1) VoWLAN call quality can be excellent, comparable to DECT and VoLTE, but is degraded by delay and jitter caused by congestion on the WLAN. 2) 802.11 contention introduces delay and jitter as devices must wait to transmit over the shared medium. WLAN QoS aims to prioritize voice packets to reduce this impact. 3) Experiments show that with medium WLAN congestion, downstream unprioritized voice is degraded but prioritized voice is protected; with heavy congestion both are affected.

1. 1
VoWLAN: Call Quality
September 2015
Brendan Kearns

2. 2
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
2

3. 3
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
3

4. 4
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?
4
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...

5. 5
802.11 Operation no
QOS
And With QOS

6. 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
6
tx
wait
wait
tx
freeze 1 0 tx
2
4
1
3
0
2
time
A
B
C

7. 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
7
tx
wait
wait
tx tx
2 1 0
2 1 0
tx 7 6 5
2 1 0
waitA
B
C

8. 8
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
8
Upstream
DELAY
DownStream
DELAY
Device
Access Point
Device Device Device

9. 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 9
tx
wait
wait
tx
freeze 1 0 tx
2
4
1
3
0
2
time n x 10 micro sec
A
B
C

10. 1
0
Experiments in the LAB
Simulated RTP Flows with background UDP & TCP Load
Extremly Accurate Latency, Jitter, Packet Loss Measurement

11. 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
11

12. 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
0
50
100
150
200
250
300
350
400
450
0
20
40
60
80
100
120

13. 1
3
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

14. 1
4
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

15. 1
5
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

16. 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

17. 1
7
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

18. 1
8
Deployment of QOS in
The Home

19. 1
9
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
19

20. 2
0
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

21. 2
1
Conclusion

22. 2
2
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
22

23. 2
3
Extra Slides

24. 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