CDPD: Cellular Digital Packet Data Daniel Grobe Sachs Quji Guo

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!

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.

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.

CDPD Market CDPD is used primarily for CDPD costs: 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)

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

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

CDPD - Layering Application Transport Network Data link Physical 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

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

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.

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.

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.

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.

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..

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

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

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

Reverse Link MAC Near/Far problem Protocol: Similar to Ethernet MAC. 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

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

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.

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

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.

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

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

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.

CDPD - SNDCP Compression Header compression Data 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

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

Problems with CDPD Limited bandwidth Security: 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.

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.

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