This Recommendation specifies a mapping function that allows narrow-band data communication applications utilizing protocol stacks which include one of the HDLC protocols also to be deployed in a B-ISDN environment. The required mapping function is called Service Specific Convergence Sublayer for HDLC Applications. This Recommendation makes use of the fact that the subset of services provided by AAL type 5 (Recommendation I.363.5) can replace the framing and bit error detection part of HDLC. This Recommendation is applicable to equipment to be attached to an ATM network when protocol stacks developed for a narrow-band environment are deployed in a B-ISDN environment.