As we design our wireless LANs, we all need to be aware of how some of the latest smartphones, tablets and laptops operate over Wi-Fi. Their speeds, their roaming behavior, rate vs. range characteristics, CPU/application performance all matter. Join us in this session to discuss with your peers To learn more, visit us at http://www.arubanetworks.com/wlan. Join the discussion at https://community.arubanetworks.com

1. Mobile Devices and Wi-Fi
Peter Thornycroft
March 2014

2. CONFIDENTIAL
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2 #AirheadsConf
Agenda
The commercial value chain
Consumer device reference models
Battery life
QoS
Location
5GHz and DFS channels
Authentication & Passpoint
Handover behavior

3. 3
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Commercial models
• What we see:
– The chain leads to the
cellular operator and
consumer
• What we want to see:
– Some recognition for the
enterprise user
Consumers (your typical Gen-Y) who
don’t care too much about Wi-Fi
performance at work
Chip vendor incorporates driver, is
really responsible for Wi-Fi
functionality, selling to …
Phone / device vendor who has cost
constraints, won’t waste time on
features not of interest to its biggest
customers who are…
Cellular Operators, for whom Wi-Fi
is a minority interest in the first place
and anyway sell to …
Mobile OS
vendor does
some
influencing

4. 4
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WLANs differ from home APs
Home AP reference model
A single AP, not doing much of interest
WLAN reference model
Many, APs with same SSID and coordinated,
seamless handover (no DHCP, common
authentication etc.)
- No point in looking for other APs because
there (usually) aren’t any
- Established (~correct) behavior is to hang
onto the AP until the signal is very weak,
then switch to cellular data if available
- There is always a ‘better’AP
- But the device needs to scan (or use neighbor
report) to be aware of the ‘better’AP.
Benefits of good WLAN client behavior…
- Devices get higher rates
- Other devices get more airtime, better network
capacity
- Less time on the air - better battery life
- Less mutual (co-channel) interference
Same effects are seen in public places, hot zones – ‘always best connected’ activity in Hotspot 2.0 ph3 groups.

5. 5
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Network reference models
• What we see:
– One dual-band home AP
– “give me battery life, and
keep me connected”
• What we want to see:
– Option for multiple-AP WLAN
The current model is the single-AP home network. In this
framework, the best thing is to hold onto your AP until the
signal is too weak to work, then hope you can switch to cellular
data. Probe requests are a waste of battery life because there’s
only one AP.
We want to see either a dual-model or a more flexible
architecture. Maybe sense that there are other APs in the same
system (spot the neighbor report?) and flip to a multi-AP
algorithm.
Under a multiple-AP network, there is always a really-good
signal (except at the edge). It’s just a question of probing
more often to find the better APs.
But it’s difficult to move device, OS and chip vendors away
from their well-established model. They are wary of breaking
what has taken several years to ‘perfect’.
We’ll also see that consumer APs still don’t offer the advanced
features we incorporated some years ago.

6. 6
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Power Save Modes
sleeping
time
beaconDTIM
Traffic for
you
give sleeping
WMM-PS
beaconDTIM
pkt
Traditional Power-Save
U-APSD (WMM-PS)
pkt
pkt
pkt
pkt
pkt
pkt
pkt
pkt pkt
pkt
buffered
time
DTIM

7. 7
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Battery life
• What we see:
– Minimum possible probing
• What we want to see:
– More probe requests in
WLAN
– Using 11k reports
– U-APSD within a beacon
interval
Mobile devices are usually unaware of better AP signals
because they don’t probe enough.
They don’t probe enough because of an over-zealous focus on
battery life, and a model that has only one AP.
Sometimes when a device has an ‘acceptable’ signal it stops
probing altogether. Later, when it starts to move, it may not re-
enable probing until too late to maintain the connection.
In fact, Wi-Fi accounts for less battery consumption than the
cellular subsystem, and far less than the display or CPU
processing app tasks and GPU.
So our focus is on showing device vendors they can ‘go
passive’… only using the 802.11 radio in receive mode.
‘WFA Voice-Enterprise light’, or a collection of features that
enable the device to be multi-AP-aware without reducing
battery life.

