Introduction

This Part describes the functions, features, protocol and services of the Cellular Digital Packet Data (CDPD) Mobile Data Link Protocol (MDLP).

MDLP is a protocol that operates within the Data Link Layer of the OSI architecture to provide logical link control services between Mobile End Systems (M-ESs) and Mobile Data Intermediate Systems (MD-ISs).

MDLP utilizes the services of the CDPD Medium Access Control (MAC) Layer to provide access to the physical channel and transparent transfer of link-layer frames between Data Link Layer entities. The MAC layer is defined in Part 402.

The term Data Link Layer is used throughout this specification to refer to the MDLP protocol entity. It is understood that this refers to the logical link control sublayer as provided by MDLP, as distinct from the MAC sublayer described in Part 402.

The terms user side or network side are used in this Part to refer to the M-ES or MD-IS endpoint of the data link, respectively.¹

The definition of MDLP takes into consideration the concepts and principles of the following:

a. [CCITT-X.200] and [CCITT-X.210]—the reference model and layer service conventions for OSI

b. [ISO-7809]—High-level Data Link Control (HDLC)—Classes of Procedures

c. [ISO-4335]—High-level Data Link Control (HDLC)—Elements of Procedures

d. [ISO-8802-2]—Logical link control

e. [ISO-10039]—MAC service definition

f. [CCITT-Q.920] and [CCITT-Q.921]—Link Access Procedure for the D-channel (LAPD)

g. [CCITT-V.42]—Link Access Procedure for Modems (LAPM).

The commands and responses of the peer protocol procedures defined in Section 6, page 403–26 are primarily derived from [CCITT-Q.921], with the addition of the selective retransmission procedures from [ISO-4335] and additional procedures specific to the CDPD Network environment.

Appendix 403–B, page 403–67 provides a comparison of MDLP to HDLC classes of procedures while Appendix 403–C, page 403–68 provides a detailed comparison of MDLP and LAPD. Examples of how the SREJ mechanism should be used is provided in Appendix 403–D, page 403–70.

1. The Mobile Data Base Station (MDBS) functions primarily as a data link relay between an MD-IS and M-ESs. In certain cases, the MDBS may also originate MDLP frames destined for M-ESs, primarily for radio resource management (see Part 405). The MDBS is never the destination for MDLP frames.