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A Tele-Education Oriented Experiment based on an Integrated Terrestrial/Satellite Network

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A Tele-Education Oriented Experiment based on an Integrated Terrestrial/Satellite Network
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  A Tele-Education Oriented Experiment based on an Integrated Terrestrial/Satellite Network *Davide Adami, **Piero Castoldi, °Mario Marchese, °°Andrea Morelli, °°°Luca Simone Ronga CNIT - Italian National Consortium for Telecommunications   * CNIT Pisa Research Unit, University of Pisa, Via Diotisalvi, 2, 56100, Pisa (Italy). ** CNIT National Photonic Networks Laboratory, Via Matteucci 34/L, 56100 Pisa (Italy). ° CNIT Genoa Research Unit, University of Genoa, Via Opera Pia 13, 16145, Genova (Italy). °° CNIT Parma Research Unit, University of Parma, Parco Area delle Scienze 181/A, 43100 Parma (Italy). °°° CNIT Firenze Research Unit, University of Firenze, Via di Santa Marta 3, 50139 Firenze, (Italy). E-Mail: davide.adami, piero.castoldi, mario.marchese, andrea.morelli, luca.ronga@cnit.it Abstract The paper presents the architectural solutions and the experimental results of the integration of two networks, a Campus terrestrial Network and a Geographic satellite Network, which have been designed and deployed by the CNIT (Italian National Consortium for Telecommunications). The two networks were developed in different contexts and use a different approach, but they have the common aim to demonstrate the feasibility of a tele-teaching system using standard and ad-hoc software application. The result of the integration is a nation-wide distributed tele-education network with a large number of clients. It is composed of an ATM/ADSL network within a Campus and of a TCP/IP satellite network, which acts as a backbone for the geographic distribution. On one hand, the ATM portion of the network allows reserving specific network resources for each flow (even if it is not necessary in the experiment performed due to the high capacity available), on the other hand, through a satellite network, whose bandwidth is limited to 2 Mbits/s, a TCP/IP suite is used. TCP/IP protocols offer a best-effort service but, in this case, a guaranteed service should be offered. There are two methods in the literature to reserve resources in a TCP/IP-based network: the Integrated Services and the Differentiated Services. The former has been chosen in this environment, on the base of previous experimental studies and of the test-bed dimension. The experiment of integration has been investigated by using both subjective and objective performance parameters. Network users (students) have been asked to fill in reports to measure the Perceived Quality of Service, in order to get a Mean Opinion Score (MOS) about the level of user perception of the overall tele-teaching service. Measurements of packet loss and jitter have been also performed in order to assess the Quality of Service of the integrated network. I.   I NTRODUCTION  There is an increasing interest for the delivery of multimedia services to the home and business user. Services fall within different frameworks such as education (tele-teaching, tele-tutoring, courseware on-line), entertainment (hi-fi radio broadcast,  jukebox music, movies, pay per view, tv channels, network games), public services and administration (e.g. tele-diagnosis, trading on line, tele-banking, tele-meeting). Many of them require a high level of interaction and involve different media as video, audio, document sharing, data broadcast from sensors. The offer of different and heterogeneous services implies the support of a suited telecommunication network. Some types of networks, e.g. ATM (Asynchronous Transfer Mode) have been designed to support QoS (Quality of Service) for specific traffic flows. A statistic investigation to verify the availability of the resources to guarantee a fixed level of service is performed before accepting a new call entering the network. Call Admission Control (CAC) mechanisms are used in this case. On the other hand, the Internet is a “best-effort” network, (TCP/IP protocols [1] have not been designed to provide guaranteed quality of service). Both the approaches offer advantages: ATM networks are "structured" to support QoS through simple and efficient methods to reserve bandwidth. TCP/IP networks have the advantage of being a standard de facto. Most applications to transmit data, voice and video are based on the TCP/IP suite and this protocol stack simplifies the use and development of commercial applications. Moreover, in more restricted contexts, as well as in private networks designed to support specific services (e.g. a tele-education network oriented to academic audience, as in this paper), it allows a simple interconnection with other existing networks, as, for instance, the Internet. In the same private environment, also different approaches to the distribution can be used. A wireless local loop can be used to serve users that are concentrated in