e-mail: apulia{dbruneo,mvillari,azaia}@ingegneria.unime.it ... bile devices to access advanced services available in the ..... The RTCP provides the sender.
VOD services for mobile wireless devices D. Bruneo, M. Villari, A. Zaia, A. Puliafito University of Messina, Department of Mathematics Contrada Papardo, Salita Sperone, 98166 Messina, Italy e-mail: apulia{dbruneo,mvillari,azaia}@ingegneria.unime.it
Abstract Wireless devices are becoming very popular and powerful enough to be commonly adopted to access distributed services. More and more sophisticated applications are being developed and a very promising market is quickly being established where mobile users may access multimedia data while roaming from a cell to another, anytime and everywhere. Such scenario requires a strong infrastructure in order to adequately manage all the different issues related with high level service provisioning. QoS represents one of the most crucial issues as it involves many different aspects and directly impacts the user satisfaction. In order to achieve this target, we propose an architecture which allows mobile devices to access advanced services available in the wired part of the network. This architecture, based on mobile agents technology, assumes the presence of a VOD Virtual Server. The strategy adopted is to hide all the basic mechanisms inside a middleware layer that provides the necessary interfaces to allow a simple interaction between the user on the one side and the network and the distributed services on the other. The overall architecture, that is currently being implemented and tested, has been used as an experimental environment to provide mobile users with a new service to access MPEG-4 flows.
1
Introduction
During the last few years, wireless devices have become more and more widely adopted, due to the users’ needs for mobility and information anywhere and at any time [6][8]. New and very powerful wireless handheld equipments are now available at reasonable costs, and times are mature to develop new services that can
be easily accessed by the final user. Video streaming over the Internet [4] is no longer a far imagination. Video On Demand (VOD)[5] should emerge in mid term, with the availability of high bandwidth access medium (ADSL or cable) directly in the final customer’s premises. The next step is to allow mobile users to access these services. Big efforts are then required to develop new and powerful mobile devices, to improve the network capacity and to produce more efficient video formats. Mobile devices are already able to support more and more functionalities and are equipped with at least one multimedia player. With regard to video streaming, MPEG-4 video [1] recently hit the radar screen of many popular and trade off publications, mainly as a mechanism for pirating movies. A reference architecture must be developed, which allows to resolve the user mobility issues, while respecting the narrow constraints of VOD-like services, mainly in terms of guaranteed Quality of Services (QoS). Mobility poses new and challenging issues to be solved [3], among which the QoS management is one of the most crucial, due to the immediate consequences it has on the user’s satisfaction. It is extremely important to develop an infrastructure enabling an effective QoS management, and allowing the mobile users to benefit from the service requested significantly, notwithstanding the device used and the user’s moves. In [2] we have proposed a reference architecture that makes wide use of the mobile agents communication paradigm. In this paper, we focus on the Access Points area and on the design and implementation of a VOD Virtual Server (VS) in particular.
2
Architecture
The proposed reference architecture includes all the software components involved in providing guaranteed QoS to mobile users. The scenario we are examining,
(a) Reference Scenario.
the data about the device, the user, the state of some device components (battery, wireless card), position, direction, and speed. The Access Area is the contact area between the Wireless Area and the Wired Area. It consists of the Access Points, which allow the mobile devices to access the services present in the Wired Area. The task of the Access Points is the management of the issues concerning the user’s mobility and his/her movements from a cell to another, by assuring a high level of transparency and continuity of the service. The Access Points need to manage the user’s mobility (handoff) and to implement the management policies of the QoS (reservation, allocation and distribution of the bandwidth). The algorithm used for the management of the QoS in the wireless infrastructure is based on the proactivereactive hybrid approach described in [9]. The following types of agents will be present in the Access Area: User Agents (UA), GeoLocation Agents (GLA), and QoS Agents (QA). Each time a user enters a cell, a UA is created in the corresponding Access Point. The UA will act as an intermediary for the user within the wired area. As soon as a specific service is requested, the UA starts communicating with the service manager, in order to make it perform the tailoring operations according to the data received from the PA. These operations are the changes needed for adapting the service features as effectively as possible. Then the UA starts the mediation with the QA present in the Access Point, in order to reserve the bandwidth needed for using the service with an adequate QoS. As we show in Figure 1(b), each time a handoff takes place (that is, each time the mobile device switches from a cell to another), the UA migrates to the new Access Point, and therefore follows the user during his/her movements. In order to know the identity and the address of the new Access Point, the UA sends the data (provided by the PA) about the user’s position, direction, and speed to the GeoLocation Agent present in the Access Point. Each GLA knows the position, the addresses, and the identity in detail for all of the Access Points present in the Access Area. Once the UA has received such data, it immediately notifies the QA that the user has moved from a cell to another. This way, the mechanisms of QoS management (bandwidth reservation in the destination cell, and reorganization of the reserved bandwidth in the wired area) are enabled. Furthermore, the UA can migrate to the new Access Point, by using the data coming from the GLA. A QA will be present in each Access Point. Its task will be the management of the QoS policies, according to the algorithm described before. Considering the
(b) User Mobility.
