1、INTERNATIONAL TELECOMMUNICATION UNION)45G134 TELECOMMUNICATIONSTANDARDIZATION SECTOROF ITU%.%2!,G0G0!30%#43G0G0/Mode 1 - 64 kbit/s 7 kHz audio according to Recommendation G.722;Mode 2 - 56 kbit/s 7 kHz audio according to Recommendation G.722 + up to 6.4 kbit/s data;Mode 3 - 48 kbit/s 7 kHz audio acc
2、ording to Recommendation G.722 + up to 14.4 kbit/s data.For both Modes 2 and 3, an additional 1.6 kbit/s capacity is reserved for service channel framing and modecontrol.Additional modes may be defined (see Recommendation H.221) having other speech bit rates, or data bit ratesup to a full 64 kbit/s
3、data path.For analogue telephone terminal it may be assumed that the speech signal is converted to PCM according toRecommendation G.711 at a digital network interface. These terminal are viewed as working in Mode 0, whenconnected to wideband speech terminal.2 Fascicle III.4 - Rec. G.7252.2 Terminal
4、typesThree types of terminals are defined thus far, according to their modes of operations:Type 0 - A digital telephone set working in Mode 0 only (or an analogue telephone set connected via a PCMinterface);Type 1 - A 7 kHz audio terminal capable of working in Mode 1 or in Mode 0;Type 2 - A member o
5、f a family of 7 kHz audio/data terminals capable of working at least in Modes 0, 1, 2 and3. Further modes may also be implemented. Dynamic mode switching between different modes must beprovided.In order to establish a mode of operation with the highest possible performance, terminals of Type 1 and T
6、ype 2must be able to identify the terminal type at the far end, and they must indicate their own type to the far end terminal.2.3 Establishment of compatible modes of operationAt the beginning of the communication phase of a call, all terminals start to work in Mode 0. Terminals ofType 1 and Type 2
7、will then begin an initialization procedure.This procedure (further described in 5) consists of:- the transmission of information concerning the capabilities of the respective terminal for audio codingand/or data transmission;- the determination of a suitable transmission mode consistent with the kn
8、own capabilities of both terminals(an example of agreed mode is given in Table 1/G.725); and- switching to this mode.The terminals connected to a call may change during the call. This may require reinitialization in order toidentify the terminal type and to re-establish the common mode of operation.
9、 In particular, this feature is used for Mode0 forcing, which is necessary in the case of a call transfer (see 7).TABLE 1/G.725Mode of operation upon completion of the initialization procedureAgreed mode of operation Identified terminal typeat the far endType 2 Type 1 Type 0Type of local terminalTyp
10、e 2 Mode 2Mode 1 Mode 0Type 1Fascicle III.4 - Rec. G.725 33 Frame structureThe frame structure described in Recommendation H.221 is used for dynamic mode switching and modeinitialization (see the following sections) and more generally to allocate sub-channels in connections of Type 2terminals.Recomm
11、endation H.221 defines a bit rate allocation signal (BAS) which is used to allocate subchannels and toindicate the audio coding algorithm. Table 2/G.725 gives the coding of the BAS for the attribute 000 (audio coding) asapplicable to terminals to Recommendation G.722. Only a BAS beginning with 000 s
12、hould be taken into account as tothe audio coding mode itself. In this respect, a BAS with another attribute does not modify the audio coding mode.4 Fascicle III.4 - Rec. G.725TABLE 2/G.725BAS code values affecting the audio coding modeP PCMS Service channelH Higher sub-bandL Lower sub-bandD Data ch
13、annelNote 1 - Attribute values 001xx imply switching to an unframed mode. In the receive direction, reverting to a framed mode can only be achieved by recovering frame and multiframealignment.Note 2 - The application channel is merged with the data channel to form a single data path.BAS codeOctet fo
14、rmat Audio coding ModeInformation ratesFramed CommentsBit position AudioApplicationDatachannel12345678channel00000100000001010000011000001000000010010001100000011001PPHHHHHPPHHHHHPPLLLLLPPLLLLLPPLLLLLPPLLLLLPPLLDLDPPLSSSSG 711-AG 711-G 722G 722G 722G 722G 7220012323646464564856480006.46.400000086.41
15、4.4NoNoNoYesYesYesYesNote 1Note 1Note 1Note 2Note 2Fascicle III.4 - Rec. G.725 5A second BAS attribute 100 (audio capability) is defined and is intended to be used for signalling terminalcapabilities to the distant terminal. When received, this attribute does not affect the current audio coding mode
16、. However, itmay lead to the initiation of a specific action to be carried out by the terminal. This feature is utilized in the mode initializationprocedure and in the Mode 0 forcing procedure (see 5). The coding of the BAS for attribute values assigned for audiocapability is shown in Table 3/G.725.
