A broadband network (such as B-ISDN) is needed as a result of recent ... ATM services can be provided rapidly over a large area, without the need of major ...
ATM Networking with VSATs Tolga ÖRS, Zhili SUN and S.P.W. JONES Centre for Satellite Engineering Research University of Surrey, Guildford Surrey GU2 5XH, UK {T.Ors, Z.Sun, Peter.Jones}@ee.surrey.ac.uk Abstract: This paper presents the preliminary results of the research to use Very Small Aperture Terminal (VSAT) systems for ATM (Asynchronous Transfer Mode) services. We consider a scenario where a large number of VSATs are interconnected by an On Board Processing (OBP) satellite with cell switching capabilities. Statistical multiplexing is used to take advantages of the burstiness of VBR traffic. A novel approach is used for maximizing the bandwidth utilization of the satellite. The total uplink rate is dimensioned higher than the switching rate and statistical multiplexing is performed on-board the satellite. The cell rate distribution of the multiplexed sources is used by the Connection Admission Control (CAC) to allocate an effective bandwidth to each source. MF-TDMA is used as satellite multiple access protocol since it takes advantage of the flexibility and statistical multiplexing capabilities of ATM. The required maximum delay is provided by careful timing of Frame Units (FUs) within the TDMA frame. Interleaving is used to make the transmission more robust to burst errors. Finally, the cell loss resulting from the limited bandwidth of the satellite link can be prevented by effective traffic control functions. A preventive control scheme has been used for this purpose. The Leaky Bucket (or GCRA) used as Usage Parameter Control (UPC) controls the source traffic parameters for conformance with the traffic contract. Furthermore a rate-based flow control is used to control ABR services. 1. INTRODUCTION A broadband network (such as B-ISDN) is needed as a result of recent developments in multimedia services. These services will have diverse traffic characteristics and Quality of Service (QoS) requirements. Asynchronous Transfer Mode (ATM), which was chosen by the ITU-T as the basis for B-ISDN, can potentially carry this heterogeneous mix of traffic in an integrated manner. ATM also offers the potential for improved bandwidth utilization through the statistical multiplexing of Variable Bit Rate (VBR) and bursty traffic. Whilst fibre optics is rapidly becoming the preferred carrier for high bandwidth communication services, satellite systems can play an important role in the B-ISDN. The main strengths of satellites are their fast deployment, global reach, very flexible bandwidth-on-demand capabilities. The satellite network configuration and capacity can be increased gradually to match the B-ISDN traffic evolution at each time. The role of satellites in high-speed networking will evolve according to the evolution of the terrestrial ATM based B-ISDN. However two main roles can be identified: • The introduction phase when satellites will compensate the lack of sufficient terrestrial high bit rate links mainly by interconnecting a few regional or national distributed broadband networks, usually called ’Broadband Islands’. • The maturation phase when the terrestrial broadband infrastructure will have reached some degree of maturity. In this phase, satellites are expected to provide broadcast service and also cost effective links to rural areas complementing the terrestrial network. At this phase satellite networks will provide broadband links to a large number of end users through a User Network Interface (UNI) for accessing the ATM B-ISDN. They are also ideal for interconnecting mobile sites. Furthermore they provide a back-up solution in case of failure of the terrestrial systems. In the first scenario, satellite links provide high bit rate links between broadband nodes or broadband islands. The interfaces with satellite links in this mode are of the NNI type. This scenario is characterized by a 37/1
relatively small number of large earth stations which have a relatively large average bit rate. The cost and the size of the earth station has a small impact on the suitability of the satellite solution. The RACE CATALYST project was a demonstrator for this scenario and showed the compability of satellite technology with ATM and the terrestrial B-ISDN. The equipment developed during the CATALYST project has been able to interconnect ATM testbeds as well as the existing networks such as DQDB, FDDI and Ethernet networks, all using ATM. A detailed explanation of the system design and performance is provided in [POLE94], [LOUV94], [HADJ94], [SUN95]. In the second scenario the satellite system is located at the border of the B-ISDN and provides access links to a large number of users. This scenario is characterized by a large number of earth stations whose average and peak bit rates are limited. The traffic at the earth station is expected to show large fluctuations. Therefore the multiple access scheme will considerably effect the performance of the system. Furthermore the cost and size of the earth station have a large impact on the suitability of the satellite solution. Very Small Aperture Terminal (VSAT) satellite systems could be used for the second scenario. The objective of this paper is to develop a system that uses on-board processing (OBP) satellites and VSATs in order to make B-ISDN access affordable for a large number of users by lowering earth-station cost and providing bandwidth on demand. The problems that need to be resolved are investigated. 