Network and MAC Issues for Mobile Satellite Multimedia Networks

Transcription

1 Network and MAC Issues for Mobile Satellite Multimedia Networks 1, E. Del Re 2, I. Mertzanis 3 1 DLR Oberpfaffenhofen, Wessling, Germany 2 University of Florence, Florence, Italy 3 University of Surrey, Guildford, UK Janez.Bostic@dlr.de, delre@lenst.die.unifi.it, I.Mertzanis@ee.surrey.ac.uk ABSTRACT The aim of this paper is to present a suitable satellite multimedia network architecture and the results of simulated medium access control (MAC) protocols. The new generation of satellite multimedia networks is going to handle different types of traffic such as multimedia traffic. The ATM technology was developed for the integrated transmission of various types of traffic. The future networks protocols should be as much as possible compatible and that is one of the main reasons to think about the use of ATM technology for transmission over satellites. Satellite networks present a lot of challenges related to quality of services (QoS) provision and the medium access control access protocols are strongly related to QoS provision. INTRODUCTION The aim of the COST-252 project is the migration from the first generation Satellite Personal Communication Networks (S-PCN) towards the new multimedia satellite networks focusing on non-geostationary satellite systems. The way considered in this action is to propose an integrated satellite/cellular system, which will use as far as possible the same procedures and protocols in the satellite network as the defined for the terrestrial cellular one. A very detailed overview over the activities of the Network Aspects Working Group has been presented in [1]. Since the satellite networks can provide a wide geographic coverage, they will play an important role in the Global Information Infrastructure (GII). This is very important for regions which are not connected by terrestrial fiber optic networks or broadband wireless networks. Two trends for the future mobile satellite networks have been considered. The first one is based on the Universal Mobile Telecommunication System (UMTS) design objectives and the second one is driven by the current research activities for the definition of the future wireless B-ISDN. Since in UMTS there is a requirement for B-ISDN protocol reusability, the approach selected by COST-252 was not explicitly the first or the second one. The design options of satellite systems include several choices. The basic ones are the choice of orbit (low earth orbit (LEO), medium earth orbit (MEO), and geostationary (GEO)), processing and switching (transparent satellites or satellites with on-board processing), inter satellite links (ISLs), and link layer technologies. In addition to the network architecture, the protocols must be developed for efficient use of the satellite radio resources. The most important protocol design issues concentrate on medium access control (MAC) techniques, routing, and traffic management. Future mobile communication systems will be characterized by the interoperability of several networks at the highest integration level. The satellite component and the terrestrial one will use as far as possible the same protocols [2]. The emphasis in this paper will be on the introduction of the ATM technology into the satellite environment. The paper begins introducing the aims and goals of the COST 252 project in the field of networking aspects. Then, the satellite ATM architectures defined by the Telecommunication Industry Association (TIA) are shortly described. The section ends by presenting a scenario where all components are discussed in detail. One of the main concerns of all systems with a shared medium are the MAC protocols. The first approaches were to integrate voice and data services. The packet reservation multiple access (PRMA) technology has shown its strength in the terrestrial wireless networks. In this paper the results for the satellite low earth orbit environment are shown. The PRMA was then upgraded to advanced PRMA for the investigations of real-time variable bit rate services transmission. These two approaches cannot guarantee the quality of service (QoS), which is the critical factor for multimedia services provision. The last section deals with the scheduling entity for the MF-TDMA MAC protocol, which can guarantee some level of the QoS. BROADBAND SATELLITE NETWORK ARCHITECTURE The ATM technology was chosen for satellite multimedia networks, since it has been already

2 standardized for fixed terrestrial networks and will play an important role in the establishing the GII. The Telecommunication Industry Association s (TIA s) Satellite Communications Division has started the standardization process of next generation satellite networks. Recently, a set of satellite based ATM network architectures has been defined, which provides architectures and guidelines for satellite ATM networks[3]. These network architectures cover both transparent and on-board switching satellites, network access and network interconnect scenarios, and mobile and fixed terminals and networks [4]. The architectures defined in the TIA document can be broadly grouped into two categories: Satellite ATM architectures for transparent satellites: - Fixed ATM Network and ATM Network Interconnect, which is intended for high-speed network access by fixed terminals, and high-speed interconnection of fixed ATM networks, - Mobile ATM Network, which is intended for ATM network access by mobile terminals, - Mobile ATM Network Interconnect, which is intended for high-speed interconnections between a mobile network and a fixed network or between two mobile networks, Satellite ATM architectures for satellites with onboard switches: - ATM Network, which is intended for ATM network access by terminals, - ATM Network Interconnect, which is intended for the interconnection of two ATM networks by high speed links, - Full Mesh ATM, which is intended for both ATM network access and network interconnectivity. The defined architectures differ in terms of offered data rates, required