CDPD Protocols - MDLP | MNRP

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:

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:

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

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.

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.

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

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:

NLPI
Network layer protocol identifier, identifies the associated network layer protocol entities defined as follows:

0
1
2
3
4-15

Mobile Network Registration Protocol
Security Management Entity
CLNP
IP
Reserved for future use

Sequence ID
Identifies the subnetwork service data unit (SNSDU) to which the segment contained in this PDU belongs. All segments belonging to the same SNSDU have the same sequence identifier.

Segment number
Each segment is assigned a segment number, which is sequentially assigned starting from zero. A complete sequence of SN-Unitdata PDUs can consist of 1 to 16 consecutive segments.

Data segment
Exactly one segment of the subnetwork service data unit. The maximum size of a data segment is 128 octets.

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MNRP

The Mobile Network Registration Protocol (MNRP) provides the following:

Exchange of routing and registration information between the M-ES and a serving MD-IS.
Notification of reachability and registration of an NEIassociated with an MES to a serving MD-IS.
Conveyance of authentication information about an M-ES.
Confirmation by an MD-IS of its willingless and ability to provide network routing services to an M-ES.
De-registration of a NEI with the serving MD-IS.
Registration and Authentication of multicast NEI.
Maintaining a location information database and routing of NPDU’s based on this information.
Cell selection - allows an M-ES to discover, select and maintain the most channel optimal available.
Location update - a distributed location information base is maintained that records the current serving area of each M-ES and is updated as a result of the cell transfer procedures.
Redirection and forwarding - messages which are destined for the M-ES are redirected from the home MD-IS to the serving MD-IS.

The structure of the header is as follows:

8	7	6	5	4	3	2	1	Octets
Type								1
Source Address Length								2
AFI								3
IDI								4
DFI								5
Address								6
								7
								8
								9
Registration Counter								10
Option Type								11
Option Length								12
Option Value								13 . . N

Type
The type, the following types are available:
1   M-ES Hello (ESH)
2   M-ES Bye (ESB)
3   MD-IS Hello Confirm (ISC)
4   MD-IS End System Query (ESQ)

Source Address Length
The Source Address Length.

Destination Address Length
The Destination Address Length

Option Type
The option type, the following options types are available:
0   Group Member ID
1   EID
2   Authentication Information
3   Authentication Update
4   Result Code
5   Configuration Timer

Option Length
The length of the option field

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