General Backoff Algorithm of IEEE 802.11 DCF

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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

1

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)