GB/T 15845.1-1995 Technical requirements for audio-visual user terminals Frame structure of 64-1920 kbit/s channels in audio-visual user terminal services
Some standard content:
ICS33.160bzxZ.net
National Standard of the People's Republic of China
GB/T15845.1—1995
Technical requirements for audiovisual terminals
Performance requirements of audiovisual terminalsFrame structure for a 64 to 1920 kbit/s channel in audiovisual teleservicesPublished on December 13, 1995
State Administration of Technical Supervision
Implemented on June 1, 1996
National Standard of the People's Republic of China
Technical requirements for audiovisual terminalsPerformance requirements of audiovisual terminalsFrame structure for a 64 to 1920 kbit/s channel in audiovisual teleservicesTTTKAONTKAca-
GB/T15845.1—1995
This standard adopts the International Telegraph and Telephone Consultative Committee (CCITT) Recommendation H.221 "Frame structure of 64 to 1920 kbit/s channels in audio-visual user terminal services" (1992 edition). 1 Subject content and scope of application
This standard specifies the frame structure for transmitting audio-visual user terminal services on a single or multiple B channels (64 kbit/s) or H0 channels (384 kbit/s), or a single H11 channel (1536 kbit/s) or H12 channel (1920 kbit/s). This standard is applicable to applications where a total transmission rate channel of 64 to 1920 kbit/s is dynamically divided into multiple low-rate sub-channels suitable for the transmission of services such as audio, video, data and telematics. 2 Reference standards
GB7610 Audio pulse code modulation characteristics
GB/T15845.2 Technical requirements for audio-visual user terminals Control and indication signals for frame synchronization in audio-visual systems GB/T15845.3 Technical requirements for audio-visual user terminals Methods for establishing communication between audio-visual user terminals using digital channels within 2 Mbit/s
GB/T15845.5 Technical requirements for audio-visual user terminals P×64 kbit/s video codec for audio-visual services CCITT G.7226
7 kHz audio coding within 64 kbit/s
CCITT G.7256
Systematic approach for using 7kHz audio codec within 64kbit/sCCITTG.728
16kbit/s speech coding using low delay code excited linear prediction (LD-CELP)5 Procedures for the allocation of member codes of the International Telegraph and Telephone Consultative CommitteeCCITTT.35
CCITTV.120 Support of integrated services digital network (ISDN) for data terminal equipment (DTE) with V series interface and taking into account statistical multiplexing
3 Technical requirements
3.1 General requirements
The total transmission channel is obtained by synchronously and sequentially transmitting 1B~6B connections, 1H0~5H0 connections, or one H11 or one H12 connection. The first connection established is the initial connection. The initial connection supports the initial channel in each direction. Additional connections support additional channels. The total rate at which information is transmitted is called the "transmission rate". The transmission rate can be set within a range less than the total transmission channel capacity, and its values are listed in Appendix A (Supplement).
A single 64kbit/s channel is constructed as 8-bit groups transmitted at a frequency of 8kHz. Each bit position of the 8-bit group constitutes an 8kbit/s subchannel (see Figure 1a). The eighth subchannel is called the service subchannel (Sc), which consists of the parts described in Articles 3.1.1 to 3.1.4.
A H0, H11 or H12 channel consists of a certain number of 64kbit/s time slots (TS) (see Figure 1b). The structure of the lowest numbered time slot is exactly the same as that of a single 64kbit/s channel, while the other time slots do not have this structure. In the case of multiple B or H0 channels, all channels have a frame structure. The frame structure of the initial channel controls most functions of the entire transmission, and the frame structure of the additional channel is used for synchronization, channel numbering and corresponding control.
