CDPD: Cellular Digital Packet Data Daniel Grobe Sachs Quji Guo.
-
Upload
nora-parker -
Category
Documents
-
view
221 -
download
1
Transcript of CDPD: Cellular Digital Packet Data Daniel Grobe Sachs Quji Guo.
![Page 1: CDPD: Cellular Digital Packet Data Daniel Grobe Sachs Quji Guo.](https://reader030.fdocuments.us/reader030/viewer/2022033105/56649dce5503460f94ac2b32/html5/thumbnails/1.jpg)
CDPD:Cellular Digital Packet Data
Daniel Grobe Sachs
Quji Guo
![Page 2: CDPD: Cellular Digital Packet Data Daniel Grobe Sachs Quji Guo.](https://reader030.fdocuments.us/reader030/viewer/2022033105/56649dce5503460f94ac2b32/html5/thumbnails/2.jpg)
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!
![Page 3: CDPD: Cellular Digital Packet Data Daniel Grobe Sachs Quji Guo.](https://reader030.fdocuments.us/reader030/viewer/2022033105/56649dce5503460f94ac2b32/html5/thumbnails/3.jpg)
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.
![Page 4: CDPD: Cellular Digital Packet Data Daniel Grobe Sachs Quji Guo.](https://reader030.fdocuments.us/reader030/viewer/2022033105/56649dce5503460f94ac2b32/html5/thumbnails/4.jpg)
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.
![Page 5: CDPD: Cellular Digital Packet Data Daniel Grobe Sachs Quji Guo.](https://reader030.fdocuments.us/reader030/viewer/2022033105/56649dce5503460f94ac2b32/html5/thumbnails/5.jpg)
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)
![Page 6: CDPD: Cellular Digital Packet Data Daniel Grobe Sachs Quji Guo.](https://reader030.fdocuments.us/reader030/viewer/2022033105/56649dce5503460f94ac2b32/html5/thumbnails/6.jpg)
Omnisky Coverage Map
Source: Omnisky (http://www.omnisky.com)
![Page 7: CDPD: Cellular Digital Packet Data Daniel Grobe Sachs Quji Guo.](https://reader030.fdocuments.us/reader030/viewer/2022033105/56649dce5503460f94ac2b32/html5/thumbnails/7.jpg)
CDPD Infrastructure
Source: A. Salkintzis, “Packet Data over Cellular Networks: The CDPD Approach”
![Page 8: CDPD: Cellular Digital Packet Data Daniel Grobe Sachs Quji Guo.](https://reader030.fdocuments.us/reader030/viewer/2022033105/56649dce5503460f94ac2b32/html5/thumbnails/8.jpg)
CDPD - Layering
Physical
MAC
MDLP
SNDCP
IP/CLNP
Physical
Data link
Network
Transport
Application
Network layer CDPD Layer
Connectionless Network Protocol
Subnetwork Dependant Convergence Protocol
Mobile Data Link Protocol
Media Access Control
Physical
![Page 9: CDPD: Cellular Digital Packet Data Daniel Grobe Sachs Quji Guo.](https://reader030.fdocuments.us/reader030/viewer/2022033105/56649dce5503460f94ac2b32/html5/thumbnails/9.jpg)
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.
Physical
MAC
MDLP
SNDCP
IP/CLNP
![Page 10: CDPD: Cellular Digital Packet Data Daniel Grobe Sachs Quji Guo.](https://reader030.fdocuments.us/reader030/viewer/2022033105/56649dce5503460f94ac2b32/html5/thumbnails/10.jpg)
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.
![Page 11: CDPD: Cellular Digital Packet Data Daniel Grobe Sachs Quji Guo.](https://reader030.fdocuments.us/reader030/viewer/2022033105/56649dce5503460f94ac2b32/html5/thumbnails/11.jpg)
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.
![Page 12: CDPD: Cellular Digital Packet Data Daniel Grobe Sachs Quji Guo.](https://reader030.fdocuments.us/reader030/viewer/2022033105/56649dce5503460f94ac2b32/html5/thumbnails/12.jpg)
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.
![Page 13: CDPD: Cellular Digital Packet Data Daniel Grobe Sachs Quji Guo.](https://reader030.fdocuments.us/reader030/viewer/2022033105/56649dce5503460f94ac2b32/html5/thumbnails/13.jpg)
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.
![Page 14: CDPD: Cellular Digital Packet Data Daniel Grobe Sachs Quji Guo.](https://reader030.fdocuments.us/reader030/viewer/2022033105/56649dce5503460f94ac2b32/html5/thumbnails/14.jpg)
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..
![Page 15: CDPD: Cellular Digital Packet Data Daniel Grobe Sachs Quji Guo.](https://reader030.fdocuments.us/reader030/viewer/2022033105/56649dce5503460f94ac2b32/html5/thumbnails/15.jpg)
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.
