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VoWLAN: Call Quality September 2015 Brendan Kearns

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Eir is former incumbent in Ireland Quad Play Operator, number 1 in fixed and two in mobile IMS in place for Enterprise SIP Trunking & Hosted Office Next is VoWLAN and VoLTE Our aim is to maximise voice quality where we can We are considering implementation VoWLAN QOS in the home network We have tested VoWLAN quality in the Lab and field My own perspective M.ENG Thesis on VoWLAN in 2006 First experimental study on the effectiveness of 802.11e

Context: VoWLAN Eir

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How good can VoWLAN quality be and what degrades it? Describe WLAN contention and its problems Does QOS on WLAN work? experiments in the Lab and the home WLAN QOS design Conclusions

Contents

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Quality can be excellent and best was fullband Opus which samples at 48Khz … which is superior to DECT, OTT services and VoLTE. WLAN introduces delay and jitter when congested Some might say that WLAN is so fast that it will never be congested?? WLAN Headline speeds are impressive but throughput is much less One device transmits at a time And must wait for acknowledgements Wait and backoff times increase exponentially as number devices increases. Radio headers are transmitted at lower speeds than user data, Handsets drop down to lower speeds when channel conditions fall off

What Determines Voice Quality on IP?

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although the network may be well dimensioned for low latency and packet loss, the end to end quality for the user is contingent

on the traffic level on the wlan...

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802.11 Operation no QOS And With QOS

WLAN Access is Contention Based

User C is transmitting User C stops transmitting

A & B pause for ‘wait time’…then….

Start random backoff….

User B starts counting at a lower number so gets medium User A freezes counter

And gets medium after B has finished transmission

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tx

wait

wait

tx

freeze 1 0 tx

2

4

1

3

0

2

time

A

B

C

WLAN Collisions

A & B are waiting for C to finish They choose the same backoff countdown (2) So they transmit at the same time Packets collide and there is no ack, thus collission detected Now backoff counter doubles up Less likely that they choose same random backoff this time User B chooses a smaller countdown so he gets the medium

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tx

wait

wait

tx tx

2 1 0

2 1 0

tx 7 6 5

2 1 0

wait A

B

C

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Universal wait and backoff times Means that short voice packets must wait behind long data frames Downstream traffic is penalised Access point is just another station It transmits more than half of the total traffic So downstream voice packets must wait longer IEEE Defined 802.11e to resolve the problems AP reduces the time voice packets wait to transmit It is implemented in all WiFi chip sets It is called WiFi Multimedia extension (WMM)

Problems With WLAN

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Upstream DELAY

DownStream DELAY

Device

Access Point

Device Device Device

802.11e WLAN QOS Shorter wait and backoff time for voice Access point broadcasts longer wait & backoff times to wireless devices So traffic transmitted by access point is prioritised Radio can be reserved For n slots And Voice quality protected At medium cong No admission control

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tx

wait

wait

tx

freeze 1 0 tx

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4

1

3

0

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time n x 10 micro sec

A

B

C

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Experiments in the LAB Simulated RTP Flows with background UDP & TCP Load Extremly Accurate Latency, Jitter, Packet Loss Measurement

Upstream Packet Delay During MEDIUM UDP Congestion

Two existing voice calls One prioritised One not Prioritised

UDP stream loads WLAN to 70% In both cases latency increases to 100mS Upstream voice is not impacted at medium congestion

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Downstream Packet Delay During Medium UDP Congestion

Prioritised voice is protected

Latency is increased to 100mS

Non Prioritised Voice is garbled

Latency goes to 400mS Buffers are filled and packets are dropped

This is because it is transmitted by the AP

And the AP is transmitting most of the traffic on the WLAN

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150200

250300

350400

450

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TCP has less impact on voice quality than UDP Both prioritised and non prioritised voice calls were little impacted

TCP Unfairness But TCP throughput was reduced A Problem? Upstream TCP ack can be blocked by prioritised upstream voice Solution: Do not implement upstream prioritisation OR engineer the access point QOS parameters

WLAN Loaded up to 80% Utilisation with TCP

0.00%

20.00%

40.00%

60.00%

80.00%

100.00%

1 5 9 13 17 21 25 29 33 37 41 45 49

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WLAN QOS in the Home Inject known audio stream into active call Audio received compared with original Gives accurate quality score UDP, TCP load generated with iPerf or torrent

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Two floor house with no internal concrete VDSL 50Mbit/s with IPTV Service, 802.11n Access Point

WLAN modulation / signal strength during VoWLAN call below 0 – 300 seconds

Call quality remained good until 30M from AP During 10Mbit/s torrent download reach reduced to 11M from AP During 35Mbit/s download downstream voice was garbled close to AP

VoWiFi Quality Urban House

Voice Quality is Protected in the Home

IMS client running on iPhone Without congestion PESQ is 4.4 When WLAN is loaded then Downstream degraded first PESQ = 1.1

Then Upstream Degraded Congestion begins at 60% WiFi QOS protects voice up 80% utilisation Reach of the WLAN through walls and distance is increased by approximately 10% when prioritised – during light to medium congestion (60-75% Utilisation)

downstream

upstream

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Two floor house with all concrete walls & floors ADSL 3Mbit/s

WLAN modulation / signal strength during VoWLAN call Downstream Call quality garbled upstairs WLAN repeater required to give full indoor coverage

Implementation of QOS QOS makes little difference as WLAN is never congested

VoWiFi Quality Rural Concrete House

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Deployment of QOS in The Home

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Voice can be prioritised on WiFi if three conditions are met (1) Voice packet is DSCP marked In the downstream by the IMS A Problem?

The AP can read layer 2 or layer three markings Eir layer two markings not aligned with 802.11e and cannot be changed Can Home Gateway be configured to ignore layer two .1p markings?

(2) Operating system of the device passes DSCP to the WiFi chipset We found that WMM itself is supported Android & IOS Access points do have configuration screens to turn WMM on / off Some can to alter the change wait times for the four classes

(3) WMM is implemented WiFi chipset All WiFi chipsets implement WMM

Requirements For WMM

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At Network level Voice is prioritised in both directions at layer two Home gateway marks upstream

Voice packets scheduled out of gateway before Internet

Passed through access network in VLAN with Pbit set

VLAN – MPLS – VLAN mapping through access and core

Only downsream WLAN traffic is prioritised SBC sets DSCP for downstream

This leads to Following Solution

HGW 7750

7750 DSCP

Marking In IMS

Layer 2

DSCP

SIP Client

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Conclusion

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Turning on WMM at access point provides significant protection for voice traffic against competing wireless data traffic

Turning on WMM at the client makes only a small difference where there are a small number of clients on the wireless LAN. This plus the “TCP Unfairness” problem means that it can be omitted.

All Home gateways support WMM but their firmware may need to be altered to prioritise on DSCP rather than layer two

Conclusion

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Extra Slides

Other measures to improve vowlan call experience

When the user is inside the “rove in” volume client registers The user has IMS calling available inside the “rove out” outside which client de-registers from the IMS.

Before de-register there is a transition volume where voice quality deteriorates, physically 1-3M wide, and where the user is warned through an audible beep that the call is about to drop.

in a low – medium loaded WLAN voice reach is approximately 15% less than data reach. During high load, which induces packet delay, voice reach shrinks faster because of its greater sensitivity to delay. QOS slows this shrinkage,

Rove In Bad

Voice

No Voice

No WLAN