ITU-T Rec. H.221 (12/1990) FRAME STRUCTURE FOR A 64 TO 1920 kbit/s CHANNEL IN AUDIOVISUAL TELESERVICES
FOREWORD
Introduction
1 Basic principle
    1.1 Frame alignment signal (FAS)
    1.2 Bit-rate allocation signal (BAS)
    1.3 Encryption control signal (ECS)
    1.4 Remaining capacity
2 Frame alignment
    2.1 General
    2.2 Multiframe structure
    2.3 Loss and recovery of frame alignment
    2.4 Loss and recovery of multiframe alignment
    2.5 Procedure to recover octet timing from frame alignment
        2.5.1 General rule
        2.5.2 Particular cases
        2.5.3 Search for frame alignment signal (FAS)
    2.6 Description of the CRC4 procedure
        2.6.1 Computation of the CRC4 bits
        2.6.2 Consequent actions
    2.7 Synchronization of multiple connections
        2.7.1 Multiple B-connections
        2.7.2 Multiple H0 connections
3 Bit-rate allocation signal
    3.1 Encoding of the BAS
    3.2 Values of the BAS
    3.3 Procedures for the use of BAS
ANNEX A Definitions and tables of BAS values
A.1 Audio command values (000)
A.2 Transfer-rate command values (001)
A.3 Video, encryption, loop and other commands (010)
A.4 LSD/MLP commands (011)
A.5 Audio capabilities (100)
A.6 Video, MBE and encryption capabilities (101)
A.7 Transfer-rate capabilities (100)
A.8 LSD/MLP capabilities (101)
A.9 Escape table values (111)
A.10 HSD/H-MLP capabilities (111) [10000]-(101)
A.11 HSD/H-MLP commands (111) [10000]-(011)
A.12 Applications within LSD/HSD channels - capabilities (111) [10010]-(101)
A.13 Applications within LSD/HSD channels - commands (111) [10010]-(011)
ANNEX B Frame structure for interworking between a 64 kbit/s terminal and a 56 kbit/s terminal
B.1 Sub-channel arrangement
B.2 Operation of the 64 kbit/s terminal
B.3 Restriction against some communication modes
B.4 Audio Command Codes (000)