Figure 1. Architecture.
which is shown in Figure 1(a), consists of the following areas: • Wireless Area (mobile devices) • Access Area (Access Point) • Wired Area (Internet, Grid) The Wireless Area is the coverage area of an Access Point, where one or more mobile devices can be found. Each mobile device is characterized by the presence of a fixed agent called Personal Agent (PA). The task of the Personal Agent is to establish a connection between the user and the agents present in the other areas of the scenario. The hardware and software features of the device used must be considered. In particular, the PA will need to: • know the user’s identity and profile • know the physical features of the device where it is resident (screen, CPU, and RAM) • monitor the ”degradable” components of the device (the battery in particular) • know the user’s actual position, by means of a GPS, for instance. The PA is automatically created during the boot stage of the device. The initialization of the PA consists of the loading of data corresponding to the user’s profile and to the device features. Once the initialization has ended, the PA monitors the network activity, in order to check the input and the output of the Wireless Area. When the device enters the Wireless Area, the PA immediately creates a User Agent (UA) in the corresponding Access Point. This User Agent will represent the PA within the Wired Area. The information exchanged between the PA and the UA concern 2
requests coming from the UAs and the QoS policies implemented, the QA performs the bandwidth reservation operations, in order to guarantee each user with the QoS required. The QAs exchange some messages, in order to manage the user’s mobility effectively. They reserve the resources appropriately, so to ”anticipate” the handoff. In the Wired Area, that is the fixed network infrastructure, we can find the servers that provide the services (MPEG-4 flows) requested by mobile users. Moving from an Access Point to another might cause considerable changes in the delays in the Wired Area. This might prejudice the QoS policies established among the Access Points. In order to try to deal with the issue of the coordination of computational and storage activities and resources present in the Wired Area of the architecture, we assume the presence of a Grid computing environment (see Figure 1(a)). The MA will have the task of monitoring the service requests and the state of the server. The Grid Agent will have the task of managing the Grid services and the movements of data and/or code, the execution of remote jobs, or the access to distributed resources. The MPEG Agents will communicate with the QAs, in order to obtain the data concerning the user’s needs for QoS and their mobility. Some nodes will be present in the Grid Area, which will make their resources available. However, they will not act as MPEG-4 servers. In order to select the most convenient MPEG-4 server, the following relation has been used:
wl, wu, wthr, and wdst are the weights assigned to the parameters load, user, throughput, and distance. Their sum will equal 1. The previous relation can be periodically evaluated in order to keep trace of changes in the system. When the user wishes to access the system for viewing an MPEG4 video (stored in the Grid zone), he/she runs the PA GUI. After an authentication phase, the PA sends the userID to the Access Point. It is checked that a UA corresponding to this userID is not already active or suspended, and then a new UA is created. If a UA with the same userID already exists, it is required to migrate from the old Access Point, where it is either active or suspended, to the new Access Point. Thus, the UA sends an acknowledge message to the PA running on the mobile device. The mobile device sends the user’s request, the information about the user’s profile, and the hardware and software features of the client. This stage is necessary, since the user might change his/her access device while moving from an Access Point to another. In general, the user’s request consists of a set of keywords representing the video he/she is interested in. According to the information received, the UA requests each MA for the list of movies that match with the user’s preferences, and creates a merge for all the lists received. This list, that indicates also the server where the movie is available, is sent to the PA. At this stage of development, the communications with the MAs are done through broadcast mechanisms managed by the agent platform. This might make the system less scalable. Some solutions are therefore being studied. They are based on peer-to-peer mechanisms, which allow to solve this problem. In order to manage the video streaming, the Virtual Server described in section 3 has been installed on each Access Point to hide the MPEG-4 server from the PA. This will help to manage frequent situations of server overload and handoff, which require to change run time the MPEG4 server used for the video streaming. Thus, the PA will always make reference to the following address: rtsp://AccesPointAddress:554/SelectedMovie.mp4.