3.1.1 Frame Alignment Signal (FAS)
The alignment signal structures the I channel 1) and other fixed 264 kbit/s channels into frames of 80 8-bit groups and multiple frames (MF) of 16 frames. Each multiple frame is further divided into 8 sub-multiple frames (SMF) containing 2 frames. "Frame Alignment Signal (FAS)" refers to bit 18 of the service subchannel (SC) in each frame. In addition to frame alignment information and multiple frame alignment information, control and alarm information are also embedded in FAS. FAS also contains error detection information, which controls the end-to-end error performance and detects the effectiveness of frame alignment. Other time slots are aligned with the first time slot. Bits are transmitted to the line in sequence with bit 1 first. When an 8kHz network clock is provided (for example, in an ISDN basic rate interface or primary rate interface), FAS is sent and received as the least significant bit of an 8-bit group every 125μs. Northern Special Edition 5
Figure 1a Frame structure of a single 64kbit/s channel (B channel) Bits
Group number
GB/T15845.1-1995
Title丨Publicly prepared channel
H11:=4
H12: ±=5
8 bits
Figure 1b Frame structure of a higher rate single channel (H0, H11, H12 channels) KAONKAca-
It should be noted that in situations where audio-visual terminals and telephone terminals are required to communicate, network timing must be used to send signals. The receiving end should search for FAS in all bit positions. If the received FAS position conflicts with the network's 8-bit group timing, the received FAS position should be used first.
When the network does not provide this timing, F AS can be used to obtain the receive octet timing. In this case, this terminal cannot transmit FAS that is correctly aligned with the octet timing portion of the network, nor can it interoperate with terminals that rely solely on network timing for octet alignment.
Note: 1) "I channel" refers to the initial or only B channel, the initial or only H0 channel time slot TS1, and the H11 and H12 channels TS1. 2) A fixed frame channel refers to a channel with a frame structure. 3.1.2 Bit Rate Allocation Signal (BAS)
Every frame Bits 9 to 16 of the business subchannel (SC) are called BAS. This signal allows the transmission of codewords that describe the terminal's capabilities. It configures the capacity of a single channel or synchronized multiple channels in various ways, and commands the receiving end to tap and use the various component signals in this frame structure. BAS is also used as a control and indication signal. 3.1.3 Encrypted Control Signal (ECS)
The encrypted control signal should have a dedicated transmission channel. When necessary, 800 is provided by allocating bits 17 to 24 of the business subchannel (SC). bit/s dedicated subchannel. Therefore, the variable data and video transmission rate is reduced by 800bit/s. This 800bit/s subchannel is called ECS channel.
3.1.4 Remaining capacity
GB/T15845.1—1995
In a single 64kbit/s connection, the remaining capacity of the bit 1 to 8 load of each 8-bit group (including the remaining part of the business subchannel) can carry various signals in the multimedia service framework under the control of BAS. For example: 56 kbit/s coded speech using a truncated form of GB7610PCM; a.
speech coded at 16 kbit/s and video coded at 46.4 kbit/s; b.
speech coded at 56 kbit/s with a bandwidth of 50 to 7000 Hz (sub-band ADPCM in accordance with CCITT Recommendation G.722); this coding algorithm can also operate at 48 kbit/s, so it can be used at up to 14.4 kbit/s rate dynamically inserted data; still images encoded at 56 kbit/s;
e. 56 kbit/s data in an audiovisual session (e.g. file transfer between personal computers). 3.2 Frame Alignment
3.2.1 Overview
A frame of length 80 octets produces an 80-bit word for the traffic subchannel. These 80 bits are numbered 1 to 80. Bits 1 to 8 of the traffic subchannel in each frame constitute the FAS (see Figure 2), which includes: a.
Complex frame structure (see 3.2.2);
Frame Alignment Word (FAW);
"Bit A";
d. "Bit E" and "Bit C" (see 3.2.6).
Note: 1) See 3.2.2 and Figure 3.
"Alignment word"
Figure 2 Allocation of bits 1 to 8 of each public subchannel in each frame 3
2) The first 7 bits of the alignment word are in the even frame. The 8th bit of FAW is the inverse of the 1st bit of FAW in the odd frame to avoid the imitation of FAW caused by the frame repetition pattern.
