Is IEEE 802 11 TSF Scalable L Huang

Is IEEE 802. 11 TSF Scalable? L. Huang, T. H. Lai, On the scalability of IEEE 802. 11 ad hoc networks, Mobi. Hoc 2002.

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 How to fix it?

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 the n(n-1)/2 links/pairs 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 If there is no beacon from fastest station in /(d*T) intervals, fastest-station out of sync occur. n T

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

Fastest-station out-of-sync (3) n = number of stations. n W + 1 = size of beacon window. n P = P(n, w) = prob(fastest station wins beacon contention) n 01 w W+1

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

P(n, w, k) = prob(F. S. succeeds | it sends at k) =? k 0 w b slot 0 i i+b-1 n-1 stations k #stations x≥ 2 y n-x-y #stations x≥ 2 n-x-y P(n-x-y, w-i-b, k-i-b) y w

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

E(L) = ? n ei: F. S. sends another beacon after i intervals. τ = /(d*T) i H 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 others 802. 11 a 54 Mbps ∆ = 224 s d = 0. 003%

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%

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)

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