The terminal shall comply with the [eapex] command specified in Recommendation H.244, 4)
5) The terminal shall comply with and shall respond to the [AggIN] command specified in Recommendation H.244. When one or more signals
NOTE: In the 1993 version and earlier versions of I1.320, 16F-H,-campl was specified instead of [SM compPl as the command to be implemented, and the (null), Lcapex and LAggIN" commands were not provided. Terminals designed in accordance with one of the above versions may interoperate with type Xr terminals via CAI at 384 khit/: rate I:, assuming that the terminal is capable of receiving a capability set containing repeated [nul) cautious. 4.3.3 Video codec
The video codec shall support Recommendation H.261 and optionally Recommendations H.262 and H.263, according to the video hierarchy specified in Annex A of Recommendation H.242.
4.3.4 Audio codec
Must support (G.711 recommendation, optional support G.722, G.728, G.723.1 and G.729 recommendations (see table). 4.3.5 Frame structure
Complies with the provisions of Recommendation II.221,
GB/T 15845, 4-2003/ITU-T H 320 :19994.3.6 Control and Indication (C&I)
Use an equivalent subset of Recommendation H.230 (see 5.4). 4.3.7 Calling procedures
Comply with the provisions of Recommendation H.242.
4.4 Call Control Arrangements
In order to establish intercommunication between various audiovisual terminals, in-band and out-of-band procedures must be performed in accordance with Recommendation H.242 and other related recommendations. The different steps of the call are determined according to the point-to-point configuration, where X is the training terminal and Y is the called terminal. 4.4. 1 The provision of communication during the audiovisual telephone call establishment process is carried out in the following main steps: Phase A: Call establishment, out-of-band signaling
Phase B1. Initialization of the initial double channel mode; 6)
Phase CA: Call establishment of additional channels if necessary: d) Phase CB1, initialization of the additional channel; Phase B2 (or CB2): Establishment of common parameters: e)
Phase C: Audiovisual terminal communication;
g) Phase D, termination phase;
h) Phase E: Call release.
4. 4. 1. 1 Phase A - Call establishment After user initialization, terminal X performs the call establishment procedure (defined in Recommendation Q.939). Once the terminal receives an indication from the network that the connection has been established, an end-to-end two-way channel is opened and the H.221 specified warning is superimposed on this channel. After the connection is established, all terminals will begin operating in the mode OF specified in Recommendation H.221 (A-law or μ-law). In-band procedures are activated.
4.4.1.2 Phase B1 - Mode Initialization
4. 4. 1. 2. 1 Phase B1-1
Using the procedures provided by Recommendation H.242, fixed-position PCM audio is transmitted bidirectionally. Terminal capabilities are exchanged after reset and reset. 4. 4. 1. 2. 2 Phase B1-2 (Terminal Procedures) Determine the appropriate mode to be transmitted. Normally this will be the highest common mode (see Table 3 for the case of B or 2B channels), but a lower compatible mode may be selected instead.
With both ends having announced their ability to operate on the additional channel, Terminal X issues a request to establish a supplementary call. An audiovisual telephone terminal using multiple connections needs to know the number to be dialed to establish the additional connection. In most cases this number is stored in advance in the terminal or can be extracted from the number dialed to establish the initial channel. When this information is not available or cannot be retrieved, the terminal can obtain the number by the procedure defined in the H.212 Recommendation. If the user at either party does not wish to make a single call to two or more channels, even if the user's terminal has this capability, it must indicate in stage B1-1 that the terminal is set to have only single channel capability. In this case, we wish to distinguish between the capabilities that the user wishes to activate and the actual capabilities of the terminal. (See Chapter 5 of Recommendation H.242)
4.4.1.2.3 Phase B1-3 (Mode Transition) If the termination has (G.728), (G.723.1), or (G.729) capabilities (or a combination of these capabilities) and is connected to a 64/56 kbit/s connection, and the received capability set includes (1B, (G.728), (G.723.1), or (G.729) audio capabilities, and H.261, H.262, and H.263 video capabilities, indicating that the peer user terminal type is X, then the end will switch to one of the following modes: a2.&3/or a. Otherwise, if the terminal type is X. And the received capability set includes (G.728), (G.723.1), or (G.729) audio capabilities (or a combination of these capabilities) and H.261, H.262, and H.263 video capability, the terminal shall switch to one of the following modes: a2, ag, or ai
In other cases, higher rates than 64/56khit/s are available, and the terminal may switch to mode ba or a higher mode in Table 1, depending on the application.