2. ATM VSAT SATELLITE SYSTEM ARCHITECTURE 2.1 ATM Networking via Satellite Computer networking has seen a tremendous growth during the last two decades, and there are today numerous global networks, using a mixture of terrestrial and satellite links. However, there is little experience concerning the interconnection of broadband networks using high-speed satellite links. The problem is made difficult from the fact that such a network must cope efficiently with both the high transmission rates of ATM/B-ISDN and the substantial satellite channel round-trip propagation delay. One of the major issues seen as affecting the rapid implementation of B-ISDN over the coming decade is the requirement to provide such a service over a wide geographical area, which is not possible without a substantial initial investment. The actual demand and traffic characteristics of potential network users are yet unknown, and future customers are likely to be skeptical about joining a network at a very high initial premium. Communication satellites, as a possible way of offering access to the broadband network appear to be a very attractive option because: • ATM services can be provided rapidly over a large area, without the need of major investment. • Satellite communication systems can be complementary to terrestrial networks, especially for widely dispersed users. • The broadcast nature of satellites can be used where the same message has to be send to a large number of stations (point-to-multipoint transmissions). • A wide range of customer bit rates and circuit provision modes can be supported. • New users can be accomodated swiftly with simply installing new earth stations at customer premises (cost of earth station will have an important impact). Thus possible network enlargement is not a significant planning problem.
The network architecture consists of a large number of VSAT earth stations which are interconnected in a mesh configuration by an cell switching on-board processing (OBP) satellite with spot beams.
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2.2 Ground Segment Figure-1 shows the functions of the VSAT earth station. The User Network Interface (UNI) constitutes the boundary between the terrestrial system and the satellite network. The arriving cells are controlled by the UPC for their conformance with the traffic contract. Only the peak rate is controlled for CBR, ABR and UBR traffic. However for VBR sources also the mean rate is controlled using a dual LB configuration shown in Figure 2. The UPC parameters for the ABR type traffic are adaptive (dynamic GCRA) and changed according to the explicit rate feedback signal from the satellite. The Cell Scheduling and Processing Module consists of the cell scheduling and cell processing parts. The cell scheduling part is responsible for scheduling CBR, VBR, ABR and UBR traffic. CBR and VBR connections have priority and maximum burst utilization is achieved by detecting the silence periods of VBR voice connections. During these periods ABR and UBR traffic is transmitted. ABR traffic has higher priority than UBR but is limited by a certain rate which is send to the ground station from the satellite. The cell processor, also performs a number of functions. First the processing function sorts the cells by VP and assigns new VPIs. Next, the ATM cells are interleaved in blocks of two. Each packet called Frame Unit (FU) shown in Figure-3 contains two cells destined for the same downlink beam. This simplifies the on-board processing. Then VPs are mapped into satellite packet addresses for on-board routing and FU headers are assigned. The packets are than scrambled before modulation for transmission. Procedure for the destination VSAT earth station primarily consists of the procedure above in reverse order. 2.3 Space Segment The most important element of the space segment is the on-board processor with cell switching capabilities for satellite ATM networking. Several concepts for on-board switching have been proposed. However, these systems have been mainly targeted at applications supporting a predominance of voice traffic, and therefore employ circuit switched techniques. Examples are the Advanced Communication Technology Satellite (ACTS) [WRIG92] and the European Space Agency (ESA) time-space-time prototype [FROM 92]. With the introduction of new services (such as providing access to the information super-highway), the question arises how to offer better satellite resource utilization in a mixed traffic (voice, data and video) environment. We propose a novel approach of statistically multiplexing the traffic on board the satellite for maximum bandwidth utilization. On the uplink the satellite is accessed via 8 Uplink Groups (UG). Each UG comprises of 16 carriers with a transmission rate of 2.048 Mbit/s. On board the satellite the 16 carriers are first demodulated and the FUs processed to extract the cells. Then all cells from the 16 carriers are multiplexed discarding empty cells. The output of the multiplexing buffer is 16 Mbit/s, the cell switch rate of one port of the OBP cell switch. In this scenario maximum utilization of the burst slots is not required (considering that VSATs don’t generate much traffic) for maximum bandwidth utilization of the satellite, by concentrating the traffic in the sky. This introduces some issues for the design and analysis of the satellite architecture. Many considerations previously the concern only of the ground segments now shift to the space segment. The on-board processor allocates bandwidth on demand and performs statistical multiplexing. This essentially changes the nature