Downloaded 321 times
Uploaded byAruba, a Hewlett Packard Enterprise company
Mobile Devices and Wi-Fi
The document outlines the challenges and opportunities in optimizing mobile devices and Wi-Fi networks, emphasizing the need for improved battery life and better user experiences with multi-access point (AP) networks. It discusses the limitations of current consumer devices, the importance of quality of service (QoS) and location-based services, and the developments in 802.11 standards such as Passpoint and fast BSS transition for seamless connectivity. The author advocates for industry collaboration to enhance wi-fi performance, enabling users to access better connections across various environments.
More Related Content
Similar to Mobile Devices and Wi-Fi
More from Aruba, a Hewlett Packard Enterprise company
Mobile Devices and Wi-Fi
- 1. Mobile Devices andWi-Fi Peter Thornycroft March 2014
- 2. CONFIDENTIAL © Copyright 2014.Aruba Networks, Inc. All rights reserved 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 CONFIDENTIAL © Copyright 2014.Aruba Networks, Inc. All rights reserved #AirheadsConf 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 CONFIDENTIAL © Copyright 2014.Aruba Networks, Inc. All rights reserved #AirheadsConf 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 CONFIDENTIAL © Copyright 2014.Aruba Networks, Inc. All rights reserved #AirheadsConf 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 CONFIDENTIAL © Copyright 2014.Aruba Networks, Inc. All rights reserved #AirheadsConf 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 CONFIDENTIAL © Copyright 2014.Aruba Networks, Inc. All rights reserved #AirheadsConf 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 CONFIDENTIAL © Copyright 2014.Aruba Networks, Inc. All rights reserved #AirheadsConf 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 CONFIDENTIAL © Copyright 2014.Aruba Networks, Inc. All rights reserved #AirheadsConf 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 CONFIDENTIAL © Copyright 2014.Aruba Networks, Inc. All rights reserved #AirheadsConf 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 CONFIDENTIAL © Copyright 2014.Aruba Networks, Inc. All rights reserved #AirheadsConf 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 CONFIDENTIAL © Copyright 2014.Aruba Networks, Inc. All rights reserved #AirheadsConf 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 CONFIDENTIAL © Copyright 2014.Aruba Networks, Inc. All rights reserved #AirheadsConf 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 CONFIDENTIAL © Copyright 2014.Aruba Networks, Inc. All rights reserved #AirheadsConf 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 CONFIDENTIAL © Copyright 2014.Aruba Networks, Inc. All rights reserved #AirheadsConf 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 CONFIDENTIAL © Copyright 2014.Aruba Networks, Inc. All rights reserved #AirheadsConf Current handover narrative A Good signal, this is dandy! Time / distance 0 sec Signal Strength
- 17. 17 CONFIDENTIAL © Copyright 2014.Aruba Networks, Inc. All rights reserved #AirheadsConf 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 CONFIDENTIAL © Copyright 2014.Aruba Networks, Inc. All rights reserved #AirheadsConf 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 CONFIDENTIAL © Copyright 2014.Aruba Networks, Inc. All rights reserved #AirheadsConf 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 CONFIDENTIAL © Copyright 2014.Aruba Networks, Inc. All rights reserved #AirheadsConf 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 CONFIDENTIAL © Copyright 2014.Aruba Networks, Inc. All rights reserved #AirheadsConf 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 CONFIDENTIAL © Copyright 2014.Aruba Networks, Inc. All rights reserved #AirheadsConf 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 CONFIDENTIAL © Copyright 2014.Aruba Networks, Inc. All rights reserved #AirheadsConf 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 CONFIDENTIAL © Copyright 2014.Aruba Networks, Inc. All rights reserved #AirheadsConf 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 CONFIDENTIAL © Copyright 2014.Aruba Networks, Inc. All rights reserved #AirheadsConf 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 CONFIDENTIAL © Copyright 2014.Aruba Networks, Inc. All rights reserved #AirheadsConf 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 CONFIDENTIAL © Copyright 2014.Aruba Networks, Inc. All rights reserved #AirheadsConf Signal Strength Proper ‘11k’ handover narrative A Good signal, this is dandy! Time / distance 0 sec
- 28. 28 CONFIDENTIAL © Copyright 2014.Aruba Networks, Inc. All rights reserved #AirheadsConf 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 CONFIDENTIAL © Copyright 2014.Aruba Networks, Inc. All rights reserved #AirheadsConf 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 CONFIDENTIAL © Copyright 2014.Aruba Networks, Inc. All rights reserved #AirheadsConf 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 CONFIDENTIAL © Copyright 2014.Aruba Networks, Inc. All rights reserved #AirheadsConf 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 CONFIDENTIAL © Copyright 2014.Aruba Networks, Inc. All rights reserved #AirheadsConf 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 CONFIDENTIAL © Copyright 2014.Aruba Networks, Inc. All rights reserved #AirheadsConf 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 CONFIDENTIAL © Copyright 2014.Aruba Networks, Inc. All rights reserved #AirheadsConf 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 CONFIDENTIAL © Copyright 2014.Aruba Networks, Inc. All rights reserved Thank You #AirheadsConf