The purpose of the amendment to this ETS is to correct the errors in the published document following the PE 82 and subsequent V99. Where the existing scope shall not be altered from This ETS defines protocols for the operation of a Channel Aggregation Unit effecting synchronisation and aggregation of multiple ISDN channels at 64 kbit/s, these channels being with a differential delay in the range of +/-1 second and with or without octet integrity structure. It also addresses the case of interworking with a digital network restricted to a rate of 56kbit/s. It is applicable to a wide range of user data transport up to 63 channels using ISO/IEC 13871, but special provision is made for audiovisual systems according to ETS 300 144, up to 24 channels; the aggregation may be network-based or associated with Customer-Premises Equipment. The available operating modes (B1, B2, B3, H2) provide options with/without transmis sion-management overhead and user-dat a rates at or less than the exact multiples of 64 kbit/s or 56 kbit/s.The number of aggregated channels may be varied dynamically during asession. Procedures are given for dealing with faults, including loss of channels and slip. Call control is outside the scope of this ETS.