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CMPE 257: Wireless and Mobile Networking SET 3p:

CMPE 257: Wireless and Mobile Networking SET 3p:. Medium Access Control Protocols. MAC Protocol Topics. Time synchronization Power saving. IEEE 802.11 Time Sync. Function. Bandwidth: Up to 54 Mbps Good for a few hundred nodes Time Synchronization Function (TSF) Not scalable

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CMPE 257: Wireless and Mobile Networking SET 3p:

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  1. CMPE 257: Wireless and Mobile NetworkingSET 3p: Medium Access Control Protocols UCSC CMPE252B

  2. MAC Protocol Topics • Time synchronization • Power saving CMPE257 UCSC

  3. IEEE 802.11 Time Sync. Function • Bandwidth: • Up to 54 Mbps • Good for a few hundred nodes • Time Synchronization Function (TSF) • Not scalable • How to fix it? • Note: Only single-hop ad hoc networks are dealt with here ([HL02]). CMPE257 UCSC

  4. IEEE 802.11 TSF • Time divided into beacon intervals, each containing a beacon generation window. • Each station: • Waits for a random number of slots; • transmits a beacon (if no one else has done so). • Beacon: several slots in length. beacon interval window CMPE257 UCSC

  5. IEEE 802.11 TSF • Beacon contains a timestamp. • On receiving a beacon, STA adopts beacon’s timing if T(beacon) > T(STA). • Clocks move only forward. 12:01 12:02 12:01 12:00 12:01 faster slower adopts not adopts CMPE257 UCSC

  6. Problems with 802.11’s TSF • Faster clocks synchronize slower clocks. • Equal opportunity for nodes to generate beacons. 1:16 1:17 1:18 1:19 1:21 1:23 1:21 1:22 1:23 1:25 1:28 1:31 1:18 1:18 1:18 1:19 1:21 1:23 1:23 1:23 1:23 1:25 1:28 1:31 1:10 1:11 1:12 1:13 1:14 1:15 1:13 1:13 1:13 1:13 1:14 1:15 +3 +4 +5 +6 +7 +8 +3 +4 +5 +6 +7 +8 CMPE257 UCSC

  7. The Out-of-Sync Problem When number of stations increases Fastest station sends beacons less frequently Stations out of synchronization CMPE257 UCSC

  8. Two Types of Out-of-Sync • Fastest-station out-of-sync– fastest station is out of sync with all others. • k-global out-of-sync– k percent of links are out of sync. • Questions: How often? For how long? CMPE257 UCSC

  9. Fastest-station out-of-sync (1) • Clock1 and Clock2: two fastest clocks • d = their difference in accuracy • T = length of beacon interval (0.1 sec.) • Clock drift: d*T per beacon interval. • In /(d*T) intervals, fastest-station will be out of sync with all others. T CMPE257 UCSC

  10. Fastest-station out-of-sync (2) • n = number of stations. w = size of beacon window. • P’(n,w) = prob(fastest station wins beacon contention) CMPE257 UCSC

  11. Prob(Fastest station sends a beacon) CMPE257 UCSC

  12. Fastest-station out-of-sync (3) • H = # beacon intervals with F.S. out-of-sync. • L = # beacon intervals between async periods. • E(R) = E(H)/[E(H)+E(L)] = percent of time in which the fastest station is out of sync with all others. H L CMPE257 UCSC

  13. How often does fastest-node get out of sync with others? CMPE257 UCSC

  14. Percentage of time fastest station out of sync with all others 802.11a 54 Mbps ∆ = 224 s d = 0.003% CMPE257 UCSC

  15. How often does 25%-async occur? CMPE257 UCSC

  16. Percentage of time with 25 percent of links out-of-sync 802.11a 54 Mbps ∆ = 224 s d = 0.01% CMPE257 UCSC

  17. How to fix it? • Desired properties: simple, efficient, and compatible with current 802.11 TSF. • Causes of out-of-sync • Unidirectional clocks • Equal beacon opportunity • Single beacon per interval • Beacon contention (collision) CMPE257 UCSC

  18. Improve fastest station’s chance • Let the fastest station contend for beacon generation more frequently than others. CMPE257 UCSC

  19. Adaptive Clock Sync Protocol • Station x participates in beacon contention once every C(x) intervals. • Initially, C(x) =1. Always, 1 < C(x) < Cmax. • Dynamically adjust C(x): x x C(x) +1 faster C(x) -1 slower CMPE257 UCSC

  20. Once the protocol converges Fastest station, C(x) =1 Other stations, C(x) = Cmax (Cmax= ?) CMPE257 UCSC

  21. What if the fastest node leaves the IBSS? • The previously second fastest now becomes the fastest. Its C(x) will decrease to 1. CMPE257 UCSC

  22. What if a new fastest node enters the IBSS? • The previously fastest now no longer the fastest. Its C(x) will increase to Cmax. CMPE257 UCSC

  23. Performance • 802.11 Performance of TSF • ATSP Performance of ATSP • TATSP Performance of Modified TSF CMPE257 UCSC

  24. Modified TSF • Divide stations into three groups: • Group 1: C(x) = Cmax1 = 1 • Group 2: C(x) = Cmax2 = a small number • Group 3: C(x) = Cmax3 = a large number CMPE257 UCSC

  25. Performance of TSF CMPE257 UCSC

  26. Performance of ATSP CMPE257 UCSC

  27. Performance of Modified TSF CMPE257 UCSC

  28. Summary • Showed: the IEEE 802.11 Timing Sync Function (TSF) is not scalable. • Proposed: a simple remedy compatible with the current TFS. • Choice of Cmax? CMPE257 UCSC

