Exploiting Multiple Wireless Interfaces in Smartphones for Traffic Offloading Sanaa Sharafeddine1 , Karim Jahed1 , Nadine Abbas2 , Elias Yaacoub3 , and Zaher Dawy2 1
Lebanese American University, Lebanon, Email: {sanaa.sharafeddine, karim.jahed}@lau.edu.lb 2 American University of Beirut, Lebanon, Email: {nfa23, zd03}@aub.edu.lb 3 Qatar Mobility Innovations Center (QMIC), Qatar, Email:
[email protected]
Abstract— Smartphones are evolving at a fast rate in terms of their computational, storage, and communications capabilities. A high-end smartphone is equipped with multiple wireless interfaces with varying bit rates, energy consumption requirements, and coverage ranges. The joint utilization of the existing wireless interfaces facilitates the development of advanced techniques to boost the performance of wireless networks and enhance the experience of mobile users. Among these techniques is deviceto-device cooperation where a smartphone receives content from a base station on a given wireless interface and distributes it to other devices in its vicinity via another wireless interface. Another technique is traffic offloading in heterogeneous network scenarios where a smartphone downloads content using multiple wireless interfaces. In this paper, we study the readiness of high-end smartphones to utilize multiple wireless interfaces simultaneously focusing on capabilities and challenges. We adopt an experimental approach using a mobile cooperative video distribution testbed to obtain and evaluate performance results with focus on energy consumption. We consider various scenarios involving a combination of wireless technologies that include Bluetooth, WiFi, WiFi-Direct, and 3G.
I. I NTRODUCTION Current smartphones are capable of providing connectivity anytime anywhere while supporting multitude of services and applications. They are equipped with multiple wireless interfaces which can be jointly exploited to develop advanced enhancement techniques that can boost throughput, improve quality of experience, minimize cost, and/or reduce energy consumption. The joint utilization of multiple wireless interfaces in smartphones facilitates: i) the implementation of cooperative content distribution strategies where a given device downloads data from an access point (AP) over a long range (LR) wireless interface and relays it to nearby devices over a higher bit rate short range (SR) wireless interface (see Fig. 1) [1], [2], [3], [4]; ii) the operation of heterogeneous networks where a mobile device uses multiple wireless interfaces simultaneously to offload traffic among different wireless access networks [5], [6]; iii) the seamless formation of personal area networks with mobile-to-mobile resource sharing among multiple personal devices in a home or office environment [7], e.g., to form a cluster with shared storage, computational power, and/or displays. In addition to supporting standard long range (LR) wireless interfaces such as WiFi IEEE802.11a/b/g/n and 3G/4G cellular, modern smartphones are becoming equipped with enhanced short range (SR) wireless interfaces to enable high bit rate device-to-device (D2D) communications such as Blue-
Fig. 1. Upper figure: traditional wireless access network scenario. Lower figure: example cooperative network scenario that shows a smartphone utilizing its WiFi and Bluetooth (BT) interfaces simultaneously to offload traffic from the wireless access network via local D2D links. A data acquisition unit is connected to selected devices in order to measure energy consumption in real time.
