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.

1:10

1:11

1:12

1:13

1:14

1:15

1:13

1:13

1:13

1:13

1:14

1:15

1:16

1:17

1:18

1:19

1:21

1:23

1:18

1:18

1:18

1:19

1:21

1:23

+3

+4

+5

+6

+7

+8

+3

+4

+5

+6

+7

+8

1:21

1:22

1:23

1:25

1:28

1:31

1:23

1:23

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.

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 = P(n,W) = ?

k w

0

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

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)

P(n,W,k) = ?

E(H) = ?

H = # beacon intervals with F.S. out-of-sync.

LH

E(L) = ?

ei: F.S. sends another beacon after i intervals.

LH

i

τ = /(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 others

802.11a54 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-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

Once the protocol converges

Fastest 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 TSF

Suppose some nodes do not implement the new protocol.

Performance

802.11 Performance of TSF ATSP ATSP.pdf

Performance of TSF

Performance of ATSP

Summary

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

Proposed: a simple remedy compatible with the current TFS.

What’s Next?

It’s 25 μs, stupid!

How to deal with MANET?

transmission range