protocol processing, mobility support provided by the network, and whether the satellites are used at the access or for transmission. For each architecture the reference architecture and protocol reference model are defined. The COST-252 Network Aspects Working Group has defined a more detailed architecture and satellite access interface and protocol stacks [5], which are described in the following paragraphs and subsections. The satellite constellation is the initial parameter that affects the design of a global network infrastructure. The number and the location of the Land Earth Stations (LES) depends on the constellation parameters as well as on the provision of Inter-Satellite Links (ISLs) and the feeder link bandwidth requirements. Another parameter that influences the selection of the satellite network architecture, is the dependency on the terrestrial network infrastructure. The satellite links are essential for interstation signaling, when there is no terrestrial infrastructure deployed. For example, most GEO systems do not need a ground station interconnection through terrestrial links, whereas non-geo systems require only few satellite links to the LESs, when ISLs are used; otherwise they highly depend on a fast backbone network. In GEO systems ISLs mainly handle the traffic between different regions of the earth and bypass the terrestrial links, whereas in most non-geo constellations ISLs are essential in order to reduce the number of LESs. Most of the future broadband satellite systems share common characteristics on the satellite network architecture, the on-board satellite processing and switching capabilities, the user terminals, the supported protocol standards, the access scheme and the interconnection to the terrestrial networks. In a typical broadband satellite system that is shown in Fig. 1, the following network entities exist: User Terminals (UT): Several different protocol standards, such as: ATM User Network Interface (ATM-UNI), Frame Relay UNI (FR-UNI), Narrowband Integrated Services Digital Network (N-ISDN) Basic Rate Interface (BRI), N-ISDN Primary Rate Interfaces (PRI), Transmission Protocol/Internet Protocol (TCP/IP). They are connected to the Satellite Adaptation Unit (SAU) through one of the supported standard interfaces. Satellite Adaptation Unit (SAU): This is a nonstandard, specially designed unit, responsible to provide access to the satellite network. It performs all the necessary user terminal protocol adaptations to the satellite protocol platform. The SAU also includes all the physical layer functions such as channel coding, modulation/demodulation, the Radio Frequency (RF) parts and the antenna section. A set of different types of terminals, with a variety of transmission capabilities, is usually offered by a satellite network. Starting from minimum transmission rates of 16 kbps of even less, they can cope with maximum transmission rates of 144 kbps or 384 kbps for personal type user terminals, or even 2,048 kbps and higher for fixed type terminals with larger antennas. All of the supported terminals, share the same access scheme and protocols stacks. Payload (P/L): Full on-board satellite signal regeneration is assumed in most of the future broadband satellite systems. The on-board satellite processing units perform multiplexing, demultiplexing, channel coding/decoding and ATM like switching, using a multi-spot beam

3 configuration. The on-board satellite switch includes only part of the functions that a ground ATM switch performs. To minimize the payload complexity, the call set-up signaling and the CAC functions are performed on the ground. Gateway stations (GTW): These are the LESs that provide connectivity to the external networks. Most of the future proposed broadband satellite systems support interconnection to the fixed B- ISDN, Frame Relay, N-ISDN, PSTN and Internet, via the proper interworking units at the gateway. The level of interworking between the GTWs and the terrestrial networks depends on the type of traffic that the satellite network carries. The SAU provides an access interface very similar to the standard ATM-UNI, therefore the required interworking at the terminal and the GTW is minimised. The signalling protocols for call and connection control can be reused (based on the ITU-T Q.2931 standard) and the traffic and network management functions can share common characteristics. All ATM service classes can be supported and directly mapped into the satellite air interface through the SAU. Finally, the interconnection interfaces with all the other public terrestrial networks should be based on standards for interworking with B-ISDN. In GEO systems the placement and number of GTWs mainly depends ISL on the traffic demand. A large number of gateways is expected in geographical areas where the traffic demand is high; these gateways are always connected with the same satellite(s). However, in non-geo systems the number and placement of the gateway stations depends on some additional system design characteristics, such as: constellation design, use (or not) of ISLs and the overall end-toend delay constraints. For example, in a global MEO system with no inter-satellite links, a total number of less than 10 gateways can provide full connectivity to the land masses most of the time. A LEO system will require tens to hundreds of gateways but this number can be reduced with the use of ISLs. Network Station (NCS): A central entity, commonly used in GEO satellite systems that provides overall control of the satellite network resources and operations. This node is responsible for allocating radio resources to the LESs/GTWs according to a long term resource planning scheme. The NCS is responsible for performing most of the resource management, call management and user/terminal mobility management functions, in addition to authentication, registration, deregistration and billing. In some systems, these operations are performed in more than one ground station in a distributed manner. ISL On-board switching UP link processor DOWN link processor On-board ler Telemetry and tracking command system Satellite coverage areas Satellite adaptation unit Single user environment TTC SAU User terminal GTW N-ISDN Multi user environment SAU Satellite adaptation unit NCS Network control station IWU Gateway station B-ISDN PSTN Internet Frame-relay Fig. 1: Satellite network architecture and connectivity with the fixed networks

