Cellular Digital Packet Data

(CDPD)

SUBMITTED TO-MR.

SUBMITTED BY-

Bhanu Pratap Singh (Roll No.)

INTRODUCTION

CDPD (Cellular Digital Packet Data)Offers low-cost, ubiquitous, wireless data

networkCan be overlaid on AMPS and IS-136, and

share its infrastructureTransmits packet data over idle cellular

voice channelsAutomatically switches to another channelDoesn’t communicate with underlying

cellular network

CDPD (Cellular Digital Packet Data)

Takes advantage of its knowledge of channel assignment algorithms for cellular system

Predicts the channels available for CDPD use

Serve as the wireless extension to a PSDN or other data networks such as the Internet

CDPD Architecture• Two basic classes of network entitiesES & IS

• Though physical location if M-ESs may change from time to time, continuous network access is maintained.

Mobile End System (M-

ES)

• Mobile Database StationMDBS

• Mobile Data-intermediate System .MD-IS

Architectures in brief

MD-IS MD-IS

AMPSMSC

M-ES

MDBS

MDBS

AMPS BS

AMPS BSAMPSBase

Station

MobileData Base

Station

Mobile Station

To other intermediate systems

CDPD Architecture

Two basic classes of network entities ◦ End system (ES)◦ Intermediate system (IS)

CDPD Architecture (2)

Mobile End System (M-ES)

◦ Though physical location if M-ESs may change from time to time, continuous network access is maintained

◦ Consists of three parts Subscriber unit

– used to access the radio interface Subscriber identity module

– contains information to identify a subscriber Mobile application subsystem

– provides M-ESs application functionality

CDPD Architecture (3) Mobile Database Station (MDBS)

◦ Control of the radio interface radio channel allocation interoperation radio media access control

◦ Consists of several modem-transceivers, each of which supports one channel pair

◦ User data received by the modem-transceiver are processed by control computer

◦ M-ES can communicate only with outside world through MDBS

Mobile Data-intermediate System (MD-IS)◦ Connects to several MDBSs via wired links or via microwave◦ Consists of frame relay switch, packet router, and

workstation◦ Receives data from one network and forwards it to another

network◦ Supports user mobility – CDPD-specific mobile network

location protocol

CDPD Air Interface

CDPD Air Interface

CDPD channel streams - 19.2Kbps raw duplex wireless link

Use idle cellular RF channels CDPD transmission must not interfere with

cellular voice Forward link - link from MDBS to M-ES

◦ MD-IS sends frames to the MDBS on the forward link

◦ MDBS broadcasts frames in its radio coverage area

◦ Only M-ESs that have valid NEI (network entity identifiers) can decode the received data

CDPD Air Interface (2)

Reverse link - link from M-ES to MDBS◦ Uses DSMA (digital sense multiple access)

MDBS broadcasts the availability of the reverse link by an idle/busy control flag

If there are no transmission on the reverse channel, MDBS sets the control flag “idle”

M-ES may transmit the data on the reverse link at idle status

◦ If M-ES detects a “busy” status Waits for a random period of time Rechecks the status of the control flag

◦ If two M-ESs detect the “idle” status, and try to access the reverse link at the same time Collision occurs Exponential backoff procedure for retransmission

CDPD Air Interface (3)

To prevent Channel-hogging situation◦ Maximum time period is defined between two bursts

When M-ES in communication moves from one cell to another◦ Radio link transfer process is required to reconnect the

M-ES in the new cell◦ CDPD link transfer process is controlled by the M-ES◦ M-ES monitors the radio link quality

If the quality falls below the thresholds M-ES initiates link transfer New MD-IS updates its registration directory

If the old MDBS and the new MDBS connected to different MD-ISs Transport layer retransmission is required to reestablish

the end-to-end connection

CDPD Air Interface (4) Sleep mode operation

◦ An idle M-ES shut off power for predefined period◦ To “wake up”

MD-IS periodically broadcasts a notification message M-ES periodically activate its receiver If its name is found in the list,

MD-IS leaves sleep mode Sends a notification message to the MD-IS

◦ TEI (temporary equipment identifier) assignment procedure TEI is dynamic assigned MDLP address Sleep mode operation is requested T203 – element inactivity timer T204 – TEI notification timer

Radio Resource Allocation

◦ CDPD use channel sniffing and channel hopping◦ MDBS periodically scans/monitors the channels

If a communication link is exit between cellular system and CDPD If an communication link is not exit – sniffing is used

◦ MDBS Generates a candidate list of available channels ◦ List is then forwarded to the MD-IS◦ MD-IS collects channel lists from all the associated MDBSs◦ Determines the CDPD channel streams based on its knowledge

of the voice channel allocation algorithm◦ Periodical channel switching timed hop or planned hop

to avoid channel sealing or channel stealing◦ Timer

Dwell timer Layout timer

Channel hopping

Channel 5

Channel 6

VOICE CDPD

VOICEChannel

7

Roaming Management

◦ Is achieved by two functions MHF (Mobile Home Function) MSF (Mobile Serving Function)

◦ M-ES is identified by a distinct NEI◦ Every NEI is associated with home MD-IS ◦Home MD-IS maintains a location directory

Is called the location directory service in the MHF Record the address of the current serving MD-IS for each

of its homed M-ESs

◦ Visited MD-IS maintains a registration directory Is called the registration directory service in the MSF Keep track of all visiting M-ESs

Roaming Management Location registration procedure

M-ES

OldMD-IS

HomeMD-IS

NewMD-IS

Old MDBSNew MDBS

Registration

Cacellation

Registration directory service in the MSF

Location directory service in MHF

Roaming Management NPDU forwarding procedure

Dataforwarding

MD-IS

HomeMD-IS

NewMD-IS

M-ES New MDBS

Step 1 Step 2

Step 3

Step 4

NPDU (network protocol data unit)

NPDU data routed

Redirection and forwarding service

Decapsulate and route :

readdress service