Is IEEE 802.11 TSF Scalable?

1
Is IEEE 802.11 TSF Scalable?

• L. Huang, T.H. Lai,
• On the scalability of IEEE 802.11 ad hoc
  networks, MobiHoc 2002.

2
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?

3
802.11s 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.

beacon interval
window
4
802.11s Time Sync Function (II)

• Beacon contains a timestamp.
• On receiving a beacon, STA adopts beacons timing
  if T(beacon) gt T(STA).
• Clocks move only forward.

1201
1202
1201
1200
1201
faster
slower
adopts
not adopts
5
Problems with 802.11s TSF

• Faster clocks synchronize slower clocks.
• Equal opportunity for nodes to generate beacons.

116 117 118 119 121 123
118 118 118 119 121 123
121 122 123 125 128 131
123 123 123 125 128 131
110 111 112 113 114 115
113 113 113 113 114 115
3 4 5 6 7 8
3 4 5 6 7 8
6
The Out-of-Sync Problem

• When number of stations increases
• Fastest station sends beacons less frequently
• Stations out of synchronization

7
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?

8
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 dT per beacon interval.
• If there is no beacon from fastest station in
  ?/(dT) intervals, fastest-station out of sync
  occur.

T
9
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)?

L
H
10
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)

0 1
w
? ? ?
W 1
11
p P(n,W) ?
P(n,W,k) prob(F.S. succeeds it sends at slot
k)
k
w
0
? ? ?
? ? ?
12
P(n,w,k) prob(F.S. succeeds it sends at k)
?
k
w
0
? ? ?
? ? ?
b
n-1 stations
k
w
i
ib-1
0
slot
? ? ?
? ? ?
? ? ?
? ? ?
stations
x2 y
n-x-y
? ? ?
? ? ?
? ? ?
? ? ?
stations
y
x2
n-x-y
P(n-x-y, w-i-b, k-i-b)
13
P(n,W,k) ?
14
E(H) ?

• H beacon intervals with F.S. out-of-sync.

L
H
15
E(L) ?

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

t ?/(dT)
i
L
H
16
E(L) ?
17
Prob(Fastest station sends a beacon)
18
How often does fastest-node get out of sync with
others?
19
Percentage of time fastest station out of sync
with all others
802.11a 54 Mbps ? 224 ?s d 0.003
20
How often does 25-async occur?
21
Percentage of time with 25 percent of links
out-of-sync
802.11a 54 Mbps ? 224 ?s d 0.01
22
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)

23
Improve fastest stations chance

• Let the fastest station contend for beacon
  generation more frequently than others.

24
Adaptive Clock Sync Protocol

• Station x participates in beacon contention once
  every C(x) intervals.
• Initially, C(x) 1. Always, 1 lt C(x) lt Cmax.
• Dynamically adjust C(x)

x
x
C(x) 1
faster
C(x) -1
slower
25
Once the protocol converges
Fastest station, C(x) 1
Other stations, C(x) Cmax (Cmax
?)
26
What if the fastest node leaves the IBSS?

• The previously second fastest now becomes the
  fastest. Its C(x) will decrease to 1.

27
What if a new fastest node enters the IBSS?

• The previously fastest now no longer the fastest.
  Its C(x) will increase to Cmax.

28
Compatible with current TSF

• Suppose some nodes do not implement the new
  protocol.

29
Performance

• 802.11 Performance of TSF
• ATSP ATSP.pdf

30
Performance of TSF
31
Performance of ATSP
32
Summary

• Showed the IEEE 802.11 Timing Sync Function
  (TSF) is not scalable.
• Proposed a simple remedy compatible with the
  current TFS.

33
Whats Next?

• Its 25 µs, stupid!
• How to deal with MANET?

transmission range