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. The reverse 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

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  Mark S. Taylor, William Waung, Mohsen Banan, “Internetwork Mobility The CDPD Approach”, June 11, 1996 URL: ity/one/main.html