1 / 38

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, …

lenore
Télécharger la présentation

Mobile Computing

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  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

More Related