Unfairness in IEEE 802.11 Access Mechanism By Li Zhifei

1. Unfairness in IEEE 802.11 Access Mechanism By Li Zhifei

2. Content Outline • Introduction • Motivation • Fixed EIFS Problem in IEEE 802.11 • Enhanced VCS (EVCS) • Conclusions • Research Direction

3. Medium Access Protocol (MAC) Basic objectives: • Maximize utilization of the limited bandwidth • Fair access of the shared wireless medium

4. Why do we need fairness in IEEE 802.11? • The medium is shared by multiple contending nodes • Every node operates in a distributed manner • IEEE 802.11 is a random access protocol.

5. Definition of Fairness There are many definitions about fairness: • Max-min fairness • Proportional fairness and so on We use the simplest one: • To allocate the bandwidth equitably

6. IEEE 802.11 Basics • Designed for Wireless LAN • Based on CSMA/CA • CS: including Physical CS and Virtual CS • CA: binary exponential back-off • Four-way handshake: RTS-CTS-Data-ACK • For example: Node A wants to send a packet to node B.

7. Fairness of IEEE 802.11 in MANETs Unfairness may be caused by: • Imprecise CS • Inappropriate CA • Wireless error • Mobility and so on.

8. Motivation and Focus • Most work focus on WLANs rather than MANETs • Most work consider unfairness caused by CA algorithms • Not much focus given to unfairness caused by CS in MANETs We focus on: • Study unfairness caused by CS • Try to achieve fairness through enhanced CS mechanism

9. A B C One simple scenario 3-nodes with two single-hop flows Flow 1: from node A to B Flow 2: from node B to C Distance between neighbors: 200 meters

10. Simulation result

11. Fixed EIFS problem • Whenever a node detects a Sensing Range (SR) frame on the medium, it defers the transmission by a fixed duration, i.e. EIFS. This duration is sometimes smaller and sometimes larger than the desired period by which the transmission should be deferred, and it is the main cause of the unfairness.

12. Enhanced Virtual CS (EVCS) • A node should distinguish among the type of frames (RTS, CTS, Data and ACK) when detecting a SR frame. • EIFS value should be directly linked to the type of SR frame detected.

13. EVCS (cont’d) • EIFS(collision or MAC error) = Standard EIFS; • EIFS(RTS) = SIFS + TxTime(CTS); • EIFS(CTS) = SIFS+TxTime(Max-Data-Length); • EIFS(Data) = SIFS+TxTime(ACK); • EIFS(ACK) = 0;

14. Results under EVCS

15. A B C D Another scenario 4-nodes with two single-hop flows Flow 1: from node A to B Flow 2: from node D to C Distance between neighbors: 200 meters

16. Simulation Results Under IEEE 802.11: Flow from A to B gets 1.4 Mbps while the other flow completely starves. Under EVCS:

17. Conclusions • IEEE 802.11 exhibits substantial unfairness caused by fixed value of EIFS • EVCS solves the problems to a great extent and thus improve fairness substantially

18. Future Research • Consider other factors’ effects on fairness,such as: mobility, hidden-terminal problem, capture and so on • Study the short-term fairness • Quality of Service (QoS) issues in MANETs

19. Thank you! Any question?