QoE-driven Wi-Fi Selection Mechanism for Next Generation Smartphones

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QoE-driven Wi-Fi Selection Mechanism for NextGeneration Smartphones

Hyun Soon Kim, Eugene Kim, Hwangnam KimSchool of Electrical Engineering, Korea University

Seoul, Republic of Korea 136075Email:{gustns2010, ekim57, hnkim}@korea.ac.kr

AbstractSmartphones and portable devices are widely usedthese days. Data transmission networks such as 3G and Wi-Fiare used for most smartphones, and LTE is quickly increasingits market share in order to support enormous network userswho cause data congestion on every part of network topology.Especially, wireless part of network suffers from starvation andcongestion due to many users who wish high quality Internetservice such as HD video streaming. In this paper, we propose anefficient mechanism of auto-connecting qualified Wi-Fi network forthe next generation smartphones in order to provide acceptableInternet service for device users. One of authentication-free access

points and others that are provided by contracted Internet serviceprovider can be automatically connected with the mechanism. Also,quality of experience (QoE) of the connected AP is guaranteed byestimating Signal to Interference and Noise Ratio (SINR) whichis derived by the number of stations sharing the same channel.We implemented the proposed mechanism in both Android phoneand also OPNET simulation, and also we conducted a performanceevaluation study. The simulation results indicate that the proposedmechanism retains the best set of candidate APs to be connectedand improves the MAC layer throughput.

I. INTRODUCTION

Desire of high quality wireless network service is unex-

pectedly arising due to the boom of smartphones and Tablet

PCs. Large screen size compared to feature phones results inthe demand of high quality video or picture so that screen

resolution is always an update issue for new models of those

hand-held devices. High quality screen resolution in turn causesthe necessity of qualified network service that supports high

definition video streaming. Many Internet service providers

strive to build up APs to cover crowded zones with too many

users (which is called offloading) and develop new generation ofwireless networks such as WiMAX, LTE and WiBro to achieve

bursting user requirements. SK Telecom and LG U+, telecom

companies in Korea, already have launched LTE data serviceon the year 2011. Huge amount of investment has been already

made to commercialize LTE networks in Korea [1].

The use of wireless Internet service is not restricted to beperformed only at cellular networks but also other networks

such as WiMAX and Wi-Fi. Telecom companies in South

Korea have already constructed millions of Wi-Fi APs [2] toserve the customers who want to use qualified Internet service.

Indeed, most of hot-spots around cities perform much better

than original 3G network service in terms of transmission speed.However, a complete auto-configuration for switching back and

forth between Wi-Fi and Cellular Network and even between

APs is not supported yet, so that the switching is done manually

by device users.

On the other hand, IEEE 802.21 protocol [3] specifies media-independent handover service to provide skeleton of inter-

networking to Internet service providers. Also, it motivates

many researchers to focus on inter-networking of cellular andISM band network to provide better wireless Internet service

to service users [4]. Inter-networking and vertical handover

of different networks have been already structured, designedand proposed in perspective of the network layer. However

all those techniques assume the AP and STA to be connected

automatically in perspective of the link layer. In addition, each

network region is covered by multiple service providers and theydo not collaborate to support all the customers. A customer Cthat uses network of a service providerSshould use APs that areprovided by the service providerS, so that the Internet service isnot guaranteed to be used anywhere. Some places might have no

AP belonging to the service provider S. On the contrary, someplaces might have APs that can support the Internet service tothe customer C but there are too many customers served bythose APs, so that the service quality cannot be guaranteed

Many smartphones connects to an AP which is recorded on the

connection history database. However, the way of automaticallyconnecting AP usually results in bad Wi-Fi performance or

the failure of service continuity, which consequently turns into

using expensive cellular network without any notice to usersAlso, unnecessary handover delay and unsatisfactory QoE of

the Internet service may occur by connecting to those bad APs.

