is ieee 802.11 tsf scalable? l. huang, t.h. lai, on the scalability of ieee 802.11 ad hoc networks,...
TRANSCRIPT
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?
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
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
Problems with 802.11’s TSF
Faster clocks synchronize slower clocks. Equal opportunity for nodes to generate beacons.
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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.
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)?
LH
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 + 1
0 1 w
P(n,w,k) = prob(F.S. succeeds | it sends at k) = ?
k w
k wi
i+b-1
b
0
x≥2 y
slot
#stations n-x-y
n-1 stations
0
x≥2 #stations n-x-y y
P(n-x-y, w-i-b, k-i-b)
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)
Improve fastest station’s 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):
x
faster C(x) +1x
slower C(x) -1
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.
Summary
Showed: the IEEE 802.11 Timing Sync Function (TSF) is not scalable.
Proposed: a simple remedy compatible with the current TFS.