of the satellite from a deterministic system to a stochastic system. In a stochastic system, the arriving traffic is random and statistical fluctuations may cause congestion. where cell loss due to buffer overflow might occur. Thus, it is necessary to incorporate a traffic and control mechanism to regulate the input traffic. Uplink transmission uses a 24 ms MF-TDMA frame. Downlink transmission uses a 24 ms TDM frame. Within a frame the data is transmitted in portions of 111 bytes. One such portion, shown in Figure-3 is called a Frame Unit (FU). Each FU contains two ATM cells, thus the design allows the assignment of capacity with a granularity of 32kbit/s. Note that the uplink transmission rate per terminal is 37 kbit/s due to the FU overhead. For a frame efficiency of 95% each carrier can support maximum 50 terminals. The traditional demand-assignment scheme using a ground terminal as control station has two important drawbacks: long set-up time (about 500ms because of two hops assuming a negligible processing time at the control station) and limited channel utility. Both are due to the long propagation delay of the satellite link. Both disadvantages can be removed by using an on-board module providing fast channel set-up 37/3
capabilities. The major issues in developing an OBP cell switch include: (i) the traffic model, (ii) traffic and congestion control, (iii) testing and evaluation. Section 3 explains the used traffic model and traffic/congestion control mechanism in detail whereas Section 4 provides simulation and theoretical results of the system performance. 3. TRAFFIC AND CONGESTION CONTROL Providing the desired QoS for various traffic categories in ATM networks is not an easy task. The design of a suitable ATM traffic and congestion control is the most important challenge for the success of an ATM based B-ISDN. Therefore it has been the subject of vigorous research over recent years. Various control mechanisms have been proposed for ATM networks. These can be classified into two categories: reactive control and preventive controls. Preventive control techniques attempt to prevent congestion by taking appropriate actions before they actually occur. A preventive flow control mechanism consists of the connection admission control (call level) and usage parameter control (cell level). Reactive control (burst level) is a technique used to recover from a congested state. This Section will first introduce the ATM service categories. Then more detailed information about the used connection admission control, usage parameter control and reactive control mechanism is provided. 3.1 Service Categories According to [ATMF95] services provided at the ATM layer consist of the following five service categories: • • • • •
CBR rt-VBR nrt-VBR UBR ABR
Constant Bit Rate Real-Time Variable Bit Rate Non-Real-Time Variable Bit Rate Unspecified Bit Rate Available Bit Rate
Service categories are distinguished as being either real-time (rt) or non-real-time (nrt). CBR and rt-VBR are real-time categories whereas nrt-VBR, UBR and ABR are non-real time categories. 3.1.1 Constant Bit Rate (CBR) Services CBR services generate traffic at a constant rate and can be simply described by their peak cell rate (PCR). The burstiness of a CBR source is equal to one, and the source is active during the duration of the connection (or the silent periods are also transmitted at the peak rate). This service category is intended for real-time applications, i.e. those requiring tightly constrained delay and delay variation, as would be appropriate for voice and video applications. Cells which are delayed beyond the value specified by Cell Transfer Delay (CTD) are assumed to be significantly reduced value to the application and might be discarded. 3.1.2 Variable Bit Rate (VBR) Services The traffic generated by a typical VBR source, in general, either alternates between the active and silent periods and/or has a varying bit rate generated continuously. The peak-to-average bit rate (burstiness) of a VBR source is often much greater than one. VBR services can be described by different sets of traffic descriptors. The used TDs are described in Section 3.2.1. The rt-VBR service category is intended for real-time applications such as voice and video guaranteeing a certain CTD. The non-real-time category is intended for non-real time applications which have bursty traffic characteristics and not so stringent delay characteristics. A bound on the mean transfer delay is however 37/4
provided. For both rt- and nrt-VBR services the required low cell loss ratio (CLR) is guaranteed by the network. 3.1.3 Unspecified Bit Rate (UBR) Services The UBR service category is intended for non-real-time applications. This category does not specify traffic related service guarantees. Specifically, UBR does not include the notion of a per-connection negotiated bandwidth. No numerical commitments are made with respect to the cell loss rate experienced by a UBR connection, or as to the cell transfer delay experienced by cells on the connection. 