Voice Traffic Performance over Wireless LAN using the Point Coordination Function

1. Voice Traffic Performance over Wireless LAN using the Point Coordination Function Wei Wei Supervisor: Prof. Sven-Gustav Häggman Instructor: Researcher Michael Hall Helsinki University of Technology Communications Laboratory April, 2004

2. Contents • Background • Objectives • Introduction to WLAN • Simulation • Results • Conclusions • Future work

3. Why WLAN? • Mobility - It brings increased efficiency and productivity. • Flexibility - Fast and easy deployment. - Can be set up where the wired networks are imposible or difficult to reach.

4. Voice over WLAN (1) • Nowadays, IEEE 802.11 WLAN standard is being accepted widely and rapidly for many different environments. • Mainly, WLAN is used for Internet based services like web browsing, email, and file transfers.

5. Voice over WLAN (2) • However, demand for supporting real-time traffic applications such as voice over WLAN has been increasing. • To meet this need, IEEE 802.11 standard defines an optional medium access protocol, Point Coordination Function (PCF).

6. Objectives • To implement the basic PCF algorithm in a time-driven simulation program written in C language. • To measure some metrics such as throughput, delay, frame loss rate, etc. • To evaluate the voice traffic performance in WLAN using PCF to investigate if PCF is capable of the real-time applications such as voice service.

7. Network architecture (1)

8. Network architecture (2) • Basically, WLAN network consists of four components: Distribution System, Access Point, Mobile Station, and wireless medium. • Distribution System (DS): - A backbone network that connects several access points or Basic Service Sets. - Wired or wireless, implemented independently. - In general, Ethernet is used as the backbone network technology.

9. Network architecture (3) • Access Point (AP): - Connected to the DS, wireless-to-wired bridging function. • Mobile Station (MS): - In general, it’s referred to laptop computer. • Wireless medium: - Frequency Hopping, Direct Sequence Spread Spectrum, Infra-red.

10. Network architecture (4) • Basic Service Set (BSS): - It consists of a group of stations that are under control of DCF or PCF. • Extended Service Set (ESS): - It consists of several BSSs via DS. - Provides larger network coverage area.

11. Network architecture (5) • IEEE 802.11 defines two operation modes: Ad-hoc mode and Infrastructure mode. • Ad-hoc mode: - A set of 802.11 wireless stations communicate directly with each other, without using access point. - Also called Independent Basic Service Set (IBSS).

12. Network architecture (6) • Infrastructure mode: - The network consists of at least one access point and a set of mobile stations. - AP bridges the wireless traffic to a wired Ethernet or the Internet. - AP can be compared with a base station used in a celluar network.

13. IEEE 802.11 MAC layer • IEEE 802.11 defines two medium access methods: the mandatory Distributed Coordination Function (DCF) for non-real-time applications, and the optional Point Coordination Function (PCF) for real-time applications.

14. DCF • Basic access method of IEEE 802.11, using Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) to access to the shared medium. • Backoff before transmission, provide fair access to the medium. • No QoS guarantees, best effort.

15. PCF • Optional access method, resides on top of DCF. • To support real-time applications. • Centralized control. • Polling based access mechanism.

16. Coexistence of DCF and PCF Taken from IEEE 802.11 standard

17. Inter-Frame Space (IFS) • Basically 3 different IFSs. • Short IFS (SIFS) • PCF IFS (PIFS) • DCF IFS (DIFS) • SIFS < PIFS < DIFS • IFS determines priority: - After a SIFS, only polled MS can send - After a PIFS, only AP can send (PCF control) - After a DIFS, every station can send according to CSMA/CA (DCF)

18. PCF operation (1) • The time on the medium is divided into two parts: Contention-Free Period (CFP) controlled by PCF and Contention Period (CP) controlled by DCF.

19. PCF operation (2) • During a CFP, at least 2 maximum size frames transmitted. • During a CP, at least 1 maximum size frame transmitted, including RTS/CTS and ACK.

20. PCF operation (3)

21. PCF polling scheme (1) • A poll list is created when the MSs supporting real-time service negotiate with Point Coordinator (PC) during the association procedure. • The MSs are put on the poll list in order. • The poll list gives the highest privilege to PCF supported MSs.

22. PCF polling scheme (2) • The polling scheme is based on Round-Robin scheduler recommended by IEEE 802.11 standard. • Only the polled MS can transmit a frame. • During one CFP, the MS can be polled once. • If the CFP terminates before all MSs on the poll list are polled, the poll list will resume at the next MS in the following CFP. • The CFP may terminate befor time, if all MSs on the poll list have no data to send. • Data frame, ACK, and poll combined to improve efficiency.

23. Simulation scenario • A single BSS in an infrastructure network configuration.

24. Simulation model assumptions (1) • Only use voice traffic during CFP, not consider data traffic during CP. • RTP/UDP/IP/MAC/PHY, this adds an overall overhead of 78 bytes to every voice packet. • G.711 PCM voice codec used, fixed traffic interval 20ms or 40ms, 160bytes or 320bytes payload, respectively. • Buffer size = 1.

25. Simulation model assumptions (2) • Power saving mode is neglected. • Foreshortened CFP is neglected. • Fragmentation/Defragmentation is neglected. • Broadcast/Multicast frames not considered. • Mobility, multipath interference, and hidden-node problem are not considered. • Basic rate used: 11 Mbps.

26. Functions included in simulation (1) • One access point and specific number of VoIP stations • Voice connections: bi-directional deterministic stream of frames with calculated duration and inter-frame interval, PCM over RTP over UDP over IP over LLC over MAC over PHY assumed • SIFS and PIFS times

27. Functions included in simulation (2) • Acknowledgement, beacon, CF-poll, and CF-end frames • Piggybacking of Ack and CF-poll information • Random generation of erroneous frames • Recording of simulation data

28. Simulation parameters

29. Metrics • Superframe size • Maximum number of VoIP MS • Throughput • Frame loss rate • Access delay

30. Results: superframe size • Normalized throughput for different SF using 160-byte payload

31. Results: superframe size • Normalized throughput for different SF using 320-byte payload

32. Results: max. number of VoIP MS for 160-byte payload

33. Results: max. number of VoIP MS for 320-byte payload

34. Results: capacity

35. Results: frame loss rate

36. Results: average access delay for different SF using 160-byte payload

37. Results: average access delay for different SF using 320-byte payload

38. Results: comparison of average access delay btw. 160 and 320-byte payload

39. Results: cumulative delay distribution for 160-byte payload

40. Results: cumulative delay distribution for 320-byte payload

41. Conclusions • The proper superframe size should be approximately similar to the traffic interval, which results in good performance. • Longer payload provides higher normalized throughput and lower frame loss rate, but longer access delay. • Maximum number of VoIP MS: for 160-byte payload, 21; for 320-byte payload, 36. • When the number of VoIP MS increases, performance degrades dramatically. PCF provides limited QoS.

42. Future works • Perform an authentic evaluation in a WLAN - Assumptions - Realistic traffic model • PCF problems - unpredictable Beacon frame delay resulting in shortened CFP - unknown transmission time of polled stations making it difficult for PC to predict and control the polling scheldule for the remainder of CFP • IEEE 802.11e introduced EDCF and HCF to support QoS

43. Q & A Thank you for your attention!