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Mobile Computing and Wireless Networking

Mobile Computing and Wireless Networking. 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 networking Mobile networking MAC, Routing, Reliable data transport, …

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Mobile Computing and Wireless Networking

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  1. Mobile Computing and Wireless Networking 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 networking • 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 More Flexible Resource Usage Mobile Computing LANs + WSs Networking Timesharing Batch Single User OS More Freedom from Collocation

  4. Challenges • Battery constraints • 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. Internet infrastructure Cellular Wireless Network

  7. Mobile Ad Hoc Network • Provide differentiated QoS levels to different wireless applications • Achievable by QoS-sensitive MAC and network layer scheduling

  8. 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

  9. Applications of 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

  10. Why Wireless Networks ? • Potential ease of deployment • Decreased dependence on infrastructure

  11. 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

  12. 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

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

  14. 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

  15. 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

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

  17. 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

  18. Link Layer

  19. Hidden Terminals&RTS/CTS Handshake

  20. 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

  21. 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

  22. Exposed Terminals&RTS, CTS, and Dual Busy Tones

  23. 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

  24. Network Layer

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

  26. Mobile Ad Hoc Networks • Mobility causes route changes

  27. Transport Layer

  28. Transport Protocols

  29. 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

  30. 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

  31. ProblemsinAd Hoc Networking

  32. Problem Space • Practical considerations • Consumer demand or lack thereof • Standardization • Government regulations • Technical issues

  33. 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

  34. Adaptive Modulation • Channel conditions are time-varying A B

  35. Choose modulation scheme as a function of channel conditions

  36. Adaptive Modulation • If physical layer chooses the modulation scheme transparent to MAC  MAC cannot know the time duration required for the transfer • Must involve MAC protocol in deciding the modulation scheme • Some 802.11-compliant implementations use a sender-based scheme for this purpose • Receiver-based schemes can perform better

  37. DATA2Mbps Sender-Based “Autorate Fallback” MAC Protocol • Sender decreases rate after N consecutive ACKS are not received • Sender increases rate after Y consecutive ACKS are received C A B D 2Mbps 1Mbps

  38. Performance of Sender-Based“Autorate Fallback” BPSK (1Mbps) QPSK (2Mbps) CCK (5.5Mbps) CCK (11Mbps) Expected ARF

  39. RTS (2) CTS (1) Receiver-Based Autorate MAC Protocol • Sender sends RTS containing its best rate estimate • Receiver chooses best rate for the conditions and sends it in the CTS • Sender transmits DATA packet at new rate • Information in data packet header implicitly updates nodes that heard old rate C 1 A B D 2 2Mbps 1Mbps

  40. Physical Layer • Several other physical layer capabilities call for changes to upper layers of protocol stack • Example: Power control

  41. Directional / Smart Antennas • Various capabilities • Sectored antennas (fixed beam positions) • Beam steering • Tracking a transmitter • MAC and routing protocols for ad hoc networks using such antennas • How to take into account antenna capabilities? • Network may be heterogeneous

  42. Physical Layer • Are ad hoc networks benefiting from the progress made at physical layer ? • Other interesting areas • Efficient coding schemes • Various diversity techniques

  43. Physical Layer: Simulation Models • Insufficient accuracy in commonly used physical layer models • Physical link state is not binary as often assumed • Reliable packet reception does not depend just on distance • Transmit power • Interference level • Fading • Need to use realistic models • Modulation scheme • Coding

  44. Link Layer

  45. Interesting Link Layer Issues • Medium access control • Retransmission mechanisms • Transmission scheduling • Which pending packet should a node attempt to transmit? • Adaptive parameter selection • Frame size • Retransmission limit

  46. QoS in Medium Access Control • Many proposals for achieving fairness • Fair scheduling schemes attempt to provide equitable sharing of channel • Unpredictable nature of transmission errors makes it difficult to make hard guarantees • Need to develop a probabilistic framework

  47. QoS in MAC • Easier in a centralized protocol (such as 802.11 point coordination function), than in a distributed protocol • Distributed MAC appears more suitable for ad hoc networks, however • Perhaps a hybrid protocol will be best • How to design such a protocol ?

  48. Transmission Scheduling • When multiple packets pending transmission, which packet to transmit next? • Choice should depend on • Receiver status (blocked by some other transmission?) • Congestion at receivers • Noise level at receivers • Tolerable delay for pending packets • Need interaction between upper layers and MAC

  49. MAC for Multiple Channels • How to split bandwidth into channels? • How to use the multiple channels ? • Dedicated channel for control ?

  50. Network Layer

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