8. 8
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The mystery of missing
smartphone QoS
Android
App Code
(QoS – unaware coder)
Driver & microcode
Multi-level QoS
priority API
(that’s OK)
Parrots the driver
API (that’s not OK)
Can’t spell QoS
anyway so it’s
inconsequential
Wi-Fi air interface
• QoS priority (~WMM)is there if
app developers want to use it
• But… it’s not documented And
anyway… app developers are
not QoS-aware
– Socket.setTrafficClass(int value) IPTos
• The OS has a hard time figuring out
the QoS Pri required by each app…
• Thus WMM priority is seldom used in
mobile device apps
Same observations apply to WMM-PS (U-APSD) for intra-beacon power save.

9. 9
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QoS
• What we see:
– WMM functionality exists in
mobile device OS
– But APIs are arcane
– No documentation or
promotion
• What we want to see:
– Better API support
– Developer guidelines
WMM QoS is enabled through the OS to the chip/driver.
But to invoke a high-priority connection, the app developer
must add some parameters to the commands that open sockets .
App developers are unaware of the need to apply Wi-Fi QoS,
and/or are not informed of the required APIs, and/or are not
technically capable of understanding that aspect of app
programming.
This includes developers of voice and video apps including
those in vertically-integrated companies.

10. 10
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Location (distance)
enhancements
RTT “Round-Trip-Time”
A standard (actually two standards
and several proprietary variants)
“802.11k”
Location Track Notification,
Modified (to finer timestamps) in
“802.11mc”
Fine Timing Measurements
Distance Calculations
Measure
with me!
Now
here are
my times
t1, t4
OK, here
t1
t3
t4
t2
Challenges:
- Need to combine/average several
frames to get a good reading.
- Averaging many frames affects
battery life, network capacity
Challenges:
- Measuring to nanoseconds
(speed of light: 1 ft per nsec)
- Setting up circuitry to
timestamp the right frame
- Calibration for time frame
leaves (arrives) at the antenna
Once all four timestamps are in one
place, subtraction and /2 gives time-
of-flight and multiply-by-speed-of-
light gives distance
Got
it
Implementation
In mobile device Wi-Fi chips late
2014
In access points 2015 (early
implementation 2014)
No Wi-Fi Alliance certification >>
may cause interoperability teething
troubles
Accuracy should be 1 – 5 metres,
depending on the number of frames
averaged & underlying hardware
Most useful in line-of-sight, but
better accuracy at longer distances
than RSSI
Many variations possible with
WLAN topologies
d = ((t4 – t1) – (t3 – t2)) * c / 2

11. 11
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Location
• What we see:
– RSSI reports
• What we want to see:
– RTT support
– Raw data for RTT, RSSI
Location and location-based-services have attracted the
attention of many commercial and technical principals across
the industry.
Current development is focused on time-based distance
(mostly Round-Trip-Time) measurements:
- 802.11mc Fine Timing Measurement
- Wi-Fi Alliance Wireless Network Management ++
- In-Location Alliance
Look for RTT announcements and features over the next 12
months.
There is a significant danger that this location technology
reverts to proprietary, closed islands rather than developing
along open, standard APIs.
For example:
- Will raw data be available via OS API calls, or mysteriously
processed within the chip/driver or OS itself?
- Will devices built on different chip families interoperate for
RTT location?

12. 12
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DFS channels – useful at last!
How many radar triggers?
frequency
insallations
0 / year 5 / hour
Usually none, but in some places
> comfortable
Devices supporting DFS
Apple > 2 years
Intel > 2 years
Samsung > 1 year
Others getting there
Most
WLANs
A few
Special concerns
No active client scanning in DFS
bands because they don’t passive-
scan for radar
- slow AP acquisition
- fixed (eventually) by neighbor
report
5GHz Channel count
13 20MHz channels, no DFS
22 20MHz channels including DFS
Channel strategy
Dot them around?
Use the spectrum!

