Chap 8. Cellular Digital Packet Data (CDPD)

1. Chap 8. Cellular Digital Packet Data(CDPD) 네트워크 컴퓨팅 연구실 석사1학기 박호진 ( sanzini96@hotmail.com )

2. Introduction • CDPD (Cellular Digital Packet Data) • Offers low-cost, ubiquitous, wireless data network • Can be overlaid on AMPS and IS-136, and share its infrastructure • Transmits packet data over idle cellular voice channels • Automatically switches to another channel • Doesn’t communicate with underlying cellular network • 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

3. CDPD Architecture • Two basic classes of network entities • End system (ES) • Intermediatesystem (IS)

4. 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

5. 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

6. 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

7. 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

8. 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 transferprocess 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

9. 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

10. 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 sealingor channel stealing • Timer • Dwell timer • Layout timer

11. Channel hopping

12. 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

13. Roaming Management • Location registration procedure Registration directory service in the MSF Location directory service in MHF

14. Roaming Management • NPDU forwarding procedure Redirection and forwarding service NPDU data routed Decapsulate and route : readdress service NPDU (network protocol data unit)