17、TABLE 3/G.725BAS code values for audio capabilityThe third bit of the H.221 frame alignment signal (FAS) in odd frames, herein called the A-bit, is set to 1 on loss offrame or multiframe alignment, and is set to 0 on acquiring both frame and multiframe alignment. Optionally, the terminal mayset the
18、A-bit to 0 on acquiring framing and before acquiring multiframing. A terminal which is receiving a frame signal withthe A-bit set to 0 can assume that the distant terminal is able to act upon a change of BAS.4 Basic sequences for in-channel proceduresThree signalling sequences are defined in this se
19、ction. These sequences are used as the building blocks for theprocedures defined in 5 and 6. The sequences are described in Figure 1/G.725.BAScodeAudio coding capability Comments1000000010000001100000101000001110000100100001011000011010000111NeutralType 0, A-lawType 0, -lawType 1Type 2ReservedReserv
20、edReserved for national useNo change of audio capability6 Fascicle III.4 - Rec. G.7254.1 Mode switching sequenceMode switching is defined in Recommendation H.221 and is done using eight-bit BAS codes, with audio codingattribute (000).In the present submultiframe the transmitting terminal sends a BAS
21、 code to signal the mode of operation in which itwill transmit during the next submultiframe.The receiving terminal decodes and validates the BAS code during the present submultiframe, and switches itsreceive mode to the signalled mode of operation at the beginning of the next submultiframe. If the
22、receiving terminal does notreceive a valid BAS code due to transmission errors, it continues its present mode of operation during the next submultiframe.Fascicle III.4 - Rec. G.725 7c) Frame recoveryTM Transmit modeRM Receive modeSMF SubmultiframeMF MultiframeBAS Bit rate allocation signalNote - Exa
23、ct timing of the A-bit indication depends on the implementation.FIGURE 1/G.725Basic sequences for in-channel procedures8 Fascicle III.4 - Rec. G.7254.2 Capability exchange sequenceThe capability exchange sequence forces framing in both direction of transmission, and the exchange of terminal typeindi
24、cation using the BAS code with the audio capability attribute (100).The terminal which initiates the capability exchange sequence sets a times T1 (value: 10 seconds) and transmits in aframed mode with the BAS signal stating its current capability.When the distant terminal decodes the audio capabilit
25、y BAS code in two consecutive submultiframes, it starts thecapability exchange sequence. One of the three cases may occur:- Within the timer expiration period, multiframe alignment1)has been achieved, the A-bit has received with avalue of 0, and the audio capability BAS code of the distant terminal
26、has been validated in two consecutivesubmultiframes. In this case the sequence completed successfully.- The timer has expired without multiframe alignment1). In this case, the sequence failed.- The times has expired with multiframe alignment1)achieved, but without either the validation of the A-bit
27、as 0or the receiving of the distant terminals audio capability BAS code (or both). In this case, the sequence isrestarted.4.3 Frame recovery sequenceWhen a terminal is transmitting in an unframed mode and wishes to institute framing in its transmitting direction, itsuperimposes the frame structure o
28、ver the transmitting information using the neutral audio capability BAS code. This audiocapability BAS code is transmitting for at least 3 multiframes (48 frames).5 Initialization and Mode 0 forcingRecommendation G.722 terminals will be connected to digital networks where other kinds of terminals wi
29、ll also beconnected, e.g. Type 0 terminals, data terminals, telematic terminals, servers, etc. When compatibility between the differentservices involving those terminals is required, initialization procedure is necessary. When automatic compatibility is required,a procedure based on the sequences de
30、fined in 4 is used.For call transfer or mode mismatch recovery, it is necessary for terminals to operate in the common Mode 0 and thusa Mode 0 forcing procedure is required, again based on the sequences defined in 4.At the commencement of the call there is a need for an initialization procedure to e
31、nsure that the two connectedterminals can operate in the most suitable common mode.5.1 Mode initialization procedureThe mode initialization procedure ensures that both terminal have been informed of the capabilities of the otherterminal, and switched to the highest common audio mode in both directio
32、ns of transmission. In case of two Type 2 terminals,both directions of transmission will be in either Mode 1, Mode 2, or Mode 3, but symmetry is not required. The procedure canbe initiated by both terminals independently. However, even if it is initiated by only one terminal, it will still achieve t
33、he sameresults as a procedure that was initiated independently, due to the symmetric operation of the capability exchange sequence._1)Optionally, frame alignment may be sufficient.Fascicle III.4 - Rec. G.725 9At the beginning of the mode initialization procedure, the terminal starts transmitting in
