1 / 157

Mobile Ad Hoc Networks

Mobile Ad Hoc Networks. Organization. Introduction and Architecture Applications and Challenges Media Access Control Routing in Ad Hoc Networks Transport Layer Issues Overarching Issues. MANETs: Introduction. MANETs are mobile nodes that form a network in an ad hoc manner

martha-horn
Download Presentation

Mobile Ad Hoc Networks

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 Ad Hoc Networks

  2. Organization • Introduction and Architecture • Applications and Challenges • Media Access Control • Routing in Ad Hoc Networks • Transport Layer Issues • Overarching Issues Prasant Mohapatra

  3. MANETs: Introduction • MANETs are mobile nodes that form a network in an ad hoc manner • The nodes intercommunicate using single or multi-hop wireless links • Each node in MANETs can operate as a host as well as a router • The topology, locations, connectivity, transmission quality are variable Prasant Mohapatra

  4. X D Z S MANETs: Operations D Y X S Prasant Mohapatra

  5. MANET: Applications • Civil • Wireless LANs/WANs – mobile and stationary • Remote data collection and analysis • Taxi/Cabs, Buses scheduling • Disaster recovery • Communications over water using floats • Vehicular Ad Hoc Network • Defense • Battlefield communications and data transfer • Monitoring and Planning Prasant Mohapatra

  6. Issues and Challenges • Operating in presence of unpredictable mobility and environmental changes • Operating in an error prone media • Low bandwidth channels • Low power devices with limited resources • Maintaining and retaining connectivity and states • Security: infrastructure and communication Prasant Mohapatra

  7. MAC for MANET • Special requirements • Avoid interferences among simultaneous transmissions • Yet, enable as many non-interfering transmissions as possible • Fairness among transmissions • No centralized coordinators, should function in full distributed manner • No clock synchronization, asynchronous operations Prasant Mohapatra

  8. Carrier-Sensing in MANET • Problems • Hidden terminal problem • Exposed terminal problem Prasant Mohapatra

  9. MACs Suitable for MANET • MACA [Karn’90] • Propose to solve hidden terminal problem by RTS/CTS dialog • MACAW [Bharghavan’94] • Increase reliability by RTS/CTS/DATA/ACK dialog • IEEE 802.11 [IEEE 802.11WG] • Distributed and centralized MAC components • Distributed Coordination Function (DCF) • Point Coordination Function (PCF) • DCF suitable for multi-hop ad hoc networking • Also use RTS/CTS/DATA/ACK dialog Prasant Mohapatra

  10. IEEE 802.11 DCF • Uses RTS-CTS exchange to avoid hidden terminal problem • Any node overhearing a CTS cannot transmit for the duration of the transfer • Any node receiving the RTS cannot transmit for the duration of the transfer • To prevent collision with ACK when it arrives at the sender • Uses ACK to achieve reliability Prasant Mohapatra

  11. IEEE 802.11 DCF • CSMA/CA • Contention-based random access • Collision detection not possible while a node is transmitting • Carrier sense in 802.11 • Physical carrier sense • Virtual carrier sense using Network Allocation Vector (NAV) • NAV is updated based on overheard RTS/CTS packets, each of which specified duration of a pending Data/Ack transmission • Collision avoidance • Nodes stay silent when carrier sensed busy (physical/virtual) • Backoff intervals used to reduce collision probability Prasant Mohapatra

  12. Disadvantage of IEEE 802.11 DCF • 802.11 DCF not considered perfect for MANET • High power consumption • Hidden terminal problem not totally solved, exposed terminal problem not solved • Cause fairness problem among different transmitting nodes • Can only provide best-effort service • Active research area in MAC for MANET Prasant Mohapatra

  13. MAC: Advanced Topics • Support for QoS provisioning • Power Efficiency • MAC for Directional Antenna Prasant Mohapatra

  14. QoS-aware MAC protocols • IEEE 802.11 Real-time extension • Black burst contention scheme • MACA/PR (Multihop Access Collision Avoidance with Piggyback Reservation) Prasant Mohapatra

  15. Extend 802.11 DCF for Service Differentiation [Campbell’01] • For high priority packets • Backoff interval in [0,CWh] • For low priority packet • Backoff interval in [CWh+1, CW] • After collision, packet with smaller CW is more likely to occupy medium earlier Prasant Mohapatra