3) Bit A: Repeated frame alignment loss indication (0\ for alignment, "1" for loss). 4) The use of bit E and bits C1 to C4 is described in Section 3.2.6 (E=0 means no error or CRC is not used, E=1 means there is an error). FAW consists of bits 2 to 8 of the even frame FAS "0011011" and bit 2 of the consecutive odd frame FAS "1". Whenever the receiving end is in the repeated frame alignment state, bit A of the I channel is set to "0", otherwise it is set to "1" (see Section 3.2.3). For additional channels, see Section 3.2.7.1.||tt| |3.2.2 Repeating structure
Next,
L1~L3 are channel numbers. The least significant bit is in L1. Channel
R: Reserved, set to 0.
A, E, C1C4, as shown in Figure 2.
GB/T15845.1—1995
The channel bit 13
N1N4: Used for the repeating number described in Article 3.2.2, When the number is not working, it is set to 0. N4
Multiframe number:
0 (number is not working)
N5: Indicates whether the multiframe number is working (N5=1) or not working (N5=0). 5
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TEA, when an internal terminal device has a fault so that it cannot receive and cannot act on the input signal, the terminal device alarm is set to "1", otherwise it is set to *0". Figure 3 Arrangement of bits 1 to 8 of each public sub-channel in a multiframe 5
GB/T15845.1—1995
Each multiframe contains 16 consecutive frames numbered 0 to 15, and each multiframe is divided into 8 sub-multiframes containing every 2 frames, see Figure 3. The multiframe positioning signal is arranged in the bit 1 position of frames 1, 3, 5, 7, 9, and 11, and its code is "001011". Bit 1 of frame 15 is reserved for future use. Temporarily set to "0".
Bit 1 of frames 0, 2, 4, and 6 is used as a modulo 16 counter to number the multiframes in a decrementing manner. The least significant bit is transmitted in frame 0 and the most significant bit is transmitted in frame 6. The receiver uses this multiframe number to equalize the different delays of the separated connections so that the received signals can be synchronized.
For communication with multiple B or multiple HO channels, multiframe numbering is necessary for both the initial and additional channels. However, for single B or single HO, or H11/H12 communication that does not require synchronization between multiple channels, the multiframe numbering can be embedded or not embedded in these channels. When the multiframe is numbered, bit 1 of frame 8 is set to "1", and when it is not numbered, it is set to "0". Bit 1 of frames 10, 12, and 13 must be used to number each channel in the multi-connection structure, so that the far-end receiver can place the 8-bit groups received every 125μs in the correct order. The information bits in a multiple frame shall be valid only after three multiple frames have been received in unison. 3.2.3 Loss and recovery of frame alignment
Frame alignment is defined as lost when three consecutive frame alignment words are received with errors. Frame alignment is defined as recovered when the following sequence is detected:
a. The first correct 7 bits of the frame alignment word appear for the first time; b. A "1" is detected in bit 2 of FAS in the next frame c. The first correct 7 bits of the frame alignment word appear in the next frame. If frame alignment is obtained but multiframe alignment cannot be obtained, frame alignment shall be searched for at another location. When frame alignment is lost, bit A of the next odd frame in the transmit direction is set to "1". 3.2.4 Loss and recovery of multiple frame alignment
Multiframe alignment is used to number and synchronize two or more channels and may also be used for encryption. Terminals with only single channel capability, which do not use the multiple frame structure, must transmit the multiple frame structure but do not need to detect multiple frame alignment on the incoming signal. When the frame alignment has been restored, such a terminal outputs A=0. Such a terminal cannot transmit TEA (see Figure 3). After the local frame alignment has been valid, other functions represented by bit 1 of the business subchannel can be used. When the remote end's multiframe alignment has been notified (A=0 is received), it can be expected that the remote end has a valid BAS code and can interpret the BAS code. When three consecutive multiframe frame alignment signals are received with errors, the multiframe alignment is defined as lost. When the frame alignment signal of the next multiframe is received without errors, it is defined as the multiframe alignment has been restored. When the multiframe alignment is lost, even when receiving an undetermined frame mode, the next odd bit A in the sender direction is set to "1". When the frame alignment has been restored, bit A is reset to "0". When the frame alignment of the additional channel and synchronization with the initial channel have been restored, its bit A is reset to "0". 