Based on these observations, we propose an efficient mecha-nism of auto-connecting a mobile device to the best Wi-Fi AP,

which guarantee the connected AP is much more qualified in

serving the Internet services than an AP that is determined bythe current connection mechanism. The proposed mechanism

consists of two schemes where the one is to discover the best

set of AP candidates for the Internet connectivity and the other

is to estimate the link quality of each AP candidate so as tochoose the best qualified AP. We implemented the proposed

mechanism in Android phone and also constructed a OPNET

simulation environment to carry out a performance evaluationstudy. The results present that the proposed mechanism can

increase the availability of network connectivity and also im-

prove the MAC throughput via qualified APs. We vision thathe proposed QoE-driven AP selection mechanism contributes

to better vertical handover solutions and the next generation

smartphones will adopt it to support complete inter-networkingamong heterogeneous networks.

The remainder of the paper is organized as follows. We

reveal default Wi-Fi connection problems in section II, and

then propose a new QoE driven AP selection mechanism in

2012 The First IEEE Workshop on Enabling Technologies for Smartphone and Internet of Things (ETSIoT)

978-1-4673-2557-8/12/$31.00 2012 IEEE 13


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section III. We present a performance evaluation results in

section IV, and finally conclude the paper with section V.

I I . DEFAULTW I-FI C ONNECTION P ROBLEMS

In this section, we briefly reveal the main drawback ofthe current AP connection in mobile devices and analyze the

underlying causes for the drawback.

A. Basic Connection Procedure

The basic procedure of Wi-Fi Connection in current mobiledevices is composed of three steps: scanning, authentication

and association. Of course, roaming is an ideal approach to

re-associate seamlessly, and so many Internet service providersare continuing to deploy this roaming capability to their APs.

Note that the ultimate ideal approach is to unify all the APs

deployed from different service providers to make devices move

around over any network freely. However, this will not come innear future because expenses and benefits for service providers

should be considered appropriately. Therefore, a complete new

connection (disconnection due to signal loss and then connec-tion trial after few seconds of continuous disconnection state)

is the basic connection procedure in these days. Smartphonesuse the following procedure of connection:

The antenna enables its receiving mode to sense beaconmessage from any AP surrounding the device;

If the antenna senses no beacon message during a time

period (usually 100 ms), it switches to another channel torepeat the process;

If the antenna receives some beacon, the physical later

checks whether the configuration of the AP is found inthe device;

If there is no history of connection which matches with the

AP, the current scanning continuously or open next beacon

message to repeat the process;

If the configuration history confirms that the AP is con-nectable, do authentication and association to complete

connection procedure;

Scanning usually takes about 300ms to a few seconds untilit finds an appropriate beacon to use for connection. At least

100msfor each completely non-overlapping channel is scannedbecause each AP usually sends its beacon frame with theperiod of 100ms. In addition to this default connection scenario,

commercialized devices use a configuration history, which saves

all information necessary to re-connect the same Service SetID (SSID), to see whether or not the received beacon contains

SSID that has been recorded in that history. Service providers

usually use the configuration history to make users automati-

cally connected to their own APs for convenience. The processof this connection scenario is optimally fast since the scanning

time is optimized. If any beacon is received and confirmed to

contain the pre-specified connectable AP information, the deviceconnects to the AP right away without carrying out any further

procedure to see which APs are available for the user. This

procedure is optimal in terms of connection delay, but yieldsserious problems described in next subsection.

Note that the above connection procedure is implemented

in many simulation engines to mimic the existing wireless

devices, and some mobile devices and simulation engine (such

Fig. 1: Partial channel search problem

as OPNET) simply use the firstly arrived beacon message forthe connection.