  16. Black Burst [Sobrinho’99] • Provides differentiation among real-time flow and best-effort flow • Provides fairness and priority scheduling among real-time flows • Fully distributed Prasant Mohapatra

  17. Black Burst A C B Media Busy A B C Carrier Sense Prasant Mohapatra

  18. Black Burst A B C DIFS Media Busy A B C Prasant Mohapatra Black Bursts

  19. Black Burst A B C DIFS Media Busy A B C Prasant Mohapatra

  20. Black Burst A B C DIFS Media Busy A B C Prasant Mohapatra

  21. Black Burst A B C Media Busy A B C Prasant Mohapatra

  22. Black Burst A B C DIFS RT A Media Busy RT A A B C Prasant Mohapatra

  23. Black Bursts • All nodes begin the priority contention phase together • Higher priority node transmit a longer burst than low priority node • After transmitting its burst, a node listens to the channel • If channel still busy, the node has lost contention to a higher priority node Prasant Mohapatra

  24. MAC: Advanced Topics • Power Efficiency • Power Saving: Make wireless interface sleep at appropriate times • Power Control: Use the appropriate transmission power Prasant Mohapatra

  25. Power Saving • Power Consumptions From Spec. of Orinoco 11b WLAN PC Card [Proxim Co. 2003] Battery Voltage: 5V Dose mode: 9 mA (= 45 mW) Receiver mode: 185 mA (= 925 mW) Transmit mode: 285 mA (= 1425 mW) Prasant Mohapatra

  26. MAC Layer Approach • Basic idea: turn on/off the radio of specific nodes at appropriate times • IEEE 802.11 • PAMAS • S-MAC • STEM • Asynchronous Wakeup patterns Prasant Mohapatra

  27. PS Mode in WLANs • ATIM (Ad hoc Traffic Indication Map) window : short interval during which the PS hosts wake up periodically. • Assume that hosts are fully connected and synchronized. • In the beginning of each ATIM window, each mobile host will contend to send a beacon frame. • Successful beacon serve for synchronizing mobile host’s clock. • This beacon also inhibits other hosts from sending their beacon • To avoid collisions among beacons, use random back-off [0-2*CWmin–1] Prasant Mohapatra

  28. PS Mode in WLANs • After the beacon, host can send a direct ATIM frame to each of its intended receivers in PS mode. • After transmitted an ATIM frame, keep remaining awake • On reception of the ATIM frame, reply with an ACK and remain active for the remaining period • - Data is sent based on the normal DCF access. Prasant Mohapatra

  29. Problems • PS mode of 802.11 is designed for single hop (fully connected) ad hoc network. • If applied for multi-hop • Clock synchronization • Communication delay and mobility are all unpredictable • network merging • Neighbor discovery • When a host in PS mode, both its chance to transmit and to hear other’s signal is reduced ==> inaccurate neighbor information • Network partitioning • Inaccurate neighbor information may lead to long packet delay or even network (logically) partitioning problem. Prasant Mohapatra

  30. PAMAS [Singh’98] • A node avoids overhearing packets not addressed to it • so, reduce power consumption of processing unnecessary packets • Use of a separate channel for signaling Prasant Mohapatra

  31. PAMAS • When to turn off? • when a node has no packets to send, it should power itself off if a neighbor starts transmitting • if at least one neighbor is transmitting and another is receiving, the node should power itself off • How long to be powered off? • It knows the duration of other’s transmission • What if the intended receiver is powered off? • Have to wait for it to wake up Prasant Mohapatra

  32. STEM [Schurgers’02] • Sparse Topology and Energy Management • Basic idea: to wake up nodes only when they need to forward data • using asynchronous beacon packets in a separate control channel to wake up nodes • latency is traded off for energy savings Prasant Mohapatra

  33. Wakeup mechanisms • On-demand wake-up • STEM, Remote Activated Switch(RAS) • Scheduled rendezvous • 802.11, Bluetooth, etc • Asynchronous wakeup • Power saving protocols [Tseng’02] • Asynchronous wake-up [Zheng’03] Prasant Mohapatra

  34. MANET Power Saving Protocols [Tseng’02] • Three asynchronous wakeup patterns • Dominating-awake-interval • Periodically-fully-awake-interval • Quorum-based Prasant Mohapatra

  35. Tseng’s Protocol • Beacon interval • For each PS host, it divides its time axis into a number of fixed length interval • Active window • On state • Beacon window • PS hosts send its beacon • MTIM window • Other hosts send their MTIM frames to the PS host. • Excluding these three windows, PS host with no packet to send or receive may go to the sleep mode. Prasant Mohapatra

