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.

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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.

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Network architecture (3)

• Access Point (AP)
• - Connected to the DS, wireless-to-wired
  bridging function.
• Mobile Station (MS)
• - In general, its referred to laptop
  computer.
• Wireless medium
• - Frequency Hopping, Direct Sequence Spread
  Spectrum, Infra-red.

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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.

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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).

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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.

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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.

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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.

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PCF

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

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Coexistence of DCF and PCF
Taken from IEEE 802.11 standard
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Inter-Frame Space (IFS)

• Basically 3 different IFSs.
• Short IFS (SIFS)
• PCF IFS (PIFS)
• DCF IFS (DIFS)
• SIFS lt PIFS lt 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)

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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.

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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.

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PCF operation (3)
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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.

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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.

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Simulation scenario

• A single BSS in an infrastructure network
  configuration.

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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.

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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.

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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

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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

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Simulation parameters
Channel rate 11 Mbps
Channel frame error rate (CFER) 0.03
Voice payload 160/320 bytes
Slot time 20 ?s
SIFS 10 ?s
PIFS 30 ?s
DIFS 50 ?s
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Metrics

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

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Results superframe size

• Normalized throughput for different SF using
  160-byte payload

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Results superframe size

• Normalized throughput for different SF using
  320-byte payload

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Results max. number of VoIP MS for 160-byte
payload
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Results max. number of VoIP MS for 320-byte
payload
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Results capacity
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Results frame loss rate
36
Results average access delay for different SF
using 160-byte payload
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Results average access delay for different SF
using 320-byte payload
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Results comparison of average access delay btw.
160 and 320-byte payload
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Results cumulative delay distribution for
160-byte payload
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Results cumulative delay distribution for
320-byte payload
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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.

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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

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Q A

• Thank you for your attention!