This Recommendation specifies the Link Access Procedures on the D-channel (LAPD) of an ISDN customer access. Implementations of this Recommendation are in use in existing networks. This Recommendation has been amended in order to add a new Annex J and a new Appendix V to describe in general terms the link access procedure for use in a symmetrical application between two Private Integrated Network Exchanges (PINXs) at the Q reference point.