A mesh topology DVB-S network architecture for node interconnection, featuring QoS capabilities. G. XILOURIS♠, A. KOURTIS♦, G. STEFANOU♠, ♠ University of Athens, Informatics and Telecommunications Dept., Panepistimiopolis, Ilisia,
15484 Athens, Greece. ♦Institute of Informatics and Telecommunications NCSR «DEMOKRITOS», Agia Paraskevi Attikis, 15310 Athens Greece. Email: {xilouris, kourtis}@iit.demokritos.gr,
[email protected] Abstract. This paper proposes a prototype mesh topology network architecture based on regenerative DVB GEO satellites, which enables the interconnection of heterogeneous terrestrial distribution nodes to each other, with minimum possible delay, offering maximum spectral efficiency of the satellite transponder. A dynamic bandwidth management mechanism, applied directly on the DVB-S stream, is proposed, which enables the provision of interactive IP based multimedia services at a guaranteed QoS. An actual transparent GEO satellite, was used in order to create an emulated DVB-S regenerative environment and validate the performance of the network architecture, the interoperability of the heterogeneous networks and the bandwidth management mechanism. Keywords: DVB-S, WLAN, DVB-T, QoS, regenerative satellites.
I. INTRODUCTION The evolution of digital video broadcasting (DVB) standard and its application over satellites (DVB-S) is a significant and widely used technological development in wireless telecommunications [1] [2] [3]. The DVB technology is able to combine digital TV programs and Internet protocol (IP) services into the same MPEG-2 transport stream through a standardized encapsulation method (Multi-Protocol encapsulation or MPE). Hence, the DVBS platform apart from being a medium for broadcasting “bouquets” of TV programmes to a large number of viewers distributed over large geographical areas, can also realize a unidirectional communication link from the service provider to the users, for the provision of IP services. However, the integration of basic TCP/IP services such as Internet, access to any Web page, e-mail, multimedia, ftp etc., does not only require the encapsulation of IP data into the MPEG-2 stream, but also the existence of an additional return channel acting as an uplink and conveying traffic from the user back to the service provider. The ISDN/PSTN was the first attempt to implement a wired and low bit rate return channel and it is currently the most widely used. The most recent technology in the return channel is via satellite (DVB-RCS), which enables the creation of an uplink from the end user’s premises to the satellite, in a widely dispersed network architecture. This solution is not addressed to single end users (home users), but to groups of users located within a small area, like a building of an enterprise (business users). It also requires a large number of uplinks to the satellite. Another significant problem in the provision of TCP/IP services through satellites is the propagation delay of the RF signal travelling the distance of 36.000 Km from the earth station to the satellite and reverse (one hop). This delay has a minimum value of about 240 msec per hop. For TCP/IP services a request or acknowledgement from the user has to travel one hop to reach the service provider and the reply has to travel the same distance, resulting an overall delay of 480 msec. Furthermore, in a large network where a significant number of users need 1
to be connected to each other, the packets in the transport layer of the MPEG-2 stream or the IP level, have to be collected to a central point and properly re-routed. Since the most convenient location of this central point is on the ground, the number of hops is doubled, because now a packet has to travel from the source to the central point (one hop) and from there to its destination (second hop). A solution to the problem of delay is the use of regenerative satellites with on board processing techniques (OBP) [4]. In this type of satellites, the uplink traffic from various users/service providers is received on the satellite where they are multiplexed, creating a single MPEG-2 stream. This is then transmitted in a common downlink, based on DVB-S technology. In this way, requests, acknowledgements (ACKs) and replies reach their destination in a single hop. Currently, OBP techniques are applied on commercial satellites, where the satellite transponder is used for the broadcasting of a digital TV bouquet, the different programs being transmitted to the satellite from different ground stations. The most important applications of this type of regenerative satellites are television services (SkyPlex) [5], where a return channel is not required. In this unidirectional link, each TV programme is transmitted to the satellite through a proprietary satellite terminal and the Program Identification (PID) is inserted to the final MPEG-2 downlink transport stream through a coordinating ground station, in order the downlink signal to correspond to the DVB-S standards. New generation satellites include the processing of MPEG-2 transport streams with IP data encapsulated. They can be categorized in those using DVB-RCS in the uplink and DVB-S in the downlink and those using DVB-S technology in both the uplink and downlink, while preserving the PID, without the use of a coordination ground center. The first type of satellites have relatively lower uplink capacity and are addressed to small and medium size users (company, enterprise). The second type has higher uplink capacity, using the satellite as an infrastructure aiming at the creation of high bit rate trunks to