A MultiplexMulticast Scheme that Improves System Capacity of VoiceoverIP on Wireless LAN by 100%

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A Multiplex-Multicast Scheme that Improves System
Capacity of Voice-over-IP on Wireless LAN by 100

• B91902058 ???
• B91902078 ???
• B91902088 ???
• B91902096 ???

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Outline

• Introduction
• VoIP Multiplex-Multicast Scheme
• Capacity Analysis
• Delay Performance
• Conclusions

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Introduction

• This paper considers the support of VoIP over
  802.11b WLAN.
• WLAN capacity can potentially support more than
  500 VoIP sessions when using GSM 6.10 codec.
• But various overheads bring WLAN capacity only 12
  VoIP sessions when using GSM 6.10 codec.

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Introduction

• 802.11b, which can support data rates up to
  11Mbps.
• A VoIP stream typically requires less than
  10Kbps.
• 11M/10K 1100, which corresponds to about 550
  VoIP sessions, each with two VoIP streams.

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Introduction

• The efficiency at the IP layer for VoIP
• A typical VoIP packet at the IP layer consists of
  40-byte IP/UDP/RTP headers.
• A payload ranging from 10 to 30 bytes, depending
  on the codec used.
• less than 50!!

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Introduction

• At the 802.11 MAC/PHY layers
• Attributed to the physical preamble, MAC header,
  MAC backoff time, MAC acknowledgement, and
  inter-transmission times of packets and
  acknowledgements.
• The overall efficiency drops to less than 3!!

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Outline

• Introduction
• VoIP Multiplex-Multicast Scheme
• Capacity Analysis
• Delay Performance
• Conclusions

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????

• Unicast
• Broadcast
• Multicast

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Multiplex-Multicast Scheme

• An 802.11 WLAN is referred to as the basic
  service set (BSS) in the standard specification.
• There are two types of BSSs
• Independent BSS and Infrastructure BSS.

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Multiplex-Multicast Scheme

• Independent(ad hoc) BSS

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Multiplex-Multicast Scheme

• Infrastructure BSS

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Multiplex-Multicast Scheme

• This paper focuses on infrastructure BSSs.
• We assume that all voice streams are between
  stations in different BSSs.
• Each AP has two interfaces, an 802.11 interface
  which is used to communicate with wireless
  stations, and an Ethernet interface which is
  connected to the voice gateway.

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Multiplex-Multicast Scheme
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Multiplex-Multicast Scheme

• Within a BSS, there are two streams for each VoIP
  session.
• M-M Scheme idea
• Combine the data from several downlink streams
  into a single packet for multicast over the WLAN
  to their destinations.

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Multiplex-Multicast Scheme
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Multiplex-Multicast Scheme

• multiplexer(MUX), demultiplexer(DEMUX)
• Add miniheader
• In miniheader, there is an ID used to identify
  the session of the VoIP packet.

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Multiplex-Multicast Scheme
Header data1
Header
MUX
Header data2
Minih.Data1Minih.data2Minih.data3
DEMUX
Header data3
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Multiplex-Multicast Scheme

• Reduce the number of VoIP streams in one BSS from
  2n to 1 n, where n is the number of VoIP
  sessions.
• The MUX sends out a multiplexed packet every T
  ms, which is equal to or shorter than the VoIP
  inter-packet interval.
• For GSM 6.10, the inter-packet interval is 20 ms.

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Multiplex-Multicast Scheme
DEMUX
DEMUX
MUX
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Multiplex-Multicast Scheme

• Problem
• Security!?

