This Recommendation covers the characteristics and parameters of a digital transmission system at the network side of the NT1 to form part of the access digital section for the ISDN basic rate access. The specification of transmission system characteristics at the network side of the NT1 does not correspond to a reference point defined in the reference configuration for ISDN user-network interfaces in Recommendation I.411.The system will support the– full duplex;– bit sequence independenttransmission of two B-channels and one D-channel as defined in Recommendation I.412 and the supplementaryfunctions of the access digital section defined in Recommendation I.603 for operation and maintenance.This Recommendation describes 4 transmission systems using different line codes and transmission methods. They are given in Appendices I to IV as examples following the understanding that only one system should be recommended and,provided in an annex to this Recommendation when this is achieved in future.