13. 13
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5GHz band
• What we see:
– Beginning to favor 5GHz
over 2.4
– Spreading DFS support
• What we want to see:
– Overweight 5GHz bias
– 100% DFS support
About 18 months ago Apple supposedly reversed from
unconditionally preferring 2.4GHz to favoring 5GHz.
Unfortunately the battery-saving imperative (see earlier) means
that when a device has an acceptable signal from its AP, it will
stop scanning for a better one. Especially scanning in other
bands.
This can cause difficulties when the WLAN seeks to move a
device to a different band: it may refuse to scan the alternate
band.
DFS support is improving, now available on all Apple devices
(since iPhone 4S) and many Android (since early 2013: e.g.
Samsung Note, Galaxy S4).
We believe this is a good time to start deploying DFS channels.

14. 14
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Passpoint
Identify a hotspot with
Internet reachability and
friendly authentication
Pre-association discovery
What
have you
got?
T-Mobile
BT
Comcast
Orange…
- Pre-association
- New GAS/ANQP protocol
- Lists service providers
- Acceptable authentication
Authenticate to home SP
T-Mobile BTOrange
Accuris
Aicent
BSG…
Hub
(settlement)
RADIUS
e.g. DIAMETER
WPA2 Options
- EAP-TLS
- EAP-TTLS
- EAP-SIM
- EAP-AKA(‘)
Make a list of available
options, decide which to use
Prioritise account options
T-Mobile home (have SIM)
BT visiting (have pwd)
Orange visiting (have pwd)
Comcast visiting (have cert)
Home AP (not Passpoint)
Local (not Passpoint) hotspot
SPs, phone designers all want a
say
- Distinction between ‘home’
and ‘visiting’ hotspot
- May have different tariffs
- Policy for time-of-day,
location…
ANDSF is a cellular protocol that can pass policy to the device to help it make offload decisions.
Passpoint phase 2 introduces se mi-automatic online sign-up and policy services.
T-Mobile SIM

15. 15
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Authentication
• What we see:
– Beginning to support HS2.0
(Passpoint)
• What we want to see:
– Passpoint with EAP-SIM
everywhere
– SPs supporting Passpoint
Passpoint (Hotspot 2.0, from 802.11u) was released as a WFA
certification in June 2012.
For the following 12 months, while SP and enterprise WLAN
equipment supported Passpoint, you could not purchase a
commercial device that was compliant.
That has changed in the last 6 months (iOS7, Samsung Galaxy
S4). Now, we realize that no SP has deployed a network with
standard HS2.0 support.
Why not?
- Actually, NTT has…
- AT&T stayed proprietary
- Cellular operators (see commercial chain above) have no
incentive to allow others (MSOs) to steal their customers
- The enterprise WLAN vendors are waiting for wider
availability
But it’s time!
Public facing vendors should take AOS 6.4, contact a hub
vendor, fire it up and advertise support.

16. 16
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Current handover narrative
A
Good signal, this is dandy!
Time / distance
0 sec
Signal Strength

17. 17
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Current handover narrative
A
Good signal, this is dandy!
OMG, the signal is getting
really low!
Time / distance
0 sec ~30 sec
Signal Strength

18. 18
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Current handover narrative
A
Good signal, this is dandy!
OMG, the signal is getting
really low!
SOS, sending 10 probe
requests on 3 channels
Time / distance
0 sec ~30 sec 35 sec 38 sec
Signal Strength

19. 19
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Current handover narrative
A
B
C D
E
Good signal, this is dandy!
OMG, the signal is getting
really low!
SOS, sending 10 probe
requests on 3 channels
Wowza, responses from 20
APs, how to choose?
Time / distance
0 sec ~30 sec 35 sec 38 sec
Signal Strength