4 Satellite Interface and Protocol Stacks Two main scenarios for the satellite access network protocol can be envisaged [6]. The first one uses ATM cell encapsulation and satellite specific protocols for establishing and managing user connections, whereas the second one provides a highly integrated solution with the ATM protocol stack. The ATM protocol considerations over the air interface and the on-board satellite switch architecture are different for each protocol platform. A more general satellite packet switching approach increases the system flexibility to accommodate any protocol standards without being restricted to adopting an ATM satellite switching solution. However, the second approach provides a highly optimized protocol architecture, if ATM is adopted as the transport mechanism for the future broadband communication systems. ATM Protocol Encapsulation Protocol encapsulation is a simple and easy way to implement the technique for passing arbitrary protocol information through the network entities that could otherwise not interpret the specific information. In this scenario, the satellite protocol platform is designed to transparently support different user terminal standards through a proprietary, satellite specific interface. The satellite access protocol is terminated at the GTWs. Therefore, no modifications to the existing protocol standards are necessary. This approach appears to be very attractive in systems that need to accommodate several different types of user terminals with a variety of protocol standards, like in the case of the Universal Mobile Telecommunications System (UMTS) concept. Circuit switching, packet switching, or even hybrid solutions for the on-board satellite processor, can be implemented for networks that use this type of protocol platform. However, in this approach it is very difficult to offer an optimum performance to any particular protocol standard, resulting in protocol inefficiencies (related to the increased packet overheads). Interface The proposed protocol stacks for both the control and the user plane in a satellite ATM network are shown in Fig. 2. The layer which replaces the standard ATM layer includes all the required modifications (in the cell header and its functions) that need to be addressed for the space segment. The fields in the S- ATM cell header carry essential routing and control information for the satellite segment. The dimensioning of the Satellite Virtual Path Identifier (S-VPI) and the Satellite Virtual Channel Identifier (S-VCI) fields depends on system parameters such as: the satellite capacity, the number of spot-beams, the satellite terminal transmission granularity and the switch architecture. The (MAC) using MF-TDMA or Code Division Multiple (CDMA) based on a Demand Assignment Multiple (DAMA) protocol and the radio physical layers reside below the. Signaling for call control is based on the Q.2931 protocol standard and is terminated in the ground segment. For mobile and portable terminals, future standards by ATM Forum or ITU-T for B-ISDN signaling and Intelligent Network (IN) concept can be supported in a highly integrated network environment. Mobility for Satellite ATM Networks For all non-geostationary satellite systems the networking must involve the rerouting of active connections in real time to support the mobility of end user devices and nodes of the network itself. This process is called handover and in satellite networks it occurs because of the following mobility interactions: end user to satellite, earth station to satellite, and satellite to satellite in the case of ISLs. In the case of a full mesh network all components have to have the mobility management capabilities. UT & SAU Network Station ATM Switch UT & SAU UNI * Signalling UNI/NNI* Signalling Gateway Station NNI/UNI Signalling APPLICATION Higher Layers User Plane Protocols APPLICATION Higher Layers SAAL Satellite SAAL ATM SAAL ATM AAL Satellite Gateway Station AAL ATM ATM Satellite Network a) ATM Network Satellite Network Fig. 2: ATM based protocol reference model: a) plane, b) User plane b) ATM End User

5 MAC PROTOCOLS FOR SATELLITE MULTIMEDIA NETWORKS For broadband multimedia mobile satellite networks various access types are foreseen. Broadly the following terminal types can be considered: earth station, group terminal and user terminal. The first two types are mostly used for interconnection of various island networks. The users have not direct access to the satellite and the traffic is multiplexed by the end point (earth station or group terminal). The access to the shared radio medium can be performed by fixed assignment techniques, such as frequency division multiple access (FDMA) or time division multiple access (TDMA). For the mobile satellite networks the direct user access to the satellite is much more important. The multiple access control (MAC) techniques must be much more flexible for efficient shared radio resources utilization. Therefore, some simulation results for the packet reservation multiple access (PRMA), which is suitable for integrated transmission of voice and data, are presented first. The PRMA with some modifications was also simulated for variable bit rate video transmission. Packet Reservation Multiple The PRMA protocol was initially proposed for terrestrial microcellular networks where it exhibits very interesting features such as [[7], [8], [9]]: management of voice and data traffics for future multimedia applications, compatibility with the ATM standard, high efficiency expressed by a high multiplexing gain value, dynamic PRMA carrier allocation to cells, transparent behaviour with respect to user mobility. The satellite segment