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  1. MAC protocols derived from IEEE 802.11 Virtual CarrierSensing Mechanism Shweta Jain Computer Science Department, SUNY@Stony Brook Research Proficiency Exam

  2. Outline • Problems in Ad hoc networks • Potentially useful techniques to alleviate problems • Directional Antenna • Rate Adaptability • Path Diversity • Overview of IEEE 802.11 • D-MAC schemes, Another directional MAC scheme, DVCS and MMAC ---- Directional Antenna • ARF,RBAR and OAR --- Rate Adaptability • Anycast --- Path Diversity Research Proficiency Exam

  3. Problems in Ad hoc networks • Dynamic Channel conditions • Poor Bandwidth Utilization • Mobile nodes • Hidden and exposed terminal problems Research Proficiency Exam

  4. Potentially useful techniques to alleviate problems • Directional Antenna: Antenna that are capable of directing signals in a particular direction • Rate Adaptation: A technique that enable wireless devices to switch between transmission rates to achieve a better SNR • Path Diversity: The number of paths available to route a packet between two stations Research Proficiency Exam

  5. Motivation • IEEE 802.11 is a widely adopted MAC layer protocol in ad hoc networks • Designed for Omni directional antenna but can be easily modified to use Directional Antenna • Supports multiple rates • Can be easily modified without changing the hardware, to support anycasting. Research Proficiency Exam

  6. Overview of IEEE 802.11 Research Proficiency Exam

  7. D-MAC Schemes Y. B. Ko, V. Shankarkumar, and N. H. Vaidya. Medium access control protocols using directionalantennas in ad hoc networks. In Proceedings of IEEE INFOCOM’2000, March. • Assumptions: • Each node has multiple directional antennae to cover all angles • Initially each node knows its neighbors location • Each node is equipped with GPS • Collision occurs if more than one antennae receive different signals simultaneously Research Proficiency Exam

  8. D-MAC Scheme 1 • Sender: Send Directional RTS (DRTS) in the direction of the receiver • Receiver: Send Omni-Directional CTS (OCTS). • Sender: If DRTS/OCTS Handshake is successful send Directional DATA • Receiver: On error free DATA reception send Directional ACK Research Proficiency Exam

  9. D-MAC Scheme 2 • Sender: • (a) If none of the antennae is blocked send an Omni RTS • (b) If atleast one antenna is blocked send a DRTS • Receiver: Respond with an OCTS • Sender: if RTS/CTS Handshake is successful send Directional DATA • Receiver: If DATA was received correctly send Directional ACK Research Proficiency Exam

  10. DMAC Schemes Optimizations • Directional Wait to Send – To stop unnecessary retry attempts. • RTS/CTS Exchange for ACK– for guaranteed delivery of DATA packets. • Otherwise separate channel for ACK transmission Research Proficiency Exam

  11. Comments • Location information is not always available • Send periodic beacons • Include location in RTS/CTS and other messages • Switch to ORTS if DRTS fails for a given number of times Research Proficiency Exam

  12. Performance Evaluation Research Proficiency Exam

  13. Another Directional MAC Protocol A. Nasipuri, S. Ye, J. You, and R. Hiromotoxi. A MAC protocol for mobile ad hoc networks using directional antennas. In Proceedings of the IEEE Wireless Communications and Networking Conference (WCNC), 2000. • Assumptions • Each radio has M directional antennae • Nodes maintain antenna orientation using simple instruments like a compass • Antennas can be switched to passive or active • Only active antennae can transmit or receive signals • More than one signal is perceived using selection diversity Research Proficiency Exam

  14. Protocol Details • Sender: Send an Omni Directional RTS • Receiver: Send an Omni Directional CTS and set the antenna that received the max signal power as active and others as passive • Sender: If RTS/CTS Handshake was successful, set the antenna that received maximum signal when CTS was received • Sender: Send DATA through the active antenna • Receiver: Receive DATA and send ACK through the active antenna • After RTS/CTS/DATA/ACK set all antennae as active Research Proficiency Exam

  15. Performance Result • 15x15 uniform grid = 225 nodes • Packet arrival is a Poisson process • Packet destination is chosen randomly from one of the neighbors Research Proficiency Exam

  16. Why does the performance improve? • Omni RTS and Omni CTS silence all neighbors so no spatial reuse here. • Nodes beyond the transmission range but within the carrier sensing range are not completely silenced. They account for the increased throughput Research Proficiency Exam

  17. Directional Virtual Carrier Sensing • Assumptions • Electrically steerable antenna capable of measuring Angle of Arrival and antenna Gain • Aims at maximizing gain not SNR because of contention at the MAC layer Mineo Takai, Jay Martin, Rajive Bagrodia, and Aifeng Ren. Directional virtual carrier sensing for directional antennas in mobile ad hoc networks. In Proceedings of the 3rd ACM international symposium on Mobile ad hoc networking and computing, pages 183–193. ACM Press, 2002. Research Proficiency Exam

  18. Features • No additional hardware required • No specific antenna configuration required • Interoperable with a network containing omni directional antenna • DVCS adds three primary capabilities to IEEE 802.11 • Angle of Arrival Caching • Beam Locking and unlocking • Directional Network Allocation Vector (DNAV) Research Proficiency Exam

  19. Protocol Details • Estimate and cache AOA information from every received or sensed signal • Sender: Send a DRTS if AOA available in the cache else send ORTS • If DRTS fails certain number of times (4) invalidate cache and send ORTS • Receiver: If ORTS or DRTS received correctly adapt beam pattern to maximize gain and lock the pattern • All other nodes set their DNAVs to reserve the channel for the ongoing transmission Research Proficiency Exam