20. 20
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Current handover narrative
A
B
C D
E
Good signal, this is dandy!
OMG, the signal is getting
really low!
SOS, sending 10 probe
requests on 3 channels
Wowza, responses from 20
APs, how to choose?
Let’s reauthenticate with
this one!
Time / distance
0 sec ~30 sec 35 sec 38 sec 40 sec reauthentication request
40.2 sec reauthenticated
Signal Strength

21. 21
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802.11 k, v, r
• Many features, most important are:
• Neighbor report from AP to client (802.11k)
• Channel report from AP to client (802.11k)
• Beacon report from client to AP (802.11k)
• BSS Transition Management from AP to client (802.11v)
• Fast Transition by client (802.11r)
• (All rolled up in 802.11-2012, 2014)

22. 22
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802.11r fast BSS transition
C D
R0 key
C
802.1X
authenticator
R0 key
S0 key
S1 key
PTK
Initial Authentication
establishes level 0 key
WLAN distributes
level 1 keys
R1 key
C D
On reassociation,
client presents level
1 key to new AP
R1 key
PTK
S0 key
S1 key
PTK
Mobility domain: A group of
APs covered by a level 0
keyholder
Over-the-air reassociation
widely adopted, over-the-DS
reassociation (via the current
AP) not used
Key suite includes:
Level 0 key (derived at initial authentication,
never exposed OTA)
Level 1 key (per-AP keys) used to derive…
Pairwise temporal keys (to encrypt
communication)
PTK
R1 key
Differences between FT and OKC? … Not much
keyscope keyscope

23. 23
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802.11k, v, r features
B
C D
E
Neighbor report
AP chan secy key beacon
scope offset
B 6 WPA2 0 45
D 52 WPA2 0 12
E 161 WPA2 0 74
Information about other APs to help
with handover candidate discovery
C
Beacon report
Client reports how it hears (RSSI)
the beacons of other APs
BSSID RSSI
AP B -65
AP D -72
AP E -65
C
BSS Transition Management
AP instructs client to move to
another AP
Move to AP D…
E
DB
D
C
Channel report
AP informs client of channels used
by the WLAN
Channel
6
52
161
Overlaps with neighbor report

24. 24
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802.11k Neighbor report
• Advertised by AP in the beacon (for all clients, non-
associated) and sent solicited per-client
• List of ‘neighbor’ APs with same SSID includes:
– BSSID
– Channel
– Beacon time offset
– PHY type
– QoS capability
– ‘Key scope’ for common authenticator
• 802.11 does not require neighbor list to be cropped or
ordered or modified per-client (but infrastructure may do so)
• Eliminates the need for active probe request-response
scanning

25. 25
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The evils of active scanning
• Takes time
– Need to probe on each selected channel in turn, wait ‘reasonable’ interval for responses
– Need to return to current channel for beacon (DTIM)
• Inaccurate results
– RSSI of a single probe response varies ~ +/- 6dB from ‘average’
– Some APs will miss probe requests, or responses are lost
– If the device returns to current channel after ~15msec, sometimes misses responses
• Consumes power
– Typical pattern is to send 2 probe requests per channel, stay awake ~15–20msec
– Each probe request generates ~6 probe responses in a ‘typical’ WLAN
– Each probe response needs an ack
• Consumes airtime, affecting others’ performance
– Frames are sent at low rates, probe responses are retried

26. 26
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Better handover performance
with ‘11k’
Current handover sequence:
1. Figure out it’s time to scan
2. Figure out channels to scan
3. Send probe requests, get responses
4. Identify best AP
5. Reauthenticate to new AP
802.11k handover sequence:
1. Periodically request neighbor report
2. Passive scan for neighbor beacons
3. Note if a neighbor AP is ‘better’
4. Reauthenticate to new AP
Probe requests & responses
Signal strength
Time, distance
Signal strength
Time, distance
Behavior c 1999 Behavior c 2013
Signal strength
Time, distance
Neighbor reports & passive scanning
Behavior c 2014 ?

