Wireless VoIP

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Wireless VoIP

• C3
• R94922096 ???
• R94922088 ???

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Outline

• Problems to use VoIP on wireless network
• Voice over WLAN
• MAC method
• 802.11e
• Dual queue scheme
• VoIP and 802.11x standards

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VoIP on Wireless Network

• Wireless network lower speed , noise
• Upgrade physical speed , reduce noises (PHY)
• Real-time packet prioritize (MAC)
• 1AP-to-many Station
• Upgrade the capacity of single AP
• Admission control
• Roaming
• Mobile device power
• Wireless security

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Voice over WLAN
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802.11 supplements glossary

• 802.11a 5GHz OFDM PHY layer
• 802.11b 2.4GHz CCK PHY layer
• 802.11c bridging tables
• 802.11d international roaming
• 802.11e quality of service MAC
• 802.11f inter-access point protocols
• 802.11g 2.4GHz OFDM PHY
• 802.11h European regulatory extensions
• 802.11i enhanced security
• 802.11n MIMO ODFM PHY

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802.11 standards
higher layers
supplements 802.11c and 802.11f
802.11 MAC
supplements 802.11d, 802.11e, 802.11i and 802.11h
802.11 PHY
supplements 802.11a, 802.11b and 802.11g

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PHY ? 802.11n

• 2.4GHz5GHz (a/b/g)
• MIMOOFDM
• MIMO (Multiple-In, Multiple-Out)

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MIMO
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IEEE 802.11 MAC
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Hidden Node Problem
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Dual Queue Strategy
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Dual Queue Strategy

• The 802.11e MAC implementation cannot be done by
  just upgrading the firmware of an existing MAC
  controller chip only
• It is difficult to Upgrade (replace) the existing
  APs

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Dual Queue Strategy

• above 802.11 the MAC controller
• Original NIC driver ? FIFO queue
• New NIC driver ? RT NRT queue
• Strict priority queuing
• Effect of MAC HW Queue

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Dual Queue Strategy
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VOIP AND ADMISSION CONTROL

• VoIP
• codec ? G.711
• 64 kbps stream
• 8-bit pulse coded modulation (PCM)
• sampling rate 8000 samples/second
• A VoIP Packet per 20ms
• 160-byte DATA 12-byte RTP header 8-byte
  UDP header 20-byte IP header 8-byte SNAP
  header
• 208 bytes per VoIP packet

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VOIP AND ADMISSION CONTROL

• VoIP Admission Control
• assumptions
• ACK Packet transmitted with 2Mbps
• Long PHY preamble
• Packet transmission MAC
• DIFS deference
• Backoff
• Packet transmission
• SIFS deference
• ACK transmission

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VOIP AND ADMISSION CONTROL

• VoIP packet transmission time ? 981µs
• VoIP MAC packet transmission time
• 192-µs PLCP preamble/header (24-byte MAC
  header 4-byte CRC-32 208-byte payload) / 11
  Mbits/s 363 µs
• ACK transmission time at 2 Mbits/s
• 192-µs PLCP preamble/header 14-byte ACK packet
  / 2Mbits/s 248 µs
• Average backoff duration
• 31 (CWmin) 20 µs (One Slot Time) / 2 310 µs

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VOIP AND ADMISSION CONTROL

• Every VoIP sessioin
• inter-active ? 2 senders
• one voice packet transmitted every 20ms
• Every 20ms time interval
• 20 ( 20 ms / 981 µs) voice packets
• Maximum number of VoIP sessions over a 802.11 LAN
  is 10

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COMPARATIVE PERFORMANCE EVALUATION

• Using the ns-2 simulator
• 802.11b PHY
• Traffic
• Voice ? two-way constant bit rate (CBR) session
  according to G.711 codec
• Data? unidirectional FTP/TCP flow with 1460-byte
  packet size and 12-packet (or 17520-byte) receive
  window size.

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COMPARATIVE PERFORMANCE EVALUATION
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EVALUATION RESULT

• Pure VoIP
• Effect of VoIP with different TCP session numbers
• Performance with Dual queue
• Unfairness of NRT Packet
• Effect of MAC HW Queue

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Observation

• Compare to our Evaluation
• packet drop rate
• 50 packets for the RT queue size
• Downlink is disadvantaged
• Simulation results are based on 11 Mbps

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EVALUATION RESULT

• Pure VoIP
• Effect of VoIP with different TCP session numbers
• Performance with Dual queue
• Unfairness of NRT Packet
• Effect of MAC HW Queue

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Observation

• Effect of queue size

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EVALUATION RESULT

• Pure VoIP
• Effect of VoIP with different TCP session numbers
• Performance with Dual queue
• Unfairness of NRT Packet
• Effect of MAC HW Queue

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Observation

• worst case delay 11ms
• Queuing delay with the single queue
• MAC HW queue wireless channel access
• NRT queues
• Size 50 or 100 ? increase as the number of TCP
  flows increases
• Size 500 ? almost no change in delay

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EVALUATION RESULT

• Pure VoIP
• Effect of VoIP with different TCP session numbers
• Performance with Dual queue
• Unfairness of NRT Packet
• Effect of MAC HW Queue

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(No Transcript)
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Observation

• Unfairness
• between upstream and downstream TCP flows with
  the queue sizes of 50 and 100
• Queue size for the AP should be large enough -
  This is good for us

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EVALUATION RESULT

• Pure VoIP
• Effect of VoIP with different TCP session numbers
• Performance with Dual queue
• Unfairness of NRT Packet
• Effect of MAC HW Queue

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(No Transcript)
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Observation

• Delay of downlink voice packets
• increases linearly proportional to the MAC HW
  queue size
• Another effect
• with the MAC HW queue size of 8, the worst delay
  is observed with a single VoIP session
• Large MAC HW queue size is still aceptable
• lt25ms

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Brief Summary

• Driver of the 802.11 MAC controller
• Strict priority queuing
• Bottleneck of TCP in WLAN ? downlink

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VoIP and 802.11e QoS standards
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Whats the difference between Wireless/Wired VoIP?

