Packet Data Over Cellular Networks:The CDPD Approach

Svet Naydenov

CS 556

Overview

What is CDPD? CDPD Overview The Physical Layer Medium Access Control Forward Link Structure Reverse Link Structure The Medium Access Procedure Logical Link Control

What is CDPD?

CDPD (Cellular Digital Packet Data) is a specification for supporting wireless access to the Internet and other public packet-switched networks. Cellular telephone and modem providers that offer CDPD support make it possible for mobile users to get access to the Internet at up to 19.2 Kbps .

CDPD support both IP and CLNP (Connectionless Network Protocol)

CDPD Overview

Overview – Elements of CDPD

M-ES consists:- mobile terminal (PC, PDA, etc)

- CDPD radio modem attached to the mobile terminal and manages the radio link and protocols

IS (Intermediate Systems) are two types:- generic IS: simple router without knowledge of CDPD and mobility issues- MD-IS: mobile data IS is a specialized IS that provides switching, accounting, registration, authentication, encryption, and mobility management functions

Overview – Elements of CDPD

MDBS-mobile data base stations:- passes data between M-ESs and MD-IS and does not perform networking functions- creates and manages the air interface the M-ESs and the CDPD backbone.

The Physical Layer in CDPD The Physical Layer in CDPD is a functional entity that

accepts a sequence of bits from the Medium Access Control (MAC) layer and transforms them into a modulated waveform onto a physical 30kHz RF channel

Communication between an MDBS and a M-ES take place over a pair of RF channels - “forward” and “reverse” channels.

The forward channel accommodates transmissions from the MDBS to M-ESs.

The reverse channel accommodates transmissions from the M-ESs to MDBS. Thereverse channel is shared among all M-ESs communicating with the same MDBS.

The Physical Layer in CDPD

The physical layer communicates with Radio Resource Management Entity, which tunes the physical layer to a specific RF channel pair, sets the transmission power level to a desired level, measures the received signal level of an RF channel and estimates its potential to offer acceptable communication.

Medium Access Control (MAC)

MAC is a functional entity logically operating between the physical layer and link layer control

Purpose of MAC: to transport information (link protocol data units - LPDU) between peer link layer control entities across the CDPD air interface.

MAC provides the following services: - encapsulates LPDUs into frame structures to

establish the LPDU boundaries - encodes LPDUs to provide error protection

- detects and corrects bit errors within received frames

Forward Link Strucrure

Reverse Link Structure

Medium Access Procedure

An M-ES can access the reverse channel using digital sense multiple access with collision detection (DSMA/CD) algorithm.

DSMS/CD make use of two flags:- busy/idle flag is a 5-bit sequence transmitted on the forward channel once every 60 bits, providing binary info whether the reverse channel is busy/idle.- decode status flag is a 5-bit sequence indicating whether the MDBS has decoded the preceding block.On successful decoding (no decoding errers) the flag is 00000, on unsuccessful decoding 11111.

Medium Access Procedure

Assume M-ES wants to transmit data. It first senses the busy/idle flag, which needs to be idle so transition can take place. If the channel is found busy, the E-MS waits a random number of seconds and than repeats the sensing of the flag.

After an M-ES has started a transmission, it checks the decode status flag in every forward channel back it receives and resumes or suspends transmission depending on the value of this flag.

Logical Link Control (LLC)

The purpose of the LLC layer is to transport information between network layer entities across the CDPD air interface.

Data links connection may be either point-to-point or broadcast. - point-to-point data links are used to transport info between single M-ES and it’s serving MD-IS.- broadcast data links are used for point-to-multipoint or multipoint-to-multipoint communications on a CDPD channel stream.

Logical Link Control (LLC)

There are two operation modes for information transfer within a data link connection:- unacknowledged mode is used for information transfer on a broadcast channel, which makes transmission unreliable (no error or flow control mechanisms are used).- acknowledged mode is used for information transfer on a point-to-point data link channel. This channel also supports unacknowledged mode for information transfer.

References

A. Salkintzis, “Packet Data over Cellular Networks: The CDPD Approach,” IEEE Communication Magazine, June 1999, pp. 152-159.

Mark S. Taylor, William Waung, Mohsen Banan, “Internetwork Mobility The CDPD Approach”, June 11, 1996 URL:http://www.leapforum.org/published/internetworkMobility/one/main.html