is ieee 802.11 tsf scalable?
DESCRIPTION
Is IEEE 802.11 TSF Scalable?. 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?. 802.11’s Time Sync Function (I). - PowerPoint PPT PresentationTRANSCRIPT
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Is IEEE 802.11 TSF Scalable?
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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?
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802.11’s 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.
window
beacon interval
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802.11’s Time Sync Function (II)
Beacon contains a timestamp. On receiving a beacon, STA adopts beacon’s
timing if T(beacon) > T(STA). Clocks move only forward.
faster adopts
12:01 12:00
slower not adopts
12:01 12:0212:01
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Problems with 802.11’s TSF
Faster clocks synchronize slower clocks. Equal opportunity for nodes to generate beacons.
1:101:111:121:131:141:15
1:131:131:131:131:141:15
1:161:171:181:191:211:23
1:181:181:181:191:211:23
+3
+4
+5
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+8
+3
+4
+5
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+8
1:211:221:231:251:281:31
1:231:231:231:251:281:31
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The Out-of-Sync Problem
When number of stations increases
Fastest station sends beacons less frequently
Stations out of synchronization
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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?
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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
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Fastest-station out-of-sync (2)
n = number of stations. w = size of beacon window. P’(n,w) = prob(fastest station wins beacon contention)
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Prob(Fastest station sends a beacon)
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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.
LH
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How often does fastest-node get out of sync with others?
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Percentage of time fastest station out of sync with all others
802.11a54 Mbps∆ = 224 s d = 0.003%
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How often does 25%-async occur?
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Percentage of time with 25 percent of links out-of-sync
802.11a54 Mbps∆ = 224 s d = 0.01%
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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)
1nProb <
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Improve fastest station’s chance
Let the fastest station contend for beacon generation more frequently than others.
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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
faster C(x) +1x
slower C(x) -1
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Once the protocol converges
Fastest station, C(x) =1
Other stations, C(x) = Cmax (Cmax= ?)
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What if the fastest node leaves the IBSS?
The previously second fastest now becomes the fastest. Its C(x) will decrease to 1.
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What if a new fastest node enters the IBSS?
The previously fastest now no longer the fastest. Its C(x) will increase to Cmax.
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Compatible with current TSF
Suppose some nodes do not implement the new protocol.
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Performance
802.11 Performance of TSF ATSP ATSP.pdf TATSP Performance of Modified TSF
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Performance of TSF
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Performance of ATSP
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Performance of Modified TSF
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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
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Summary
Showed: the IEEE 802.11 Timing Sync Function (TSF) is not scalable.
Proposed: a simple remedy compatible with the current TFS.
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What’s Next?
IBSS: single-hop MANET: multihop
transmission range