Is IEEE 802.11 TSF Scalable?

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Is IEEE 802.11 TSF Scalable?. L. Huang, T.H. Lai, On the scalability of IEEE 802.11 ad hoc networks , MobiHoc 2002. IEEE 802.11: how large can it be?. Bandwidth: Up to 54 Mbps Good for a few hundred nodes Timing Synchronization Function Not scalable How to fix it?. - PowerPoint PPT Presentation

Transcript of Is IEEE 802.11 TSF Scalable?

  • Is IEEE 802.11 TSF Scalable?L. Huang, T.H. Lai, On the scalability of IEEE 802.11 ad hoc networks, MobiHoc 2002.

  • IEEE 802.11: how large can it be?Bandwidth: Up to 54 MbpsGood for a few hundred nodesTiming Synchronization FunctionNot scalableHow to fix it?

  • 802.11s Time Sync Function (I)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.windowbeacon interval

  • 802.11s Time Sync Function (II)Beacon contains a timestamp.On receiving a beacon, STA adopts beacons timing if T(beacon) > T(STA).Clocks move only forward.

    faster adopts12:0112:00slower not adopts 12:01 12:0212:01

  • Problems with 802.11s TSFFaster clocks synchronize slower clocks.Equal opportunity for nodes to generate beacons.1:101:111:121:131:141:151:131:131:131:131:141:151:161:171:181:191:211:231:181:181:181:191:211:23+3+4+5+6+7+8+3+4+5+6+7+81:211:221:231:251:281:311:231:231:231:251:281:31

  • The Out-of-Sync ProblemWhen number of stations increases

    Fastest station sends beacons less frequently

    Stations out of synchronization

  • Two Types of Out-of-SyncFastest-station out-of-sync fastest station is out of sync with all others.k-global out-of-sync k percent of the n(n-1)/2 links/pairs are out of sync.Questions: How often? For how long?

  • 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.If there is no beacon from fastest station in /(d*T) intervals, fastest-station out of sync occur.T

  • Fastest-station out-of-sync (2)How often may it occur? Once occurs, how long may it last?H = # beacon intervals with F.S. out-of-sync.L = # beacon intervals between async periods.E(H) = ? E(L)?


  • Fastest-station out-of-sync (3)n = number of stations. W + 1 = size of beacon window.P = P(n,w) = prob(fastest station wins beacon contention)

    W + 10 1 w

  • p = P(n,W) = ?kw 0P(n,W,k) = prob(F.S. succeeds | it sends at slot k)

  • P(n,w,k) = prob(F.S. succeeds | it sends at k) = ?kwkwi i+b-1b0x2 yslot#stationsn-x-yn-1 stations0 x2 #stationsn-x-yyP(n-x-y, w-i-b, k-i-b)

  • P(n,W,k) = ?

  • E(H) = ?H = # beacon intervals with F.S. out-of-sync.


  • E(L) = ?ei: F.S. sends another beacon after i intervals.

    LHi = /(d*T)

  • E(L) = ?

  • Prob(Fastest station sends a beacon)

  • How often does fastest-node get out of sync with others?

  • Percentage of time fastest station out of sync with all others802.11a54 Mbps = 224 s d = 0.003%

  • How often does 25%-async occur?

  • Percentage of time with 25 percent of links out-of-sync 802.11a54 Mbps = 224 s d = 0.01%

  • How to fix it? Desired properties: simple, efficient, and compatible with current 802.11 TSF.Causes of out-of-syncUnidirectional clocks Equal beacon opportunity Single beacon per intervalBeacon contention (collision)

  • Improve fastest stations chance

    Let the fastest station contend for beacon generation more frequently than others.

  • 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):xfasterC(x) +1xslowerC(x) -1

  • Once the protocol convergesFastest station, C(x) =1 Other stations, C(x) = Cmax (Cmax= ?)

  • What if the fastest node leaves the IBSS?

    The previously second fastest now becomes the fastest. Its C(x) will decrease to 1.

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

  • Compatible with current TSFSuppose some nodes do not implement the new protocol.

  • Performance802.11 Performance of TSFATSP ATSP.pdf

  • Performance of TSF

  • Performance of ATSP

  • SummaryShowed: the IEEE 802.11 Timing Sync Function (TSF) is not scalable.

    Proposed: a simple remedy compatible with the current TFS.

  • Whats Next?Its 25 s, stupid!How to deal with MANET?transmission range