Slow Start Backoff Algorithm for Wireless Ad Hoc Networks

1. Slow Start Backoff Algorithm for Wireless Ad Hoc Networks Der-Jiunn Deng Department of Computer Science and Information Engineering National Changhua University of Education

3. Ad Hoc Networks • Ad hoc network: mobile stations can dynamically form network without AP • Applications: • “laptop” meeting in conference room, car • interconnection of “personal” devices • battlefield • IETF MANET (Mobile Ad hoc Networks) working group

4. CSMA/CA CSMA: sender - if sense channel idle for DISF sec. then transmit entire frame (no collision detection) -ifsense channel busy then binary backoff CSMA receiver: if received OK return ACK after SIFS

5. CSMA/CA CSMA: others • NAV: Network Allocation Vector • 802.11 frame has transmission time field • others (hearing sata) defer access for NAV time units

6. DIFS DIFS DIFS DIFS DIFS DIFS DIFS DIFS CW Busy Medium Busy Medium Busy Medium Busy Medium Busy Medium Busy Medium SIFS Basic Access Method (CSMA/CA) Free access when medium is free longer than DIFS Source Data Select a CW size and decrement backoff as long as medium is idle Destination ACK Other CW NAV Defer access

7. DIFS DIFS DIFS initial attempt 31 first retransmission 63 second retransmission 127 third retransmission 255 forth retransmission 511 0 1023 CW min CW max CW SIFS Binary Exponential Backoff (BEB) Busy Medium Busy Medium Data ACK time slot

8. Congested Scenario • The usage of backoff algorithm avoids long access delays when the load is light because it selects an initial (small) parameter value of contention window (CW) by assuming a low level of congestion in the system. • This strategy might allocate initial size of CW, only to find out later that it is not enough when the load increased, but each increase of the CW parameter value is obtained paying the cost of a collision (bandwidth wastage) • After a successful transmission, the size of CW is set again to the minimum value without maintaining any knowledge of the current channel status.

9. LILD

10. EIED

11. Slow Start Backoff Alg.

12. SSB

13. SSB

14. Impact of Different Threshold

15. Impact of Different Threshold

16. Slow-Start Backoff Alg. 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1

17. Saturation Throughput Analysis 0 1 2 x m-1 m

18. Saturation Throughput Analysis Let

19. Saturation Throughput Analysis

20. X X X X X X X X 1st 2nd 3rd xnd p 1-p 1-p 1-p Geometric Distribution Consider a sequence of Bernoulli trails with the probability of success on being P. Let r.v. X denote the number of trials up to and including the first success

21. Geometric Distribution Consider a sequence of Bernoulli trails with the probability of success on being P. Let r.v. X denote the number of trials up to and including the first success

22. DIFS DIFS DIFS CW P-Persistent CSMA/CA SIFS Busy Medium Busy Medium Data ACK decrement backoff as long as medium is idle defer access Data

23. Saturation Throughput Analysis

24. Saturation Throughput Analysis

25. Optimal CW size

26. Simulation Results

27. Simulation Results

28. Simulation Results

29. IEICE (Jan. 1999)

30. IEEE JSAC (Jun. 2005)

31. IEEE TWC (Dec. 2008)

32. IEICE (Jan. 1999)

33. IEEE JSAC and IEEE TWC

34. Forward Looking G stuff Internet WLANs PSTN now • Based on the history • Telephone network (PSTN) • Computer network and mobile phone network • Internet and the G stuff (GSM/GPRS/3G) • Wireless networks (WLANs) 10‘s 80‘s 90’s 00‘s

35. G stuff Internet WLANs PSTN now 10‘s 80‘s 90’s 00‘s The Past: You already knew!

36. G stuff Internet WLANs PSTN now 10‘s 80‘s 90’s 00‘s The Present: You are watching it!

37. G stuff Internet WLANs PSTN now 10‘s 80‘s 90’s 00‘s The Future: No one can tell!