interconnect terrestrial remote networks. Satellites of the first type already exist in orbit (AMERHIS). A satellite of the second type had been developed, but failed to be placed in orbit (STENTOR) [6]. This paper proposes a prototype mesh topology network architecture based on regenerative DVB-S satellites, which is able to interconnect heterogeneous terrestrial distribution nodes (DN) to each other, with minimum possible delay. These DNs are considered to be large scale access networks covering metropolitan areas and offering high bit rate connection links to their corresponding end users. Furthermore, DNs host a number of services, which can be provided not only to the users located in their coverage area, but also to users belonging to other DNs. End users can be connected to their neighbouring DN either via wired (i.e. ISDN, xDSL) or wireless (i.e. WLAN, DVB-T, UMTS) access technologies, leading to a heterogeneous network environment, where time varying traffic with different properties is generated and routed among the connected nodes through the satellite. An actual transparent satellite in loop back mode was used, for the development of an emulated regenerative DVB-S environment, demonstrating the interconnection of two DNs, each based on different access network technologies. The former serviced users through a WLAN and the latter through a DVB-T platform. The feasibility of interconnecting heterogeneous networks (WLAN, DVB-S, DVB-T) was demonstrated. The overall network performance was measured, for the provision of interactive IP services, from one node to a user of the other node. The paper examines the issue of providing guaranteed levels of quality of service (QoS) and proposes a prototype bandwidth allocation mechanism, based on policy strategies applied directly on the MPEG-2 transport stream. The proposed mechanism manages dynamically the uplink bandwidth of each DN and enables the provision of interactive IP based multimedia services at a guaranteed QoS.
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II. REGENERATIVE SATELLITE ARCHITECTURE A mesh topology network architecture based on a regenerative DVB satellite, is illustrated in Figure 1. This architecture enables the direct connection of each node directly to any other in a mesh topology. It consists of two core subsystems: a) the space segment and b) the terrestrial distribution nodes. The former makes use of an on board processing (OBP) satellite with regenerative capabilities, able to interconnect the terrestrial distribution nodes via DVB-S based technology communications channels, in both the uplink and downlink. The latter comprises of a number of heterogeneous distribution nodes. Each DN covers a medium to large area, like a city and implements an access network offering connectivity to its corresponding users. Each DN may use a different technology in its access network, either wired (i.e. ISDN, xDSL, etc) or wireless (i.e. WLAN, DVB-T, UMTS, etc). It is also considered to be connected to a content provider, thus hosting a number of services, which can be provided not only to the neighboring users within its coverage area, but also to users belonging to other DNs.
Figure 1. Overall mesh topology network architecture. Referring to figure 1, the uplink traffic stemming from each terrestrial DN, is encapsulated into an MPEG-2 transport stream (TS) and is transmitted to the satellite using different frequencies F1, F2,…,FN. The uplink transport streams from all the terrestrial DNs are multiplexed by the on board DVB-S processor, to produce a single transport stream, which is transmitted at frequency F0 (downlink). In this way, the DNs transmit in different frequencies, but all receive in a common one. Furthermore, each node is able to communicate with any other over a single hop satellite channel. The combination of minimum equipment and shortest possible delay (single hop) are only due to the use of a regenerative satellite. 3
a. Space Segment The block diagram of the space segment is depicted in figure 2. According to this figure the uplink traffic from each terrestrial distribution node, is received at the corresponding frequency (F1, F2,…,FN) by the appropriate receiving module and demodulated (MPEG-2 transport stream). Each baseband MPEG-2 transport stream (TS) (where the IP traffic from the corresponding node is encapsulated) is forwarded to a remultiplexing device, which multiplexes all incoming MPEG-2 TS and produces the final DVB-S MPEG-2 TS. Finally, the remultiplexed MPEG-2 TS is modulated (QPSK), amplified and broadcasted via the common downlink, at frequency F0, to all distribution nodes located within the broadcasting area.
Figure 2. Space segment configuration. b. Ground Segment The block diagram of each DN is illustrated in figure 3. It consists of the satellite up and down link chains, the core network of the DN (where a possible content provider can be connected), the access network adaptor at the DN site, the actual access network, the Network Interface Unit (NIU) at the user’s site and finally the user. The satellite uplink chain consists of an IP to MPEG-2 TS encapsulator according to the MPE (MultiProtocol Encapsulator) standard, a QPSK modulator, an upconverter (from the IF of the modulator to the Ku band), an HPA (High Power Amplifier) and the dish antenna. The satellite downlink consists of the LNB (Low Noise Block), to down convert the received signal from the Ku band to L-band, the QPSK demodulator and the MPEG-2 TS to IP de-encapsulator, which extracts the IP datagrams from the TS and forwards them to the core network of the DN.