of future global-coverage mobile communication systems will be based on non-geo satellites; in particular, Low Earth Orbit - Mobile Satellite Systems (LEO-MSSs), represents an interesting solution, since they are characterized by low propagation delays and low propagation attenuation that allow the use of low-power hand-held terminals. This study focuses on the investigation of the PRMA protocol suitability in LEO-MSSs. The main problem in using PRMA in a LEO satellite system is the inherent Round Trip Delay (RTD) that typically varies from 5 to 30 ms, depending on the satellite constellation altitude and the minimum elevation angle acceptable by mobile terminals for reliable communications [[10], [11]]. The PRMA protocol utilizes a Slotted-ALOHA (S- ALOHA) access and a Time Division Multiple (TDMA) transmission mode on a reservation basis. The transmission time on a PRMA carrier is divided in slots with duration T s ; N slots are grouped together to form a frame with duration T f. Every message from a terminal is divided into packets. Each packet has a header with H bits which contains synchronization data, control data and the identification of both the source terminal and the destination one. R c denotes the bit-rate for a PRMA carrier, whereas R s denotes the bit rate for a traffic source. The efficiency of the PRMA approach in managing resources relies (i.e., slots of the TDMA frame) on the use of a voice activity detector, which separates silent periods and talking periods (talkspurts) within a conversation (i.e., ON/OFF voice source model). During a call, a terminal transmits one packet per frame on a given slot, only during a talkspurt. When there is a silent pause, this slot can be destined to another active source. Therefore, the assignment of time slots to terminals is not fixed, but it is dynamically handled on the basis of the presently active terminals. When a time slot has been assigned to a terminal, it is marked as unavailable, otherwise it is idle. A feedback channel broadcast by the controller (i.e., the satellite with on- board processing capabilities) informs the terminals about the state of each slot and about the results of their transmission attempts (if any). The access to an idle slot is of the S- ALOHA type: as soon as a terminal needs to transmit its packets, it enters the contending state, CON, where it attempts to transmit a packet in the first available slot, according to a permission probability, p. If two or more terminals have attempted to send their packets on the same slot there is a collision; if we neglect the capture effect (the ability of the access receiver to detect the strongest packet, when two or more packets contend for the same time slot), the satellite can not recognize any terminal and no reservation is obtained. Therefore, the involved terminals re-schedule randomly their transmission attempts on free slots, according to their specific permission probabilities. If the transmission attempt of a terminal on a slot has collided, the terminal attains a reservation for the exclusive use of this slot in subsequent frames. In order to grant a real-time voice transmission, a voice terminal discards the first packet in its buffer, when the access delay exceeds a maximum value, D max ; a common assumption is D max = 32 ms. When a packet is discarded, the voice terminal tries to obtain a reservation with the next packet. The quality of voice transmissions with PRMA is measured by the probability P drop that a packet is dropped from the buffer of a generic voice terminal due to an excessive delay. It is required, that P drop 1%, in order to have a minimal degradation in the perceivable speech quality. In contrast to the terrestrial micro-cellular systems where the RTD value is in the order of few µs (hence,

6 we may assume that a terminal immediately knows the outcome of its transmission attempt), in the LEO scenario the RTD is in the order of ms and it is not negligible. Therefore, the RTD imposes a limitation to the number of attempts that a voice terminal may issue for a packet within D max (i.e., before clearing this packet from its buffer). For PRMA in LEO-MSSs, we have considered RTD always equal to its maximum value for a given satellite constellation, RTD max (conservative assumption). Moreover, we have assumed RTD max T f. Hence, when a terminal makes a successful transmission attempt on an idle slot it knows the outcome of its transmission before the beginning of the same slot in the next frame as shown in Fig. 3. A PRMA carrier: (TDMA structure) LEO Satellite Transmission attempt by user a.. T f = RTD max.. User a knows (before the homologous slot in the subsequent frame) that it has obtained a reservation. = slot available (information broadcast by the satellite in the feedback channel) = slot reserved (information broadcast by the satellite in the feedback channel) Fig. 3: Description of the access phase for the PRMA protocol in LEO-MSSs In spite of the limitations due to RTD of LEO-MSSs, the results obtained in this study have shown that the capacity of a PRMA carrier is minimally affected by the RTD in LEO systems [10]. In particular, we have shown that the PRMA scheme in LEO-MSSs maintains the capacity improvements exhibited in terrestrial microcellular networks, with only a slight performance degradation as presented Fig. 4. Moreover, an improvement of the PRMA protocol, called PRMA-HS (Packet Reservation Multiple with Hindering States), which allows that a terminal attempts transmissions also during the time interval needed to receive the outcome of a reservation attempt (waiting time) [10], [11]. The first successful attempt of a terminal is recorded by the satellite in a data base in order to ignore any successive successful transmission attempt made by the same terminal during the waiting time. After the first successful attempt, the terminal enters a block of hindering states, because any successive transmission attempt is useless and may disturb the access attempts of other terminals (hindering contention). However, we