Physical
MAC
MDLP
SNDCP
IP/CLNP
![Page 16: CDPD: Cellular Digital Packet Data Daniel Grobe Sachs Quji Guo.](https://reader030.fdocuments.us/reader030/viewer/2022033105/56649dce5503460f94ac2b32/html5/thumbnails/16.jpg)
Forward Link Structure
Source: A. Salkintzis, “Packet Data over Cellular Networks: The CDPD Approach”
![Page 17: CDPD: Cellular Digital Packet Data Daniel Grobe Sachs Quji Guo.](https://reader030.fdocuments.us/reader030/viewer/2022033105/56649dce5503460f94ac2b32/html5/thumbnails/17.jpg)
Reverse Link Structure
Source: A. Salkintzis, “Packet Data over Cellular Networks: The CDPD Approach”
![Page 18: CDPD: Cellular Digital Packet Data Daniel Grobe Sachs Quji Guo.](https://reader030.fdocuments.us/reader030/viewer/2022033105/56649dce5503460f94ac2b32/html5/thumbnails/18.jpg)
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.
Physical
MAC
MDLP
SNDCP
IP/CLNP
![Page 19: CDPD: Cellular Digital Packet Data Daniel Grobe Sachs Quji Guo.](https://reader030.fdocuments.us/reader030/viewer/2022033105/56649dce5503460f94ac2b32/html5/thumbnails/19.jpg)
Reverse Link MAC
Source: J. Agostsa et al., “CDPD: Cellular Data Packet Standards and Technoloy”
![Page 20: CDPD: Cellular Digital Packet Data Daniel Grobe Sachs Quji Guo.](https://reader030.fdocuments.us/reader030/viewer/2022033105/56649dce5503460f94ac2b32/html5/thumbnails/20.jpg)
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.
![Page 21: CDPD: Cellular Digital Packet Data Daniel Grobe Sachs Quji Guo.](https://reader030.fdocuments.us/reader030/viewer/2022033105/56649dce5503460f94ac2b32/html5/thumbnails/21.jpg)
Mobile Data Link Protocol
Physical
MAC
MDLP
SNDCP
IP/CLNP
Source: J. Agostsa et al., “CDPD: Cellular Data Packet Standards and Technoloy”
![Page 22: CDPD: Cellular Digital Packet Data Daniel Grobe Sachs Quji Guo.](https://reader030.fdocuments.us/reader030/viewer/2022033105/56649dce5503460f94ac2b32/html5/thumbnails/22.jpg)
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.
![Page 23: CDPD: Cellular Digital Packet Data Daniel Grobe Sachs Quji Guo.](https://reader030.fdocuments.us/reader030/viewer/2022033105/56649dce5503460f94ac2b32/html5/thumbnails/23.jpg)
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.
![Page 24: CDPD: Cellular Digital Packet Data Daniel Grobe Sachs Quji Guo.](https://reader030.fdocuments.us/reader030/viewer/2022033105/56649dce5503460f94ac2b32/html5/thumbnails/24.jpg)
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?
Physical
MAC
MDLP
SNDCP
IP/CLNP
![Page 25: CDPD: Cellular Digital Packet Data Daniel Grobe Sachs Quji Guo.](https://reader030.fdocuments.us/reader030/viewer/2022033105/56649dce5503460f94ac2b32/html5/thumbnails/25.jpg)
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.
![Page 26: CDPD: Cellular Digital Packet Data Daniel Grobe Sachs Quji Guo.](https://reader030.fdocuments.us/reader030/viewer/2022033105/56649dce5503460f94ac2b32/html5/thumbnails/26.jpg)
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
![Page 27: CDPD: Cellular Digital Packet Data Daniel Grobe Sachs Quji Guo.](https://reader030.fdocuments.us/reader030/viewer/2022033105/56649dce5503460f94ac2b32/html5/thumbnails/27.jpg)
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
![Page 28: CDPD: Cellular Digital Packet Data Daniel Grobe Sachs Quji Guo.](https://reader030.fdocuments.us/reader030/viewer/2022033105/56649dce5503460f94ac2b32/html5/thumbnails/28.jpg)
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.
![Page 29: CDPD: Cellular Digital Packet Data Daniel Grobe Sachs Quji Guo.](https://reader030.fdocuments.us/reader030/viewer/2022033105/56649dce5503460f94ac2b32/html5/thumbnails/29.jpg)
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.
![Page 30: CDPD: Cellular Digital Packet Data Daniel Grobe Sachs Quji Guo.](https://reader030.fdocuments.us/reader030/viewer/2022033105/56649dce5503460f94ac2b32/html5/thumbnails/30.jpg)
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