3.2.5 Process of recovering 8-bit group timing from alignment When the network does not provide 8-bit group timing, the terminal can recover the 8-bit group timing in the receiving direction from the bit timing and from the frame alignment, and the 8-bit group timing in the transmitting direction can be obtained from the network bit timing and the internal 8-bit group timing. 3.2.5.1 General principles
The receive 8-bit group timing is usually determined by the FAS position. However, before the call starts and the frame alignment is obtained, the receive 8-bit group timing can be set to the same as the internal 8-bit group timing. Once the first alignment is obtained, the receive 8-bit group timing starts at the new bit position, but It is not yet valid. The received 8-bit group timing is valid only when the frame alignment is not lost during the next 16 frames. 3.2.5.2 Special cases
a. At the beginning of a call, when the terminal is in forced receive mode, or when frame alignment is not obtained, the terminal may temporarily use the sent 8-bit group timing:
b. When frame alignment has been obtained and then lost, the received 8-bit group timing will not be changed until the frame alignment is restored; c. Once the frame alignment and restored frame alignment have been obtained, the 8-bit group timing is considered valid during the call inactivity period unless the frame alignment is lost and a new frame alignment is obtained at another bit position; 6
GB/T15845.1—1995
HTYKAONKAca| |tt||d When the terminal switches from fixed frame mode to unfixed frame mode (with the help of BAS), the previously obtained 8-bit group timing must be maintained; e. When a new frame alignment is obtained at a new position, which is different from the previously valid frame alignment, the received 8-bit group timing will be reinitialized to the new position, but it is not yet valid. At this time, the previous bit position should be stored. If the frame alignment is not lost during the next 16-second period, the new position is valid, otherwise the stored previous bit position is reused. 3.2.5.3 The search for the frame alignment signal (FAS) can be carried out in two ways, the sequential method or the parallel method. In the sequential method, the 8 possible bit positions of the FAS are searched bit by bit. When the FAS is valid and then lost, the search must start from the previously valid bit position. In the parallel sequence method, a sliding window can be used, which shifts 1 bit per bit period. In this case, when the frame alignment is lost, the search must start from the next bit position of the previous valid bit position.
3.2.6 Description of CRC4 method
In order to provide end-to-end quality monitoring of the connection, a 4-bit cyclic redundancy check (CRC4) method can be used. The 4 bits C1, C2, C3 and C4 calculated at the source end are embedded in the bit 58 position of the odd frame. In addition, the bit E of the odd frame is used to transmit an indication whether the most recent CRC block received in the input direction contains errors. When the CRC4 method is not used, the sender sets bit E to 0 and sets bits C1, C2, C3, and C4 to \1". After the receiver receives 8 consecutive CRCs set to all "1", the receiver temporarily prohibits CRC error reporting. After receiving two consecutive CRCs containing "0" respectively, the receiver allows CRC error reporting.
3.2.6.1Calculation of CRC 4 bits
Calculate CRC 4 bits (C1, C2, C3, C4) for every block of two frames of each B/H0/H11/H12 channel. Each CRC block consists of an even frame followed by an odd frame. For B/H0/H11/H12 channels, the size of the CRC block is 160/960/3840/4800 8-bit groups. For 128/192/256/512/768/1152/1472 kbit/s channels, the size of the CRC block is 320/480/640/1280/1920/2880/3680 8-bit groups. Calculate 50 times per second. Note: 1) If the transmission rate is part of any H0/H11/H12 channel and is not fully occupied, only the part covered by the transmission rate is calculated. 3.2.6.1.1 Multiplication-Division Processing
The polynomial represented by the given C1~C4 in block N is the remainder after the polynomial of block (N-1) is multiplied and divided by the generating polynomial + 1 (modulo 2).