  20. Directional NAV (DNAV) Research Proficiency Exam

  21. Comments • DVCS transmits and receives all frames directionally. • It increases spatial reuse of the channel thereby increasing throughput • It reduces the energy consumption by reducing the input power in the antenna in order to achieve the same Gain. • Simulation results show that DVCS increases the network capacity 3 to 4 times. Research Proficiency Exam

  22. Performance Evaluation Research Proficiency Exam

  23. Interoperability with Omni directional nodes Research Proficiency Exam

  24. Multi-Hop RTS MAC (MMAC) • Observation: • Power radiated by a non isotropic antenna in the direction of the main lobe is higher than that radiated by a non directional (isotropic or omni directional antenna) Romit Roy Choudhury, Xue Yang, Nitin H. Vaidya, and Ram Ramanathan. Using directional antennas for medium access control in ad hoc networks. In Proceedings of the 8th annual international conference on Mobile computing and networking, 2002. Research Proficiency Exam

  25. Features • Utilize the higher transmission range obtained due to directionality. • Solves new problems caused by Directional antenna: • New hidden terminals due to asymmetry in gain, unheard RTS/CTS • Higher directional interference • Deafness Research Proficiency Exam

  26. Network Model • Two kinds of neighbors • Direction-Omni Neighbors (DO) • Direction-Direction neighbor (DD) • Directional Omni (DO) route is larger than Direction Direction (DD) route Research Proficiency Exam

  27. Protocol Details Research Proficiency Exam

  28. Disclaimer • Performance depends upon • Network topology • Flow or route configuration • Random topologies provide the best performance results Research Proficiency Exam

  29. Performance results Research Proficiency Exam

  30. Flows with aligned routes Research Proficiency Exam

  31. Flows with less aligned routes Research Proficiency Exam

  32. Randomly selected routes Research Proficiency Exam

  33. Auto Rate Fallback (ARF) • Assumption: The receiver and transmitter hardware is capable of switching between transmission and reception rates Ad Kamerman and Leo Monteban. Wavelan-II: A high-performance wireless LAN for the unlicensed band, Bell Labs Technical Journal 1997. Research Proficiency Exam

  34. Features • Transmission rates are switched between 2Mbps and 1Mbps. • Rate switching changes the SNR making it easy to cope with poor channel conditions • Also capable of exploiting favorable channel condition by increasing transmission rate • Enables temporal reuse of the wireless channel Research Proficiency Exam

  35. Protocol Details • Sender: • Initially transmits at 2Mbps • Keeps track of MAC layer Acknowledgements. • If two consecutive ACKs are missed, decrease the data rate to 1Mbps and start a timer • After either the timer expires or 10 successive ACKs are received correctly, upgrade to the higher rate • If the next ACK fails, immediately reduce the rate. Research Proficiency Exam

  36. Disclaimer • Slow in responding to changing channel conditions • Not capable of taking advantage of god channel condition immediately Research Proficiency Exam

  37. Receiver Based Autorate (RBAR) • Assumption: The hardware is capable of switching between rates • The network is modeled to include Ricean fading. Gavin Holland, Nitin Vaidya, and Paramvir Bahl. A rate-adaptive mac protocol for multihop wireless networks. In Proceedings of the 7th annual international conference on Mobile computing and networking, 2001. Research Proficiency Exam

  38. Features • Capable of learning the instantaneous channel condition from the receiver • Capable of quickly adapting to the channel condition • Receiver makes the choice of data rate • Increases temporal reuse of the channel Research Proficiency Exam

  39. Protocol Details Research Proficiency Exam

  40. Comments • Responds to channel conditions faster than ARF • Overhead due to Reservation Sub header (RSH) • RSH may be lost due to collision or deep fade Research Proficiency Exam

  41. Impact of Lost RSH Research Proficiency Exam

  42. Performance results Research Proficiency Exam

  43. Opportunistic Auto Rate (OAR) • Observation: Channel coherence time is the time for which the channel condition is better than average. This condition usually persists for more than one packet transmission. A.Sabharwal B. Sandeghi, V.Kanodia and E.Knightly. Oppurtunistic media access for multirate ad hoc networks. In ACM MOBICOM, September 2002. Research Proficiency Exam

  44. Features • Exploits channel coherence time transmit multiple back to back data packets at higher rate. • Enhances RBAR to produce better throughput • Reduces the time spent in contention compared to RBAR • Achieves the same temporal fairness as IEEE 802.11. • Exploits the fragmentation procedure prescribed and mandated in 802.11 for large data packet sizes. • The network model includes multipath fading. Research Proficiency Exam

  45. Fragmentation in 802.11 Set the “More Fragment” field in the frame control in the MAC header Set the “fragment number” field to indicate the fragment number Research Proficiency Exam

  46. Multiple transmissions in OAR Set the “More Fragment” field in the frame control in the MAC header Set the “fragment number” field to 0 so that the receiver does not actually treat the next packet as a fragment. Research Proficiency Exam

  47. Comments • Sender might not have more data to utilize the channel. • OAR reduces to RBAR • Channel conditions may change during the transmission. • Rate is changed after notifying the receiver through RSH messages Research Proficiency Exam

  48. Performance results Research Proficiency Exam

  49. Anycast • Observation: Received signal strength in a wireless network varies at a rapid rate and the variations are transient. S Jain, S Das, and Yi Lv. Exploiting path diversity in the link layer in wireless ad hoc networks, Submitted to MOBIHOC 2004 December 2003. Research Proficiency Exam

  50. Features • MAC layer is provided with multiple link options for packet delivery. • MAC layer exploits this path diversity to produce better packet delivery fraction • Both analytical and experimental results prove the benefit of exploiting path diversity at the link layer Research Proficiency Exam