have obtained that despite this drawback, the PRMA-HS protocol outperforms the classical PRMA protocol, since it allows a lower access delay. Packet dropping probability, Pdrop Satellite PRMA (T f RTD max = 5 ms) Terrestrial PRMA (T f = 5 ms) Satellite PRMA (T f RTD max = 15 ms) Terrestrial PRMA (T f = 15 ms) Permission probability, p Fig. 4: Behavior of P drop versus p for the PRMA protocol in both LEO satellite cases, continuous line (T f = RTD max = 5 ms and T f RTD max = 15 ms), and terrestrial ones, dashed lines (T f = 5 ms and T f = 15 ms) with 30 terminals/carrier, R c = 765 kbps R s, = 32 kbps, H = 64 bit. For the investigation of the performance of the PRMA- HS protocol we assume T f = n RTD max, with n greater than or equal to 1. For the sake of simplicity, we will consider only integer values of n which are divisors of N; then, RTD max will contain an integer number of slots. In the PRMA-HS protocol a terminal stays in the contending state (CON) until it obtains a reservation. If only one terminal has attempted to transmit on a given slot, after the RTD max time (i.e., N/n slots) the terminal will know the positive outcome of its access attempt. During the waiting time, the terminal continues to attempt transmissions on each idle slot according to the permission probability. Of course, any successful reservation attempt after the first one will be ignored by the satellite. Fig. 5 shows the comparison between the PRMA-HS protocol and the PRMA scheme in terms of P drop in LEO-MSSs for p = 0.4 and T f = 15 ms with both n = 3 (i.e., RTD max = 5 ms) and n = 1 (i.e., RTD max = 15 ms). We note that in this figure the performance difference between PRMA and PRMA-HS is slight for n = 3 and significant for n = 1. Hence, we may state that the performance of PRMA strongly depends on n: an increase in RTD leads to a worse behavior of the PRMA, whereas the PRMA-HS is less sensitive to variations of RTD. This is an interesting result that makes the PRMA-HS protocol quite insensitive to the variations of RTD experienced in LEO systems during

7 call lifetime, mainly due to the motion of LEO satellites. Finally, we have investigated the suitability of the PRMA for the support a mixed traffic where voice sources and data sources share the access to the same carrier [12], [13], [14], [15]. Different permission probabilities have been considered for voice and data terminals, p v and p d, respectively. The quality of service parameter for voice sources is P drop, whereas for data sources we consider the mean message delay, T msg, which means the mean time from the messages arrival instant to the data terminal data buffer to the instant when those messages have been completely sent. We assume that data sources produce messages according to independent Poisson processes; messages have a length geometrically distributed in packets; an exhaustive discipline is used to manage the packets in the buffer of data sources when these hold a reservation. Packet dropping probability, Pdrop PRMA RTDmax 15 ms PRMA RTDmax = 5 ms PRMA-HS RTDmax = 5 ms PRMA-HS RTDmax 15 ms Number of terminals per carrier Fig. 5: Comparison in terms of P drop between the PRMA-HS protocol and the original PRMA scheme in LEO-MSSs with p = 0.4, T f = 15 ms, R c = 765 kbps R s, = 32 kbps, H = 64 bit, N = 21 slots/frame, both n = 3 (i.e., RTD max = 5 ms) and n = 1 (i.e., RTD max 15 ms). In Fig. 6 we compared a terrestrial system and a satellite one as a function of the input data traffic, r d (i.e., the mean number of data packets arrived per slot), in order to evaluate the impact of RTD on the performance of the PRMA protocol in terms of both P drop and T msg [15]. We have observed small differences between the terrestrial and the satellite scenario: the maximum r d value is 0.45 pkts/slot in the terrestrial case, and 0.43 pkts/slot in the LEO-MSS case. Correspondingly, parameter T msg is on the order of hundreds of slots with a difference of few tens of slots (practically, RTD) between the satellite case and the terrestrial one. Hence, the PRMA protocol manages voice and data traffics in LEO-MSSs with a quality of service very close to that obtained in terrestrial systems. P drop T msg [slots] Terrestrial PRMA Satellite PRMA Input data traffic, r d [pkts/slot] Satellite PRMA Input data traffic, r d [pkts/slot] Terrestrial PRMA Fig. 6: Behavior of P drop and T msg as a function of r d, in a terrestrial microcellular system, dashed line, and in a LEO-MSS, continuous line (R c = 720 kbp R s, = 32 kbp, H = 64 bit, T f = 16 ms, N = 20 slots/frame, 16 voice terminals/carrier and 16 data terminals/carrier, p v = 0.35, p d = 0.15, 20 pkts/msg, n = 1 in the satellite case). Therefore, we can conclude that PRMA-HS is a good solution as a unified MAC protocol for the terrestrial and satellite components of future mobile communication systems. Advanced PRMA for Real-time VBR Traffic Category The advanced PRMA (A-PRMA) [16] is similar to PRMA, only the transmission does not depend on the permission probability as in the original PRMA. It is also based on frames consisting of N time slots each. Let us have a vector with N components. The components are zeros and user identity numbers, where zero indicates an unreserved time slot, and an identity number specifies a reserved slot. Every user unit X produces from this vector two new vectors with identity numbers of time slots: (1) a vector of reserved slots for user X, and (2) a vector of unreserved slots as shown in Fig. 7. Then, if the user source has more packets in the buffer than reserved slots, it randomly selects the difference number of slots from the vector of unreserved slots. These slots are then put into vector of unsafe (unreserved) slots, while these are not reserved yet. This procedure takes place after each time slot, and therefore the contents of the last two vectors is highly dynamic. This technique is still contention based. The user source then always transmits a packet, when the actual time slot number and the time slot number in the reserved or the unsafe vector are identical.