a limited area; a satellite coverage can be deployed to serve scattered users on a geographic area; solutions based on coax cable or fiber optics can be used for users within a campus metropolitan area. Actually, a satellite backbone offers some advantages, with respect to a cable network, as scalability, wide land coverage and multicast service. Due to this, satellite links are also seen as interesting solutions to provide service to the terminal user [2]. On the other hand, many terrestrial networks are already operative and they offer a good service to the connected users. In this  framework, a very important point is to provide an efficient integration among networks based on different technology also from the point of view of the Quality of Service. ATM, as already said, provides QoS thanks to its design [3]. There is a definition of traffic classes and a traffic contract guarantees the final users. Concerning TCP/IP protocols, two main models appear in the literature: Integrated Services [4] and Differentiated Services [5]. The former allows assigning a certain bandwidth to a specific traffic flow. The mechanism uses a signalling protocol called ReSerVation Protocol (RSVP) [6] to transmit the bandwidth needs. The strategy is aimed at transforming a best-effort network into a QoS-guaranteed network (e.g. ATM) by imposing a virtual planning over the physical link. Differentiated Services use the characteristics of the IP packet (e.g. the priority fields) to provide a fixed degree of service to a group of traffic flows. The Differentiated Services approach is very attractive and, for its scalability, it is the ideal candidate to be the QoS-oriented solution in the next generation of Internet. On the other hand, the possibility to provide QoS to a single traffic flow is attractive too. Especially in small private networks, where each customer deserves attention, as in tele-education systems, the chance to distinguish each user by using the source/destination IP address and the source/destination TCP-UDP port is really useful. Moreover, most commercial routers offer the Integrated Services solution, while only few experimental tools provide the Differentiated Services solution, for now. Two other concepts, together with the already mentioned points about QoS and network integration, complete the keywords of the paper: tele-education and P-QoS (Perceived - Quality of Service). From the point of view of the network, the former represents an application including data, audio and video traffic requiring a specific quality. Actually, it is also a user service that deserves a special attention. Tele-education had a rapid evolution in these last few years. From the non-interactive systems of the beginning, essentially based on videocassettes and CDs, the tele-learning services are now heavily using interaction as a teaching means. The concept of P-QoS matches the problem of evaluating the quality "seen" by the final user. The QoS provided by the network in terms of lost packets, delayed packets and jitter, does not have an explicit connection with the level perception of the users. P-QoS by means of a tool of measure called Mean Opinion Score (MOS) allows to know the real opinion of the users regarding a service and to measure it. The MOS method makes available a group of marks, ranging from 1 (very poor) to 5 (very good), to express opinion. The paper is structured as follows. The next session contains a short description about the evolution of tele-education in dependence of the technology available. Section III describes the problem of the QoS over IP networks. Section IV presents the network test-bed: it focuses on the integration of an IP based satellite geographic network and a Campus ATM network that uses trunks of passive optical fibers (a PON, Passive Optical Network). Section V contains some indications about the implementation of the multicast service, the QoS support and the application to transmit audio and video. Some results, measured over a real tele-education session, are presented in Section VI. Section VII contains the conclusions. II.   E VOLUTION OF THE T ELE - EDUCATION S ERVICE  The first vision of tele-education was essentially based on non-interactive services. Lectures were distributed through videocassettes, CDs and also special TV channels. The students had no possibility to interact with the teacher; they could not make interventions and ask questions on-line. The possibility of asking explanations was relegated to the mail, the phone or, when available, the e-mail. The few tele-learning systems, which included interaction, even if limited, presented too many drawbacks, as the low number of sites, the high cost of the bandwidth and a service composed only of audio and video (voice and video of the teacher along with a video camera showing a blackboard). The reference technology was ISDN, for private networks [7] or the already mentioned television [8] and dedicated technology as video cameras and coders/decoders were used. A first step in the evolution was due to the diffusion of the