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Mobile Computing. Assistant Professor: Jenhui Chen Office number: 5990 Homepage: Textbooks and References. W. Stallings, Wireless Communications & Networks, Prentice Hall, August 2001.

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Mobile Computing

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Mobile computing l.jpg

Mobile Computing

Assistant Professor: Jenhui Chen

Office number: 5990


Dr. Jenhui Chen

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Textbooks and References

  • W. Stallings, Wireless Communications & Networks, Prentice Hall, August 2001.

  • D.J. Goodman, Wireless Personal Communications Systems, Addison-Wesley, 1997.

  • Selected journal articles and conference papers

Dr. Jenhui Chen

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  • Midterm Report

    • Oral presentation- over 15 pages (30%)

  • Final Project

    • Oral presentation- over 15 pages (30%)

    • Report 10 pages (40%)

Dr. Jenhui Chen

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Selected Papers

  • Journals:

    • IEEE Transactions on Mobile Computing

    • IEEE Journal on Selected Areas in Communications

    • IEEE Transactions on Vehicular Technology

    • ACM Computing Surveys

  • Magazine

    • IEEE Pervasive Computing

    • IEEE Wireless (Personal) Communications

    • IEEE Communications

    • Communications of the ACM

  • Conferences:



Dr. Jenhui Chen

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Related Topics

  • Sensor Networks

  • Pervasive Computing (Wireless LAN)

  • Internet Computing

  • Personal Communication System

    • UMTS (Universal Mobile Telecommunications System)

    • GPRS (General Packet Radio Service)

Dr. Jenhui Chen

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Chapter 1


Why and who will be mobile?

Dr. Jenhui Chen

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Telecom Networks/Systems

  • Wired Networks (Internet)

    • Local Area Networks (LANs)

    • Public Data Networks, i.e., WANs

  • Wireless Networks

    • Wireless LANs

    • Mobile Data Networks

  • Fixed Telephone Networks, i.e., Public Switched Telephone Networks (PSTNs)

  • Mobile Phone Systems

Dr. Jenhui Chen

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(1) Telephone (M)

(2) Mobile phone (K)

(3) Radio paging (K)

(4) Internet (K)
















































Telecom Statistics

1995 1996 1997 1998 1999 2000

Note: According to Jan. 2001 statistics, total mobile phone subscribers are

18.3M in which CHT: 4.7M(25.7%), and others: 13.61M(74.3%)

Dr. Jenhui Chen

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Some Forecasts

  • In 3 years, Internet traffic will grow to 10,000 times its current level.

  • Global e-business revenue will grow 86% per year to $ 1.4 Trillion in 2003.

  • Bandwidth consumption will grow by a factor of 100 to 200 over the next four years.

Dr. Jenhui Chen

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What is Mobile Computing?

  • Mobile Computing is also known as “Ubiquitous Computing” (anywhere, anytime and any device)

  • The scope covered by Mobile Computing roughly includes: Mobile Data, Wireless LANs and Ad Hoc Networks, etc.

Dr. Jenhui Chen

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Mobile Computing Chart




Operating systems

Mobile operation systems








Wireless networks

Dr. Jenhui Chen

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Application Layer

  • Vertical applications: those apply to a function part of an industry such as field sales and field service, or to specific market segment such as banking or health care

  • Horizontal applications: apply to many people across most market segments

Dr. Jenhui Chen

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Operation Systems Layer

  • This layer provides tools for application programmers to access different mobile devices and different wireless networks

  • A key layer to rapid growth of wireless networking and proliferation of applications

Dr. Jenhui Chen

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Device Layer

  • All the mobile devices we carry with us:

    • Notebooks (NBs)

    • Personal Digital Assistants (PDAs)

    • Cellular phones

    • Personal communicators

    • Combination devices

  • Combination devices are now rapidly emerging

Dr. Jenhui Chen

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Wireless Networks Layer

  • The Wireless Wide Area Network (WAN) is also called “Mobile Data” including:

    • Packet networks: RAM/Mobitex, ARDIS/Modacom

    • Paging networks

    • Data over cellular: CDPD (over AMPS), GPRS (over GSM)