tooth 3.0+HS, WiFi-Direct, and LTE-Direct which is currently under consideration for 3GPP standardization (Release 12). WiFi-Direct provides new capabilities for WiFi devices to connect to each other in order to form direct connection groups without requiring the availability of an infrastructure AP; the WiFi-Direct technical specifications define procedures that facilitate distributed device discovery and association. On the other hand, LTE-Direct relies on the LTE physical layer to enable the discovery of peers and relevant applications in an autonomous and energy efficient manner in order to facilitate synchronized D2D communications on licensed spectrum [8]. In this paper, we study the readiness of selected high-end smartphones to utilize multiple wireless interfaces simultaneously using an experimental approach. We present the design and implementation of a mobile cooperative video distribution testbed that supports D2D communications to offload traffic and reduce energy consumption of smartphones. In the testbed, a selected master device streams video frames over a LR interface (e.g., WiFi AP or 3G cellular base station), and relays them to nearby devices in real time over an efficient SR wireless interface (e.g., Bluetooth or WiFi-Direct). Due to
the proximity among cooperating devices and the efficiency of SR wireless technologies, notable performance gains can be achieved. The obtained experimental results are used to extract insights related to exploiting multiple wireless interfaces in smartphones for traffic offloading. This paper is organized as follows. Section II presents a brief discussion on the D2D cooperation capabilities of smartphones using multiple wireless interfaces. Section III presents the design details for a cooperative multimedia content distribution testbed that utilizes D2D communications for traffic offloading and energy consumption reduction. In Section III, experimental results are discussed and analyzed for various scenarios with combinations of Bluetooth, WiFi, WiFiDirect, and 3G technologies. Finally, conclusions are drawn in Section IV. II. S MARTPHONES : C APABILITIES AND L IMITATIONS Smartphones support a range of wireless interfaces that enable LR connectivity over a WiFi IEEE802.11a/b/g/n or a 3G/4G cellular interface in addition to SR D2D connectivity over a Bluetooth 3.0+HS/4.0 or a WiFi-Direct interface. This support varies between different device brands and classes with constraints on utilizing multiple interfaces simultaneously due to reasons that include operating system configuration or RF limitations. For example, most high-end smartphones cannot run 3G and WiFi simultaneously with priority always given to the WiFi interface irrespective of signal strength or access network congestion levels. Moreover, smartphones that support WiFi-Direct vary in terms of the combinations of wireless interfaces that can be used simultaneously. For example, a Galaxy Samsung SII does not allow turning on both WiFi and WiFi-Direct together, whereas a Galaxy Samsung SIII requires turning the WiFi interface on to be able to run WiFi-Direct; as a result, 3G on the LR and WiFi-Direct on the SR is supported by Samsung SII and not SIII, whereas WiFi on the LR and WiFi-Direct on the SR is supported by Samsung SIII and not SII. These examples highlight the need to further enhance the capabilities of high-end smartphones to be able to develop advanced solutions for traffic offloading and D2D cooperation in heterogeneous network scenarios. Table I presents experimental energy consumption measurement results for a Samsung SII smartphone in idle mode with different combinations of wireless interfaces switched on without any signaling or download/upload activities. These sample results demonstrate that switching multiple interfaces on consumes less energy than the sum of the individual energy increment of each interface, e.g., having Bluetooth, WiFi, and 3G switched on together consumes 6.6 J which is less than the sum of 8.4 J (1.2 J + 3.4 J + 3.8 J). In general, the requirements for smartphones to utilize multiple wireless interfaces simultaneously include a combination of the following factors depending on the target solution whether cooperative content distribution, heterogeneous network offloading, or personal area network sharing: RF and processing capabilities, distributed D2D discovery
Active Interfaces None Bluetooth WiFi WiFi-Direct 3G Bluetooth and WiFi 3G and WiFi-Direct Bluetooth, WiFi and 3G
Total Energy 47.7 J 48.9 J 51.1 J 51.1 J 51.5 J 51.7 J 52.9 J 54.3 J
Interface Energy – 1.2 J 3.4 J 3.4 J 3.8 J 4.0 J 5.2 J 6.6 J
TABLE I E NERGY CONSUMPTION OF THE WIRELESS INTERFACES IN IDLE MODE OVER A DURATION OF ONE MINUTE .
and association mechanisms, limit on the number of possible collaborators, availability of channel state information among devices, sensitivity to mobility, impact on quality of service due to additional delays, etc. The achieved performance gains span multiple metrics that include energy consumption reduction, traffic offloading and network throughput enhancement, range extension, financial cost reduction, quality of experience improvement, etc. III. C OOPERATIVE V IDEO D ISTRIBUTION T ESTBED : A RCHITECTURE AND I MPLEMENTATION In this section, we present the design of the various components of a mobile cooperative video distribution testbed. The design is divided into two main modules: server module and client module which in turn is divided into cluster head sub-module and peer sub-module. The testbed supports 3G and WiFi for LR connectivity in addition to Bluetooth and WiFi-Direct for SR connectivity. In terms of content, we focus on video distribution due to the popularity of mobile multimedia streaming applications. In terms of performance assessment, we focus on energy consumption from the batteries of smartphones due to the limited battery capacities in handheld devices. A. Client Module Design A client mobile device will first perform device discovery over the selected SR wireless interface to scout for any existing clusters of cooperating devices. In case none was found, then the device has no choice but to connect directly to the application server over the LR interface, acting as a cluster head. On the other hand, if clusters were discovered, then the device selects one based on a predefined criterion that can include a cluster allocation decision from the application server or access network side. For example, one possible selection criterion is to select the cluster head with minimum cluster size to maintain load balancing or to select the cluster head with maximum willingness to cooperate based on some incentives. 1) Cluster head operation: A cluster head mobile device acts as the master node. It has the following two main functionalities: to receive and play the video from the server, and to re-stream the video to other peers in its cluster. The cluster head must first connect to the server over the selected LR interface. After initiating the connection, it can signal the server to start streaming at anytime. Meanwhile, other devices
(peers) can begin to initiate a connection with the master over SR D2D connections. When the master signals the “play” event, the server will begin to send the video. A typical video file is normally composed of a muxed container of a number of streams. The process of reading the file and separating those streams is known as demuxing. A stream can be an audio, video, or data (subtitles) stream. The master then demuxes the stream container, decodes the video stream, displays the video frames, re-encodes it with appropriate parameters, muxes it into a streamable format and relays it to all connected peers (see state diagram in Fig. 2). 2) Peer operation: A peer mobile device has a relatively simple design. Its main functionality is to play the video as it receives it from the master cluster head device. It will first contact its master over the selected SR interface. Upon handshaking, the peer will enter a ready state while waiting for the stream to start. When the master starts streaming, the peer will have to demux the container, decode the video stream, and display the video frames (see state diagram in Fig. 3). Fig. 3.
Peer (slave mobile device) state diagram.
B. Server Design The video streaming server is up and ready waiting for incoming requests from mobile devices. The server has a main function to stream the video frames to the cluster heads in addition to control functions such as video segmentation, cluster formation, mobility handling, etc. A cluster head needs to connect to the server over the LR wireless interface. Upon handshaking, the server holds until a “play” signal is received. For realtime video streaming applications, streaming immediately starts to all registered cluster heads that in turn deliver content to their associated peers. Upon receiving a “play” signal, the server initiates the streaming process that starts by demuxing the video file, decoding the video stream, re-encoding it with appropriate/negotiated parameters that suit the cluster head’s characteristics, muxing it into a streamable format, and finally transmitting it over a LR interface to the cluster heads (see state diagram in Fig. 4). Fig. 4.
Server state diagram.
C. Implementation Details
Fig. 2.
Cluster head (master mobile device) state diagram.
The testbed is implemented using a modular approach which facilitates enhancements and extensions to implement and test various protocols, design alternatives, or intelligence options. The client application is implemented on the Android platform and the server application on a GNU/Linux platform, both using Java and C programming languages. The testbed allows 3G or WiFi to be used as the LR wireless interface for serverto-mobile connectivity and Bluetooth or WiFi-Direct as the SR wireless technology for D2D connectivity. 1) Server modules: The server implementation is divided into two main modules. One module is responsible for maintaining the connections with the clients, exchanging control messages, and organizing data transfer; the server’s main thread always listens to connection requests and implements
the protocol design presented in Fig. 4. The second module is responsible for the video processing and streaming functions. Due to the lack of an adequate open source Java multimedia library, the C/C++ FFmpeg library was used, in addition to libx264 for encoding H.264 video streams. 2) Client control module: The client implementation on the smartphones is challenging since the Android API does not provide a media library with the required low-level access. Moreover, the built-in media library codecs support is rather limited. Fortunately, the Android platform provides a native development kit based on Java’s native interface, which facilitates the use of other libraries to develop the video processing part of the client using the C language. The client implementation can be divided into three modules: control, video playback, in addition to video re-streaming in case the client is assigned a master role. The client control module is responsible for gathering the required parameters from the mobile devices in addition to maintaining the connections with the server and other peers. The application first allows setting the SR interface to be used. Then, a service discovery is performed to scout for cluster head (master) devices over the selected interface. During this service discovery phase, several parameters are exchanged between the device and other masters and/or the server. Bluetooth and WiFi-Direct both provide distributed service discovery features. Depending on the outcome of the discovery phase, the device will either act as a cluster head or a peer. If the device acts as a cluster head (master role), it will wait for other peers to connect over the SR interface. When ready, the master can signal the server to start streaming and control is moved to the video playback module. On the other hand, if the device acts as a peer, it will connect to the selected master over the SR interface and wait for the D2D streaming to start. Once started, control is moved to the video playback module. 3) Client video playback module: This module is responsible for receiving the video packets from the server, decoding them, constructing a video picture and displaying it on the mobile device’s screen. Both master devices and peer devices can execute this module. Two threads are used within this module: a parsing thread for reading the incoming video packets from the server, and a decoding thread for reading the packets from the video queue and decoding them into raw frames. Moreover, if the client is a master device then the decoding thread will also place a copy of the raw frame in a raw-frame queue to be used later by the re-streaming module. The video rendering part is responsible for displaying the received pictures on the screen; it is implemented using OpenGL ES (Android’s native renderer). 4) Client video re-streaming module: This module is executed only by cluster heads (master devices) and it is responsible for encoding the raw frames provided by the decoder and streaming them over the SR interface to all other peers in the master’s cluster. For each raw frame, the re-streamer will encode it using the H.264 codec, packetize it, and then transmit the output packet to each peer using SR D2D connectivity.
IV. E XPERIMENTAL E VALUATION We have conducted a set of experiments using the implemented testbed; the aim is to analyze the performance of selected smartphones while utilizing multiple wireless interfaces simultaneously to offload video traffic from a LR WiFi or cellular interface via SR D2D links. Several combinations of LR and SR wireless technologies have been considered in order to provide comparative analysis and extract insights of practical importance. The smartphones used in the experiments included combinations of Galaxy Samsung SII and SIII. For the energy consumption measurements, we used a National Instruments data acquisition unit (NI USB-6251) to capture in realtime the voltage drop across a resistor connected to the battery. Figure 5 presents sample energy consumption results while downloading a given video over a WiFi interface versus a 3G interface. The figure presents the power consumed from the smartphone’s battery over time. It can be seen that the 3G interface has a denser plot with notably higher peaks than the WiFi interface; this in turn indicates higher energy consumption using 3G for the considered scenario.
Fig. 5. Upper plot: WiFi power consumption over time. Lower plot: 3G power consumption over time.
Table II presents selected energy measurement results for downloading a given video content with/without D2D offloading assuming WiFi is used on the LR interface and Bluetooth is used on the SR interface. All devices are placed close to each other and to the AP, processing energy consumption is included as part of the transmission/reception energy consumption, and the energy consumed in idle mode due to basic device activities has been subtracted systematically from all values. The duration to download the video was around one minute in all experiments. Table II has five columns that represent the following. Column 1 is the number of devices requesting the same video content. Column 2 is the energy consumed per device when all the devices receive the content on the LR WiFi interface (no D2D cooperation); it can be seen that the energy consumption increases notably with the number of devices due to resource contention on the WiFi access network. Columns 3, 4, 5 present the energy consumed with D2D cooperation where one
Devices 1 device 2 devices 3 devices 4 devices
LR only 27.5 J 32.0 J 43.9 J 54.1 J
master Rx – 63.7 69.4 71.9
and Tx J J J
master Tx – 36.2 J 41.9 J 44.4 J
peer Rx – 31.5 J NA 37.0 J
TABLE II E XPERIMENTAL RESULTS FOR A GIVEN SCENARIO WITH W I F I ON THE LR INTERFACE AND B LUETOOTH ON THE SR D2D LINKS . Devices 1 device 2 devices 3 devices 4 devices
LR only 27.5 J 32.0 J 43.9 J 54.1 J
master Rx and Tx – 68.8 J 83.1 J 84.3J
master Tx – 41.3 J 55.6 J 56.8 J
peer Rx – 39.4 J 41.9 J NA
TABLE III E XPERIMENTAL RESULTS FOR A GIVEN SCENARIO WITH W I F I ON THE LR INTERFACE AND W I F I -D IRECT ON THE SR D2D LINKS . Devices 1 device 2 devices 3 devices 4 devices
LR only 62.5 J NA NA NA
master Rx and Tx – 97.3 J 102.2 J 109.2 J
master Tx – 34.8 J 39.7 J 46.7 J
peer Rx – 31.5 J NA 37.0 J
TABLE IV E XPERIMENTAL RESULTS FOR A GIVEN SCENARIO WITH 3G ON THE LR INTERFACE AND B LUETOOTH ON THE SR D2D LINKS .
device acts as a master to stream the video on its WiFi interface and then relay it in realtime to the other devices (peers) via SR Bluetooth links. Column 3 is the energy consumed by the master smartphone including reception on LR and transmission on SR; the energy consumption increases with the number of devices due to contention and additional signaling on the SR wireless interface. Column 4 is the energy consumed by the master smartphone including only transmission on SR (this is obtained by subtracting 27.5 J from the values in Column 3). Column 5 is the energy consumed by each peer smartphone while receiving via Bluetooth on the SR; this also increases with the number of devices due to contention and signaling on the SR links. The following insights can be extracted from Table II. The master device consumes notably higher energy than the peers, with the energy consumed for transmission on the SR higher than the energy consumed for reception on the LR. Moreover, the energy consumption does not increase linearly with the number of devices that the master device relays the content to on the SR links (relaying to one device consumes 36.2 J whereas to three devices consumes 44.4 J); this is an important observation that favors forming larger cooperation clusters and that demonstrates the potential gains of D2D traffic offloading. To quantify the gains of cooperative content distribution, we consider an example scenario with four devices. If all devices receive on the WiFi LR without offloading, the total energy consumption is 4 ∗ 54.1 = 216.4 J. With D2D offloading (1 master and 3 peers), the total energy consumption is reduced to 71.9+3∗37 = 182.9 J which corresponds to a gain of 15%. Moreover, the scenario with offloading utilizes one fourth of the LR resources compared to the traditional scenario.
Tables III and IV present a similar set of results assuming WiFi/WiFi-Direct and 3G/Bluetooth on the LR/SR links, respectively. Comparing all tables together, we can note that Bluetooth is more energy efficient than WiFi-Direct on the D2D links and WiFi is more energy efficient than 3G on the LR link for the considered experiments. For 3G, it is not possible to control the LR link quality and to capture the impact of contention on the access network as the number of devices increases. The presented results cannot be generalized to all scenarios since they depend on the link qualities, device characteristics, device relative locations, etc. However, they present typical values that can be encountered in real scenarios and, thus, help extract insights of practical relevance. V. C ONCLUSION We studied capabilities and limitations of smartphones to utilize multiple wireless interfaces simultaneously for advanced wireless networking techniques such as cooperative content distribution with D2D communications, heterogeneous network offloading,or personal area network sharing. To this end, we explained the design and implementation of a testbed for cooperative multimedia streaming that support multiple long range and shore range wireless technologies such as Bluetooth, WiFi, WiFi-Direct, and 3G. Experimental energy consumption results are presented and analyzed for various scenarios and practical insights are extracted. ACKNOWLEDGEMENTS This work was made possible by NPRP grant 09-180-2-078 from the Qatar National Research Fund (a member of The Qatar Foundation). The statements made herein are solely the responsibility of the authors. R EFERENCES [1] M. Ashraful Hoque, M. Siekkinen, and J. K. Nurminen, “Energy-efficient multimedia streaming to mobile devices - A survey,” IEEE Communications Surveys and Tutorials, accepted for publication, 2013. [2] E. Yaacoub, L. Al-Kanj, Z. Dawy, S. Sharafeddine, F. Filali, and A. AbuDayya, “A utility minimization approach for energy-aware cooperative content distribution with fairness constraints,” Wiley Transactions on Emerging Telecommunications Technologies, vol. 23, pp. 378–392, June 2012. [3] K. Jahed, M. Younes, and S. Sharafeddine, “Energy measurements for mobile cooperative video streaming,” in Wireless Days 2012, November 2012. [4] L. Al-Kanj, Z. Dawy, and E. Yaacoub, “Energy-aware cooperative content distribution over wireless networks: Design alternatives and implementation aspects,” IEEE Communications Surveys and Tutorials, accepted for publication, 2013. [5] X. Bao, U. Lee, I. Rimac, and R. Choudhury, “Dataspotting: Offloading cellular traffic via managed device-to-device data transfer at data spots,” ACM SIGMOBILE Mobile Computing and Communications Review, vol. 14, pp. 37–39, July 2010. [6] B. Han, P. Hui, V. Kumar, M. Marathe, J. Shao, and A. Srinivasan, “Mobile data offloading through opportunistic communications and social participation,” IEEE Transactions on Mobile Computing, vol. 11, pp. 821– 834, May 2012. [7] N. Vallina-Rodriguez, V. Erramilli, Y. Grunenberger, L. Gyarmati, N. Laoutaris, R. Stanojevic, and K. Papagiannaki, “When David helps Goliath: The case for 3G onloading,” in 11th ACM Workshop on Hot Topics in Networks (HotNets), October 2012. [8] Qualcomm, “LTE Direct: The case for device-to-device proximate discovery,” in Qualcomm White Paper, February 2013.