When the contents of a block are represented as a polynomial, the first bit of the block should be the most significant bit. Similarly, C1 is defined as the most significant bit of the remainder, and C4 is the least significant bit of the remainder. This process can be implemented with a four-level register and two XOR gates. 3.2.6.1.2 Encoding Process
a. Initialize the CRC bit position of the odd frame to "0", that is: C1=C2-C3=C4=0; b. Perform the multiplication-division process on this block as described in Section 3.2.6.1.1; C. Store the remainder derived from the multiplication-division process and prepare to embed it into the position corresponding to the CRC of the next odd frame. Note: These CRC bits do not affect the calculation of the CRC bits of the next block, because the corresponding positions have been set to "0" before the calculation. 3.2.6.1.3 Decoding process
a. After extracting the CRC bits of a received block and replacing them with "0", the multiplication-division process described in Section 3.2.6.1.1 is performed;
The remainder derived from the multiplication-division process is then stored and then compared bit by bit with the CRC bits received in the next block; c. If the remainder calculated by decoding accurately corresponds to the CRC bits sent by the encoding end, the detected block is considered to be error-free. 3.2.6.2 Subsequent processing
3.2.6.2.1 Operation on bit E
If the receiving direction finds an error (at least one bit error) in the process of detecting C1 to C4 of the most recent CRC block, the sending direction sets bit E of block N to "1", otherwise it is set to "0". 3.2.6.2.2 Monitoring of erroneous frame positioning
GB/T15845.1—1995
In the case of long-term FAW spoofing, the CRC4 information can be used to re-request the positioning search. For this purpose, the number of erroneous CRC blocks is calculated during 2s (100 blocks) and compared with 89. If the number of erroneous CRC blocks is greater than or equal to 89, the frame positioning search will be restarted.
The values 100 and 89 are selected for:
a. For a random transmission bit error rate of 10-3, the probability of erroneously restarting the frame positioning search due to the number of erroneous blocks being greater than or equal to 89 should be less than 10-;
b. In the case of frame positioning spoofing, the probability of not restarting the frame positioning search after a 2s period should be less than 2.5%. NOTE: The values in this clause and the example in clause 3.2.6.2.3 are for 64 kbit/s channels. The details are different for H0, H11 and H12 channels, but the principle remains applicable.
3.2.6.2.3 Error performance monitoring
The quality of a 64 kbit/s connection can be monitored by counting the number of erroneous CRC blocks during a 1 s period (50 blocks). To provide information on the errors, Table 1 shows the error rate P for random errors as a function of the proportion of erroneous CRC blocks. Table 1
Proportion of erroneous CRC blocks
The quality of the connection can be monitored in the reverse direction by counting the received bits E. 3.2.7 Synchronization of multiple connections
Some audiovisual terminals are capable of communicating on multiple B or HO connections. In this case, a single B or HO initial connection is first established, the possibility of multiple connections being determined by the transmission rate capability BAS in Annex A (Supplement), and then additional connections are established and synchronized by the terminals using a multiple frame structure.
3.2.7.1 Multiple B connections
FAS and BAS are transmitted on each B channel.
FAS operates as follows:
Multiframe numbers are used to determine the relative transmission delays between B channels as described in 3.2.2; a.
Transmit the channel numbers as described in 3.2.2, with the initial connection having the channel number 1 and up to 5 additional connections; c.
Whenever the received additional channel is not synchronized with the initial channel, set the output bit A to "1" in the additional B channel of the same connection,
d. After obtaining reception synchronization between the initial channel and the additional channels by rearranging the associated repeating signals by introducing a delay, the transmitted bit A is set to "0";
e. The bit E of each additional B channel is transmitted on the additional B channel of the same connection because it is related to the physical conditions of the transmission path.
The BAS operation of the additional connection is limited to the transmission of the additional channel number (so that the channel number of any additional connection must be transmitted in the BAS in accordance with Appendix A (Supplement) and the FAS as described in Section 3.2.2, while the channel number of the initial channel is only transmitted in the FAS). Once the remote end receives the bit A set to "0" associated with the sequence numbered channel, it adds the capacity of these additional connections to the capacity of the initial connection by sending the BAS at the transmission rate of Appendix A (Supplement). In these The order of bits transmitted in the channel is consistent with the example given in Figure 4.
