1/24 voip prioritisation over wlan for fixed mobile convergence brendan kearns, eircom séan murphy...
TRANSCRIPT
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VoIP Prioritisation over WLAN for Fixed Mobile Convergence
Brendan Kearns, eircom
Séan Murphy Performance Engineering Laboratory, University College Dublin
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WLAN Congestion Dominates Voice Quality
Light Load
Network Core
Access Point
Network Access
Congestion
Upstream DELAY
DownDELAY
Devices
Access Point
5/24
WLAN speed increasing so congestion unlikely?
11Mbit/s 802.11b6 calls
54Mbit/s 802.11g18 calls
100Mbit/s 802.11n40 calls
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In reality, capacity is not so high ….and
5Mbits Max 802.11b
<<6 calls
28Mbit/s Max 802.11g
<<18 calls
50Mbit/s Max 802.11n
<<40 calls
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Use Cases: Home and Enterprise
• Devices clustered around AP
• Good channel conditions
• High utilisation
Access Point
LaptopRadio
• Multiple devices give longer wait times
• Dispersal means lower bitrates
• Encryption reduces efficiency
• Congestion at lower utilisation
Access Point
LaptopRadio
The Home
The Enterprise
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Contents
• Contrast the WLAN access protocol, without and with QOS
• Test bed description
• How are voice / data are discriminated
• Experimental Results
• Conclusions
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Behaviour without QOS
• AP & devices contend for access
1 0 tx
1
3
0
2
time
tx
busy
busy
• As we start the AP is transmitting
2
4
• Handset, laptop pause for ‘wait time’
• Then start random backoff countertx
freeze
• Laptop gets medium
• Handset freezes counter
• Voice packet is delayed
AP
Upstream DELAY
DownDELAY
Devices
Access Point
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Behaviour without QOS…problems
• Universal wait and backoff times, thus
• voice can get trapped behind data traffic
• all traffic equally degraded in congestion
• No priority for access point, thus
• downstream traffic is penalised
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802.11 operation with QOS
• 802.11e & WMM
• WMM certification
• Shorter wait & backoff counter for voice
• AP brodcasts wait & backoff counter to wireless devices
• So downstream traffic is prioritised
• Now handset gets the radio
tx
wait
wait
tx
freeze 1 0 tx
2
4
1
3
0
2
time
AP
Upstream DELAY
DownDELAY
Devices
Access Point
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Test Bed
• VoIP traffic generation code
• Tx and Rx are time synchronised
Access Point
VoIP Call Generator
Server
Wireless Device Client 1
Call 1
Time Synch
Wireless Device Client 8
Call 8
• Eight calls sufficient to overload .11b
• Thus one way delay measured
Upstream DELAY
DownDELAY
Devices
Access Point
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QOS Switched off: all traffic is legacyAccess Point with WMM Switched Off
All Device Types
Legacy
Legacy
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QOS on: four traffic classes
LegacyUp
Best Effort*Down
VoiceDown
VoiceUp
1. Voice Downstream
2. Voice Upstream
3. Best Effort Downstream
4. Best Effort or Legacy Upstream
Access Point with WMM Switched On
LegacyDevice
WMM Device
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[EXPT 1] Enterprise: WMM switched off at AP
Calls started at 10 sec intervals
Each call lasts 90 sec
Graph of utilisation Vs time
It shows utilisation building up until seven calls are connected
Seventh Call
First Call
Congestion
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[Expt 1] Enterprise: WMM switched off at AP
Now view from latency perspective
Latency on vertical axis Vs Time
Good quality at six calls
On seventh call
Downstream garbled
Constant 400mS delay
Large packet loss
Upstream minimally impacted
Degraded Voice
0
100
200
300
400
500
600
Access Category = Legacy
Upstream Latency
For a Call
Downstream Latency
For a call
Seventh Call
WLAN user cannot hear B party
0
50
100
150
200
250
300
Time 0 - 90sec
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[Expt 2] Enterprise: Some handsets have QOS Mixture of legacy and WMM calls
Good quality at six calls
On seventh, legacy calls impacted
Upstream garbled
Massive delay
No packet loss
High latency because its got lowest priority
Downstream is not impacted
0
400
800
1200
1600
0
50
100
150
200
Legacy Downstream Latency
Legacy Upstream Latency
B party cannot hear
WLAN user7th
Calladded
Prioritised voice quality is protected
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[Expt 3] Enterprise: File Download
Latency shown for legacy call
Large UDP download for 20 sec
Downstream voice quality degraded
400ms delay
20% packet loss
Upstream is not impacted
Download Starts
WLAN user cannot hear B party
Legacy
Downstream
Latency
0
100
200
300
400
Legacy Upstream Latency
0
100
200
300
400
Repeat expt with prioritised call in progress
Now there is no impact on voice quality
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Summary
Downstream LatencyUpstream Latency
EXPT 1No WMM
Degraded Voice
Constant 400mS DelayLarge Packet Loss
EXPT 2Latency for Legacy Devices Competing
with Prioritised Voice
Degraded Voice
Massive DelayNo Packet Loss
EXPT 3Latency for Legacy Devices During Data
Download
Degraded Voice
Constant 400mS DelayLarge Packet Loss
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Conclusions
• WMM protects voice quality during congestion
• Absolute guarantees not possible with WMM
• QOS case is not compelling for home
• QOS can deliver significant benifits to the enterprise
This is the first published experimental study of QOS on commercially available hardware. It was configured with realistic QOS parameters …….
so the results presented are applicable to real implementations.……...
Dziekuje.……...
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Personal details: (1/2)
Name:
Surname:
Company:
Phone:
E-mail:
Brendan
Kearns
eircom
+353 1 701 7842
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Personal details: (2/2)
Name:
Surname:
Company:
Phone:
E-mail:
Sean
Murphy
UCD, Dublin
+353 1 716 0000