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Figure 3. Block diagram of a distribution node. A number of end users can be connected to the core network of the DN, through a variety of wired or wireless access networks. At the DN site, an access network adaptor interfaces the core network of the DN to the actual access network. Correspondingly, at the user site an appropriate Network Interface Unit (NIU) interfaces the user to the access network. In the case of a DVB-T access network the network adaptor can be divided in the forward and the return channel adaptors (see figure 4). The forward channel adaptor consists of an IP to MPEG-2 TS encapsulator, a COFDM (Coded Orthogonal Frequency Division Multiplexing) modulator, an RF power amplifier in the UHF band and the appropriate antenna. In the case of ISDN, the return channel adaptor consists of a modem bank. At the user site, the NIU consists of two parts: the first part (forward channel) is implemented by a DVB-T PCI card (or an external DVB-T to IP module), which demodulates the COFDM signal, deencapsulates the IP packets and forwards them to the user’s PC. The second part (return channel) is implemented by an appropriate ISDN modem. In the case of a WLAN access network, the forward and return channel adaptor at the DN site are implemented through an Access Point and at the user’s site, the NIU is implemented by a Station Adapter. According to the configuration of figure 3, upon a user’s demand for multimedia services provision, the data requests are forwarded to the core network of the DN through the access network. Depending on the location of the requested service (in the neighbouring DN or in a remote one), a router takes the responsibility to forward the request either to the local content provider, or to the satellite uplink chain. Similarly, IP reply data from the DN to requests from
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other DNs are forwarded to the uplink chain. Uplink signals from all DNs are received on the regenerative satellite and a common downlink DVB-S signal is generated and transmitted to the earth.
Figure 4. Block diagram for a DVB-T Access network. This down link signal is received by all DNs in frequency F0, where it is demodulated and passed to the MPEG-2/IP de-encapsulation module. As long as this common downlink also carries the IP datagrams (encapsulated in the MPEG-2 TS) that are generated by (or destined to) every DN within the satellite coverage area, the MPEG-2/IP module is initially responsible to filter out all IP traffic that is not destined to the corresponding DN (users, local server). Then, IP data packets that are destined to this DN, are de-encapsulated and forwarded to a router. Received IP packets may be either requests from user of another DN, or replies from other DNs destined to users of this node. An appropriate router forwards the received IP packets either to the local content provider or to the user via the corresponding access network.
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III. DYNAMIC BANDWIDTH MANAGEMENT The interconnection of DNs through a mesh topology network architecture, using a regenerative DVB satellite leads to a heterogeneous and complicated network environment. Furthermore, as DNs represent a metropolitan area, a large number of users will have to be served by the network. QoS models and strategies have been extensively studied for IP based core networks. The most widely known are Integrated Services (IntServ) and Differentiated Services (DiffServ), as defined by IETF (Internet Engineering Task Force). However, IntServ and DiffServ and are not straight through applicable to DVB networks. Instead, an alternative technique is proposed, which matches best to a mesh topology network architecture over a regenerative satellite. In this respect, a dynamic bandwidth management technique is proposed, which is based on policy strategies applied directly on the MPEG-2 transport stream. The proposed mechanism manages dynamically the uplink bandwidth of each DN and enables the provision of interactive IP based multimedia services at a guaranteed QoS. The bandwidth management mechanism operates within the IP/MPEG-2 TS encapsulator and it is based on the dynamic reallocation of the available bandwidth of the uplink following a predefined priority policy mechanism. This is achieved by an add on software, which enables the partitioning of the total bit rate of the uplink transport stream into virtual channels each one assigned one PID, where the different QoS strategies can be applied. The assignment of an IP data flow at a virtual channel is achieved through a filtering mechanism which is able to monitor traffic and based on some pre defined filters (IP source and destination address, source and destination port, TOS, protocol type) encapsulates that traffic to a specific virtual channel. DVB-S Uplink Transport Stream for IP services (e.g. 3 Mbps) Modified by the Bandwidth Manager
Virtual IP Channel 1 (e.g. 2 Mbps)
Virtual IP Channel 2 (e.g. 700 Kbps)
Virtual IP Channel 3 (e.g. 300 Kbps)
Figure 5. Bandwidth slicing principle in a DVB-S uplink. Referring to figure 5, the MPEG-2 transport stream is sliced into virtual IP channels, each one defining the bit rate that can be assigned to a specific service. This assignment can be based on a number of policies. For the needs of this paper, the following policy types have been implemented: ¾ Static guaranteed: this policy guarantees a static bandwidth to each virtual channel. A minimum bit rate value must be specified so that the actual bit rate is guaranteed up to this boundary value. The unused bandwidth (minimum bit rate - instant bit rate) is reserved and cannot be allocated to other virtual channels. ¾ Dynamic guaranteed: this policy guarantees a dynamic bandwidth to each virtual channel. A minimum bit rate value must be specified so that the actual bit rate is guaranteed up to this boundary value. On the contrary of the static guaranteed policy, the unused bandwidth (minimum bit rate -instant bit rate) is not lost, but can be allocated to other virtual channels .
