TDM-based Coordination Function (TCF) in WLAN for High Throughput

1. TDM-based Coordination Function (TCF) in WLAN for High Throughput Chaegwon Lim and Chong-Ho Choi School of Electrical Engineering and Computer Science and ASRI Seoul National University, Seoul, Korea IEEE Global Telecommunications Conference (Globecom) 2004 (Acceptance rate: 37.7%)

2. Overview • Introduction • Previous work • Protocol Descriptions • Performance Evaluation • My Personal Comments • Future work • Conclusion

3. Introduction - 1 • The demand for high throughput of IEEE 802.11 is increasing as most applications require a wide bandwidth. • However, the most popular protocol on IEEE 802.11 networks, Distributed Coordination Function (DCF), cannot meet the expectation due to its contention-based nature.

4. Introduction - 2 • Collision resolution scheme in DCF, the Binomial Exponential Backoff (BEB), doubles the size of the contention window up to a maximum value (1024) when there is a collision. • A significant number of redundant idle slots are introduced due to successive collisions before a successful data transmission. • Therefore, the actual throughput is far below the theoretical one.

5. Introduction - 3 • Factors considered in IEEE 802.11 networks’ performance analysis: • The number of active stations. • Bit Error Rate (BER) • Contention window (CWin) size… • The number of active stations is directly related to the competition level in seizing the radio resource. • This germinates the idea of TCF.

6. Introduction - 4 • Assumptions: • Propagation delay is negligible. • all stations are within the radio transmission range  no hidden node. • TCF uses information on the number of active stations explicitly to eliminate the contention period in DCF of IEEE 802.11. • It makes each station adjust the starting time of its radio transmission according to the number of active stations in order to avoid collisions.

7. Introduction - 5 • It can be employed in both infrastructure and ad hoc modes and implemented distributively. • The overall throughput is improved as it • is always higher than that of DCF • approaches the maximum throughput as the number of active stations increases. • It also guarantees fairness among active stations.

8. Previous work • Kwon et al. proposed a fast collision recovery (FCR) algorithm. • FCR uses a smaller sized contention window (CW) and reduces the value of backoff counter (BC) exponentially when idle slots are detected consecutively for a fixed number of times. • Whenever a station detects the transmission of another station, it increases the value of its CW and picks the value of BC randomly.

9. Protocol Specification – Terminology (1) • Service Period (SP) • A period during which each active station transmits frames in round-robin manner. • Join Period (JP) • New stations can join during this period. • The duration of JP (AD) is fixed and known to all stations.

10. Protocol Specification – Terminology (2) • Backoff Counter (BC) • It linearly depends on the number of active stations. • It is decreased by one • for each DIFS (50 microsec) after a successful transmission or • for each ACKTimeout after sending a data frame or • for each idle slot time (20 microsec) after DIFS or ACKTimeout. • The station transmits data when the BC becomes zero DATA SIFS ACK DIFS Slot1 Slot2 BC: 5 5 4 4 3 3 2 5 5 5 5 5 5 DATA ACK timeout Slot1 Slot2 BC: 5 5 5 5 4 4 3 3 2

11. Protocol Specification – Terminology (3) • The Number of Active Stations (NS) • It increments NS by one when it detect a transmission • Each station sets the NS to zero when the BC is reset.

12. Protocol Specification – State Transition Diagram

13. Protocol Specification – STANDBY State (1) • When a station enters the STANDBY state, it estimates the number of active stations by just counting the number of transmissions between two consecutive JPs. • How can a station to identify a JP ? • If it detects idle time that is equal to AD, it assumes that this instant is the end of the JP. AD=5 New node’s view SP SP JP

14. Protocol Specification – STANDBY State (2) • The station sets its NS as the number of active stations and updates NS continuously until it enters the JOIN state. • If a station in this state has data to send and knows the number of active stations, it enters the JOIN state at the end of the JP. New node’s view SP JP SP JP

15. Protocol Specification – STANDBY State (3) The time duration of the column where all square are white = SlotTime (20 microsec) The time duration of the column containing a grey sqaure = Tdata + SIFS (10 microsec) + Tack + DIFS (50 microsec) if successful = Tdata + ACKTimeout (30 microsec) otherwise AD = 5 The end of the JP

16. Protocol Specification – JOIN State (1) • Immediately after the end of the current JP, the station sets • BC = NS + X where X = 0,1, … , AD-1 Purpose: avoid collisions among several new stations • NS = 0 • The station updates NS continuously until BC becomes 0.

17. Protocol Specification – JOIN State (2)

18. Protocol Specification – JOIN State (3) • Once a station receives ACK in JP, it jumps to the ACTIVE state. • BC and NS are reset: • BC = NS + 1 + AD – X • Purpose: A slot alignment operation to ensure that there are no idle slots in between active stations. • NS = 0 • The latest station which enters the ACTIVE state becomes the last sender in the SP.