3.2.7.2 Multiple H0 connections
FAS and BAS are transmitted in the first time slot of each HO. The operation of FAS is the same as that of Section 3.2.7.1, but the channel number is also used to determine the order of the 6 8-bit groups received every 125μus, which is related to the 6 8-bit groups received on other channels. The BAS operation of the additional channel is the same as that of Section 3.2.7.1. 8
3.3 Bit Rate Allocation Signal (BAS)
3.3.1 Coding of BAS
GB/T15845.1—1995
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The bit rate allocation signal occupies bits 9 to 16 of each traffic subchannel. An 8-bit B The AS code (bo, b1, b2, bs, bu, bs, bs, br) is complemented by 8 error correction bits (Po, P1P2Ps*P4P5P6P7) to implement a (16, 8) double error correction code. This error correction code is obtained by shortening the (17, 9) cyclic code and has a generating polynomial (as shown in equation (1)): ga) = a++r+4+2+r+1
The error correction code bits are calculated as coefficients of the remainder polynomial in the following equation (see equation (2)): Pr+P+r+z++++P=
RESg（)bor16+b1+b18+bgz12+ba1+b10+br9+b8）Where: RESg（r）[f(r) - the remainder obtained by dividing f by g(r)]. (1)||tt| |The BAS code is transmitted in an even-numbered frame, and its corresponding error correction bits are transmitted in subsequent odd-numbered frames. The BAS code or error correction bits are transmitted in the following order to avoid spoofing of the frame alignment word. Bit position
The decoded BAS value is valid only if the receiver is in frame and subframe alignment and the FAW in the same subframe is received with less than or equal to 2 bit errors. Otherwise, the decoded BAS value is ignored. When the receiver actually loses frame alignment, it is reasonable to discard any changes caused by the first three decoded values as if they had errors (even after error correction). 3.3.2 BAS values
The encoding of BAS is carried out in a hierarchical attribute method: including attribute classes (8 classes), attribute families (8 families), attributes (8 attributes) and values (32 values). The first 3 bits of an attribute represent its number describing a general command or capability, and the last 5 bits indicate the "value" - a specific command or capability.
The following attributes are defined under the conditions of Class (000), Family (000): Attributes
Audio Coding Command
Transmission Rate Command
Video and Other Commands
Data Command
Terminal Capability 1
Terminal Capability 2
GB/T15845.1—1995
The values of these attributes are listed and defined in Appendix A (Supplement). They provide the following possibilities: a.
Transmission on single or multiple channels at various aggregate rates; Audio transmission, digitally coded with algorithms conforming to various standards; Video transmission, digitally coded with algorithms conforming to standards; Low-speed data (LSD) in TS1 of the I channel or higher-speed initial channel; High-speed data (HSD) in the highest numbered 64 kbit/s channel or time slot (except the I channel); Data transmission in multi-layer protocols either on the I channel (MLP) or on non-I channels (H-MLP); Encrypted control signals;
Network-oriented loopback for maintenance purposes; Signaling for control and indication;
System for conveying messages related to the manufacturer and type of equipment. j
Command BAS attribute has the following meaning: Based on the BAS command code received in one frame (even frame) and its error correction code received in the next frame (odd frame), the receiver is ready to accept the state mode change starting point from the next frame (even frame): thus, the mode change can take effect within 20 ms. The command is mandatory until it is cancelled. The bit positions occupied by the combination of BAS commands are shown in Figures 4a to 4g. The capability BAS attribute has the following meaning: it indicates the ability of the terminal to receive and correctly process various types of signals; based on a set of capability values received from the remote end Y, the terminal X shall not transmit signals outside the declared capability range. The values [0 to 7] of attribute (111) are reserved for setting classes, and [8 to 15] for setting families; the default values are all (000). College number
8Beite
Group number
14.4kbit/sLSD bit numbering and position Figure 4a
GB/T15845.1—1995
Bit number
56kbit/sLSD bit numbering and position
State number
62.4kbit/sLSD bit numbering and position
RBit number
Bit number
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