Voice traffic performance over wireless lan using the point coordination function
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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. Contents. Background Objectives

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Voice traffic performance over wireless lan using the point coordination function

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


Contents
Contents Coordination Function

  • Background

  • Objectives

  • Introduction to WLAN

  • Simulation

  • Results

  • Conclusions

  • Future work


Why wlan
Why WLAN Coordination Function?

  • 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.


Voice over wlan 1
Voice over WLAN (1) Coordination Function

  • 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.


Voice over wlan 2
Voice over WLAN (2) Coordination Function

  • 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).


Objectives
Objectives Coordination Function

  • 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.


Network architecture 1
Network architecture (1) Coordination Function


Network architecture 2
Network architecture (2) Coordination Function

  • 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.


Network architecture 3
Network architecture (3) Coordination Function

  • 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.


Network architecture 4
Network architecture (4) Coordination Function

  • 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.


Network architecture 5
Network architecture (5) Coordination Function

  • 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).


Network architecture 6
Network architecture (6) Coordination Function

  • 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.


Ieee 802 11 mac layer
IEEE 802.11 MAC layer Coordination Function

  • 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.


DCF Coordination Function

  • 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.


PCF Coordination Function

  • Optional access method, resides on top of DCF.

  • To support real-time applications.

  • Centralized control.

  • Polling based access mechanism.


Coexistence of dcf and pcf
Coexistence of DCF and PCF Coordination Function

Taken from IEEE 802.11 standard


Inter frame space ifs
Inter-Frame Space (IFS) Coordination Function

  • 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)


Pcf operation 1
PCF operation (1) Coordination Function

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


Pcf operation 2
PCF operation (2) Coordination Function

  • 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.


Pcf operation 3
PCF operation (3) Coordination Function


Pcf polling scheme 1
PCF polling scheme (1) Coordination Function

  • 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.


Pcf polling scheme 2
PCF polling scheme (2) Coordination Function

  • 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.


Simulation scenario
Simulation scenario Coordination Function

  • A single BSS in an infrastructure network configuration.


Simulation model assumptions 1
Simulation model assumptions (1) Coordination Function

  • 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.


Simulation model assumptions 2
Simulation model assumptions (2) Coordination Function

  • 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.


Functions included in simulation 1
Functions included in simulation (1) Coordination Function

  • 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


Functions included in simulation 2
Functions included in simulation (2) Coordination Function

  • Acknowledgement, beacon, CF-poll, and CF-end frames

  • Piggybacking of Ack and CF-poll information

  • Random generation of erroneous frames

  • Recording of simulation data


Simulation parameters
Simulation parameters Coordination Function


Metrics
Metrics Coordination Function

  • Superframe size

  • Maximum number of VoIP MS

  • Throughput

  • Frame loss rate

  • Access delay


Results superframe size
Results: superframe size Coordination Function

  • Normalized throughput for different SF using 160-byte payload


Results superframe size1
Results: superframe size Coordination Function

  • Normalized throughput for different SF using 320-byte payload




Results capacity
Results: capacity Coordination Function


Results frame loss rate
Results: frame loss rate Coordination Function







Conclusions
Conclusions 320-byte payload

  • 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.


Future works
Future works 320-byte payload

  • 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


Q & A 320-byte payload

Thank you for your attention!


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