3.1.4 Available Bit Rate (ABR) Services ABR is a service category for which the limiting transfer characteristics (such as PCR) provided by the network may be changed subsequent to connection establishment. A flow control mechanism is specified which supports several types of feedback to control the source rate in response to changing ATM layer transfer characteristics. If the end-system adapts its traffic in accordance with the feedback it is guaranteed a certain CLR and it will obtain a fair-share of the available bandwidth according to a network specific allocation policy. ABR service is not intended to support real-time applications. On establishment of an ABR connection, the end system specifies both a maximum required bandwidth (PCR) and a minimum usable bandwidth (MCR) to the network. The MCR may be specified as zero. The bandwidth available from the network may vary, but shall not become less than MCR. Only the rt-VBR and ABR traffic categories are considered since they represent integrated transport of realtime and non-real time traffic flows. CBR traffic and UBR traffic can easily be integrated in the traffic management scheme. The bandwidth assignment to CBR services is the peak rate and traffic control of the peak rate is not difficult. Thus CBR traffic will only have the effect of background traffic. Handling UBR traffic introduces the issue of protecting the QoS objectives of the ABR connections. 3.2 Connection Admission Control (CAC) The CAC decides whether or not a connnection can be accepted. Network resources are allocated according to the traffic contract which is negotiated between the user and the network. Parameters which form this contract are the Traffic Descriptors, QoS requirements, Conformance Definition and the Service Category. A connection request is accepted only when sufficient resources are available to establish the call through the whole network at its required QoS and maintain the agreed QoS of existing calls. This also applies to re-negotiation of connection parameters within a given call. There are two alternative approaches for bandwidth multiplexing: deterministic or statistical multiplexing. In deterministic multiplexing each connection is allocated its peak bandwidth. Although this can eliminate cell level congestion, it goes against the philosophy of ATM, which offers the potential for improved bandwidth utilization through statistical multiplexing of variable bit rate and bursty traffic. There is a need, specially for satellite links which are bandwidth-limited compared to optical fibre links, to fully use the statistical multiplexing capabilities of ATM. 3.2.1 Traffic Descriptors and Parameters The Connection Traffic Descriptors (CTD) play an important role in the preventive control scheme. The Connection Admission Control (CAC) has to consider the CTD in order to allocate the necessary network resources for the connection. The Usage Parameter Control (UPC) then monitors the conformance of cells to the negotiated CTD. This ensures that the unintentional or malicious behaviour of some users will not result in a performance degradation for other users. An important issue is the set of traffic parameters to be included in the CTD. Only the peak cell rate and Cell Delay Variation (CDV) tolerance has been standardized by the ITU-T. Some other widely used traffic 37/5
parameters have also been proposed: mean rate and mean burst length [VAKI91], sustainable cell rate and burst tolerance [ATMF94]. In this paper we will use the peak rate, CDV, mean cell rate and mean burst duration as traffic parameters. 3.2.2 Source Characterization and Source Model The traffic model, which is widely used for the characterization of ATM sources, is the on-off source model. This model has been successfully used to realistically model packetized speech, still picture and interactive data services [ONVU94]. Using the on-off model the ATM cell stream from a single source is modelled as a sequence of alternating burst periods and silence periods as shown in Figure-4. a and b are the transition rates between the on and off states. The duration of each burst is exponentially distributed with mean 1/a ms. During such a period ATM cells are transmitted with constant interarrival time T ms, where T = 1/PCR. After generation of the ATM cells an exponentially distributed silence period with mean value 1/b ms follows. If N identical on-off sources are multiplexed, [WEIN78] found that the number of active sources i can be modelled very well by a continous-time birth-death (b-d) proccess, shown in Figure-5. The interrarival time of cells in state i is 1/(i PCR). This b-d model greatly simplifies the simulation by reducing the simulation time since for N sources 2N states are required for the on-off model and N states for the b-d model. The parameters used by both models governing the transition rates are: mean burst duration = a-1, mean silence duration = b-1. 3.2.3 CAC Algorithms A variety of CAC algorithms have been proposed. The aim is to use an algorithm that is simple (in terms of processing and storage requirements) and efficient (to allow statistical multiplexing gain). We use a modified binomial model. The binomial formula was used in [SYKA92] with success, as the buffer size to burst length ratio, for the source types considered there, was close to zero. The maximum number of identical sources in the on-state without cell loss is equal to the capacity C=link rate/PCR. As the number of active sources becomes larger than C, the output link is not able to carry the required bandwidth (bw) and cell loss occurs. Our modification in the binomial model is that we assume that cell loss will occur in case of an overload situation where i > C+2. This assumption is justified when many connections are multiplexed and the buffer size is large enough, compared to the burst length, to absorb one source in temporary overload. The equilibrium probability of i sources being active Pbi is given by the binomial distribution
(1)
Dividing the rate of lost ATM cells with the maximum number of generated cells N1PCR, the cell loss rate (CLR) is obtained:
(2)
where
1=a
-1
/(a-1+b-1).
