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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
  • Background
  • Objectives
  • Introduction to WLAN
  • Simulation
  • Results
  • Conclusions
  • Future work
why wlan
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.

voice over wlan 1
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.
voice over wlan 2
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).
objectives
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.
network architecture 2
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.

network architecture 3
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.

network architecture 4
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.

network architecture 5
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).

network architecture 6
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.

ieee 802 11 mac layer
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.
slide14
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.
slide15
PCF
  • 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

Taken from IEEE 802.11 standard

inter frame space ifs
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)

pcf operation 1
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.
pcf operation 2
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.
pcf polling scheme 1
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.
pcf polling scheme 2
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.
simulation scenario
Simulation scenario
  • A single BSS in an infrastructure network configuration.
simulation model assumptions 1
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.
simulation model assumptions 2
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.
functions included in simulation 1
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
functions included in simulation 2
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
metrics
Metrics
  • Superframe size
  • Maximum number of VoIP MS
  • Throughput
  • Frame loss rate
  • Access delay
results superframe size
Results: superframe size
  • Normalized throughput for different SF using 160-byte payload
results superframe size1
Results: superframe size
  • Normalized throughput for different SF using 320-byte payload
conclusions
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.
future works
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
slide43
Q & A

Thank you for your attention!

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