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¾ Best effort: the bit rate allocated to this virtual channel(s) depends on the available bandwidth. This type of virtual channel is not considered as priority regarding the guarantee of the bit rate. A priority value can be assigned to each virtual channel in order to prioritise it among other virtual channels. The priority value enables the prioritisation of the allocation of the possible excess bandwidth. Priority-based policy ranges from 1 (the highest) to 99 (the lowest).
IV. NETWORK IMPLEMENTATION THROUGH ATLANTIC BIRD-II SATELLITE An actual satellite (Atlantic Bird II) was used in order to develop an emulated DVB-S regenerative environment to demonstrate and validate the performance of the network architecture, the interoperability of the heterogeneous networks and the bandwidth management mechanism. Atlantic Bird II (AB-II) is a transparent satellite and for the needs of this paper it was used in loop back mode, i.e. the transmission and reception were performed through the same earth station. Two laboratory DNs were implemented, the first based on WLAN technology and the second based on a DVB-T platform. Finally, a dynamic bandwidth management software was installed on the satellite uplink chain. For the needs of the experiments, a bandwidth equal to 3 MHz was rented from the AB-II satellite, through its administrator. The transmission and reception parameters applied in the experiments are depicted in the following table 1. Uplink frequency Downlink frequency Modulation FEC Useful Bit Rate Baudrate
14.041GHz 12.541GHz QPSK 3/4 3072 Kbps 2222.298 Kbaud/s
Table 1. Characteristics of the DVB-S Link During the transmission, Eb/No was equal to 12.2 dB and BER (after Viterbi) equal to 10-8. The overall network configuration, which was implemented, is depicted figure 6. It realises the basic characteristics of an actual regenerative satellite i.e. reduced delay (one hop for data request and ACKs and one hop for replies), common downlink frequency for all nodes and existence of packets for requests and replies in the same downlink MPEG-2 transport stream. Its main difference is that the creation of the downlink MPEG-2 TS is not performed on the satellite, but on ground station and then transmitted to the AB-II, but this has no significant impact on the overall network performance and on the dynamic bandwidth management mechanism. In the WLAN node, users are connected to the DN through wireless links, which are based on IEEE 802.11b standard and offer TCP/IP connections. The communication between the DN and each user is a half duplex point-to-point RF link, in the international scientific and medical (IMS) frequency band (2400 – 2483.5 GHz). The physical layer is based on Spread Spectrum techniques, and more specifically on Direct Sequence (DS). In the implementation level, a WLAN Access Point (AP) is installed at the DN site and a Station Adapter (SA) at each user’s site. The maximum link capacity for DS devices is 11 Mbps, offering an
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aggregate throughput of about 5.5 Mbps to all users in the coverage area. The gateway for this node is router B. In the DVB-T node, the downlink is implemented through a DVB-T channel. The COFDM parameters that were set for the experiments (see table 2) lead to a maximum downlink stream of 30,74 Mbps [9]. In the uplink, ISDN is the most commonly used, although other technologies have been proposed [7], [8].
