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 .
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 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)

6. Omnisky Coverage Map
Source: Omnisky (

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

8. 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

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 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

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

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) 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.

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 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

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 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.

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
D. Saha and S. Kay, “Cellular Digital Packet Data Network,” IEEE Transactions on Vehicular Technology, August 1997, pp
A. Salkintzis, “Packet Data over Cellular Networks: The CDPD Approach,” IEEE Communication Magazine, June 1999, pp
A. Salkintzis, “Radio Resource Management in Cellular Digital Packet Data Networks,” IEEE Personal Communications, December 1999, pp