34、Mode 0, while initiating thecapability exchange sequence ( 4.2). The receive part is in frame search and the receive audio is in Mode 0. If the capabilityexchange sequence has terminated successfully, the mode switching sequence ( 4.1) is executed in order to switch to acommon working mode. The init
35、ialization procedure is completed when both terminals have switched to their commonworking mode(s), according to their capabilities.If the capability exchange sequence failed, i.e. no framing was detected on the incoming path within the timerexpiration period, the terminals shall continue transmitti
36、ng in Mode 0, but without framing.5.2 Mode 0 forcing procedureWhere it is necessary to ensure that both terminals are operating in Mode 0 (for instance before call transfer), thisprocedure is used.For the forcing terminal, BAS 10000010 (Type 0 terminal) is sent to the terminal which is to be forced
37、to Mode 0,using the capability exchange sequence (see Figure 1b/G.725). When this is complete, both terminals execute the modeswitching sequence (see Figure l a/G.725) with the BAS value of 00000100 or 0000101 to switch transmission to Mode 0,since PCM is now the only common mode of operation.At the
38、 completion of the mode-switching sequence, both terminals are operating in Mode 0. Changes of networkconfiguration can now be achieved (see 7).5.3 Mode mismatch recovery procedureIn the case where mode mismatch between Mode 0 and Mode 1 has been detected in the receiver (e.g. by examiningthe energy
39、 level or by monitoring the statistics of the decoded output), the Mode 0 forcing procedure may be used to establisha common working mode. The audio output has to be muted starting from the detection of the mismatch until theestablishment of the common Mode 0. Following this procedure, reinitializat
40、ion can be achieved by using the modeinitialization procedure.5.4 Recovery from unexpected loss of frame alignmentIf a terminal unexpected loses frame alignment on its receive path, a timer T2 is set (value: 0.1 seconds). During thistime the status of the framing in the receive direction is monitore
41、d:- if framing is recovery before the time expires, the normal operation is continued;- if framing is not recovered before the timer expires, the terminal goes to the Mode 0 forcing procedures. Theaudio output should be muted starting from the expiry of the timer until the Mode 0 forcing procedure i
42、scompleted. Following this procedure, reinitialization can be achieved by using the mode initialization procedure.6 Dynamic mode switching procedure for Type 2 terminalsThe dynamic mode switching procedure makes use of the frame structure specified in Recommendation H.221 asdescribed in 3, and of th
43、e sequences defined in 4.When the terminal is receiving in a framed mode, and is capable of decoding the A-bit, mode switching should bedelayed if the A-bit is set to 1; eventually the mode mismatch recovery procedure as described in 5.3 might be used.10 Fascicle III.4 - Rec. G.7256.1 Dynamic mode s
44、witching from a framed mode to another framed modeThe mode switching sequence (see Figure l a/G.725) is used.At the transmitting terminal, of a BAS is transmitted to signal a new audio mode, the audio encoder must operate inthe appropriate audio encoding mode (Recommendation G.711, A- or -law; Recom
45、mendation G.722, mode 1) from the firstoctet of the next submultiframe.Similarly, at the receiving terminal, if the received BAS signals a new audio-mode, the audio decoder must operate inthe appropriate audio decoding mode (Recommendation G.711, A- or -law; Recommendation G.722, Modes 1, 2 or 3; mu
46、tedaudio) from the first octet of the next submultiframe.6.2 Dynamic mode switching from a framed mode to an unframed modeAs in 6.1, the mode switching sequence is used.However, as the BAS for signalling an unframed mode is transmitted for a single submultiframe, a mode mismatchmay occur in severe e
47、rror conditions. Optionally, two methods may be alternatively or simultaneously used to improve thereliability of the switching:i) If the distant terminal is transmitting in an unframed mode, the capability exchange sequence (seeFigure 1b/G.725) is used first in order to be able to decode the A-bit
48、in the receive direction and to be sure thatmultiframing is aligned in the distant terminal. The mode switching sequence may then be transmitted.ii) The new BAS value in the mode switching sequence may be repeated several times. This will cause a temporarymismatch on the least significant bit.6.3 Dy
49、namic mode switching from an unframed mode to a framed modeThe frame recovery sequence (see Figure 1c/G.725) and the mode switching sequence are sequentially transmitted.Alternatively, another method may be used. The capability exchange sequence is used, followed by the modeswitching sequence. This requires a bi-direction link and provides a more reliable procedure. However it causes an unneededmode change in the distant terminal.6.4 Dynamic mode switching from an unframed mode to another unframed modeThe frame recovery sequence and mode switchin