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GB/T 15845.4--2003/ITU-TH.320:1999If the terminal has type XX. Or a higher type, and the received capability set contains {2B} or higher rate, while waiting for the establishment of additional connections, whether to switch the mode to open the video or switch the audio coding to another mode is the terminal's own choice. Mode switching uses the procedures defined in Recommendation H.242. If neither terminal accepts the common mode, asymmetric communication may result (this situation is not excluded) - see 4.41.5
Table 3 Common modes for different types of audiovisual telephone communication using B or 2B channels (default) x
(G. 711.G, 728)
(G. 711, G. 722)
(G. 711,G. 728)
(G. 711,G. 722.
Note 1 that "\b,/b" indicates whether b or beb is used: b gives higher quality audio, b gives higher quality video, terminal type
(with audio codec)
(G. 711,G. 728)
(G, Note 2: The communication modes listed in this table include the possibility of using the H.261CIF format as well as the H.261QCIF format. The SIF, 2SIF and 4SIF formats recommended by H.262 and the SQCIF, QCIF, CIF, 4CIF and 1GCIF formats recommended by H.263 may optionally be used. The choice of the operating format is left to the processing of the entire end according to the capabilities of the exchange; it is not necessary to be symmetrical in both directions. Each terminal may use the "minimum decodable picture interval" in its transmission direction, and it will make the best use of the capabilities indicated by the other terminal. This table does not include the case of five-up communication between audio-visual telephones and telephone terminals. If the audio-visual telephone terminal is connected to a telephone, the selected mode is used for communication.
4. 4. 1. Phase 3 CA - Call establishment for additional channels After phases B1-3 and B2, the communication phase is entered on this channel. If an additional channel has been requested, phase A is executed again (herein termed "phase CA") and is strictly followed. The additional call establishment is done by the terminal. 1I. 221 is superimposed on each established channel.
Note: During phase CA, an intermediate viewing pattern should be provided for the initial channel to be initialized until the initialization phase is fully completed. 4. 4. 1. 4 Phase CB1-
Pattern initialization of a pair of additional channels
4. 4. 1. 4. 1 Phase CB1-11
Acquire the pitch and pause alignment according to the procedure provided in Recommendation H.242. 4. 4. 1. 4. 2 Phase CB1-12
Acquire channel synchronization,
4. 4. 1. 4. 3 Phase CB1-2 (Terminal Procedure) determines the appropriate mode to be transmitted: Since the appropriate operating mode was determined in phase B1-2, this phase can be skipped. 4.4.1.4.4 Phase CB1-3 (Mode Change) Both terminals switch to the mode F that was confirmed in phase B1-2 according to the procedure specified in Recommendation H.242. GB/T 15845.4—2003/ITU-T H.320:1999 It should be noted that if Recommendation G.728 or G.722 is available, G.711 mode should be avoided after initialization, because these two modes can provide more bandwidth for video signals, and G.722 can provide better speech quality. Complete symmetry is not required, that is, it is not necessary to provide equal audio and video transmission in both transmission directions, and clause 8.2 of Recommendation H.242 does not require the output mode to be automatically selected as the input mode, but rather lists better methods of mode control. Audiovisual telephony applications require symmetry in transmission rates, but the settings for the audio and video coding of the transmission are made by the user or preset in the terminal.
4.4.1.5 Phase B2 (or CB2) - Establishment of common parameters After the completion of Phase B1, this phase establishes the common operating parameters specific to audiovisual telephony (e.g. encryption). First, the capabilities or requirements of the receiving end are indicated, and then the transmitting end determines the operating parameters and controls the receiving end. The PAS codes for this purpose are defined in Recommendation II.221.
Sometimes the user may wish to indicate a preferred reception mode (e.g., between mode a in Table 3 that provides better voice quality and mode b that provides better video quality). This can be done using the "mode preference" indication defined in clause 9.5 of Recommendation H.242. Principle It is recommended that audiovisual telephony have the ability to send and respond to these codings. 4.4.1.6 Phases - Audiovisual Telephone Communications
Where more than one channel is used, there are intermediate phases CA, CB1 and CB2 as described in this clause. Similarly, if additional channels are dropped during a call, there are intermediate phases CD and CE as described in clause 4.4.4. The measures of this clause shall apply to any channel - initial or additional - for which phases B1 and B2 have ended and for which phase D has not yet begun. 4.4.1.6.1 Mode Changes
Depending on the action taken by either user (e.g., activating a fax machine), a mode other than the highest common mode may be more appropriate. Changes to this mode are made according to the procedures specified in Recommendation H.242. 4.4.1.6.2 Capability Changes
A user may change the capabilities of its terminal during a call (e.g., connect or switch auxiliary information communication equipment), in which case the terminal shall perform the capability exchange procedure specified in Recommendation H.242. 4.4.1.7 Phase - Termination Phase
4.4.1.7.1 Phase D1 (Termination Process)
When the user hangs up, the terminal directly enters Phase D2. 4.4.1.7.2 Phase D2 (Mode Conversion)
According to the provisions of H.242 Recommendation, it is forced to switch to 0F mode (or considering the results of Phase 1, if it does not enter 0F mode, it needs further development).
