Is IEEE 802 11 Scalable IEEE 802 11
Is IEEE 802. 11 Scalable?
IEEE 802. 11: how large can it be? n Bandwidth: • • n Up to 54 Mbps Good for a few hundred nodes Timing Synchronization Function • Not scalable
Basic Service Set (BSS) BSS: building block of 802. 11 LAN n Infrastructure BSS n Independent BSS (IBSS) -- Ad Hoc n AP Infrastructure BSS IBSS
802. 11 Timers (Clocks) n n n Timer: 64 bits, ticking in microseconds. Accuracy: within + 0. 01%, or +100 ppm. Time synchronization needed for: n n n Frequency hopping Power management ∆ = max tolerable difference between clocks.
Frequency Hopping f 1 f 2 f 3 f 4 f 5
Power Saving Beacon interval sleep Beacon window ATIM window time
802. 11’s Time Sync Function (I) Time divided into beacon intervals, each containing a beacon generation window. n Each station: n waits for a random number of slots; n transmits a beacon (if no one else has done so). n n Beacon: several slots in length. beacon interval window
802. 11’s Time Sync Function (II) Beacon contains a timestamp. n On receiving a beacon, STA adopts beacon’s timing if T(beacon) > T(STA). n Clocks move only forward. n 12: 01 12: 00 12: 01 12: 02 faster adopts slower not adopts
Problems with 802. 11’s TSF Faster clocks synchronize slower clocks. n Equal opportunity for nodes to generate beacons. n 1: 10 1: 11 1: 12 1: 13 1: 13 +3 1: 14 1: 15 +7 +4 +5 +6 +8 1: 16 1: 17 1: 18 1: 19 1: 18 1: 19 +3 1: 21 1: 23 +7 +4 +5 +6 +8 1: 21 1: 22 1: 23 1: 25 1: 23 1: 25 1: 28 1: 31
The Out-of-Sync Problem When number of stations increases Fastest station sends beacons less frequently Stations out of synchronization
Two Types of Out-of-Sync Fastest-station out-of-sync – fastest station is out of sync with all others. n k-global out-of-sync – k percent of links are out of sync. n Questions: How often? For how long? n
Fastest-station out-of-sync (1) Clock 1 and Clock 2: two fastest clocks n d = their difference in accuracy n T = length of beacon interval (0. 1 sec. ) n Clock drift: d*T per beacon interval. n In /(d*T) intervals, fastest-station will be out of sync with all others. n T
Fastest-station out-of-sync (2) n n n = number of stations. w = size of beacon window. P’(n, w) = prob(fastest station wins beacon contention)
Prob(Fastest station sends a beacon)
Fastest-station out-of-sync (3) H = # beacon intervals with F. S. out-of-sync. n L = # beacon intervals between async periods. n E(R) = E(H)/[E(H)+E(L)] = percent of time in which the fastest station is out of sync with all others. n H L
How often does fastest-node get out of sync with others?
Percentage of time fastest station out of sync with all others 802. 11 a 54 Mbps ∆ = 224 s d = 0. 003%
Global Asynchronism Lower bound on E(H) n Upper bound on E(L) n H L
How often does 25%-async occur?
Percentage of time with 25 percent of links out-of-sync 802. 11 a 54 Mbps ∆ = 224 s d = 0. 01%
Percentage of channels out of sync
How to fix it? n Desired properties: simple, efficient, and compatible with current 802. 11 TSF. n Causes of out-of-sync n n Unidirectional clocks Equal beacon opportunity Single beacon per interval Beacon contention (collision) Prob < 1 n
Improve fastest station’s chance n 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. n Initially, C(x) =1. Always, 1 < C(x) < Cmax. n Dynamically adjust C(x): n x faster C(x) +1 x slower C(x) -1
Once the protocol converges Fastest station, C(x) =1 Other stations, C(x) = Cmax (Cmax= ? )
What if the fastest node leaves the IBSS? n The previously second fastest now becomes the fastest. Its C(x) will decrease to 1.
What if a new fastest node enters the IBSS? n The previously fastest now no longer the fastest. Its C(x) will increase to Cmax.
Compatible with current TSF n Suppose some nodes do not implement the new protocol.
Performance of Modified TSF 802. 11 Performance of TSF n ATSP. pdf n TATSP Performance of Modified TSF n
Performance of TSF
Performance of Modified TSF
Summary n Showed: the IEEE 802. 11 Timing Sync Function (TSF) is not scalable. n Proposed: a simple remedy compatible with the current TFS.
What’s Next? n IBSS: single-hop n MANET: multihop ? ? transmission range
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