B. Problem Analysis

There are three problems of the aforementioned original

Wi-Fi connection procedure. First, the partial channel search

sometimes incurs serious lack of service quality problem shownin Fig. 1. The figure shows that experimental device is connected

to myLGnet. However, AndroidHotspot1597 has much better

signal strength because of the short distance which makes highprobability of the better service if the APs functionalities are

assumed to be equal to each other. This unreasonable connection

is caused by a partial search, which means the channel search isnot completely done for the rest of available channels after the

connection is established with the first beacon message. Even if

there exists an AP that has better quality compared to others, the

device does not connect to that AP because the corresponding

to beacon frame has not arrived first.The second problem is that the device never tries to connect

to an AP that requires no authentication process, so called

authentication-free AP. This AP can be connected by any device,

and therefore the configuration history is unnecessary for theconnection process when authentication-free APs are available

However, since IEEE 802.11 protocol [5] does not specify the

way of connecting authentication-free AP, many devices are notcapable of connecting to such open APs. This leads to reducing

the number of choices for connecting to the Internet, and thus,

some places which are not covered by APs from contracted

service providers let users to manually search for an AP toconnect.

The last problem is that there is no specific service levelmetric at the moment of AP connection. Most of applications

that aid the connection of Wi-Fi define the quality of AP

in the increasing order of RSSI strength in terms of dBm[6]. It is true that the RSSI value goes higher as closer the

distance between nodes in a link, but this common sense does

not properly guarantee the actual quality of network servicedue to interference caused by other competing links. Fig. 2

shows the configuration of a simple experiment that presents

the phenomenon. In the experiment, an AP and a laptop is

composed as one communicating pair, and distance between

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Fig. 2: Channel interference problem

them is about a meter without any obstacle between them.One pair surrounded by solid line box is configured to use

the channel 1 and the other two pairs surrounded by dashed

line box is set to use the channel 6 in common. No other linkexists in the proximity of this experimentation and only one

smartphone eavesdrops signal to measure RSSI for each beacon

of AP. Each laptop communicates with the server by sending

iperfmessage for 1000 seconds using the established link with

the closest AP. The laptop using the channel 1 alone showed

throughput of 21 Mb/s, whereas the other two pairs showed13 to 14 Mb/s individually. This is because the channel 6 isused by two laptops within the same hop and therefore each

station interferes with each other. The pair with the channel 1

is not interfered because the channel 1 and 6 are completelynon-overlapping channels. However, RSSI values for all APs

beacon messages were about -40 to -45dBm in perspective ofthe eavesdropping smartphone device. This experiment clearlyshows that RSSI is a not good indicator to be used as a metric

for wireless Internet service quality. And it is trivial to expect

that Wi-Fi connection that neglects quality metric is expected

to result in even worse Internet services to device users.

Above three problems show that there are three importantaspects that should be addressed for optimal AP selection:

A full scanning is required; device should collect all beaconmessages by fully scanning all channels in order to select

the best AP within the sensing range of the device;

Authentication-free APs are candidates in selecting suit-able APs; more AP choices bring higher probability of

connecting the Internet and selecting qualified AP;

Medium access control (MAC) protocol should be used as

a selection criterion regardless of whether it is contentionbased or contention-free; the number of stations using

the same channel should be counted to see how many

communications are made with the same channel.

Considering above aspects, we propose the optimized APsearch and selection mechanism, and we also validate the per-formance improvement that the proposed mechanism achieves.

III. QOE- DRIVENA P SELECTION M ECHANISM

The proposed AP selection mechanism for maximizing usersquality of experience (QoE) consists of two important schemes.

The first scheme is the ISP & Authentication based AP can-

didate selection scheme that is used to increase the probability

of connecting a qualified AP. The second scheme is the SINR-

Fig. 3: A simulation topology for ISP & Authentication search

based AP selection scheme that is used to connect the optimal

AP among connectable candidate APs.

A. ISP & Authentication based AP Candidate Selection

Since smartphones usually connect to an AP that is already

configured in its small database, some service providers inscribeits configuration data into the phone database before customers

use it, and thus, APs of contracted service provider are automat-

ically connected by the device without any notification to users.

However, authentication-free APs cannot be automatically usedby the device, and therefore AP candidate selection should take

into account those APs as candidates for the connection.