    • Data over satellite

  • Wireless LANs: with much higher rate but smaller coverage than Mobile Data networks

Dr. Jenhui Chen

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Applications Market by 2005

Dr. Jenhui Chen

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Field Sales Application

  • Sales quotation

  • Inventory check

  • Order entry

  • Credit authorization

  • Invoicing

Dr. Jenhui Chen

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Field Service Application

  • Obtaining a maintenance history of the item requiring service

  • Performing complex diagnostics that require access to databases and applications at other locations

  • Checking parts inventory if required

  • Updating the maintenance database after the service is done

  • Invoicing for the job

  • Real-time dispatching of the field engineer

Dr. Jenhui Chen

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Transportation Application

  • The oldest application

  • Including:

    • Automatically locating the vehicle

    • Dispatching the vehicle to the next job

    • Routing the vehicle if required

    • Capturing data from the vehicle

Dr. Jenhui Chen

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Personal Communications Application

  • Messaging

  • Calendaring

  • Directories

  • Info Systems

  • Fax

Dr. Jenhui Chen

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Mobile Office Application

  • Fax

  • E-mail

  • LAN access

  • File transfer

  • Database access

Dr. Jenhui Chen

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Field sales




Retail stores

Stock exchanges




Rental car agencies


Vertical Market Examples

Dr. Jenhui Chen

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User authentication

Data privacy

Privacy of user location


Limited bandwidth (10K~10M)


Still in its infancy

The main/ biggest reason for late data implementations


Radiation is harmful to human beings

Trends: low power, thus less radiation


Dr. Jenhui Chen

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Application/Technology Matrix

Dr. Jenhui Chen

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Horizontal Application Examples

  • Near term horizontal applications (LAN app.)

    • Dynamic work environment

      • Trade show

      • conference

    • Difficult to wire areas

    • New employees who need immediate service

  • Broad-based horizontal applications (WAN app.)

    • Wireless meeting

    • Wireless traveler

    • Interactive TV

Dr. Jenhui Chen

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Positioning of Wireless Networking

  • Comparison between wireless data & wired data capacities

  • Position of wireless networks relative to wired networks:

    • Not a replacement but an extension to wired networks

Dr. Jenhui Chen

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CDPD:Cellular Digital Packet Data

Daniel Grobe Sachs

Quji Guo

Dr. Jenhui Chen

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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!

Dr. Jenhui Chen

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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.

Dr. Jenhui Chen

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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.

Dr. Jenhui Chen

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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)

Dr. Jenhui Chen

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Omnisky Coverage Map

Dr. Jenhui Chen

Source: Omnisky (

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CDPD Infrastructure

Dr. Jenhui Chen

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

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Data link


CDPD - Layering


Connectionless Network Protocol

Subnetwork Dependant Convergence Protocol

Mobile Data Link Protocol

Media Access Control






Network layer CDPD Layer

Dr. Jenhui Chen

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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.





Dr. Jenhui Chen


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  • 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.

Dr. Jenhui Chen

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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.

Dr. Jenhui Chen

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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.

Dr. Jenhui Chen

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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.

Dr. Jenhui Chen

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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..

Dr. Jenhui Chen

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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.





Dr. Jenhui Chen


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Forward Link Structure

Dr. Jenhui Chen

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

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Reverse Link Structure

Dr. Jenhui Chen

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

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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.





Dr. Jenhui Chen


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Reverse Link MAC

Dr. Jenhui Chen

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

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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.

Dr. Jenhui Chen

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Mobile Data Link Protocol





Dr. Jenhui Chen


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

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  • 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.

Dr. Jenhui Chen

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  • 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.

Dr. Jenhui Chen

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  • 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?





Dr. Jenhui Chen


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  • 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.

Dr. Jenhui Chen

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  • 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

Dr. Jenhui Chen

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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

Dr. Jenhui Chen

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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.

Dr. Jenhui Chen

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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.

Dr. Jenhui Chen

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  • 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

Dr. Jenhui Chen

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