8 Frame with N time slots Vector of reserved slots for Source 2 Vector of reserved slots for Source 3 Vector of unreserved slots for all Sources increases almost linearly over the load range from 0 to 0.8, since all VBR sources have the same mean bit rate; it then turns into saturation reaching a throughput of between 0.8 and Vector of unsafe slots for Source 2 Vector of unsafe slots for Source 3 Fig. 7: Creation of reserved and unreserved vectors within each source in A-PRMA The supported elementary channel bit rate is R e = 16 kbps. The 26.5 ms frame contains 125 identical time slots. The payload of one slot has been assumed to be an ATM cell including the header, i.e. 53 8=424 uncoded bits. This corresponds to an uplink carrier information bit rate of 2 Mbps. The rt-vbr source is modeled using an autoregressive model with parameters valid for a typical video CODEC according to [17]. Throughout the simulations, a mean bit rate of 128 kbps has been used, and the reference value for the standard deviation σ has been 57 kbps; this latter parameter has been varied in a certain range for specific investigations. In the reference case, the VBR source simulations have revealed a peak rate of approximately 300 kbps corresponding to a 0.43 burstiness value. The A-PRMA protocol including the used source traffic models has been specified in SDL, and the simulations have been performed using the SDL tool SDT. The comparison of the original and advanced PRMA with respect to the cell loss probability is shown in Fig. 8(a). The superior performance of A-PRMA is obvious. It is also interesting that the difference for original PRMA protocols with different permission probabilities is very small, and it is almost independent from the delay in contrast to A-PRMA, as can be seen in Fig. 8(b). The curves in the latter show the cell loss probability for different number of VBR user sources with different (one-way) delays. The results are compared with the TDMA curve, where each user source has fixedly allocated 1/N of the carrier capacity, where N is the number of user sources. One obtains that up to the average load of approximately 0.5 the TDMA protocol is more efficient. Beyond this load value A- PRMA clearly outperforms the fixed allocation scheme. However, the performance gain decreases with growing delay. The average throughput of A-PRMA with respect to the average load shows the typical behavior: the curve Cell loss probability TDMA Delay 0 PRMA q=0.2 PRMA q=0.3 Advanced PRMA PRMA Delay (10.6 ms), q= Number of VBR users Normalized average load a) Cell loss probability TDMA Delay 0 Delay 25 (5,3 ms) Delay 50 (10,6 ms) Delay 60 (12,72 ms) Delay 75 (15,9 ms) Delay 100 (21,2 ms) Number of VBR users b) Normalized average load Fig. 8: Cell loss probability performance of advanced PRMA: a) comparison of original and advanced PRMA; b) dependency on load and (one-way) propagation delay values (in slots and ms) Fig. 9 displays the relation between throughput and cell loss probability. The graph confirms that with increasing throughput also the cell loss probability raises, since higher throughput reflects more competing user sources, consequently causing more collisions.

9 Fig. 10 finally shows the comparison of the cell loss probability for a system with (one-way) delay 12.6 ms and different standard deviations σ of source bit rates. The reference curves are again for the fixed allocation TDMA scheme. It can be clearly obtained that for both the reference scheme and A-PRMA the cell loss probability raises with the average load and standard deviation. Comparing the three pairs of schemes, one also finds that the A-PRMA tends to outperform the corresponding TDMA at already lower load values with a higher source bit rate dynamics, i.e. higher standard deviation. Cell loss probability Cell loss probability (Delay 50 (10.6 ms)) Throughput Fig. 9: Cell loss probability vs. Throughput of advanced PRMA σ = 101 kbit/s TDMA, σ = 101 kbit/s σ = 57 kbit/s TDMA, σ = 57 kbit/s σ = 28.5 kbit/s TDMA, σ = 28.5 kbit/s Number of VBR users Normalized average load Fig. 10: Cell loss probability performance of advanced PRMA for different bit rate dynamics of rt-vbr MAC with QoS for Satellite Multimedia Networks For multimedia networks the quality of service (QoS) is essential. The multimedia applications bit rates are normally variable. For efficient use of transmission resources the statistical multiplexing should be used. At the call or connection level the connection admission control (CAC) algorithm decides if a new connection can be established or not. The approach can be similar to the one for terrestrial fixed ATM networks. At the cell level an entity called scheduler redirects the cells or packets to the correct outgoing links. In the case of wireless or radio access the scheduler has to also manage the uplink access (terminal to access point, which is base station or satellite). The MAC protocol for the satellite broadband multiservice systems must achieve the following goals: Support of different kinds of traffic categories such as constant bit rate (CBR), real-time variable bit rate (rt-vbr), non-real-time variable bit rate (nrt- VBR), unspecified bit rate (UBR) and available bit rate (ABR), Guarantee QoS, which means that the MAC protocol will guarantee the connection parameters negotiated at the connection setup for the time of the connection. The real-time services are very sensitive to cell transfer delay (CTD) and cell delay variation (CDV) QoS parameters, whereas the data services are very sensitive to cell loss ratio (CLR) QoS parameter. Fairness, i.e. the MAC protocol must serve the terminals of the same priority class with equal probability, Efficiency, i.e. the MAC protocol must minimize the network bandwidth usage while guaranteeing QoS, Small signalling overhead of the MAC protocol functions, i.e. the information flow between the access point and terminal should be as small as possible. The PRMA, PRMA-HS and A-PRMA do not support all of these requests. Especially the QoS is a problem. The entities which are mainly responsible to guarantee the QoS parameters in fixed networks are the usage parameter control (UPC) and the traffic scheduler. Therefore, we will first look into packet scheduling in fixed networks and the differences for scheduling in radio networks. Scheduling of Packets in fixed Networks In fixed networks the scheduling is associated only with the outgoing links as shown in Fig. 11. The scheduling function, which redirects the cells according the pre-

