1. Chap 8. Cellular Digital Packet Data (CDPD) 네트워크 컴퓨팅 연구실 석사 1 학기 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)  Intermediate system (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 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

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 sealing or 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 NPDU (network protocol data unit) NPDU data routed Redirection and forwarding service Decapsulate and route : readdress service