We conducted a simple simulation to show the impact of

having more choices for connection. The network configuration

consists of 12 APs and one hand-held device in the way that

12 APs surrounds the hand-held device. As for the device, itreceives data from a server through one of the APs. As for

APs, one fourth of those APs belong to a contracted serviceprovider, another one fourth of APs are free to be used. The

rest of APs are unusable because they are secured with some

authentication mechanism or belong to other service providers

The server sends 1 Mb/s downlink IP flow to the device andthe connected AP is supposed to relay the flow to the device

Fig. 3 describes the simulation setting visually.

In the simulation, we compared two results where the one

comes from the original connection mechanism built in the

OPNET simulator and the other is obtained when we useauthentication-free APs. In order to see the pure effect of the

authentication-free AP considered mechanism, we do not useany scheme that qualifies the link status of each AP. Recalthat as mentioned early, the first arrived beacon message that

contains configured AP information triggers the connection

establishment process in the original mechanism.

We employed MAC layer throughput as a performance metric

to see the impact of including free APs as candidates for thenetwork connectivity. When we used authentication-free APs for

the connectivity, we observed that 51.9 % of MAC throughput

has increased in average compared to the original connection

mechanism.

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It is general that most of APs require an authentication

process to network users for access authorization but cafesand restaurants around public places do provide the Internet

service to customers by opening APs without any authentication

requirements. This simulation results show that Internet service

providers are not only those telecom companies but also cafe& restaurant owners that provide Internet access while selling

their products. The reason of providing APs is because many

customers now use hand-held devices to access the Internet.Therefore, neglecting authentication-free APs as a candidate is

not a good idea in those places.

B. SINR based optimal AP Selection scheme

After gathering candidates via the above scheme, the opti-

mized AP selection scheme is conducted to connect the best

qualified AP. The original connection mechanism does not useany quality metric to select an AP, and some of the Wi-Fi related

applications refer to RSSI value and suggest the user to connect

an AP with the highest RSSI value. However, as mentionedearlier, RSSI-based network selection sometimes results in bad

QoE. For example, even if an AP with the channel 1 shows

the largest RSSI of all, meaning the AP is positioned nearby auser, the channel 1 can be possibly used by many other APs

and wireless devices that communicate to those APs. Based on

this observation, we proposes SINR based optimal AP selection

scheme that considers the number of stations communicating ateach channel in order to make an wireless device connect into

the least crowded channel. We use the following equation to

measure the quality of each AP or estimate SINR for each AP:

SINR =RSSIcur

xSLxRSSIx +N

Note that RSSI value in the equation is initially measured indBm but it is translated intomiliwatts[7]. Nominator in equa-tion III-B shows the RSSI value of AP to be currently evaluated,

whereas denominator indicates the summation of RSSIs fromAPs that use the same channel. x indicates an AP within the

set of S and S is the set of APs that use the same channel. L

indicates the number of stations that communicate through theAP x. N indicates the noise level measured while receiving a

beacon frame from the current AP. Information of the channel

number and the number of communicating stations is containedin the beacon frame received from each AP, and the RSSI value

can be measured while receiving beacon physically. Note that

the number of stations connected to each AP is described in the

22nd element of the beacon body named BSS Load. The BSSLoad value contains the number of stations using corresponding

AP and the channel utilization value provided as percentage of

how many busy time slot is counted among all time slots. TheBSS load is frequently used in articles that focused on load

balancing among nearby APs [8], [9]. The channel information

can be earned by PHY parameter sets or catching the momentof receiving beacon and see what channel is scanned. Also, the

channel number of each AP can be easily earned by Android or

iOS SDK using scan or Wi-Fi related libraries. The noise level isalso available at devices antenna, and some user level programs

like wavemon support the functionality of showing noise level

of specified AP in real time. The available AP set Salso includes

the current AP, so that nominator of equation indicates the

signal power of the link which can be possibly generated by

the downlink flow from the AP to the device that attempts toconnect. This algorithm is designed in the assumption that most

data flows stream from AP to devices. It means that the RSSI

value of AP can be represented as the power generated by that

AP because APs are the ones that take over channel the mostin case of downlink flow. In this paper, SINR is defined as the

smallest possible SINR that can happen to the users device if

it is connected to the AP. The SINR based selection providesa new priority based metric that can reinforce the RSSI metric

to provide an optimal AP for users according to the degree

of crowd of each channel. However there are several issuesrevealed as follows:

