CDPD: Cellular Digital Packet Data

1. CDPD:Cellular Digital Packet Data Daniel Grobe Sachs Quji Guo

2. What is CDPD? • Motivation: Packet data over AMPS • AMPS is unsuited for packet data • Long call setup times • Modem handshaking required • Analog providers have AMPS allocation. • Use AMPS channels to provide data service. • “Cellular digital packet data” • Can’t interfere with existing analog service. • CDPD is cheap: no new spectrum license needed!

3. Design Goals • Goals: • Low speed, high latency data service • Primarily intended for paging and email. • Provide broadcast and multiple-access service. • Dynamically shared media, always online. • Share channels with AMPS allocation • Transparency to existing AMPS service.

4. CDPD History • Standard released Jan, 1995 (v1.1) • Initially used by police (~1996) • Wide service availability around 2000 • Omnisky, Verizon Wireless, others. • Covers most US population centers • Champaign-Urbana now covered. • Rural area coverage poor.

5. CDPD Market • CDPD is used primarily for • Law enforcement • Handheld/laptop IP access • Main competition: “Wireless Web” phones. • CDPD costs: • Wireless modems: ~$300 (Omnisky Palm V) • Service: $30-$40 per month (handheld) • $40-$80 per month (laptop)

6. Omnisky Coverage Map Source: Omnisky (http://www.omnisky.com)

7. CDPD Infrastructure Source: A. Salkintzis, “Packet Data over Cellular Networks: The CDPD Approach”

8. Application Transport Network Data link Physical CDPD - Layering IP/CLNP Connectionless Network Protocol Subnetwork Dependant Convergence Protocol Mobile Data Link Protocol Media Access Control Physical SNDCP MDLP MAC Physical Network layer CDPD Layer

9. CDPD Physical Layer • 30KHz BW channels, shared with AMPS • Separate forward and reverse channels • Forward channel is continuous • Reverse channel is multiple access. • Gaussian Minimum-Shift Keying-GMSK • GMSK compromises between channel bandwidth and decoder complexity. • 19.2kbps per channel. IP/CLNP SNDCP MDLP MAC Physical

10. AMPS and CDPD • CDPD runs alongside AMPS • AMPS system is unaware of CDPD system • CDPD system watches AMPS behavior • AMPS generally has unused channels. • Blocked calls when all channels are allocated. • 1% block probability => all channels used only 1% of the time.

11. CDPD Channel Usage • CDPD uses unused AMPS channels. • Usually are several available. • Each 30KHz channel = 19.2kbps up and down • CDPD channel hopping. • Forced: AMPS must be vacated within 40ms of allocation for voice use. • Planned: Regular hops prevent AMPS system from identifying channel as unusable.

12. Channel Scanning • 1. Check signal levels from nearby cells. • Use a list of reference channels distributed by the CDPD infrastructure to find levels. • 2. Select cell with best signal. • If non-critical and no cell is significantly better than current, no handoff is done (hysteresis) • 3. Scan RF channels in cell for CDPD. • Stop when an acceptable channel is found.

13. Handoff in CDPD • Critical handoffs: Must choose new channel • High error rate is observed or BS signal lost. • Received signal strength below a threshold. • Base station does not receive data from mobile. • Noncritical handoffs • Channel rescan interval expires. • Signal strength changes significantly.

14. CDPD effects on AMPS • CDPD logically transparent to AMPS • Can reduce AMPS service quality • More channel usage => increased interference. • If AMPS system is close to SIR margin, CDPD can push it below. • Full CDPD usage can push SIR down ~2dB • 19 channels/cell, Pblock = 0.02, 12.3 Erlangs • Limiting channels used reduces SIR cost..

