CONTENTS

 1     Scope
 2     Basic frame structures
        2.1     Basic frame structure at 1544 kbit/s
                  2.1.1     Frame length
                  2.1.2     F-bit
                  2.1.3     Allocation of F-bit
        2.2     Basic frame structure at 6312 kbit/s
                  2.2.1     Frame length
                  2.2.2     F-bits
                  2.2.3     Allocation of F-bits
        2.3     Basic frame structure at 2048 kbit/s
                  2.3.1     Frame length
                  2.3.2     Allocation of bits number 1 to 8 of the frame
                  2.3.3     Description of the CRC-4 procedure in bit 1 of the frame
                  2.3.4     Synchronization Status: S_an
        2.4     Basic frame structure at 8448 kbit/s
                  2.4.1     Frame length
                  2.4.2     Frame alignment signal
                  2.4.3     Service digits
        2.5     Basic frame structure at 44 736 kbit/s
                  2.5.1     Multiframe length
                  2.5.2     Multiframe overhead bits
                  2.5.3     Allocation of the multiframe overhead bits
 3     Characteristics of frame structure carrying channels at various bit rates in 1544 kbit/s
        3.1     Interface at 1544 kbit/s carrying 64 kbit/s channels
                  3.1.1     Frame structure
                  3.1.2     Use of 64 kbit/s channel time slots
                  3.1.3     Signalling
        3.2     Interface at 1544 kbit/s carrying 32 kbit/s channel time slots (see Note)
                  3.2.1     Frame structure
                  3.2.2     Use of 32 kbit/s channel time slot
                  3.2.3     384 kbit/s 12-channel time slot grouping
                  3.2.4     32 kbit/s signalling grouping channel multiframe structure
                  3.2.5     Signal grouping channel unused bits
                  3.2.6     Loss and recovery of signalling channel multiframe alignment
        3.3     Interface at 1544 kbit/s carrying n ´ 64 kbit/s
 4     Characteristics of frame structures carrying channels at various bit rates in 6312 kbit/s interfaces
        4.1     Interface at 6312 kbit/s carrying 64 kbit/s channels
                  4.1.1     Frame structure
                  4.1.2     Use of 64 kbit/s channel time slots
                  4.1.3     Signalling
        4.2     Interfaces at 6312 kbit/s carrying other channels than 64 kbit/s
 5     Characteristics of frame structures carrying channels at various bit rates in 2048 kbit/s interfaces
        5.1     Interface at 2048 kbit/s carrying 64 kbit/s channels
                  5.1.1     Frame structure
                  5.1.2     Use of other 64 kbit/s channel time slots
                  5.1.3     Signalling
        5.2     Interface at 2048 kbit/s carrying n ´ 64 kbit/s
                  5.2.1     One n ´ 64 kbit/s signal on the tributary side of a multiplex equipment
                  5.2.2     One or more n ´ 64 kbit/s signal on the multiplexed signal side of a multiplexing equipment
 6     Characteristics of frame structures carrying channels at various bit rates in 8448 kbit/s interface
        6.1     Interface at 8448 kbit/s carrying 64 kbit/s channels
                  6.1.1     Frame structure
                  6.1.2     Use of 64 kbit/s channel time slots
                  6.1.3     Description of the CRC procedure in 64 kbit/s channel time slot 99
                  6.1.4     Signalling
        6.2     Interface at 8448 kbit/s carrying other channels than 64 kbit/s
Annex A – Examples of CRC implementations using shift registers
        A.1     CRC-6 procedure for interface at 1544 kbit/s (see 2.1.3.1.2)
        A.2     CRC-5 procedure for interface at 6312 kbit/s (see 2.2.3.2)
        A.3     CRC-4 procedure for interface at 2048 kbit/s (see 2.3.3.5)
Annex B – Alphabetical list of abbreviations used in this Recommendation