General Backoff Algorithm of IEEE 802.11 DCF

1. General Backoff Algorithm of IEEE 802.11 DCF 電機所一年級 694415026 吳豐州

2. Outline • Introduction of 802.11 BEB in DCF • EIED • General Backoff Algorithm • Result of Simulation in NS2 • Conclusion

3. 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 )

4. Introduction of 802.11 BEB

5. Collision ! Success! Collision ! Collision ! Collision ! Collision ! Collision ! Introduction of 802.11 BEB Converge too fast when load is heavy !

6. Success !! Collision ! Success !! Collision ! Success !! Collision ! Success !! Collision ! Collision ! Success !! The same as 802.11 Collision ! EIED Converge too slow when load is light !

7. Introduction of GB Backoff 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 ) BEB EIED

8. Threshold λ

9. Collision ! Success! Collision ! Collision ! Threshold λ Collision ! Collision ! Success! Collision ! General Backoff Algorithm BEB EIED

10. Packet size 256 bytes Result of simulation Packet interval 0.008 sec  256kbps

11. Result of simulation

12. 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 λThreshold • 找出最佳的α值 • 將程式修改成能與理論完全配合