V al% (Si , Aj ) = 100 ∗ (wl ∗ (load(Si )/100)+ (1) +wu ∗ (user(Si )/M AXuser(Si )) + +wthr ∗ (throughput(Si )/M AXthroughput(Si )) + +wdst ∗ (distance(Si , Aj )/M AXdistance)) where: • V al% is the percentage value assigned to the ith MPEG-4 server (Si ) at the jth Access Point (Aj ); • load is the CPU load of Si . This value is included between 1 and 100;
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• throughput denotes the current throughput of Si . This is normalized through the maximum possible throughput (M AXthroughput) for Si ;
VOD Virtual Server
The VOD Virtual Server (VS) shown in Figure 2, is a software layer interposed between multimedia servers and clients. It manages the connection (setup and maintenance) and the data transfer in a wireless environment. In order to combine the VS into the architecture described in section 2, it has been designed according to the agents technology. Whenever a user asks for a video, the request is sent, through the Personal Agent -
• user is the current number of users of Si . This value is normalized through the maximum number of users (M AXuser) accepted by the server; • distance denotes the distance (in terms of latency) between Si and Aj measured in milliseconds and normalized through the maximum acceptable value (M AXdistance); 3
used, some tailoring operations may be necessary on the video. In fact, if the user switches from a PDA to a notebook, the quality of the screen will be better. This will enable the user to see a higher resolution video stream. As described in the architecture, these operations will be done by the Grid Nodes present in the Wired Area. However, such operations will be done in background, in order to guarantee the continuity of the service. The VS will switch the streaming only when the video is ready in the correct format. This will be done transparently for the player. The case of a user who disconnects and then connects again in a different Access Area within the deadline can be considered the same way as we have described in [7]. Let us see what happens during the handoff operation. The UA anticipates the user’s roaming, and migrates to the new AP for allocating the resources needed. Furthermore, the UA asks the QoS Agent to create a SA, and to put it in contact with the SA present in the former AP. The SA will be informed about the state (MPEG-4 server, video, and progress of the view), and will get in contact with the MPEG-4 server, in order to obtain a streaming of the video requested, starting from the point reached by the user. Furthermore, the SA will wait for the user to pass. Once the handoff takes place, the stream to the client starts. From the user’s point of view, this allows to view the video continuously. In the worst of the cases, some frames may be lost, due to the synchronization problems. In fact, even if the exact time of the user’s roaming to another cell can be anticipated, in a real case this can happen later or earlier. This can be solved by designing the cells so to make them partially overlapped. The VS plays an important role during the load balancing, as well as during the selection of the most appropriate MPEG-4 server. Such operations take place in the Wired Area, as we have described in section 2. The Grid Zone will try to make a new distribution of the load among the MPEG-4 servers either periodically or as a reaction after a handoff. This operation may move the video from a MPEG-4 server to another, and consequently disconnect the player present in the mobile device. The VS (which is appropriately notified by the Grid Zone through the chain Grid Agent - Streaming Agent) can make the operation of server change transparent for the user, by buffering several packets, so to assure the continuity of service while switching from an MPEG-4 server to another. Furthermore, the VS enables to synchronize the MPEG-4 servers, by notifying the new MPEG-4 server with the exact point the streaming must be resumed from. The VS is designed to be run in wireless 802.11b APs. Multimedia communications are synchronized and managed by the Real
Figure 2. VOD Virtual Server. User Agent chain, to the QoS Agent located in the corresponding Access Point. Once the request is accepted, the QoS Agent interacts with the VS, activating all the tasks needed for the video streaming (video search, MPEG-4 server selection, etc.) on the one hand, and creating an agent called Streaming Agent (SA) on the other hand. The SA interacts with the UA during the whole video session, and takes care of the user movements and of his/her requests such as video starting, stopping, pausing, restarting, and rewinding. The VS operates as an intermediary between the MPEG-4 server and the client dealing with the following situations: • the user suddenly loses the connection • the user disconnects in order to reconnect with a different device • the user moves from an AP to another Let consider the first scenario of a sudden disconnection. In this case, the SA, advised by the UA, begins to bufferize video packets till a maximum buffer size is reached. Such buffer size has to be selected according to the available memory and to the number of users to be managed. Once the buffer is full, the SA activates the videopausing function in order to stop the video streaming and sets a timer, waiting for a possible user reconnection. If a user reconnection appears by the given deadline, the UA asks the SA to restart the video streaming, flushing the buffer and retrieving new video frames from the MPEG-4 server. We need to notice that in this case the VS is hiding the client disconnection to the server, which otherwise would have stopped the video streaming. If the deadline expires, the VS empties the buffer and tears down the video streaming. Any reconnection of the client will therefore be managed as a new request for service. If a user reconnects by using a different device (for instance, by switching from a PDA to a notebook), the PA present in the device will recognize the UA present in the Access Point, and the VS will therefore need to repeat the same operations to be done in the previous case. Due to the different devices 4
