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Mobile Computing

Mobile Computing. Chengzhi Li University of Virginia chengzhi@cs.virginia.edu www.cs.virginia.edu/~cl4v. What is Mobile Computing. Building distributed system with mobile computers and wireless communications Mobile networking MAC, Routing, Reliable data transport, …

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Mobile Computing

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  1. Mobile Computing Chengzhi Li University of Virginia chengzhi@cs.virginia.edu www.cs.virginia.edu/~cl4v

  2. What is Mobile Computing • Building distributed system with mobile computers and wireless communications • Mobile networking • MAC, Routing, Reliable data transport, … • Mobile information access • Disconnected operation, … • Adaptive applications • Proxies, transcoding, … • Energy aware systems • Goal-directed adaptation, … • Location sensitivity • GPS, …

  3. Evolution of Computing Mobile Computing LANs + WSs Networking More Flexible Resource Usage Timesharing Batch Single User OS More Freedom from Collocation

  4. Challenges • Poor local resources due to size, weight, and battery • CPU, Memory, and Channel bandwidth limited • limited wireless transmission range • limited life time • Broadcast nature of the wireless medium • Hidden & exposed terminal problems • Ease of snooping on wireless transmissions (security hazard) • Mobility • route changes • packet losses • network partitions

  5. Upper layers Transport Network Link Physical Problem Space

  6. Mobile Ad Hoc Networks • Mobile distributed multiple-hop wireless network • Formed by wireless hosts which may be mobile • Without necessarily using a pre-existing infrastructure • Routes between nodes may potentially contain multiple hops

  7. A Real Ad Hoc Network • NTDR (Near Term Digital Radio) is the only “real” (non-prototypical) Ad Hoc network in use today. • NTDR use clustering and link state routing and self-organized into a two tier ad hoc network

  8. Many Applications • Personal area networking • cell phone, laptop, ear phone, wrist watch • Military environments • soldiers, tanks, planes • Civilian environments • taxi cab network • meeting rooms • sports stadiums • boats, small aircraft • Emergency operations • search-and-rescue • policing and fire fighting

  9. Physical Layer • Traditionally, not much interaction between physical layer and upper layers • Many physical layer mechanisms not beneficial without help from upper layers • Example: Adaptive modulation

  10. Power Control • Transmit power determines • “Range” of a transmission • Interference caused at other nodes A B C D

  11. Benefits of Power Control • Transmit a packet with least transmit power necessary to deliver to the receiver • Save energy: Important benefit to battery-powered hosts • Reduce interference • Can allow greater spatial reuse

  12. Power Control • Power control introduces asymmetry • D transmits to C at low power, but B uses high transmit power to transmit to A • B may not know about D-to-C transmission, but can interfere with it A B C D

  13. Power Control • Transmit power determines • “Range” of a transmission • Interference caused at other nodes A B C D

  14. Power Control • Proposals for medium access control and routing with power control exist • Do not solve the problem satisfactorily • Ideal solution will • Reduce energy consumption, and • Maximize spatial reuse

  15. Link Layer

  16. A B C D Hidden Terminal Problem • Node B can communicate with A and C both • A and C cannot hear each other • When A transmits to B, C cannot detect the transmission using the carrier sense mechanism • If C transmits to D, collision will occur at B

  17. A B C D Exposed Terminal Problem • Node C can communicate with B and D both • Node B can communicate with A and C • Node A cannot hear C • Node D can nor hear B • When C transmits to D, B detect the transmission using the carrier sense mechanism and postpone to transmit to A, even though such transmission will nor cause collision

  18. RTS (10) CTS (10) RTS/CTS Handshake • Sender sends Ready-to-Send (RTS) • Receiver responds with Clear-to-Send (CTS) • RTS and CTS announce the duration of the transfer • Nodes overhearing RTS/CTS keep quiet for that duration • RTS/CTS used in IEEE 802.11 C 10 A B D 10

  19. Network Layer

  20. Mobile Ad Hoc Networks • May need to traverse multiple links to reach a destination

  21. Mobile Ad Hoc Networks • Mobility causes route changes

  22. Transport Layer

  23. TCP • TCP performance degrades in presence of route failures • TCP cannot distinguish between packet losses due to route change and due to congestion • Reduces congestion window in response • Unnecessary degradation in throughput

  24. TCP • TCP performance degrades in presence of route failures • TCP cannot distinguish between packet losses due to route change and due to congestion • Reduces congestion window in response • Unnecessary degradation in throughput

  25. Address Assignment • How to assign addresses to nodes in an ad hoc network ? • Static assignment • Easier to guarantee unique address • Dynamic assignment • How to guarantee unique addresses when partitions merge? • Do we need to guarantee unique addresses ?

  26. Other Issues

  27. Security Issues

  28. What’s New ? • Wireless medium easy to snoop on • With ad hoc networking, hard to guarantee connectivity • Easier for intruders to insert themselves into network

  29. Authentication • How to authenticate a node ? • May not have access to a certification authority

  30. Resource Depletion Attack • Intruders may send data with the objective of congesting a network or depleting batteries U intruder B C A D T Bogus traffic intruder

  31. Routing Attacks • Intruders may mis-route the data • not delivering it to the destination at all, or • delaying it significantly • How to detect such attacks ? • How to tolerate such attacks ?

  32. Traffic Analysis • Despite encryption, an eavesdropper can identify traffic patterns • Traffic patterns can divulge information about the operation mode • Traffic analysis can be prevented by presenting “constant” traffic pattern • Insert dummy traffic • How to make this cheaper ?

  33. Other Issues

  34. Incentives for Ad Hoc Routing • Why should I forward packets for some other nodes ? • Need some incentive mechanism • Policies to determine reward for performing each operation

  35. Applications • New applications for ad hoc networks ?

  36. Hybrid Environments • Use infrastructure when convenient • Use ad hoc connectivity when necessary or superior infrastructure BS1 BS2 E A Z Ad hoc connectivity X

  37. Summary • Plenty of interesting research problems • Research community disproportionately obsessed with routing protocols

  38. Upper layers Transport Network Link Physical Summary • Interesting problems elsewhere at the two ends of the protocol stack • How to design algorithms and applications ? • How to exploit physical layer techniques ? • Increase interaction between physical layer and upper layers

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