3.3 Usage Parameter Control (UPC) UPC is defined as the set of actions taken by the network to monitor and control traffic in terms of conformity with the agreed traffic contract at the user access. The main purpose is to protect network resources (in particular the satellite link capacity) from misbehaviour that could affect the QoS of other 37/6
established connections. The Leaky Bucket (LB) is generally agreed to achieve the best performance compromise of the mechanisms studied for UPC [ONVU94] [CHEN95]. It was first introduced in [TURN86]. Since then a number of variants have been proposed. The basic idea behind this approach is that each incoming cell needs a token to enter the network. Tokens are generated at constant rate r. The size of the bucket imposes an upper bound on the burst length and determines the number of cells that can be transmitted back to back, controlling the burst length. Provided that the burst is short, the bucket will not empty and no action will be taken against the cell stream. However, if a long burst of higher-rate cells arrives, the bucket will empty and the UPC function will take actions against cells in that burst. The tolerance allowed for the connection depends on the size of the token buffer (M) and the token generation rate (r) which are also the parameters of the LB. Conceptually, the tokens can be viewed as arrivals to a finite-capacity, single-server queue with deterministic service time. It is also obvious that the LB enforces the rate r and allows temporary bursts above the rate r depending on the bucket size (M). The implementation requires a simple up/down counter to reflect the contents of the token bucket. A number of variations of the basic LB are possible. Instead of discarding or marking cells when the token bucket is full, arriving cells can be allowed to queue in a data buffer (BD). The input or data buffer smooths the burst by spacing the cells at the cost of introducing some delay. The two types of enforcement action that can be taken within the LB scheme (cell discarding or marking) , and whether or not there is a user buffer, gives rise to different versions of the LB. 3.3.1 Control of the peak cell rate The peak rate of the source is effectively controlled by setting the token generation rate near the peak cell rate and allowing some margin for the CDV. Previous work [WILT94] has analyzed the CDV caused by various topologies in the access network and has given some typical values that could be used as default for the buffer size required to achieve a CLP within a CDV tolerance. 3.3.2 Control of the mean cell rate The mean cell rate can be very easily controlled if the burst tolerance (maximum burst size) is known. Then the dimensioning of the LB is very straightforward. However if only the mean burst duration is known than the mean cell rate is not so easy to control. This is due to the fact that a long observation time is required before detecting any non- conforming cells. A large token buffer has therefore to be chosen in order to minimize the probability of dropping/marking cells which conform to the negotiated traffic parameters. This however causes a long reaction time to increases in the mean cell rate whereas fast detection of violations is required. This problem can be solved by introducing a data buffer (B D). In order to distribute the buffer to obtain the best performance, in terms of reaction time and queuing delay, the buffered LB was analyzed in [ORS95]. 3.4 Reactive Congestion Control Although preventive control tries to prevent congestion before it actually occurs the satellite system may experience congestion due to multiplexing buffer or switch output buffer overflow. In this case, where the network relies only on the UPC and no feedback information is exchanged between the network and the source, no action can be taken once congestion has occurred. Many applications, mainly handling data transfer, have the ability to reduce their sending rate if the network requires them to do so. Likewise, they may wish to increase their sending rate if there is extra bandwidth available within the network. This kind of applications are supported by the ABR service class. The bandwidth allocated for such applications is dependent on the congestion state of the network. Rate-based control was recommended for ABR services, where information about the state of the network is conveyed to the source through special control cells called Resource Management (RM) cells [ATMF94]. Rate 37/7
infromation can be conveyed back to the source in two forms: • Binary Congestion Notfication (BCN) using a single bit for marking the congested and not congested states. • Explicit Rate (ER) indication, with which the network notifies the source of the exact bandwidth share it should be using in order to avoid congestion. BCN uses only a single bit to inform sources to reduce or increase their traffic. Although this scheme is particularly appealing for a satellite-based network because of it’s inherent broadcasting capability, it may take several round trips before the source will adjust to the right rate. This is unaccaptable for satellite links with long propagation delays. A better strategy is for the on-board switch to send RM information to the stations which send RM cells to the source containing the rate it should change to. Various algorithms have been proposed for the calculation of the fair bandwidth share per connection [JAIN95]. When the satellite is sending RM information to one station, all the active ground stations covered by the same beam can obtain the same information. The source model used for the ABR service is the persistent source which always sends with the maximum permitted rate. Different simulations [BARN95] have shown that this model imposes the heaviest constraints on the network and is therefore very appropriate for testing the throughput and cell loss of the ABR service. The switch can determine congestion either by measuring the traffic arrival rate or by monitoring the buffer status. We will use the former method where the on-board switch measures the current load by counting the number of cells received during a fixed averaging interval. Based on the known capacity of the link, the switch can determine whether it is overloaded or underloaded. 4. THEORETICAL AND SIMULATION RESULTS First we compare the accuracy of the N-state birth-death (b-d) model in representing N on-off sources. Figure-6 shows the cell rate distribution for N=50 multiplexed voice sources. The traffic parameters for the voice source are PCR=32kbit/s, mean burst duration=0.352 ms, mean silence duration=0.65 ms. The simulation results using the on-off and b-d model are compared to theoretical results (1) showing that the N-state b-d model can be used instaed of N identical on-off sources obtaining obtaining a very similar cell rate distribution (see Figure-6). The only problem with the b-d model is to decide how long the process should stay in a certain state. The logical solution would be to generate i exponential random numbers with mean a-1 and b-1 and chose the smaller number to determine the duration of state i. As the number of states increases for large N, the simulation time spend in generating random numbers also increaes. We thus limited the maximum number of generated random numbers to twenty. Figure-6 shows that the impact of this was negligible saving valuable simulation time. The cell rate distribution is presented as a relative frequency histogram where Pbi (Number of cells generated in state i / Total number of generated cells) is a function of i/N. As Pbi is proportional to the CLR for i > C+1, C has to be chosen so that Pb i is very small. Figure-7 shows the cell rate distribution for different N. For N=800 voice sources, it can be observed that the probability of 40% of the sources being active is very small whereas the contrary is true for N=50 and N=100. Thus for a certain CLR the effective bw of each source decreases as the number of multiplexed sources increases. The accuracy of the binomial model was already verified by [SYKA92] [ORS94]. It is however worth noting that the small modification resulted in more accurate results which were verified by simulations. Figure-8 shows the normalized capacity (C/N) for a certain CLR if 800 (maximum number of voice sources which can be supported by one UG with 16 carriers) or 400 voice sources are multiplexed. It can be seen that for N=800 an effective bandwidth of 0.4375 32 kbit/s=14 kbit/s is sufficient for a CLR of 1.45 10 -9. This is 43.75% of the peak rate resulting in an througput increase by 228%. The remaining bandwidth (16Mbit/s-bandwidth allocated to voice sources) can be allocated to UBR and/or ABR sources by taking advantage of the silence periods of the voice source. In the worst case when 800 terminals are transmitting voice and also want to transmit ABR traffic the fair share for each terminal is 6 Kbit/s. 37/8
The CLR experienced by the ABR traffic was observed as very low for a satellite network utilization of 95% and a multiplexing buffer size of 100 cells. The reson for chosing such a high utilization is the fact that the binomial model slightly overestimates the cell loss, assigning too much bandwidth to a source. The MCR for each ABR source was assumed zero so that the bandwidth assigned to each source was the same. Further simulations are required to obtain the CLR of ABR traffic on-board the satellite under different load conditions. 5. CONCLUSIONS As user demand is becoming more complex, VSAT satellite networks, which have so far been successfully used in providing specific communication services are expected to provide a much wider range of services (such as multimedia). An on-board cell swotching satellite with spot beams is considered for the scenario where a high number of VSATs are interconnected in a mesh configuration. In our proposal maximum utilization of the burst slots using complex DAMA schemes, which introduce high delay, is not required. Maximum bandwidth utilization of the satellite is achieved by statistical multiplexing the traffic on-board the satellite. Greater connectivity, improved spectral efficiency, improved use of the switch capacity and greater e.i.r.p. per VSAT terminal are the main advantages of using a state-of-the-art satellite system. These advantages enable new services while potentially reducing the cost of earth stations. However development of an OBP cell switched satellite communication network introduces some issues like traffic and congestion control which have been addressed in this paper. It has been shown that rt and non-rt traffic can be integrated while still achieving increses in throughput up to 228% for a very low CLR. We conclude that the use of VSATs for ATM services is possible, but careful system design and dimensioning is very important to provide the required quality of service.