Figure 6. Experimental network configuration. For the needs of the experiments, an Ethernet (IEEE 802.3) connection of 10 Mbps was used in the return channel. Modulation IFFT FEC Guard Interval Frequency
64QAM 8K 7/8 1/16 566MHz
Table 2. COFDM Parameters. The performance of the DVB-T network for TCP/IP services was individually tested. The maximum bit rate that was achieved for an ftp session was equal to 19 Mbps. This upper limit is due to the inherent delays of the encapsulation and de-encapsulation processes. Comparing
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this value with the maximum uplink bit rate to the satellite (~3 Mbps), it is evident that the DVB-T node is able to exploit the whole capacity of the satellite link. The gateway for this node is router C. Referring to figure 6, the bandwidth management software runs within the IP/MPEG-2 TS encapsulator. Also, the gateway for the satellite station is Router A, which has been appropriately configured to route incoming IP packets from any interface to the appropriate destination. In order to clarify the routing scheme, suppose that a user connected to the WLAN node requests a service from the server of the DVB-T node. The IP request packet passes through the SA and the AP to router B, from where it is forwarded to interface 3 of router A. From there it is routed to satellite uplink chain, through interface 4 and transmitted to AB-II. The downlink stream is received, demodulated and the de-encapsulated IP packet arrives at interface 1 of router A. From there, it is routed to interface 2 and then to router C from where it is directed to the server of the DVB-T node. The generated reply through router C is directed to interface 2 of router A and then to interface 4, from where it is transmitted to the satellite. The reply packet arrive at interface 1 of router A and is forwarded to interface 3. Finally, through router B and the WLAN network, it reaches the user. Corresponding routes are followed in the other direction, where a user in the DVB-T node requests data from the server of the WLAN node. It should be noted that during a TCP data transfer from one node to the other, the acknowledgement (ACK) packets create a traffic in the reverse direction. Thus, in the uplink of each node a static guaranteed virtual IP channel, equal to 270 Kbps, was anticipated to host ACK packets.
V. PERFORMANCE MEASUREMENTS The experimental platform that has been implemented was used to measure the overall network performance, during the provision of interactive IP services, from one node to a user of the other node. Optimised TCP parameters were used to enable efficient TCP data traffic performance [10], [11]. For the following measurements, the TCP window size was calculated from the well known formula: TCPWindowSize = Bandwidth * RTT where RTT is the Round Trip Time. The “Bandwidth” value in the above formula is equal to about 3 Mbps (see table 1). The RTT was experimentally measured equal to about 700 ms, which refers to a user in the DVB-T node. This is the worst case, because DVB-T exhibits higher delay than WLAN. The above formula gives a “TCPWindowSize” value equal to 231 Kbytes and the actual TCP window size, used during the experiments, was 256 Kbytes. The server and user PCs were appropriately configured to support RFC1323 TCP extension options (SACK, Window scaling, window size more than 128Kbytes etc). The Maximum segment size (MSS) used throughout the experiments was 1460bytes (Ethernet). The behaviour and performance of TCP was tested during a bulk data transfer from the data server to the Client PC generated by Iperf [12].
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a. Interconnection of Distribution Nodes. The first set of experiments, were conducted to demonstrate and validate the provision of IP services from a server located at the WLAN node to a user connected to the other node through a DVB-T access network. Using the iperf, the results of this experiment are shown in figure 7(a), which depicts the instant bit rate during a bulk data transfer. Provided that there were no other active sessions between the two nodes, figure 7 shows that the maximum bit rate that can be achieved, with the optimised TCP parameters, was around 2.7 Mbps. Considering that 270 Kbps were reserved for ACK packets, the uplink bit rate reaches 2.97 Mbps. Using the monitoring tool of the multiplexer, it was verified that the actual utilisation of the satellite channel was 96.8 %, which is quite satisfactory, considering the overhead of the MPE encapsulation at both the satellite and the DVB-T encapsulator. The average RTT during the transfer was measured equal to 769ms, with stdev: 10ms, max: 895ms, min: 676ms.
Figure 7. Bit rates of (a) a DVB-T user, downloading from the WLAN node, (b) a WLAN user, downloading from the DVB-T node. Similar measurements were taken in the case of a user connected to the WLAN node, requesting a service from the DVB-T node. The bit rate, as shown in figure 7(b), was around 2.7 Mbps. The average RTT was equal to 833ms, with stdev: 138ms, max: 1088ms, min: 639ms. From this series of experiments the validity of the proposed mesh topology network architecture has been verified and figure 7 shows that maximum spectral efficiency of the satellite bandwidth, used for the experiments (~3 Mbps), is achieved. Furthermore, the feasibility of interconnecting heterogeneous networks (WLAN, DVB-S, DVB-T) is demonstrated.