Time Streaming Protocol (RTSP) which uses the Real Time Protocol (RTP) as transport layer. The RTSP protocol uses the TCP protocol to communicate with its peer. The RTSP and HTTP protocols have similar syntax and operations, even if the RTSP semantically introduces some new methods and has a different protocol ID. Since the TCP is a transport protocol with connection, if the wireless device temporarily disconnects, the VS needs to restore a connection through the setup phase. The RTP protocol enables to send multimedia data, and uses the UDP as transport protocol. Consequently, all the weaknesses in the management of multimedia communication are overcome by using the RTP on the UDP. The RTP Control Protocol (RTCP) is often used together with the RTP. The RTCP provides the sender with a feedback about the quality of the current transmission. If the client disconnects, the RTP packets are buffered. When the client connects again, the packets are sent from the VS to the client. In order to make the client transparent to the server and vice versa, the VS receives the incoming RTSP packets, then processes and sends them. The RTP packets do not need any processing, since no track of the sender and the recipient is contained in them. The VS therefore receives the UDP datagram and sends it. Basically, the VS can be placed one level higher than the RTSP. It implements the features needed for making the video streaming reliable in a wireless environment. In conclusion, the VS is a device that emulates a multimedia MPEG-4 server providing a good QoS for the wireless systems that want to interact with multimedia applications.
4
We have used some clients based on a Pentium architecture, as well as the OS Linux Red Hat, Windows 2000 and XP, equipped with the following Players compatible with the MPEG-4 streaming: • MPEG4IP Player (Windows/Linux). • QuickTime Player 5.0.2 (Windows). • RealNetworks RealOne Player + Envivio Plugin (Windows). The multimedia flow has been tested by a Compaq iPAQ 3870 equipped with Windows PocketPC 2002 OS, Wireless card 802.11b, InterObject SMIL Player, and RealOne Real Player. The APs we have used are based on a Pentium MMX @ 166 MHz architecture, 128 MB of RAM and 64 MB DOM (Disk On Module) of mass storage, and are equipped with an embedded version of Linux based on the kernel 2.4, and with KaffeVM. Several movies of different resolution and duration, codified according to the standard MPEG-4 and bitrate up to 370 Kbps, have been used. The VS has shown to be able to work also with other types of coding such as Real Media, MP3, MPEG1, and SWF. In order to check the correct operation of the VS, two experimental trials have been organized: the first one aims to show the correct operation of the player in a single user configuration, while the second one tests the behavior of the VS in a multi-user configuration. The results of the trials are shown in Figures 3(a)/3(b) and 4(a)/4(b) respectively. In the single user session, we have measured the flow of the RTP and RTSP packets in a high mobility scenario. The client, while frequently roaming among the areas covered by the three APs, has produced a large amount of RTSP packets, which are necessary for the control phases of the flow and for redirecting the flow to the various APs. In Figure 3(a) we can see the delays introduced by the VS on the flow of RTP packets. In the upper figure we can see that the VS introduces a delay shorter than 70ms for almost all of the packets, while in the lower figure we observe that more than 60% of the RTP packets are processed by the VS in less than 15ms. Another interesting consideration concerns the packets related to the multimedia flow. In fact, we have observed that most of the packets with a delay longer than 30ms are the first RTP packets processed immediately after the connection phase. In the other phases, the delays are almost constant and shorter than 30ms. This affects the view of the movies (and of the multimedia flows in general) with a small initial delay introduced by the VS, but it does not influence the correct use of the stream during the view. Conversely, with reference to the RTSP packets, in Figure 3(b) we observe that more
Implementation and Experimental Results
Our measurements concern the delays introduced by the use of the VS with reference to both RTSP control packets and RTP data packets. The testbed consists of the following components: • A MPEG-4 Server. • Some Clients equipped with Multimedia Players. • Three APs on which the Virtual Server is running. The MPEG-4 streaming server, based on a Pentium IV architecture with Red Hat Linux (Kernel 2.4.18-3) OS, has been tested with two Software Suites: 1. RealNetworks Helix Universal Server Basic. 2. Apple Darwin Streaming Server 4.1.2. 5
(a) RTP Packets.
