CDPD

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The basic structuring of the Cellular Digital Packet Data (CDPD) Network is along the lines of the 7-layer OSI model. Each layer within CDPD may be further partitioned into a similar sequence of sub-layers. Each layer or sub-layer in the CDPD network communications architecture is defined with:

• Layer service access points.
• Layer service primitives.
• Layer protocol.
• Layer management entity.

The CDPD network specifications define a number of subprofiles as building blocks that may be selected and combined to define a particular CDPD network element. Subprofiles define the specific multi-layer protocol requirements for a CDPD network element or a CDPD network service. Three major classes of subprofiles are defined:

• Application subprofiles.
• Lower layer subprofiles.
• Subnetwork subprofiles.

The following diagram illustrates the CDPD protocols in relation to the OSI model:

MDLP

CDPD System Specification release 1.1, part 403

The Mobile Data Link Protocol (MDLP) is a protocol that operates within the data link layer of the OSI model 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 MAC layer to provide access to the physical channel and transparent transfer of link-layer frames between data link layer entities.

The purpose of MDLP is to convey information between network layer entities across the CDPD Airlink interface. It supports multiple M-ESs sharing access to a single channel stream. The channel stream topology is that of a point-to-multipoint subnetwork. In such a subnetwork, direct communication is possible only between the user side and the network side of the channel stream. Direct communication between two M-ESs on the same channel stream is not possible.

The frame format of MDLP is as shown in the following illustration:

Address
Variable number of octets as shown in the following illustration:

C/R
Command/response field bit identifies a frame as either a command or a response. The user side sends commands with the C/R bit set to 0 and responses with the C/R bit set to 1. The network side does the opposite.

TEI
Temporary equipment identifier. The TEI for a point-to-point data link connection is associated with a single M-ES. An M-ES may contain one TEI used for point-to-point data transfer. The TEI for a broadcast data link connection is associated with all user side data link layer entities. Values are encoded as unsigned binary numbers in a variable length field of a maximum 27 bits in length.

Control
Identifies the type of frame. Possible types are: I Numbered information transfer S Supervisory functions U Unnumbered information transfers and control functions.

Information
Integer number of octets containing the data.

SNDCP

CDPD System Spoecification release 1.1, part 404

The Subnetwork Dependent Convergence Protocol (SNDCP) provides a number of services to the network layer:

• Connectionless-mode subnetwork service.
• Transparent transfer of a minimum number of octets of user data.
• User data confidentiality.

The SN-Data PDU is conveyed over the acknowledged data link service in the DL-Userdata field of a DL-Data primitive. The format of the SN-Data PDU is as shown in the following illustration:

The SN-Unitdata PDU is conveyed over the unacknowledged data link service in the DL-Userdata field of a DL-Unitdata primitive. The format of the SN-Unitdata PDU is shown in the following illustration: