A Scalable Cooperative Wireless Grid Architecture and Associated Services for Future Communications

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1 A Scalable Cooperative Wireless Grid Architecture and Associated Services for Future Communications Frank H.P. Fitzek 1 and Morten V. Pedersen 1 and Marcos Katz 2 1 Aalborg University, Denmark, [ff mvpe]@kom.aau.dk 2 VTT Finland, marcos.katz@vtt.fi Abstract In our previous work we have advocated the need for cooperative behaviour among mobile devices, mainly focusing on the benefits and potentials of a novel architecture approach referred to as cellular controlled peer to peer networking. However, some important aspects of cooperative networking we not discussed in detail, and therefore this paper deals with the questions on a scalable cooperative architecture for wireless networking exploiting the available cellular and short range air interfaces and well as how such cooperative networks can be formed based on the request services. This sort of service discovery for mobile cooperative devices has to deal with the trade off between power consumption of the battery driven mobile device and the need for sufficient advertising. The paper aims also at answering the question whether state of the art service discovery mechanisms can be reused. I. THE NEED FOR A NEW COMMUNICATION ARCHITECTURE In the process of developing the new wireless/mobile generation, referred to as the fourth generation or 4G, some of the key problems to solve include enhancing spectral and power/energy efficiency, keeping the complexity of the end system reasonably low, and developing appealing services for the users. In this section we shortly introduce a new wirelessly scalable architecture introducing cooperative principles for cellular networks and aimed at solving the mentioned challenges. The architecture is basically a composite or hybrid approach combining centralized and distributed architectures, typical of cellular and ad hoc networks, respectively. The interested reader is referred to [1] for further details on this composite architecture. In order to offer more services and capabilities to the users, mobile devices are crammed with several functionalities. Such multiplicity of capabilities introduces two major problems: firstly, the mobile device becomes very expensive and secondly, the battery will be drained very quickly reducing the operational time of the device. As monolithic (or single-piece) architecture will exhibit many of the aforementioned drawbacks, the very first idea is to enable cooperation among several closely located mobile devices with their different functionalities forming an heterogeneous wireless grid. This is a clear departure from today s use where a given air interface (and corresponding architecture) is used in a given scenario or for a particular service. Dynamic interaction between and across air interfaces are not possible in such conventional approach. Clearly cooperation will enable resource sharing or augmentation among the collaborating terminals, with the cellular network being basically the service entry point and the short range links the gluing element allowing such a wireless grid to have a wirelessly scalable architecture. As given in Figure 1 a mobile device has many different capabilities and functionalities. We have grouped those of a mobile (smart) phone into three classes, namely user interfaces, communication interfaces and built-in resources. The user interfaces comprise typically the speaker, microphone, camera, display, built-in sensors, and keyboard capabilities. The built-in terminal resources include the battery, the central processing unit and the data storage. From our point of view we highlight particularly the communication interfaces, which typically include cellular and short range capabilities. Fig. 1. A mobile device broken up into the several capabilities grouped into the user interface, communication interface and the built-in resources. Instead of having one mobile device hosting all functionalities with the best possible quality, the concept presented here considers to use and share cleverly the capabilities of several, in principle different, mobile devices. Indeed, in such heterogeneous scenario each terminal could feature a particular specialization, from a simple mobile phone for voice calls to advanced terminals with music and imaging devices. In the proposed wirelessly scalable architecture, a wireless grid of terminals is formed where the connecting links among cooperating terminals are implemented by the short-range air interfaces. In general terms, such a scalable concept is not

2 totally new as Bill Joy, the visionary figure behind Jini Technology and co funder of Sun Microsystems argued already in 1999 for cooperative software: Jini technology is a dislocation in the context of cooperation of subspecies of devices. Once you have lots of different kinds of devices combining in different ways, you can not do monolithic software anymore. Each of these devices has a certain set of functions, and if we have to assume when we build them what they are going to be used for, it is not very flexible. Bill Joy Inspired on the prominent architect Frank Lloyd Wright s theory of architecture, Bill Joy rephrased the thought A brick should be a brick. A wall should be a wall. A column should be a column. for mobile devices as (see Figure 1): A cell phone has a microphone. It has a numeric keypad. It has a display. It has a transmitter and receiver. That is what it is. It has a battery with a certain lifetime. It has a processor and some memory. If it is a European model it has a stored value card. It is a collection of things. It is some collection. The fact that it is all one bundle, well, that is pretty artificial. Bill Joy In our context the main idea is to identify core functionalities of the cooperating mobile devices and suitably re assemble them according to the service to be provided; without holding the full set of functionalities in one mobile device. An example is given in Figure 2. A user may have two different terminals, where the devices have a subset of different capabilities (e.g., cellular communication facilities for the mobile phone) and a subset of the same functionality but a different realization level (e.g., the display of the table PC has a bigger size and higher resolution). the short range communication link. In the future the short range link will become far more important than it is today. The number of applications is numerous such as using remote devices for storage, camera, microphones, etc. This kind of cooperation is envisioned for terminals owned by the same customer and therefore referred to as altruistic cooperation, as we know it from bees or ants. In [1] such concept is brought to the next level with respect to two main properties. The first difference is that cooperation is envisioned also for wireless terminals of different users (known to each other or not). As each customer is basically following egoistic interests and customers may not know about the neighboring mobile device capabilities, this kind of heterogeneous cooperation seems to be more difficult to establish. The second difference is that capabilities are not simply accumulated but negotiated beforehand among the devices to then dynamically adapt/change them according to the upcoming needs. One example for such an adaptation is the air interface that needs to support cellular as well as short range communication. In [2], [3] we have shown that both communication parts can be realized within the same spectrum using a software defined radio (SDR) approach, where the ratio between cellular and short range depends on the actual needs for cooperation. Our novel wirelessly scalable architecture is an extension of a cellular network, where the mobile devices communicate with each other and with the serving base station. This basic architecture is shown in Figure 3. Here the main functionalities to group and share are the built-in resources and the communication interfaces. The wireless grid can give rise to new virtual functionalities with enhanced performance, such as a high virtual data rate and a virtual battery, a virtual processing unit and a virtual storage. The virtual functionalities can in principle be used in a more efficient way than the stand alone functionalities. Fig. 2. Example of cooperative devices of the same user for a mobile phone and a table PC. Fig. 3. Cellular Controlled Peer to Peer Network. In order to support a wirelessly scalable architecture the devices need to locally communicate with each other, which is done by wireless communication links or more precisely In [4] we proposed for the first time the idea to group mobile devices within each others proximity and let them communicate among each other over the short range communication

3 link and at the same time each device communicates with the base station. By grouping the devices, we form a new cooperative cluster, where each device can contribute with its own capabilities as given in Figure 2. These capabilities may be either homogeneous or heterogeneous in nature. It can be shown that the newly formed cluster has much better communication performance in terms of bandwidth usage, end system complexity and energy consumption. This is based on the fact that the whole cooperative cluster as such is more than the simple sum of its capabilities. We refer to [5], [6], [7], [8], [9] for examples of such cooperation benefits. In Figure 4 the comparison of monolithic and cooperative communication systems with respect to a requested service and the related costs is given. For the monolithic case the cellular communication link has to serve with a given data rate to support the requested service. This may result in a very complex architecture design. In the cooperative case the same service can be supported as the same data rate is obtained through a less complex cellular air interface and short range links. As the short range link supports higher data rates and a better energy efficiency, the overall costs are less compared to the monolithic one. On the other side there is a need for a cooperative peer device. Fig. 4. Comparison of monolithic and cooperative communication systems with respect to a requested services and the related costs. What makes the proposed approach different from many other cooperative techniques is that each interacting entity or device is in fact gaining at the same time it cooperates as shown later in Section III. This is different from many cooperative approaches such as multi hop, relaying, or others. Therefore this kind of cooperation is referred to as egoistic cooperation, inspired by the cooperation strategies found in nature by monkeys or vampire bats. In most of our previous work we claimed that the benefits of cooperation helps all players in the mobile device business including terminal manufactures, network operators, service providers, and the customer. Throughout this paper we will see such benefits and map them to the entity receiving the benefit. After our discussions on the cooperative architecture, we will focus now on service discovery for such architecture. Service discovery has basically two stages. In the first stage we would like to identify possible situations in which mobile devices are available for cooperation. We propose some ideas on how to find such situations and how to report on them. The next stage concentrates on how to encourage cooperation for certain services. Therefore we highlight some cooperative services, such as cooperative video broadcast, cooperative web browsing, and afterwards give some ideas on how to promote them within the own cooperative cluster. II. FINDING COOPERATING MOBILE DEVICES Before any cooperative service can be realized, we need to find cooperative mobile devices within our neighborhood, that is within the capability of the short range links. In the future, when cooperation will be well established, many of such cooperating entities could be involved. In order to get an initial estimate on the actual number of collaborating entities, we have implemented a mobile application for Symbian based on current commercial terminals (S60 phones) that scans the neighborhood for potential partners using Bluetooth. Currently many users switch off their Bluetooth devices for security reasons but as users become aware the benefits of cooperation this would encourage them to keep open their short-range links. From our previous findings in [1], we could report that the benefit of cooperation increases with the number of cooperating devices, but that the largest gain is attained already with a first cooperating entity is added. The application will scan within a given time period for neighboring devices. In each scanning process the number of cooperative devices, the mobile device type, the time stamp when scanning was activated and, if available, a GPS position are logged on the mobile device. The data is stored in a XML file format. A short excerpt is presented here: <device> <name>client2</name> <addr>000a3a66e726</addr> <time>16:49:06</time> <date>13/01/2007</date> <servicerecord> <recordhandle>0x1000a</recordhandle> <attribute><id>servicerecordhandle</id> <value type = "Uint">0x1000a</value> </attribute> <attribute><id>serviceclassidlist</id> <group><value type = "UUID">SerialPort</value> </group> </attribute> <attribute><id>protocoldescriptorlist</id> <group><group><value type = "UUID">L2CAP</value> </group> <group><value type = "UUID">RFCOMM</value> <value type = "Uint">0x1</value> </group>

4 The XML output shows some information of the explored device, including the device name and the time and location when the contact has been established. To make an thoroughly investigation we list even the type and capabilities of the device found. The application is made available on our web page [10] and volunteers are welcome to generate their own traces, which they can send back to us. From the trace we generate the following statistics: number of contacts per device mean number of contacts with at least one contact mean duration of cooperation identifying cooperative hot spots (only with GPS data) The submitted traces and results will be made available on our web page [10] too. This initial investigation is ongoing but gives us a first estimate if cooperation can be established at all among mobile devices. III. A FIRST APPROACH TO COOPERATIVE SERVICES Once the cooperative cluster is established different services can be provided. Obviously multicast or broadcast services are well suited for cooperative clusters. For such services the cooperative cluster can accumulate their cellular bandwidth to open up a virtual big pipe. Each terminal is receiving only a part of the content which it makes available over the short range communication link to all cooperative partners. Also on the short range communication link every collaborating terminal would receive the disjoint information, thus the missing parts. Certainly, such a scenario is ideal for file downloading or video reception. As an example of video services, the benefit of cooperation for DVB-H services was investigated in [5]. The main reason why DVB-H was introduced even DVB-T was already available are the video format and the battery consumption. In DVB-H the air interface is only active a fraction of the receiving time to receive high data rate bursts. Battery savings are available as the air interface is switched off from time to time. The main idea in [5] is to virtually increase the times the RF/BB is switched off. This is done as one mobile device of the group receives the broadcast data and forwards it to its local neighborhood, which has deactivated the cellular air interface and listens only on the short range link. As the short range link is much more energy efficient than the cellular link, energy can be saved for all remote devices. The forwarding device is investing slightly more energy as it has to activate two interfaces. Therefore for fairness reasons the role of the forwarder is changed within the group from time to time. In this scenario each cooperating entity is more than welcome as it helps to decrease the power consumption while viewing the video service. Here the benefit for the customer is clearly visible. An extension to this scenario is the usage of multiple description coded (MDC) video. Here the network provider and the service provider agree to transmit a broadcast video stream in small sub streams, where each sub stream is decodable. The more sub streams are available at the mobile device the better the video quality becomes. To improve the video quality the customer needs a high data rate connection. One possible way is to achieve a high class mobile device. This device will cost considerably more than a basic device and will also consume more energy as the realization of high data rate is quite complex over the cellular link. The second option is to exploit cooperation. Here, even basic devices will receive one or a small subset of the overall sub streams and they will exchange them cooperatively within the group. The customers will gain in power consumption and it will even result in a smaller price, if the network and service providers have agreed on such a cooperative service. The reason why the network provider and the service provider should do so, is the fact that they increase their market share. High class services can only be used in a stand alone fashion by high class mobile devices. The number of those devices may be small compared of the basic omnipresent devices. To sell those high class services even to the largest group of terminals, the network and service provider should agree that the grouping will pay off in some way. Besides multicast services also unicast services can be exploited in a cooperative manner. In [5] the DVB-H scenario is extended also for unicast IP services. As the unicast IP services are conveyed in a broadcast fashion with the DVB- H cell, the concepts derived above for video services are still valid. Among aother possibilities, we consider now the possibility of cooperative web surfing. We take the example of two mobile devices with a given cellular data rate and we assume that the data rate over the short range link is much larger than that on the cellular one. For illustrative purposes we assume a GPRS connection for the cellular and a Bluetooth connection for the short range. The communication architecture is now assumed to be as given in Figure 3. If both customers start to surf the web, their traffic can be modelled as ON OFF traffic. If they would not cooperate their cellular link would be idle for most of the times and active once a link is clicked or inserted directly. The time they have to wait until the full page can be displayed depends on the content size and the available data rate. In the cooperation case the traffic can be split in two parts. One part is the direct cellular part and the second part is relayed by the neighboring mobile device. Note that each web page is divided already now in many TCP sub streams and that therefore this scenario can be easily applied. In the cooperative scenario with two mobile devices the data rate is nearly doubled and therefore the waiting time should be broken into half. It may happen that both stations click request the medium at the same time. In such a case the performance is as bad as in the non cooperative scenario. The probability of double booking reduces with a larger number of cooperating mobile devices. How the system deals with unequal cooperation awards (if one mobile device uses more resources of the other than vice versa) depends on the network costs and the relationship of those terminals to each other.

5 IV. SERVICE DISCOVERY WITHIN THE COOPERATIVE CLUSTER In the previous section we presented some example for cooperative services. It is clear that the of list cooperative services is not complete due to the space limitations, but it gives a first insight on how such services can be applied for unicast as well as multicast services and for uplink as well as downlink. This section focuses on techniques for service announcement within a cooperative cluster. We believe that the announcement should be done by the entities themselves instead of the cellular base station. For sure, the base station may enhance the service announcement, but the main task should be done by the mobile devices themselves. On each mobile device there should be an application running showing which cooperative services are currently active. In Figure 5 such an application is depicted. As seen, the user gets an overview of the ongoing cooperative services. Furthermore the application has to inform the user about the possible costs in case he or she joins the group. Costs are composed out of the network and the service provider costs. The information needs to be given by the network and/or service provider. Fig. 6. Increased market potential for cooperative services. service before. The second reason lies in the fact that more terminals can consume even high class services. While high class services can be consumed by high class devices in a stand alone fashion, the same service can be used by commercial mobile devices exploiting cooperation. V. CONCLUSION In this paper we have presented some of our recent work advocating for a scalable cooperative wireless grid architecture and its associated services for future communications. We argue that a cooperative wireless gird composed out of commercial omnipresent mobile device can offer already now the same services as envisioned for 4G devices, at a lower complexity. The lower complexity goes in line with the need for less power consumption in the mobile device. Future advanced networks and terminals will, by design, exploit cooperative principles providing even more advanced services and applications. REFERENCES Fig. 5. Possible cooperative application on the mobile device. Such an application benefit the network operator and the service provider as not only services that are initialized by the customer himself are requested, but already ongoing services attract other users to join. As illustrated in Figure 6 the market potential will increase due to new feature announcing ongoing services. Note that this kind of advertisement differs from those that could be launched by the network operator. Here the surrounding customers are using the service and indicate that it might be interesting for them. The customer may spontaneously join the service. This will lead to a significant increase of the market potential. We stress that cooperation increases the market potential due to two main facts. The first is based on the fact that the cooperative clusters advertise their services and may convince other customers to join spontaneously. A good service or a service that is used by others intensively may be frequented also by customers that would not have considered such a [1] F.H.P. Fitzek and M. Katz, Eds., Cooperation in Wireless Networks: Principles and Applications Real Egoistic Behavior is to Cooperate!, ISBN X. Springer, April [2] J.M. Kristensen and F.H.P. Fitzek, Cooperative Wireless Networking Using Software Defined Radio, in International OFDM Workshop, Aug [3] J.M. Kristensen and F.H.P. Fitzek, The application of software defined radio in a cooperative wireless network, in 2006 Software Defined Radio Technical Conference, Orlando, Florida, USA, November 2006, SDR Forum. [4] F.H.P. Fitzek, G. Schulte, and M. Reisslein, System Architecture for Billing of Multi-Player Games in a Wireless Environment using GSM/UMTS and WLAN Services, in Proceedings of the First Workshop on Network and System Support for Games (NetGames 2002), Braunschweig, Germany, Apr. 2002, pp [5] Q. Zhang, F.H.P. Fitzek, and M. Katz, Cooperative Power Saving Strategies for IP-Services Supported Over DVB-H Networks, in IEEE Wireless Communications and Networking Conference (WCNC) Networking, Hong Kong, Mar [6] Q. Zhang, F.H.P. Fitzek, and Marcos Katz, Evolution of heterogeneous wireless networks: Cooperative networks, in 3nd International Conference of the Center for Information and Communication Technologies (CICT) - Mobile and wireless content, services and networks - Shortterm and long-term development trends, Nov [7] M. Katz and F.H.P. Fitzek, Cooperative Techniques and Principles Enabling Future 4G Wireless Networks, in The International Conference on EUROCON 2005, Nov. 2005, vol. 1, pp

6 [8] J.M. Kristensen, F.H.P. Fitzek, P. Koch, and R. Prasad, Reducing computational complexity in software defined radio using cooperative wireless networks, in International Symposium on Wireless Personal Multimedia Communications (WPMC 05), Aalborg, Denmark, Sept [9] F.H.P. Fitzek, M. Katz, and Q. Zhang, Cellular Controlled Short-Range Communication for Cooperative P2P Networking, in Wireless World Research Forum (WWRF) 17, Heidelberg, Germany, Nov. 2006, WWRF, vol. WG 5. [10] Mobile Device Group,