is ieee 802.11 scalable?. ieee 802.11: how large can it be? bandwidth: up to 54 mbps good for a few...
Post on 18-Dec-2015
214 Views
Preview:
TRANSCRIPT
Is IEEE 802.11 Scalable?
IEEE 802.11: how large can it be?
Bandwidth: • 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 Infrastructure BSS Independent BSS (IBSS) -- Ad Hoc
AP
IBSSInfrastructure BSS
802.11 Timers (Clocks)
Timer: 64 bits, ticking in microseconds. Accuracy: within + 0.01%, or +100 ppm. Time synchronization needed for:
Frequency hopping Power management
∆ = max tolerable difference between clocks.
Frequency Hopping
f1
f2
f4
f3
f5
Power Saving
sleep
Beacon window ATIM window
Beacon interval
timesleep
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 links 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. In /(d*T) intervals, fastest-station will be out
of sync with all others.
T
Fastest-station out-of-sync (2)
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. L = # beacon intervals between async periods. E(R) = E(H)/[E(H)+E(L)] = percent of time in
which the fastest station is out of sync with all others.
LH
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%
Global Asynchronism
Lower bound on E(H) Upper bound on E(L)
LH
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%
Percentage of channels out of sync
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)
1nProb <
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 of Modified TSF
802.11 Performance of TSF ATSP ATSP.pdf TATSP Performance of Modified TSF
Performance of TSF
Performance of Modified TSF
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?
IBSS: single-hop
MANET: multihop ??
transmission range
欲知後事如
何 ,
且待下回分
解 .
top related