  29. What’s Next? • IBSS: single-hop • MANET: multi-hop transmission range CMPE257 UCSC

  30. Comments • Need simulations with data traffic • Some data transmissions may go beyond the Target Beacon Transmission Time (TBTT) • More realistic analysis • Nodes may be still in defer state when in beacon window time: independent, uniform assumption doesn’t hold. CMPE257 UCSC

  31. Power Saving Protocols • Various aspects of solution for saving power • Transmission power control • Power aware routing • Low-power mode • Power saving modes in IEEE 802.11 • Active mode • Power saving mode (PS) • Protocols under Infrastructure and ad hoc network are different CMPE257 UCSC

  32. Power Saving at MAC Layer Beacon interval awake sleep Beacon window ATIM window CMPE257 UCSC

  33. Challenges • MANET (Mobile ad hoc networks) • Multi-hop, unpredictable mobility, no plug-in power, no clock synchronization • Clock synchronization • Radio interference • Variable packet delay (unpredictable mobility) • Lack of central control • Neighbor discovery • Because PS host will reduce its transmitting/receiving activity • Routing problem • Network partitioning and merging CMPE257 UCSC

  34. Design guidelines • More beacon • A PS host should not inhibit its beacon in ATIM window even if it has heard other beacons • Inaccurate-neighbor problem prevention • Multiple beacon in a ATIM window • Overlapping Awake interval • No clock synchronization • Overlapping of Wake-up pattern of two PS host • Wake-up prediction • PS host’s wake-up pattern based on their time difference CMPE257 UCSC

  35. Infrastructure and Ad Hoc Protocols • Access Point (AP) monitors each host • PS mode host wakes up periodically for incoming packet from AP. • Periodic beacon frames. • In each beacon frame, a Traffic Indication Map(TIM) will be delivered, which contains ID’s of those PS host with buffered unicast packet in the AP. • PS hosts wakes up periodically • ATIM window : short interval that PS hosts wake up • In the beginning of each ATIM window, each mobile host will contend to transmit a beacon frame. • Successful beacon synchronizes mobile host’s clock and prevents other hosts from sending their beacon CMPE257 UCSC

  36. Dominating-awake-interval • PS host stay awake sufficiently long so as to ensure that neighboring host can know each other. • Dominating awake property • AW >= BI/2 + BW • Alternatively labeled odd and even sequence of beacon intervals CMPE257 UCSC

  37. Periodically–fully-awake-interval • Two types of beacon interval • Low power intervals • Length of active window is reduced to minimum • Starts with an active window which contains a beacon window followed by a MTIM window AW = BW + MW, in the rest of the time , the host can go to the sleep mode. • Fully awake intervals • Length of active window is extended to the maximum • Arrives periodically, interleaved between low power intervals • AW = BI, rest of the time must remain awake • Suitable for slowly mobile environments CMPE257 UCSC

  38. T (=3) Beacon Interval Host A Host B Beacon Window MTIM Window Periodically–fully-awake-interval Rest of active window CMPE257 UCSC

  39. Properties • Each PS host’s beacon window overlaps with any neighbor’s fully-awake intervals in every T beacon intervals. • More power saving than previous protocol when T > 2. • Remark: Every node chooses the same T. CMPE257 UCSC

  40. Quorum-based • Quorum • A set of identities one need to obtain before doing sth. • Two quorums have non-empty intersection to ensure atomicity. 1 4 16 1 4 16 1 2 3 4 • 2 3 4 • 6 7 8 • 10 11 12 • 13 14 15 16 5 6 7 8 • 10 11 12 • 13 14 15 16 CMPE257 UCSC

  41. Quorum-based • PS host only needs to send beacon O(1/n) of all the beacon intervals • Quorum interval • Beacon + MTIM, AW = BI • Non quorum intervals • Starts with an MTIM window, after that, host may go to sleep mode, AW=MW • Amount of awaking time is less than 50%, provided n >=4 • Suitable for expensive transmission cost CMPE257 UCSC

  42. Communication with PS hosts • Unicast • Predict PS host’s wakeup time and send MTIM packet during that time • Broadcast • Divide them into groups • Hosts within the same group have overlapping MTIM window • Need multiple transmissions CMPE257 UCSC

  43. Summary • Three power saving protocol for asynchronous MANETs: • Dominating awake interval • Most power consumption, • Lowest neighbor discovery time. • Periodically-fully-awake interval • Balance both power consumption and neighbor discovery time. • Quorum based • The most power saving • Longest neighbor discovery time CMPE257 UCSC

  44. Comments on Simulations • A custom-built simulator • Many details omitted • Carrier sensing and transmission range • Star-topology • Packet delivery delay (tradeoff?) CMPE257 UCSC

  45. Future Work • More MANET scenarios • Adaptive beacon intervals? CMPE257 UCSC

  46. References • [HL02] Lifei Huang and Ten-Hwang Lai, On the Scalability of IEEE 802.11 Ad Hoc Networks, in ACM MobiHoc 2002. • [THH02] Tseng et al., Power-Saving Protocols for IEEE 802.11-Based Multi-Hop Ad Hoc Networks, in IEEE INFOCOM 2002. CMPE257 UCSC

  47. Acknowledgments • Parts of the presentation are adapted from the following sources: • Moses Pawar, USC, http://www.isi.edu/~weiye/teaching/cs558sm04/slides/Mac_protocols.ppt • Ten H. Lai, Ohio State University • http://www.cse.ohio-state.edu/~lai/788-Au03/2-scalibility.pdf • http://www.cse.ohio-state.edu/~lai/788-Au03/4-Power%20Saving.ppt CMPE257 UCSC

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