When the call is disconnected for some reason, if the local terminal waits for the response of the far end, it will take a considerable time, which will hinder the reception or initiation of the next call. Therefore, ISDV disconnection should take precedence over the call termination process specified in H.242 Recommendation. It is recommended that the waiting time should not exceed 2 seconds.
4.4.1.8 Phase E - Call Termination (Release) The terminal that has hung up sends a message on the D channel to all related channels to make these channels idle (this means that no more messages will be sent).
Other terminals will idle all channels after receiving the first disconnection message. When the disconnect message is received, the actual disconnection occurs. 4.4.2 Exception procedures for phases A and B In the event that phases A and B are not completed successfully (for a number of reasons), exception procedures need to be provided in order to ensure a suitable service. The content of this is for further study. 4.4.3 Exception procedures during phase C
During the actual exchange of audio and video data, some channels may have problems. In this case, a tallback procedure managed by the terminal needs to be activated. The description of this procedure and the appropriate indication are for further study. 4.4.4 Addition and deletion of channels during an audiovisual telephone call 4.4.4.1 Addition
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GB/T 15845. 4—2003/ITU-T H.320:1999 The provision of one or more auxiliary channels is requested based on an action taken by the user (e.g. activation of auxiliary equipment). This process proceeds as described in phases CA and CB1.
4. 4. 4. 2 Elimination
It is envisaged that there will be two phases.
4.4. 4.2. 1 Phase CD1
Select a common mode suitable for the remaining channels. 4. 4. 4. 2. 2 Phase CI2
According to the mode switching procedure specified in Recommendation H.242. The mode confirmed in phase CI>1 is adopted; the remaining channels are the channels used for initialization (see phase A). This phase also supports a suitable fallback mode, the content of which is left for future study. 4.4.5 Transmission and display of audiovisual information at the start of an audiovisual telephone call 4. 4.5.1 Audio
Audio signals shall be transmitted as defined in clause 9.1.1.2 of Recommendation H.242. For videophones, the audio signal shall not be turned off at the transmitting end.
According to the process defined in clause 9.1.1.3 of Recommendation H.242, and the initial channel is established and the received audio code is known (A-law or μ-law), the audio signal can be sent to the user, and the exchange of voice signals can begin as in ordinary telephone communication. 4. 4.5.2 Video
Depending on the terminal process selected, the initialization of the interface is completed and the users at both ends may or may not see the picture. In the case that a public mode including video has been activated in phases B1-3 or phase CB13, the user can see the picture. The following summarizes a procedure that can be used to suspend video display until the video is reduced in size by the user (by mutual agreement or other means).
a) No video transmission: The mode selected during phases B1-2 and (if relevant) phase CB1-2 includes a "video off" mode. During phase C, either user can unidirectionally switch to a "video-on" mode (H.26, H.262, or H.263): On the other hand, the terminal can send a VIR (video activation ready indication) with a C&IBAS code, but will not switch to video operation unless a packet is received from the other terminal for video or VIR. While the input video is off, the audio-visual phone screen should display a symbol or message to indicate this state (e.g., a glitch). As described in phase B12 in clause 4.4.1, depending on the terminal procedures, the request for additional channels may be delayed during the video off state. The user action to activate the video will cause the procedures CA1, CB1 (if required CB2) to occur. b) Transmit video pattern: Transmit electronically generated or other patterns instead of the usual video camera signal. The C&IBAS code VIS (Video Inhibit Indication) is used to indicate this to the far end. Transmit but not display video: The terminal processes act only locally, they do not display the input signal but display an interpretation symbol or message. User operation will cause the input signal to be displayed. However, if this depends on the interaction of the two users, a new C&IBAS code VRI must be defined (Video is ready). This is for further study. 4.5 Optional Enhancements
4.5.1 Data Ports
The data ports, which are the physical I/O ports of the terminal used for telematic and other equipment interfaces, are activated/deactivated by BAS commands. Various bit rates are obtained on these ports according to the transmission capabilities of a connection, such as multiple B/H channels, etc. The allocation of the bit stream to these ports is done by in-band signaling. Data transmission on these ports is transparent. The data transmission rates are given in Annex A of the H.221 recommendation.
For data enhancements of conforming audiovisual telephone terminals, the following provisions apply at higher levels of interworking: 1) Each terminal with at least one "audio-graphic" application, such as still picture exchange, application sharing, screen annotation, text conversation, etc. as specified in Recommendation T.81 shall use the standardized applications using the MI.P/H-MIL.P channels provided by Recommendation T.120 (e.g.
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