Interference caused by other stations can be diminished ifdevices that cause uplink flows are out of carrier sensing

range; If all devices and APs are within one hop range, the

SINR metric is applicable to both downlink and uplink butthis rarely happens in reality;

WLAN channels are partially overlapped with each other

so that the number of completely non-overlapping channelis only three, compared to total 13 channels in 2.4GHz

band; Adjacent-channel interference [10] can cause smallerSINR value estimated by the proposed equation;

AP with very low RSSI that has no competitors in the samechannel can possibly have larger SINR compared to other

APs with much larger RSSI and some competitors.

Uplink flows are lightly considered in this paper since most

of the Internet service data flow from servers to users. Adjacent-channel interference can be solved by AP installers in the way

that service providers can install APs that possess completely

non-overlapping channel to minimize interference. AP with very

low RSSI problem can be solved by giving a threshold of RSSIvalue to filter out useless APs. IEEE 802.11 protocol release at

the year of 2007 specifies receiver performance requirements

and these requirements give the minimum RSSI value for usageof each modulation technique. RSSI value less than -82 dBm

can not be used for any modulations used by 2.4GHz Wi-Fi in

current technology. Threshold values of -70 to -82 dBm resultedin good performance of using SINR metric. Simulation results

described in this paper used -75dBm as lower bound threshold

of using as a connection candidate.

IV. PERFORMANCEE VALUATION

In this section, we implemented the proposed qualified APselection mechanism in both a Android phone and the OPNET

simulation. Then we carried out a performance evaluation study

to verify the effectiveness of the proposed scheme.

A. Simulation Study

The simulation configuration and scenarios are configured

as follows to verify if the proposed qualified AP selectionmechanism improves the network throughput as follows:

We placed in the simulation network twelve pairs of AP &

STA using channel 1, 6 or 11; channel number is uniformly

distributed to APs and STAs;

We applied the proposed mechanism to the 13-th STA

and there is no pair AP to this STA at the beginning of

simulation; therefore,this STA has 12 connection choices

at that time;

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(a) The original mechanism (b) The proposed mechanism

Fig. 4: MAC throughput comparison between the original and proposed mechanism

Fig. 5: Simplified simulation topology

We configured 6 out of 12 APs to be connectable to the

STA, which are either APs of contracted Internet service

provider or authentication-free APs; rest of APs are notcandidates for connection;

We conducted 25 simulation runs with different combina-

tions of AP & STA positions and channel assignments:

We used 5 different positioning of AP & STA location;

We used 5 different channel assignment to each pair.

Fig. 5 presents a simplified simulation environment. Note thatthe real deployment in the OPNET simulation is not as regular

as shown in the figure. As shown in the figure, each device

are not guaranteed to be connected to the nearest AP and eachAP can contain more than one wireless device or none. Noise

level is constant through all channels since there is no jammer

installed in the environment. Fig. 4 shows the comparison ofperformance between two mechanisms: the original mechanism

and the proposed mechanism. Original mechanism produces bad

MAC throughput generally since the number of connectablecandidates is small and no qualification metric is used for the

connection. On the other hand, the proposed QoE driven Wi-

Fi selection mechanism produces much greater performance

in some cases. We have the following observations from the

simulation results: (i) The proposed mechanism improved MAC

throughput by 255% in average. Even if extraordinary perfor-

mance improvement in the channel assignment 5 is not included,

the MAC throughput is still improved by 89.9% in averageThe reason why the channel assignment 5 makes the best

improvement is because the original mechanism connected the13th device to an AP that has most number of stations, while

the proposed mechanism connected the device to an AP with

the least number of stations. (ii) There were cases that shows

no difference in usage between the original or the proposedmechanism because the original mechanism sometimes connects

to the optimal AP (that the proposed mechanism decides)

accidentally. Also, (iii) the performance degradation happenedat some cases. This is not because the mechanism failed to

select the optimal AP but because IEEE 802.11 MAC protocol

makes serious starvation in a particular receiver. The issue is

out of scope of this paper.

