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.
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–
–
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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
– 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
Network layer
IP/CLNP Connectionless Network Protocol
SNDCP Subnetwork Dependant Convergence Protocol
MDLP
Mobile Data Link Protocol
MAC
Media Access Control
Physical Physical
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
– Mobile may not detect a faraway transmitter.
– Base station must report busy status.
• Protocol:
– Digital Sense Multiple Access
– Nonpersistant: Checks once for busy state. IP/CLNP
– Slotted: Can only start when BS reports state. SNDCP
• Similar to Ethernet MAC.
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)
– 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)
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–
–
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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
• 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
– 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