10 negotiated QoS parameters to outgoing links, is realized within the ATM switch. In this context, it is assumed that the access links from the sources and the input buffers are dimensioned in such a way that they do not impose any constraints on the traffic, i.e. cell rate. The scheduling can also be seen as the mechanism that determines which queue is given the opportunity to transmit a cell. In general, the queues can be organized as per-group queuing or per-virtual channel/virtual path (per-vc/vp) queuing [18]. With per-group queuing a number of connections share the same queue in a firstin-first-out (FIFO) arrangement. In this queuing structure, the connections can be categorized according the service category, service class or conformance definition into groups. On the other hand, with the per- VC/VP queuing the cells of each VC or VP are queued independently. A very simple queue structure scheduling technique is the priority based scheduling, which assigns a priority to each queue and serves them in order of priority. Under this scheme, the QoS support is limited. Better techniques are those based on fair share scheduling, where for each queue is guaranteed that it gets a share of link bandwidth according to a defined weight. The weight is the criterion for connections with equal priority but different traffic and QoS parameters. In this way, they guarantee a certain minimum rate allocated among the queues and are further classified as rate allocation (work conserving) and rate-controlled (nonwork conserving) schedulers. With a work conserving scheduling a server is never idle when there is a packet to send. On the other hand, with a non-work conserving scheduling the server may be idle even when there are packets waiting to be sent [19]. Fixed Network The entities that control and guarantee that the traffic parameters are in accordance with the pre-negotiated at the connection setup are called usage parameter control (UPC) and network parameter control (NPC) and are not part of scheduling [20]. They have similar functions but at different interfaces: UPC is done at UNI, whereas NPC is done at the NNI. Their functions include monitoring cell streams, checking the conformity between the actual cell stream and the nominal cell stream (the traffic descriptor values) and taking necessary actions when unconformity is detected. The actions that can be taken are discard of cells immediately or tagging of them for discard at a congestion point when the network is congested. The cell loss priority (CLP) bit is used for tagging. Scheduling of Packets in Radio (Satellite) Networks In radio networks, the constraint is on the total bandwidth available to all users before the access point, i.e. satellite or base station.. Because of that, for the exploitation of the statistical multiplexing in a TDMA system, a scheduler is needed to organize the frame structure in the satellite uplink channels. The information on the slot assignments, as decided by scheduler, is broadcast to the user terminals via a feedback channel as shown in Fig. 11. The connection QoS parameters are negotiated during the connection setup. The entity which decides if a new connection can be accepted or not runs a connection admission control (CAC) algorithm. This entity can be placed on the on-board processing satellite or in the ground network control station. However, the parameters of the accepted connection must be transmitted to the scheduler which uses them for the process of slots allocation. Wire Multiplexing/Buffering/Scheduler Outgoing link Bandwidth constraint As already mentioned the big difference between the scheduler in fixed networks and the scheduler in radio networks is that the latter one does not schedule the traffic based on arrived cells in queues but on arrived requests for the bandwidth capacity (number of time slots in case of TDMA-based MAC). Feedback channel ( MAC frame structure) Air interface Uplink Scheduler Radio (Satellite) Network The first criterion for the scheduling decision is the priority of the service category. As shown in Table 1 the highest priority is assigned to CBR service category which is then followed by other service categories. Uplink bandwidth constraint Rec eiver From other carrier From ISL ISL Downlink Fig. 11: Scheduling in fixed and satellite networks ISL The UPC for the uplink could be performed in the terminal or within the scheduler entity on on-board the satellite. The downlink does not need UPC because this traffic has already gone through the uplink UPC and the traffic is conformed to traffic descriptors. The UPC functions implemented in the terminal can only use discard principles since the traffic over the air interface

11 has also to be conformed to traffic descriptors. On the other hand, the scheduler implemented UPC functions could also use the tagging principle if there is enough capacity. In this case the scheduler could allocate additional slots if there are non-conforming requests and when the cells would reach the access point, the CLP bit of cells in non-conforming slots could be set to lower priority like in fixed networks. Priority number Service category 5 CBR 4 Rt-VBR 3 Nrt-VBR 2 ABR 1 UBR Table 1: ATM traffic categories priorities MF-TDMA based Multiple Technique The MF-TDMA access scheme has been proposed for different GEO and LEO systems. The MF-TDMA frame is normally divided into two areas: the first one is intended for synchronization and signaling information transmission, whereas in the second one the data is transmitted. The requests for bandwidth allocations can be transmitted via out-of-band request slots, which are part of the first area. However, the in-band (implicit) requests can be sent together with data packets. The first task of the scheduler is to calculate the number of slots which will be allocated to the specific connection. This has been presented for hierarchical round robin (HRR) scheduling in [21]. Then, the allocated number of slots has to be assigned to actual slots in MF-TDMA frame. This process is much more complex and needs also the time component, such as virtual arrival times. For this first approach, we decided to implement a very simple strategy