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General Backoff Algorithm of IEEE 802.11 DCF. 電機所一年級 694415026 吳豐州. Outline. Introduction of 802.11 BEB in DCF EIED General Backoff Algorithm Result of Simulation in NS2 Conclusion. Introduction of 802.11 BEB. Collision Avoidance Backoff timer ： - PowerPoint PPT Presentation
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General Backoff Algorithm of IEEE
802.11 DCF
電機所一年級694415026 吳
豐州
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Outline
• Introduction of 802.11 BEB in DCF• EIED• General Backoff Algorithm• Result of Simulation in NS2• Conclusion
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Introduction of 802.11 BEB• Collision Avoidance• Backoff timer ：
Backoff time = Random() × SlotTime Random()=[0,CW], CWmin(31)≤ CW ≤CWmax(1023)
Collision CWnew = CWold × 2 Success CWnew = CWmin
CW0 = CWmin, ( state 0 ) CWm= CWmax ( state m )
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Introduction of 802.11 BEB
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Introduction of 802.11 BEB
FrameFirst
Second Frame
DIFS ……
32 Slot time
DIFS ……
64 Slot time
Third Frame DIFS ……
128 Slot time
FrameFourth
Fifth Frame
DIFS ………
256 Slot time
DIFS ……………
512 Slot time
Sixth Frame DIFS ………………………
1024 Slot time
Collision !
Collision !
Collision !
Collision !
Collision !
Collision !
Success!
Converge too fast when load is heavy !
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EIED
FrameFirst
Second Frame
DIFS ……
32 Slot time
DIFS ……
64 Slot time
Third Frame DIFS ……
128 Slot time
FrameFourth
Fifth Frame
DIFS ………
256 Slot time
DIFS ……………
512 Slot time
Sixth Frame DIFS ………………………
1024 Slot time
Collision !
Collision !
Collision !
Collision !
Collision !
Collision !
The same as 802.11
Success !!
Success !!
Success !!
Success !!
Success !!
Converge too slow when load is light !
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Introduction of GBBackoff time = Random() × SlotTime
– Random()=[0,CW], CWmin ≤ CW ≤ CWmax
• λ, add a threshold state (original BEB state)– 0 < CWstate ≤ λ – λ < CWstate ≤ m
• α, add a new state when λ < CWstate ≤ m– CWnew = CWold × 2 × α– α = ( 0.5 , 1 )
BEBBEB
EIEDEIED
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1,0 1,1
2,0 2,1
3,1
λ-1,0
3,0
λ,0 λ,1
λ+1,0 λ+1,1
m-1,0
0,0 0,1 0, 0 1W
1 1W
2 1W
1 1W
3 1W
1W
λ-1,1
2,
1,
3,
λ-1,
m,1m,0
m-1,1
λ,
λ+1, 1 1W
m-1, 1 1mW
1mW m,
1
1
1
1
1
1
1
1
1
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1
[1 ( )]y p
W
2
[1 ( )]y p
W
2
( )y p
W
4
( )y p
W
m
p
W
0
(1 )p
W
0
(1 )p
W
0
1 p
W
(1 )
x
p
W
2
(1 )
m
p
W
(1 )
x
p
W
1
1
1
1
1
1
1
1
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Threshold Threshold λλ
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General Backoff Algorithm
Frame0
1 Frame
DIFS ……
CW0 Slot time
DIFS ……
CW1 Slot time
2 Frame DIFS ……
CW2 Slot time
Frameλ
m-1 Frame
DIFS ………
CWλ Slot time
DIFS ……………
CWm-1 Slot time
m Frame DIFS ………………………
CWm Slot time
. . .
. . .
Collision !
Collision !
Collision !
Collision !
Collision !
Collision !
Threshold Threshold λλ
Success !
EIEDEIED
BEBBEB
Success !
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Result of simulationPacket size 256 bytesPacket interval 0.008 sec
256kbps
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Result of simulation
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Conclusion• 理論上 GB 可以改善 loading 輕、 loading 重時
的缺點 , 節省不必要的後退等待時間和減少碰撞機會
• Future work ：– α 參數配合 λ Threshold– 找出最佳的 α 值– 將程式修改成能與理論完全配合
EIED Extend EIED
BEB Extend BEB
GB22 , 1,
2
mj j
m 0
0 (0.5,1)