• Mobility
• Roaming
• Security
• Hidden UA
• Quality of Service
• Guarantee of voice quality

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Hidden Node Problem
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Quality of Service

• QoS problems
• 802.11e QoS standard
• A non-standard solution
• Dual Queue Strategy

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QoS Problems

• Dropped Packets
• Delay
• Jitter
• Out-of-order Delivery
• Error
• VoIP requires strict limits on jitter and delay

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Quality of Service

• QoS problems
• 802.11e QoS standard
• A non-standard solution
• Dual Queue Strategy

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IEEE 802.11e

• A draft standard of July 2005
• It defines a set of QoS enhancements for WLAN
  applications
• and enhances the IEEE 802.11 Media Access Control
  (MAC) layer

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Coordination Function

• For stations to decide which one has the right to
  deliver its packets
• 802.11 DCF PCF
• 802.11e EDCF HCF

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Original 802.11 MAC

• Distributed Coordination Function (DCF)
• Point Coordination Function (PCF)

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Distributed Coordination Function (DCF)

• Share the medium between multiple stations
• Rely on CSMA/CA and optional 802.11 RTS/CTS

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How DCF works?
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DCF Limitations

• When many collisions occur, the available
  bandwidth will be lower
• No notion of high or low priority traffic
• A station may keep the medium
• If the station has a lower bitrate, all other
  stations will suffer from that
• No QoS guarantees

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Original 802.11 MAC

• Distributed Coordination Function (DCF)
• Point Coordination Function (PCF)

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Point Coordination Function (PCF)

• Available only in "infrastructure" mode
• Optional mode, only very few APs or Wi-Fi
  adapters actually implement it
• Beacon frame, Contention Period, and Contention
  Free Period

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How PCF works?
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802.11 MAC Layer Framework
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802.11e MAC Protocol Operation

• Enhanced DCF (EDCF)
• Hybrid Coordination Function (HCF)

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Enhanced DCF (EDCF)

• Define Traffic Classes
• High priority traffic has a higher chance of
  being sent than low priority traffic
• A "best effort" QoS
• Simple to configure and implement

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802.11e MAC Protocol Operation

• Enhanced DCF (EDCF)
• Hybrid Coordination Function (HCF)

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Hybrid Coordination Function (HCF)

• Works a lot like the PCF
• Main difference with the PCF Define the Traffic
  Classes (TC)
• Stations are given a Transmit Opportunity (TXOP)
• The most advanced (and complex) coordination
  function
• QoS can be configured with great precision

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Whats the difference between Wireless/Wired VoIP?

• Quality of Service
• Security
• Mobility

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IEEE 802.11i

• The draft standard was ratified on 24 June 2004
• Supersede Wired Equivalent Privacy (WEP)
• Wi-Fi Protected Access (WPA) is a subset
  implementation
• And WPA2 is the full implementation

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IEEE 802.11i (cont.)

• 802.1X for authentication (entailing the use of
  EAP and an authentication server)
• RSN for keeping track of associations
• AES-based CCMP to provide confidentiality,
  integrity and origin authentication
• Authentication process four-way handshake

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The Four-Way Handshake
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Whats the difference between Wireless/Wired VoIP?

• Quality of Service
• Security
• Mobility

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IEEE 802.11f 802.11r

• 802.11f Inter-Access Point Protocol
• 802.11r Fast roaming

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Conclusion
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Paper References 1

• Jeonggyun Yu, Sunghyun Choi, Jaehwan Lee,
  Enhancement of VoIP over IEEE 802.11 WLAN via
  Dual Queue Strategy
• Moncef Elaoud, David Famolari, and Ahbrajit
  Ghosh, Experimental VoIP Capacity Measurements
  for 802.11b WLANs
• Mustafa Ergen, I-WLAN Intelligent Wireless
  Local Area Networking
• Gyung-Ho Hwang, Dong-Ho Cho, New Access Scheme
  for VoIP Packets in IEEE 802.11e Wireless LANs
• Sai Shankar N, Javier del Prado Pavon, Patrick
  Wienert, Optimal packing of VoIP calls in an
  IEEE 802.11a/e WLAN in the presence of QoS
  Constraints and Channel Errors

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Paper Reference 2

• Experimental VoIP capacity measurements for
  802.11b WLANs
• Enhancement of VolP over IEEE 802.11 WLAN via
  dual queue strategy
• An experimental study of throughput for UDP and
  VoIP traffic in IEEE 802.11b networks
• Admission control for VoIP traffic in IEEE 802.11
  networks
• How well can the IEEE 802.11 wireless LAN support
  quality of service

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Web Site References

• http//www.ieee.or.com/Archive/80211/802_11e_QoS_f
  iles/frame.htm
• http//en.wikipedia.org/wiki/IEEE_802.11
• http//www.cs.nthu.edu.tw/nfhuang/chap13.htm13.1
  
• http//www.eettaiwan.com/ART_8800360909_675327_3f3
  ffd7b_no.HTM
• http//it.sohu.com/2003/12/11/09/article216750985.
  shtml