19. Protocol Specification – JOIN State (4) Service Period

20. Protocol Specification – JOIN State (5) • More than one new stations

21. Protocol Specification – JOIN State (6) • If the number of stations in the JOIN state is greater than AD, they will compete with each other to occupy one of AD slots during JP. • New stations which failed to transmit a data frame during the JP return to a STANDBY state.

22. Protocol Specification – ACTIVE State • An ACTIVE station uses two variables, BC and NS, to transmit a data frame without contention. • After transmitting a data frame, it • sets BC to (NS + AD) and • resets NS to zero

23. Protocol Specification – ACTIVE State

24. Protocol Specification – ACTIVE State • The ACTIVE state is composed of two sub-states: ACTIVE1 and ACTIVE2.

25. Protocol Specification – ACTIVE State • When a station enters the ACTIVE state, it goes into the ACTIVE1 state.

26. Protocol Specification – ACTIVE State • When the sender station notices a collision by observing the absence of ACK, the sender station goes into the ACTIVE2 state. • It tries to send the data frame in the next SP.

27. Protocol Specification – ACTIVE State • If a station in the ACTIVE2 state suffers from a collision again during the next SP, it enters the STANDBY state.

28. Protocol Specification – ACTIVE State • When a station in the ACTIVE2 state sends a data frame successfully during the next SP, it goes into the ACTIVE1 state again.

29. Protocol Specification – ACTIVE State • The reason for using two sub-states: • Provide TCF with more robustness for possible hidden stations or other stations which are not under TCF.

30. Protocol Specification – ACTIVE State • If a station in the ACTIVE state has no more data to send, it will exit the ACTIVE state and transit to the STANDBY state.

31. Protocol Specification – ACTIVE State • If several station exit concurrently, a station in the JOIN state may determine the start of JP incorrectly and may send a data frame during SP. • It is possible that collisions may occur between stations in ACTIVE state and stations in the JOIN state. 5 active actions exit AD=5 NS=13 BC=13+5=18 3 2 1 0 17 16 15 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0 Active node’s view SP SP JP 4 3 2 1 0 JP JP SP SP New node’s view NS=3, X = 2 BC=3+2-1=4

32. Protocol Specification – ACTIVE State • Collision resolution scheme: • Stations in the ACTIVE1 state transit to the ACTIVE2 state. • Stations in the ACTIVE2 state transit to the STANDBY state. • Stations in the JOIN state return to the STANDBY state.

33. Protocol Specification – Deactivation

34. Performance Evaluation – Setup (1) • Several simulations are performed using ns-2 (version 2.26) simulator • TCF was compared with DCF and FCR. • FCR (CWmin=3) is selected because it is implemented distributively and provides high throughput due to a small CWmin and exponential reduction of BC.

35. Performance Evaluation – Setup (2) • All the 802.11b stations are in a region of 70 meters x 70 meters. • The transmission range of each station is 100 meters. • Each station always has enough data to send to one of the other stations selected randomly

36. Performance Evaluation - Setup (3) • The bit error rate (BER) is 10−5, which is the worst environment for an Orinoco PC card. • The traffic sources send data at a constant bit rate (CBR) and the size of a data packet is 1000 bytes. Tricky setting! • Time taken for simulations = 100 sec

37. Performance Evaluation – Overall Throughput (1) • Obtain a maximum throughput of DCF as a reference point • There are two stations(a sender and a receiver). • The sender station always has data to send. • The propagation delay is negligble. • The wireless network is lossless.

38. Performance Evaluation – Overall Throughput (2) • Obtain a maximum throughput of DCF • Expected time consumed to send a data frame successfully (Ttransmit) = DIFS + TE[BC]+Tdata +SIFS + Tack where TE[BC] is the expected contention period, Tdata andTack represent the transmission times of a data frame and an ACK respectively. • Throughputmax= TotalUsefulDataSize/ Ttransmit where TE[BC] is set to zero i.e Ttransmit= DIFS +Tdata +SIFS + Tack • Throughputmax = 6.4617 Mbps

39. Performance Evaluation – Overall Throughput (3)

40. Performance Evaluation - Delay

41. Performance Evaluation - Fairness where n is the number of active stations, and Xi is the measured throughput of flow i.

42. Performance Evaluation – Load Variation

43. My Personal Comments • Simulation scenarios are not comprehensive. • The value of the constant bit rate (CBR) is not given in this paper. • The value may be “tailor-made” to produce expected results. • If the outgoing data arrival rate is much smaller than the data transmission rate, a sender needs to join and exit frequently. The overhead incurred may increase the delay and hence decrease the overall throughput. • Performance under variable bit rate (VBR) traffic is not evaluated. • No analytical model.

44. Future work • Extend the operation of TCF in the environment where there are hidden stations. • Study delay-stringent versions of TCF to cope with VBR multimedia traffic.

45. Conclusions • TCF is a simple and distributed MAC scheme for IEEE 802.11 network to increase throughput using the information on the number of active stations. • It eliminates the contention by increasing BC to exceed the number of active stations and giving each station the opportunity to transmit a data frame in a round robin fashion. • It can be used in places where there is no hidden station such as conference rooms and coffee shops.