mobility and information anywhere and at any time. In order to manage the challenging problems related to the management of such new environments, in this paper we have defined and partially implemented a reference architecture for effective QoS management, which allows mobile users to access multimedia services (in our case, an MPEG-4 flow), notwithstanding the device used and the user’s moves. In order to achieve this purpose with an adequate level of transparency, and considering the heterogeneity of the devices and of the networks, mobile agents have been assumed as the underlying technology, thus exploiting their interesting features of mobility, asynchronicity and autonomy. We have designed and implemented a VOD Virtual Server that acts as a intermediary level between the client and the MPEG-4 server. Thanks to the VS, while roaming from a cell to another, a mobile user is constantly connected to the most convenient resource, which is dynamically identified in order to guarantee adequate levels of QoS. Some experimental trials have been conducted in order to test the validity of the proposed solution and its applicability in a real scenario.
(b) RTSPb Packets.
Figure 3. Number of RTP and RTSP Packets in a single-user/high mobility session.
References (a) RTP Packets - 5 Players.
(b) RTSP Packets - 5 Players.
[1] Iso/iec 14496-1 final draft international standard mpeg4 systems (oct. 1998). [2] D. Bruneo, M. Villari, A. Zaia, and A. Puliafito. Qos management for mpeg-4 flows in wireless environment. to be published in Microprocessors and Microsystems, Elsevier Science. [3] D. Chalmers and M. Sloman. A survey of quality of service in mobile computing environments. IEEE Communications Surveys, pages 1–10, Second Quarter 1999. [4] P. P. R.-t. S. o. t. I. EURESCOM. [5] J. Y. B. Lee. On a unified architecture for video-ondemand services. IEEE Transaction on Multimedia, 4(1):38–47, MARCH 2002. [6] M. Shafi. Wireless comminications in the twenty-first century: A perspective. In Proceedings of the IEEE Vol 85, 1997. [7] O. Tomarchio, L. Vita, and A. Puliafito. Nomadic users’ support in the MAP agent platform. In Proceedings of the Second International Workshop on Mobile Agents for Telecommunication Applications (MATA 2000), pages 233–242, Paris, France, 2000. [8] U. Varshney and R. Vetter. Emergin mobile and wireless networks. Communications of the ACM, 43(6):73– 81, 2000. [9] J. Ye, S. Papavassilou, A. Puliafito, and G. Anastasi. Strategies for dynamic management of the QoS of mobile users in wireless networks through software agents. In (ISCC 2002) International Symposium on Computers and Communications, pages 369–374, 1-4 july 2002.
Figure 4. Number of RTP and RTSP Packets in a multi-user/normal mobility session.
than 70% of them are processed by the VS with a delay of less than 20ms. This is definitely acceptable for control packets. In the multi-user session we have measured the flow of the packets in a normal mobility situation with 5 users. Figures 4(a) and 4(b) show that 50% of RTP packets are processed in less than 15ms, while more than 99% of RTP packets require less than 80ms to be processed. With regard to the RTSP packets, we can observe that, in a normal mobility situation, the VS can process them with a delay of less than 20ms in 80% of the cases. In conclusion, during the testing we have realized that the QoS perceived by a user is practically the same for the multimedia flows analyzed.
5
Conclusions and future development
During the last few years, wireless technology has become mature enough to satisfy the users’ needs for 6