which is based on priority and firstcome-first-serve (FCFS) strategies. In this way, when the new connection is setup, the scheduler writes the connection parameters into a linked list. This list is ordered according to priorities of the traffic classes of the connections. The scheduler begins on the top of the list and allocates the slots to services with equal priority in a round robin manner as shown in Fig. 12. First, it serves all CBR connections, then rt-vbr ones and if capacity is available also other connections, until all time slots are allocated. In such a way every connection of the same priority gets a minimum capacity. Connection 1,Priority: 5 Connection 2,Priority: 5 Connection 3,Priority: 4 Connection 4,Priority: 4 Connection 5,Priority: 3 Round-robin ordering strategy Scheduler Fig. 12: Scheduler operation Simulation Setup and Results Slots allocation Request for one time slot Con 1 Con 2 Con 1 Con 3 Con 4 Con 3 Con 5 The simulation setup consists of VBR traffic sources, which were described in the section about Advanced- PRMA, and satellite as access point with scheduler as described in the previous section. The simulation is realized only on cell level, that means that the sources are active all the time. The traffic is multiplexed onto one uplink carrier according the requests for time slots and scheduler allocation of time slots. The reservation information on time slots is broadcast to all traffic sources on the frame basis. In Fig. 13 the results of the simulation for mean cell delay of one traffic source are shown. The delay gradually increases with the number of traffic sources similarly for the different channel delays. Because of the scheduler technique the traffic source which has first established the does not have the problems with the capacity until the number of sources is much higher than 100 % load (the number of the traffic sources multiplied by the mean bit rate of the traffic sources is exactly the carrier capacity). That is not true for the sources which established the connection last. The reason is that the scheduler always allocates the first slot in the frame to the first traffic source in the linked list of traffic sources as described in the previous section. The delays for the sources at the end of the linked list begin to increase very fast when the load is about 100 % and over. Mean cell delay (ms) RTD 9 Frames RTD 7 Frames RTD 5 Frames RTD 5 Frames RTD 1 Frame Number of VBR traffic sources Fig. 13: Mean delay of cells for various number of VBR traffic sources and channel delay

12 Fig. 14 shows the CDV for various number of traffic sources and channel delays. It can be obtained that the CDV increases with the channel delay. CDV (ms) RTD 9 Frames RTD 7 Frames RTD 5 Frames RTD 3 Frames RTD 1 Frame Number of VBR traffic sources Fig. 14 : CDV for various number of VBR traffic sources and channel delay CONCLUSIONS In this paper, parts of the research work performed by the Network Aspects Working Group of COST 252 was presented. First, a short overview of the possible satellite multimedia network was presented. Then the simulations results for PRMA, PRMA-HS and A- PRMA were evaluated. These three techniques are appropriate for transmission of integrated voice and data applications, but the requests for transmission of multimedia applications are not supported. The paper continues with the aspects of scheduling for uplink MAC protocol. The scheduling can be combined with the MF-TDMA technique, since it allows implicit and explicit reservations of needed capacity. The implemented scheduler technique guarantees the CLR, but the CDV still cannot be guaranteed and therefore new approaches are needed. However, the presented scheduling technique with some modifications, such as introduction of weights, is very suitable for non-real time service categories. The presented paper has shown how problematic is the rt-vbr service category for transmission over radio or satellite networks. Future work should focus on further elaboration and definition of interesting multimedia services for satellite multimedia broadband networks. REFERENCES [1] I. Mertzanis, A. Sammut, R. Tafazolli, B. G. Evans, Network Aspects of dynamic Satellite Multimedia Systems: An Overview of COST- 252 Activities, COST 252/259 Joint Workshop, University of Bradford, April [2] A. Guntsch, M. Ibnkahla, G. Losquadro, M. Mazella, D. Roviras, A. Timm, EU s R&D Activities on Third-Generation Mobile Satellite Systems (S-UMTS), IEEE Comm. Mag., Vol. 36, No. 2, pp , February [3] TIA/EIA Telecommunication Systems Bulletin 91 (TSB-91), Satellite ATM Networks: Architectures and Guidelines, May [4] P. Chitre, F. Yegenoglu, Next-Generation Satellite Networks: Architectures and Implementations, IEEE Comm. Mag., Vol. 37, pp , March [5] I. Mertzanis, G. Sfikas, R. Tafazolli, B. G. Evans, Satellite-ATM Networking and Call Performance Evaluation for Multimedia Broadband Services, COST 252 TD(98)38, November [6] I. Mertzanis, G.Sfikas, R.Tafazolli, B.G.Evans, "Protocol Architectures for Satellite-ATM Broadband Networks", IEEE Communications Magazine, Vol.37, No.3, Special issue on Satellite-ATM Network Architectures, March 1999 [7] D. J. Goodman, R.A. Valenzuela, K.T. Gayliard, B. Ramanurthi, Packet Reservation Multiple for local wireless communications, IEEE Trans. on Comm., Vol. 37, pp , August [8] D. J. Goodman, Trends in Cellular and Cordless Communications, IEEE Comm. Mag., Vol. 29, pp , June [9] S. Nanda, D. J. Goodman, U. Timor, Performance of PRMA: a Packet Voice Protocol for Cellular Systems, IEEE Trans. on Veh. Tech., Vol. 40, pp , August [10] E. Del Re, R. Fantacci, G. Giambene, S. Walter, Performance Evaluation of an Improved PRMA Protocol for Low Earth Orbit Mobile Communication Systems, COST 252 TD(97)12, November [11] E. Del Re, R. Fantacci, G. Giambene, S. Walter, Performance Evaluation of an Improved PRMA Protocol for Low Earth Orbit Mobile Communication Systems, Int. J. Sat. Comm., Vol. 15, pp , [12] E. Del Re, R. Fantacci, G. Giambene, C. Cerboni, Performance Evaluation of the PRMA Protocol for Voice and Data Transmissions in Low Earth Orbit Mobile Communication Systems, COST 252 TD(98)2, February [13] E. Del Re, R. Fantacci, G. Giambene, T. Pecorella, Performance Evaluation of the PRMA Protocol for Voice and Data Transmissions in Low Earth Orbit Mobile Communication Systems, COST 252/259