B. Android Application

The proposed mechanism has been implemented in AndroidOS based Smartphones to determine AP candidates with esti-

mated SINR values. Android SDK provides the result of Wi-

Fi scanning and each scanned AP contains SSID information,capabilities and RSSI values. However, BSS Load information

is not shown since the SDK can only distribute a small part of

beacon frame. Therefore, the current implementation uses rather

the number of APs in the same channel instead of the numberof stations in the channel. Fig. 6 shows the process of using the

application ((a), (b)) and a map that shows location spots where

an experiment was conducted ((c)). Four buttons are distributedon the screen. User clicks the SSID button to setup SSID for ISP

identification and clicks the start button to begin AP scanning.

Screen continuously updates the result of scanning but user canclick the stop button to stop the scanning. Screen presents the

number of scanned APs, the number of APs that exceed signal

level of -80 dBm, the number of connectable AP candidates,and list of candidate APs with information of channel, SINR and

RSSI. Application suggests the user to connect an AP with the

highest SINR value and shows what the order of APs prioritized

by SINR is. User can manually select an AP to connect, or the

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(a) SSID input interface (b) Scanning result view (c) Scanned locations

Fig. 6: Android experiment

TABLE I: Android experiment results

Location Index Nscanned Nsignal Ncandidates

1 12 6 22 22 10 33 21 11 44 10 5 15 12 3 36 8 2 2

application can automatically connect to the AP with the highestSINR because the configuration history of APs of contracted

service provider has been already saved to the phone, and free

APs can be connected without any additional requirements.

Fig. 6 (c) shows 6 Locations which are evaluated in terms

of the number of AP candidates. Application first filters outAPs with too low beacon signals, and it makes a candidate list

of APs of contracted service provider and authentication-free

APs. Table I shows the results of this experiment. Note thatlocation index in the table starts from the bottom of the map

(Fig. 6 (c)). We can observe that a few APs are connectable

even though there are a lot of APs installed around device user.

V. CONCLUSION

We proposed a QoE driven Wi-Fi selection mechanism in this

paper. The proposed mechanism is conducted with two stages:ISP & Authentication based AP candidate selection is carried

out at the first stage to increase the availability of network

connectivity by considering authentication-free APs, and thenSINR based AP selection is performed at the second stage to

choose the best qualified AP. To validate the effectiveness of the

proposed mechanism, we realized it in a Android Smartphoneand also implemented a OPNET simulation environment, and

then we conducted a performance evaluation study. The perfor-

mance results indicate that the proposed mechanism can supportan optimal Wi-Fi connection and also provide better Internet

service for wireless users. Based on the results, we vision that

the proposed mechanism can be equipped at the next generation

smartphones and it can aid vertical handover among various

networks. We have several research plans as future work. We

will relax the downlink assumption for the SINR estimationWe also would like to carry out an extensive empirical study to

verify if the proposed mechanism can handle various networktopologies and traffic patterns.

ACKNOWLEDGMENT

This research was supported in part by the KCC (Korea Communications

Commission), Korea, under the R&D program supervised by the KCA (Korea

Communications Agency) (KCA-2012-08-911-05-001), and in part by the

National Research Foundation of Korea (NRF) Grant funded by the Korea

government (MEST) (No. 2010-0014060).

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QoE-driven Wi-Fi Selection Mechanism for Next Generation Smartphones