b. Evaluation of the bandwidth management mechanism. The second set of measurements was conducted, in order to demonstrate the dynamic bandwidth management capabilities of the proposed configuration. Two channels where assigned to the total uplink of 3.070 Mbps. The first channel was allocated a static bit rate of
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270Kbps, and the input to that channel contained the ACK packets from the users to the server. The second channel of total 2.8Mbps contained two virtual channels (VC). The first VC was assigned as “best effort” with priority 1 (allocated to a DVB-T user) and the second one as a “dynamic guaranteed” with a guaranteed bit rate of 1.2Mbps and priority 2 (allocated to a WLAN user). The priority affects the way the users share the available bit rate inside the same channel. The Best Effort VC is able to utilise the whole of the total bit rate minus the guaranteed bit rate. The results are illustrated in figure 8. The solid line indicates the bit rate achieved by the DVB-T user, while the dashed line, by the WLAN user. Initially, as figure 8 depicts, the only active session is from the best effort user, who consumes the whole of the satellite link (2.7 Mbps). About 50sec later, the dynamic guaranteed user starts its session. As depicted in figure 8, the guaranteed user consumes about 1.2 Mbps, while the best effort consumes the remaining 1.5 Mbps. When the session of the guaranteed user ends, the bandwidth management mechanism allocates to the best effort user the totally permitted bandwidth of 2.7 Mbps.
Figure 8. Performance of the dynamic bandwidth management mechanism. V. CONCLUSIONS A mesh topology network architecture is proposed, based on regenerative DVB-S GEO satellites, which is able to interconnect heterogeneous terrestrial DNs to each other, with minimum possible delay. An actual transparent satellite (AB-II) in loop back mode was used, in order to develop an emulated regenerative satellite environment and demonstrate the interconnection of two DNs, based on WLAN and DVB-T technologies, respectively.
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A dynamic bandwidth management mechanism, appropriate for the proposed satellite network architecture is implemented, enabling the provision of interactive IP based multimedia services at a guaranteed QoS. Network performance measurements for interactive IP services, under real satellite transmission conditions, show that the proposed architecture offers maximum spectral efficiency of the satellite transponder. The capabilities of the dynamic bandwidth management mechanism are demonstrated and its performance is evaluated.
Acknowledgements The work described in this paper was carried out in the frame if the IST REPOSIT, (Real Time Dynamic Bandwidth Optimisation in Satellite Networks, IST-2001-34692) project. The partners of this project are : Temagon, NCSR “Demokritos“, Thales Broadcast & Multimedia, Centre National d'Etudes Spatiales, Optibase, Alcatel Space Industries, Ecole des Mines de Nantes, University of Cantabria. The authors would like to thank Prof. Ch. Mantakas for his useful comments and suggestions. References [1] A. Jamalipour, T. Tung, “The role of satellites in global IT: Trends and Implifications”, IEEE Personal Communications [see also IEEE Wireless Communications], Volume: 8 Issue: 3, Jun 2001, pp. 5 –11. [2] P. Chrite and F. Yegenoglu, “Next-Generation satellite Networks: Architectures and Implementations”, IEEE Communications Mag., vol. 37, no. 3, Mar. 1999, pp.33-36. [3] H.Skinnenmoen, H. Tork, “Standardization activities within broadband satellite multimedia”, Communications, 2002, ICC 2002, IEEE International Conference on, Volume: 5, 2002, pp. 3010-3014. [4] ESA Telecommunications: AHG-RSAT, http://telecom.esa.int/telecom/www/object/printfriendly.cfm?fobjectid=951. [5] Skyplex::Digital Services – How Skyplex works, http://www.eutelsat.com/products/2_1_1_6a.html. [6] The DVB processor on STENTOR, A. Duverdier et al, ECSS conference Toulouse, September 1999. [7] Xilouris, G., Gardikis, G., Pallis, E., and Kourtis, A., “Reverse Path Technologies in Interactive DVB-T Broadcasting”, in Proc. IST Mobile and Wireless Telecommunications Summit, Thessaloniki, Greece, June 2002, pp. 292-295. [8] Rauch, C. and Kelleler, W., "Hybrid Mobile Interactive Services combining DVB-T and GPRS", In Proc. EPMCC 2001, Vienna, Austria, February 2001. [9] ETS 300 744: “Digital Video Broadcasting (DVB): Framing structure, channel coding and modulation for Digital Terrestrial Television (DVB-T)”, 1997. Rev.1.4.1, 2001 [10] M.Allman, D. Glover, L. Sanchez, “Enhancing TCP over satellite channels using standard mechanisms”, RFC 2488, January 1999. [11] V. Jacobson, R.T. Braden, D.A. Borman, “TCP extensions for high performance”, RFC 1323, May 1992. [12] Iperf Version 1.7.0, http://dast.nlanr.net/Projects/Iperf/.
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