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Outline

• Introduction
• VoIP Multiplex-Multicast Scheme
• Capacity Analysis
• Delay Performance
• Conclusions

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Capacity Analysis

• consider the continuous-bit-rate(CBR) voice
  sources
• voice packets are generated at the voice codec
  rate
• focus on the GSM 6.10 codec
• the payload is 33 bytes
• the time between two adjacent frames is 20 ms

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Capacity Analysis

• n maximum number of sessions that can be
  supported
• Tdown Tup transmission times for downlink and
  uplink packets
• Tavg average time between the transmissions of
  two consecutive packets in a WLAN
• NP number of packets sent by one stream in one
  second
• 1/Tavg number of streams NP

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Capacity of Ordinary VoIP over WLAN

• OHhdr HRTP HUDP HIP HMAC
• OHsender
• if unicast packetOHreceiver
• Tdown Tup (Payload OHhdr) 8 / dataRate
  OHsender OHreceiver

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Capacity of Ordinary VoIP over WLAN

• n downlink and n uplink unicast streams
• Tavg (Tdown Tup) / 2
• 1/Tavg 2n Np
• n 11

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Capacity of Multiplex-Multicast Scheme over WLAN

• the RTP, UDP and IP header of each packet is
  compressed to 2 bytes
• Tdown (Payload 2) n HUDP HIP HMAC
  8 / dataRate OHsender
• Tavg (Tdown n Tup) / (n 1)
• 1/Tavg (n 1) Np
• n 21.2

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VoIP Capacities assuming Different Codecs
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Simulations

• increase the number of VoIP sessions until the
  per stream packet loss rate exceeds 1
• system capacity max number of sessions
• assume that the retry limit for each packet is 3

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Simulations

• for ordinary VoIP over WLAN, the system capacity
  is 12
• exceeding the system capacity leads to a large
  surge in packet losses for the downlink streams

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Analysis vs. Simulation

• Capacity of Ordinary VoIP and Multiplex-
  Multicast Schemes assuming GSM 6.10 codec

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Outline

• Introduction
• VoIP Multiplex-Multicast Scheme
• Capacity Analysis
• Delay Performance
• Conclusions

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Delay Performance

• voice qualitypacket-loss rates delay
  performance
• with ordinary VoIP
• local delay only the access delay within the
  WLAN
• at the AP time between the packets arrival
  until its successfully transmitted or dropped
• at the client time from when the packet is
  generated until it leaves the interface card

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Delay Performance

• with the M-M scheme
• local delay access delay the MUX delay
  incurred at the VoIP multiplexer (only downlink)
• MUX delay time from the packets arrival until
  the next one is generated
• we set a requirement no more than 1 of packets
  should suffer a local delay of more than 30 ms

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Access Delay

• ordinary VoIP scheme (12 sessions)
• in the AP average delay and delay jitter are 2.5
  ms and 1.4 ms
• in the wireless station average delay delay
  jitter are 1.2 ms and 1.0 ms
• if normally distributedless than 0.27 of the
  packets would suffer local delays larger than 30
  ms

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Access Delay

• Access Delays in AP and a Station in Original
  VoIP over WLAN when there are 12 Sessions

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Access Delay

• M-M scheme (22 sessions)
• in the AP average delay and delay jitter are 0.9
  ms and 0.2 ms
• in the wireless station average delay delay
  jitter are 2.0 ms and 1.5 ms
• no link layer retransmissions for the packets
  when collisions occur

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Access Delay

• Access Delay in AP and a Station in M-M Scheme
  when there are 22 Sessions

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Extra Delay Incurred by the Multiplex-Multicast
Scheme

• when a VoIP packet waits for the MUX to generate
  the next multiplexed packet
• we set the multiplexing period to be at most one
  audio-frame period
• 20 ms if GSM 6.10 codec is used
• random variable M the MUX delay
• assume M to be uniformly distributed between 0
  and 20 ms

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Delay Distribution for Ordinary VoIP

• When System Capacity of 12 is Fully Used

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Delay Distributions for Multiplex-Multicast Scheme

• When System Capacity of 22 is Fully Used

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Outline

• Introduction
• VoIP Multiplex-Multicast Scheme
• Capacity Analysis
• Delay Performance
• Conclusions

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Conclusions

• M-M scheme can reduce the large overhead when
  VoIP traffic is delivered over WLAN
• it requires no changes to the MAC protocol at the
  wireless end stations
• more readily deployable over the existing network
  infrastructure.
• it makes the voice capacity nearly 100 higher
  than ordinary VoIP