Internet. Fig. 1 contains an example of Internet-based tele-education network. Some Local Area Networks (LANs), located remotely (e.g. in different Universities), can be connected through the Internet. The LANs used for the experiment described in the paper are represented in Fig. 1 but it is just an example. The great advantage is the possibility of using TCP/IP-based applications, often already available on the market, to send audio/video, to prepare material of support, documentation and presentations. The students may receive the material directly in their computer, both at the Campus and at home. The teacher can give the lecture directly from his office, without using a special tele-teaching classroom, even if the presence of this type of classroom is always a guarantee of audio/video quality. The concept of tele-education is not limited to audio and video but it includes also the presence of data, which can be utilised both on-line (e.g. the presentation of the teacher, explanations, material of support that can be sent as a file) and off-line (e.g. access to an educational data-base where books, papers and other material that can integrate the content of the lectures may be stored). The drawback of this approach is represented by the Internet technology available, which, on one hand, offers the mentioned opportunities, but, on the other hand, has a limited bandwidth and does not implement any algorithm to reserve bandwidth and to guarantee a fixed level of quality to the users. The effect is the limited possibility to have actual interactive services and the spreading of audio/video streaming services. In practice, the opportunities given by a full TCP/IP internetworking cannot be taken because of the lack of QoS guarantees.  LAN Genova HUBRouter LAN Firenze RouterHUBRouterRouterHUBHUBHUBHUBRouter Internet LAN ParmaLAN BolognaLAN Pisa  Fig. 1. Internet-based tele-education system. The last step of the evolution is the integration of different technological platforms along with the wide range of services offered. The integration of LANs located by the sites interested, ISDN, ATM and satellite networks, along with the introduction of new technologies and algorithms has allowed a real interaction, a high number of involved sites and the application of a new vision of tele-education. Among the new technologies and algorithms it is worth mentioning: large availability of bandwidth, implementation of new bandwidth reservation mechanisms in IP networks (Integrated and Differentiated Services), modification of the transport layer [9, 10, 11, 12, 13], multicast protocols, multicast applications, high quality data/audio/video applications. They allow to improve the performance of file transfers, to reserve bandwidth for specific flows, to protect the "most important" information from the congestion, as, for instance the teacher audio and data. The result is a tele-education system that can efficiently reach remote users, can traverse portions of networks based on different technologies and that it is based on audio-video interaction, guaranteed QoS and utilisation of didactic WEBs containing text, audio, video, images, on-line and off-line lectures. III.   ATM,   DIFFSERV,   INTSERV:   T HREE D IFFERENT W AYS OF G UARANTEEING Q O S ATM (Asynchronous Transfer Mode) technology has been designed and standardised more than ten years ago. The aim was to create a new connection oriented technology offering support to the QoS, flexible and efficient to manage bursty traffic (in contrast to circuit switching). ATM has been designed just for high-speed multimedia networking and it can carry different type of traffic. High performance is reached through a simplification of the switching characteristics implemented in hardware [14]. It allows the definition of class-of-services and offers support for it by using proper control mechanisms, which ranges from the node level to the network level. Concerning the former, there are traffic shaping mechanisms, dynamic bandwidth and buffer allocation algorithms for multimedia traffic. The latter includes network routing, flow control and Call Admission Control (CAC) systems, based on sophisticated concepts as, for instance, the equivalent bandwidth [15, 16, 17, 18, and 19] or the feasibility region [20]. ATM is currently often used as a backbone technology and also as a last mile technology, as in the case of ATM PONs (Passive Optical Networks), which are a very efficient broadband access platform for provisioning advanced multimedia services and, even if the presence of suited protocols is a guarantee, they have a so large amount of bandwidth available that, without using any additional mechanism, the QoS is often assured by the bandwidth abundance itself, as in the case of the Campus network used in the experiment presented. The Internet Engineering Task Force (IETF) has introduced two different approaches to approach the QoS issue in the IP world: Differentiated [21, 22] and Integrated [23, 24, 25] Services. The Differentiated Services approach is essentially based on the following concept: use of the priority fields of the IP packet header to differentiate the service offered. Each packet is classified when it enters the network; the concept of per connection service is dropped; a small number of service classes are considered; the traffic with similar characteristics is aggregated. Differentiated Services have interesting scaling properties. No mechanism of control signalling to detect the current state of the network and the current per-application requirements should be used, but this statement may be by-passed if a real QoS has to be offered. To provide a high level of quality there is the need of: a proper algorithm to differentiate the services and guarantee the proper quality of service; QoS-enabled applications that know service requirements in advance; possibly an efficient Call Admission Control (CAC) and a measure of the QoS. Integrated Services offer a per connection approach; they pre-allocate resources and reserve a portion of the overall bandwidth for a specific traffic flow (in this sense there is a similarity with ATM). Integrated Services-based schemes use a signalling protocol, called RSVP [6], to notify the bandwidth reservations. The reservations remain in force until the application explicitly requests the termination of them, or the network communicates that it is unable to maintain them. The approach is 'all-or-nothing'. Either the network can perform the reservations and offer the required quality or the network denies the access. The Integrated Services approach introduces problems and does not appear to be viable with large-scale networks, such as the global Internet but it is suitable for small private networks designed for a specific audience. It has been implemented within the test-bed to optimise the network resource utilisation. The Integrates Services architecture and the use of RSVP within this framework have received a  great attention in the literature for a long time. Reference [26] defines an extension of the TCP/IP architecture and protocol to support both QoS guaranteed and non-guaranteed services. An example of design and implementation of protocol architecture based on the Integrated Services is contained in [27], which lists also many references related to the topic, in a dedicated section. To support the Integrated Services model, an IP router must be able to provide an appropriate QoS to each flow. The router function that provides different qualities of services is called traffic control module, and consists of the following components.   The  Packet Scheduler: it is a software module that manages the forwarding of different packet streams in hosts and routers, based on their service class. It uses queue management techniques and various scheduling algorithms. The packet scheduler must ensure that the packet delivery service is carried out so to meet the QoS parameters requested by each flow. A scheduler can also police or shape the traffic to conform it to a certain level of service. The  Packet Classifier: it is a module that identifies the packets belonging to an IP flow that will receive a certain type of service in hosts and routers. Each incoming packet is mapped by the classifier into a specific class, based on the source and destination IP address, source and destination TCP/UDP port contained in the header of the IP packet. The  Admission Control: it implements the decision algorithm that a router uses to determine if there are enough routing and network resources to accept the reservation request coming from a new flow without damaging the service level guaranteed to the flows already accepted. If the QoS request of a new flow is accepted, the reservation instance in the router assigns the necessary resources to guarantee the requested QoS to the flow. Thus, two key features lie at the heart of an Integrated Services architecture: each router is required to know the amount of resources (buffer, link bandwidth) already reserved for the on-going sessions; a session requiring QoS guarantees must be able to reserve enough resources at each network router along the source-destination path to ensure that its QoS requirements are met. In order to determine whether the available resources are sufficient to meet the QoS requirements, a session must declare its QoS requirements and characterise the traffic it will send into the network. Even if admission control is part of the Integrated Service model and it is reported for the sake of completeness, no admission control has been performed in this work. RSVP (ReSerVation Protocol) is the signalling protocol designed by the IETF IntServ Working Group to allow applications to reserve network bandwidth dynamically. It enables RSVP capable applications, running on end-system hosts to send resource reservation requests to the destination system and to specify the QoS parameters for a specific data flow. Within the test-bed, taking advantage of some router features, video and audio applications have not been modified to support the RSVP signalling protocol: the routers directly connected to the LAN of the transmitting hosts have been configured as if they were receiving signalling messages from them. IV.   