  36. Dominating-Awake-Interval • Dominating awake property • AW >= BI/2 + BW • This guarantees any PS host’s beacon window to overlap with any neighboring PS host’s active window. • In every two beacon interval, PS host can receive all its neighbor’s beacon  short response timesuitable for highly mobile • The sequence of beacon intervals are alternatively labeled as odd and even interval Prasant Mohapatra

  37. Periodically–fully-awake-interval • Two types of beacon interval • Low power intervals • AW is reduced to the minimum • PS host send out its beacon to inform others its existence • Fully awake intervals • AW is extended to the maximum • Arrives periodically every T intervals • PS hosts discover who are in its neighborhood, and can predict when its neighboring host will wake up. Prasant Mohapatra

  38. Quorum-based • PS host only picks 2n-1 intervals (one column and one row) out of the n x n quorum • Quorum interval • Beacon + MTIM, AW = BI • Non quorum intervals • Start with an MTIM window, after that, host may go to sleep mode, AW=MW Prasant Mohapatra

  39. Asynchronous Wakeup: Formalized[Zheng’03] • Formalize the “asynchronous wakeup” schedule as a block design problem in combinatorics • Give theoretical analysis and an optimal design • Three components: • neighbor discovery • neighbor prediction • neighbor reservation Prasant Mohapatra

  40. Slot Assignments 124 235 346 457 561 672 713 SLOTS 1 2 3 4 5 6 7 Slot assignment for (7,3,1) design: The schedule repeats after 7 slots, has three “ON” slots, and any two schedules overlap at least 1 slot. Prasant Mohapatra

  41. MAC for Directional Antenna • Benefits of Directional Antenna • More spatial reuse • With omni-directional antenna, packets intended to one neighbor reaches all neighbors as well • Increase “range”, keeping transmit power constant • Reduce transmit power, keeping range comparable with omni mode • Reduces interference, potentially increasing spatial reuse Prasant Mohapatra

  42. More Spatial Reuse Omni-directional antenna Directional antenna A B A B C D C D Both A and C can transmit simultaneously While A is transmitting to B, C cannot transmit to D Prasant Mohapatra

  43. Antenna Model 2 Operation Modes: OmniandDirectional A node may operate in any one mode at any given time Prasant Mohapatra

  44. Antenna Model In Omni Mode: • Nodes receive signals with gain Go • While idle a node stays in omni mode In Directional Mode: • Capable of beamforming in specified direction • Directional Gain Gd(Gd > Go) Symmetry: Transmit gain = Receive gain Prasant Mohapatra

  45. Directional Packet Transmission B A D-O transmission B’s omni receive range D-D transmission A B B’s directional receive beam Prasant Mohapatra

  46. MAC Designs for Directional Antenna • Most proposals use RTS/CTS dialog • They differ in how RTS/CTS are transmitted • Omni-directional transmit: ORTS, OCTS • Directional transmit: DRTS, DCTS • Current proposals: • ORTS/OCTS [Nasipuri’00] • DRTS/OCTS [Ko’00] • DRTS/DCTS [Choudhury’02] Prasant Mohapatra

  47. ORTS/OCTS • Sender sends omni-directional RTS • Receiver sends omni-directional CTS • Receiver also records direction of sender by determining the antenna on which the RTS signal was received with highest power level • Similarly, the sender, on receiving CTS, records the direction of the receiver • All nodes overhearing RTS/CTS defer transmissions • Sender then sends DATA directionally to the receiver • Receiver sends directional ACK Prasant Mohapatra

  48. ORTS/OCTS – cont. • Protocol takes advantage of reduction in interference due to directional transmission/reception of DATA • All neighbors of sender/receiver defer transmission on receiving omni-directional RTS/CTS  spatial reuse benefit not realized Prasant Mohapatra

  49. D-MAC • Uses directional antenna for sending RTS, DATA and ACK in a particular direction, whereas CTS sent omni-directionally • Directional RTS (DRTS) andOmni-directional CTS (OCTS) Prasant Mohapatra

  50. DMAC: DRTS/OCTS A B C E D DRTS(B) OCTS(B,C) OCTS(B,C) DRTS(D) OCTS(D,E) DATA DATA ACK ACK Prasant Mohapatra

More Related