27. 27
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Signal Strength
Proper ‘11k’ handover narrative
A
Good signal, this is dandy!
Time / distance
0 sec

28. 28
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B
C
D
Signal Strength
Proper ‘11k’ handover narrative
A
B
C D
E
Good signal, this is dandy!
Check neighbor report
every ~10sec
Identify ‘best’ AP and check
for beacon (passive scan)
Time / distance
0 sec ~10 sec 20 sec 30 secB
C
C
D

29. 29
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Signal Strength
Proper ‘11k’ handover narrative
A
B
C D
E
Good signal, this is dandy!
Check neighbor report
every ~10sec
Identify ‘best’ AP and check
for beacon (passive scan)
Signal is low, but I have
already identified the best AP
Time / distance
0 sec ~10 sec 20 sec 30 secB
C
B
C
D
C
D

30. 30
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B
C
B
C
D
C
D
D
C
Signal Strength
Proper ‘11k’ handover narrative
A
B
C D
E
Good signal, this is dandy!
Check neighbor report
every ~10sec
Identify ‘best’ AP and check
for beacon (passive scan)
Signal is low, but I have
already identified the best AP
Reauthenticate
Time / distance
0 sec ~10 sec 20 sec 30 sec 30 sec reauthentication request
30.2 sec reauthenticated

31. 31
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Client Match
Client Match forms a virtual Beacon Report:
1. APs measure RSSI from client
2. APs receive beacon reports from the
client
3. Estimate the ‘best’AP
4. If client is _far_ from ‘best’AP…
5. Redirect (force handover) to ‘best’
AP
B
C D
E
A
track
-50
-60
-70
-80
AB E
Signal strength
distance

32. 32
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If 11k, why Client Match ?
• ‘11k’ makes information available to the client
– Neighboring APs, channels, beacon offsets…
• ‘11k’ cannot confirm that the client receives information or how it prioritises
the information
– Neighbor report information may not be used
• Transmitting (or receiving) ‘11k’ does not guarantee that the client will act on
the information
– Handover decisions may not be improved
• Client Match uses information from the infrastructure and the client (if
supports beacon reports)
– The infra knows more about the client’s situation than the client does
• Client Match completes the task by forcing a handover

33. 33
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Handover
• What we see:
– Not much
• What we want to see:
– More probe requests when
in WLAN
– Or… use passive 11k
reports
– Reauthenticate with
802.11r or OKC
Most people think inter-AP handovers take ~1second.
In fact, inter-AP handovers take 30msec, or 250msec, or 7sec
depending on the syndrome.
7sec outages occur when a device (not probing) does not
realize until too late that the signal from its serving AP is
dropping fast. By the time it starts to probe, it has lost the AP
and has to go into cold-start mode. More frequent probes (or
using passive measures as above) would eliminate 7 sec
outages.
Full WPA2 MSCHAPv2 re-authentication takes 200-250msec
to exchange ~50 frames (including acks). This is a stable
figure in the absence of very weak signals due to poor choice
of target AP (mobile devices usually make good AP choices
when aware of their environment through probing). This
outage will be barely noticeable to the user.
But faster re-authentication is possible, through old-school
OKC (from 802.11i) or 802.11r (now available on iPad).
… The ‘bad’ handover syndrome can be solved if the mobile
device is more aware of its surroundings (neighbor report) or
responds to BSS transition management frames (directed
handover from the AP).

34. 34
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Aruba Utilities shows behaviour
• What we see:
– Frequent long outages
around handover events
• What we want to see:
– More awareness of
environment
– Faster reaction to losing
signal
Aruba Utilities shows very graphically what goes on
when a mobile device moves around an enterprise
WLAN.

35. 35
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Thank You
#AirheadsConf