T HE N ETWORK T EST - BED  The network test-bed used for the experiments is composed of two main components: an ATM-PON (Passive Optical Network) and a TCP/IP-based satellite network. The interconnection between the two portions is assured via ISDN at 384 Kbits/s, as well as the connection with sites neither connected to the PON or to the satellite network. The network is part of the national network owned by the Italian National Consortium for Telecommunications (CNIT), which constitutes the backbone for the distribution of information within the framework of the project "TeleDOC" [28], oriented to support education of PhD students. The PON has been developed within the project “User Access Interactive Multimedia Network” funded by the National Research Council (CNR), which has the primary target of showing the advantages of an ADSL (Asymmetric Digital Subscriber Line) user access in a scenario employing tele-teaching and distance learning applications. It is applied in the Campus Area of the University of Parma (about 1 km 2 ), where there is a large amount of dark fibers, which connect a videoconference centre and several multimedia laboratories. The configuration employed is shown in Fig. 2. The main components of the configuration are: •   An ATM/Ethernet (10/100 Mb/s) switch, which is the core switch and allows the interconnection of the ATM backbone and of the ADSL users to the multimedia source and the ASI/CNIT satellite network through a Router. •   The switch is connected to the OLT (Optical Line Terminator), which drives the PON tree using an ATM connection base on the SONET OC-3 physical interface. •   The PON is in-between the OLT and two ONUs (Optical Network Units), which handle the traffic to/from the group of clients connected to that branch of the PON tree. •   The user access, which is physically obtained by employing Network Terminators (NTs), also known as ADSL modems, whose upstream and downstream bit rates are 1Mps and 8Mps, respectively. The two branches of the PON are terminated at the CCE (Electronic Computing Center) and at the CEDI (Center for Education and Distance Interaction), respectively, while the root of the PON is located in the vicinity of the videoconference room. The following table (Table 1) reports the length of the PON branches. There are two classrooms by CCE and by CEDI with 25 and 16 clients available, respectively; each of them consists of a PC connected through Ethernet and an NT to an ADSL line multiplexed to an ONU stream. The videoconference room hosts the OLT, the ATM switch, and a VoD (Video on Demand) server.  The geographic network interconnects remote LANs by using a satellite backbone. This portion of the network is reported in Fig. 3. Two remote LANs, located in Genova and Prato (near Firenze), are connected via satellite through a 2 Mbits/s channel; the LAN located in Pisa and Parma are connected using 3 ISDN lines each, for an overall bandwidth of 384 Kbits/s. The latter is the site where the PON is located. The former has no satellite access available. The satellite system employs the ITALSAT II (13 °  EST) satellite, providing a countrywide coverage in the single spot-beam on the Ka band (20-30 GHz). The overall bandwidth is 36 MHz. Each station can be assigned a full-duplex dedicated traffic channel with a bit-rate ranging from 32 Kbits/s to 2 Mbits/s, and it is made up of the following components. Traffic Modem: multi-rate QPSK satellite modem with speed from 32 Kbits/s to 2 Mbits/s. IP Router: a router equipped with LAN, ISDN and RS 449 interfaces for the connection with the satellite modem. Application PCs: a number of local and/or remote PCs as source of the audio/video service. to from CCE CEDI Videoconference Room 1073 m + 30m 196 m + 30m Table 1. Length of the PON branches. Fig. 2. Configuration of the ATM/ADSL Campus Network. Fig. 3. Configuration of the Geographic Satellite Network. The Geographic Satellite Network has been designed and implemented within the framework of the Project "Integration of Multimedia Services over Heterogeneous Satellite Networks" [29, 30], carried on by CNIT and funded by ASI. The applications used may be divided into two groups: •   Locally tested (on the Campus network). 1.   Collaborative environment for tele-teaching purposes (collaborative editor, shared whiteboard, collaborative browsing) to deliver hypermedia content with on-line tutoring of the students in the multimedia laboratories. 2.   Video-on-demand sessions running on the clients in a completely asynchronous fashion to see video-lectures stored in the video-server. •   Geographically tested. Interactive video-lectures by using a videoconference tool and by employing traditional transparencies filmed by high quality video cameras. The videoconference sessions involved the participation of all the local multimedia laboratories and all the remote clients connected by the satellite ASI-CNIT network.
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