15. Data Transmission Format • All links are base to mobile. • Mobile to mobile goes through base station. • Full-duplex; separate forward and reverse links. • Forward link • Continuous transmission by BS • Reverse link • Shared multiple access for mobiles. • Reverse link activity indicated by BS. IP/CLNP SNDCP MDLP MAC Physical

16. Forward Link Structure Source: A. Salkintzis, “Packet Data over Cellular Networks: The CDPD Approach”

17. Reverse Link Structure Source: A. Salkintzis, “Packet Data over Cellular Networks: The CDPD Approach”

18. Reverse Link MAC • Near/Far problem • Mobile may not detect a faraway transmitter. • Base station must report busy status. • Protocol: • Digital Sense Multiple Access • Nonpersistant: Checks once for busy state. • Slotted: Can only start when BS reports state. • Similar to Ethernet MAC. IP/CLNP SNDCP MDLP MAC Physical

19. Reverse Link MAC Source: J. Agostsa et al., “CDPD: Cellular Data Packet Standards and Technoloy”

20. Reverse Link MAC • Reverse link idle => can transmit. • Busy status checked before transmission starts.. • Continue burst unless error is indicated. • If BS indicates error, assume collision; exponential backoff is used. • Reverse link busy: • Delay for a random number of slots. • Check busy status again.

21. Mobile Data Link Protocol IP/CLNP SNDCP MDLP MAC Physical Source: J. Agostsa et al., “CDPD: Cellular Data Packet Standards and Technoloy”

22. CDPD - MDLP • Mobile Data Link Layer Protocol (MDLP) • High-level data link control (HDLC) • Similar to ISDN HDLC. • Mobile (M-ES) to Infrastructure (MD-IS) • In this layer, air link and BS become transparent • Connection oriented • MDLP Frame (message structure) • Address, control field, information field • No checksum; MAC discards incorrect packets.

23. CDPD - MDLP • Temporary equipment identifier (TEI) • Identifies destination mobile - virtual address. • Assigned by infrastructure. • Packet types • Unacknowledged information • Sequenced information • Sequence number, ack, timeout • Sliding window • Selective rejection supported.

24. CDPD - SNDCP • Subnetwork-Dependent Convergence Protocol (SNDCP) • Between IP or CLNP and MDLP • In both mobile and infrastructure (MD-IS) • Segmentation, compression, encryption • Questions: • Where and how to segment data? • Where and how to compress data? IP/CLNP SNDCP MDLP MAC Physical

25. CDPD - SNDCP • Segmentation • Goal: to fit the size of underlying frames • Two type of headers • Sequenced headers: • For compressed, encrypted, and segmented user data. • Unnumbered headers: Control information. • Efficiency consideration (similar to X.25) • Which layer should segment/assemble messages? • Use “More” indicator to avoid IP fragmentation.

26. CDPD - SNDCP • Compression • Header compression • To send the “Delta” information • Data compression • V.42bis – a dictionary-based compression • Which layer should compress data? • Source-dependent compression – higher layer • Source-independent compression – lower layer

27. CDPD - Registration • Low-level protocols ignore authentication. • Registration and Authentication • M-ES, serving MD-IS, home MD-IS • Base station (MDBS) has no network function. • Network Equipment identifier (IP, etc.) • Forwarding database in home MD-IS • Deregistration • Table maintenance timer

28. Problems with CDPD • Limited bandwidth • 19.2kbps shared per channel • Modern applications demand more bandwidth. • Security: • “Man in the middle” identity theft attack • IP network attacks • Denial of Service attacks easy.

29. Potential Improvements • Multichannel / multicarrier transmission • Would allow faster rates with AMPS compatibility. • Security Improvements • Secure against “man-in-the-middle” attacks. • Switch to CDMA/GSM. • Digital cellular services are more able to accommodate data services.

30. References • J. Agosta and T. Russell, CDPD: Cellular Packet Data Standards and Technology, McGraw Hill, 1996. • Y. Frankel et al., “Security Issues in a CDPD Wireless Network,” IEEE Personal Communications, August 1995, pp. 16-26. • D. Saha and S. Kay, “Cellular Digital Packet Data Network,” IEEE Transactions on Vehicular Technology, August 1997, pp. 697-706. • A. Salkintzis, “Packet Data over Cellular Networks: The CDPD Approach,” IEEE Communication Magazine, June 1999, pp. 152-159. • A. Salkintzis, “Radio Resource Management in Cellular Digital Packet Data Networks,” IEEE Personal Communications, December 1999, pp. 28-36