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Figure-6 Cell rate dist. for 50 voice sources
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Figure-2 UPC to control mean rate Frame Unit (111bytes) Frame Unit Header
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Figure-5 Birth-Death Process Model 37/10
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REFERENCES [ATMF94] ATM-Forum, ATM User-Network Interface Specification Version 3.1, September 1994 [ATMF95] ATM-Forum, Traffic Management Specification Version 4.0, 95-0013R8, October 1995 [BARN95] BARNHART A.W., "Baseline Model for rate-control simulations", ATM Forum 940399,Sept.1994 [CHEN95] CHEN T.M. and LIU S.S., ATM Switching Systems, Artech House, 1995 [FROM92] FROMM H.H. and BELLA L., "System architecture and design criteria for the ESA OBP development",pp.57-62, 9th Int. Conf. on Digital Sat. Com., Denmark, 1992 [HADJ94] HADJITHEODOSIOU M.H., KOMISARCZUK P., COAKLEY F., EVANS B., and SMYTHE C., "Broadband Island Interconnection via Satellite-Performance Analysis for the RACE II-CATALYST Project", International Journal of Satellite Communications, V.12, pp.223-238, 1994 [HADJ96] HADJITHEODOSIOU M.H. and COAKLEY F.P., "Networking with VSATs: From low bit rate systems to ATM service provision", AIAA 16th International Comms Satellite Systems Conference, pp. 854-864, 1996 [IVAN94] IVANCIC W.D., CHU P., and SHYY D., "Technical Issues regarding Satellite Packet Switching", International Journal of Satellite Communications, V.12, pp.257-267, 1994 [JAIN95] JAIN R., "Congestion Control and Traffic Management in ATM Networks: Recent Advances and A Survey", Computer Networks and ISDN Systems, February 1995 [LOUV94] LOUVET B. and CHELLINGSWORTH S., "Satellite integration into broadband networks", Electrical Communications, 1994 [ONVU94] ONVURAL Raif O. Asynchronous Transfer Mode Networks: Performance Issues, Artech House, 1994 [ORS94] ORS T., An Investigation of the Leaky-Bucket for Preventive Control in ATM Networks, Master’s thesis, University of Surrey, September 1994 [ORS95] ORS T. and JONES S.P.W., "Performance Optimization of ATM Input Control using Multiple Leaky-Buckets", Third IFIP Workshop on Performance Modelling and Evaluation of ATM Networks, July 1995 [POLE94] POLESE P, MORT R., and COMBAREL L., "Satellites in UMTS and B-ISDN: status of activities and perspectives", Electronics and Communication Engineering Journal, pp. 297-303, December 1994 [RACE93] RACE Common Functional Specifications D751, "Satellites in the B-ISDN, General Aspects", Issue D, December 1993 [SUN95] SUN Z., COAKLEY F.P., and EVANS B.G., "Satellite ATM for Broadband ISDN Telecommunication Sytems", V.4, pp.119-131, 1995 [SYKA92] SYKAS E.D., PASCHALIDIS I.Ch., and VLAKOS K.M., "Congestion Avoidance in ATM Networks", IEEE INFOCOM, 1992 [TUCK88] TUCKER R.C.F., "Accurate Method for analysis of a packet-speech multiplexer with limited delay", IEEE Transactions on Communications, V.36 No.4, pp.479-483, April 1988 [TURN86] TURNER J.S., "New Directions in Communicationss (or Which Way to the Information Age ?)", IEEE Communications Magazine, October 1986 [VAKI91] VAKIL F. and SAITO H., "On Congestion Control in ATM Networks", IEEE LTS, pp55-65, August 1991 [WEIN78] WEINSTEIN C.J., "Fractional speech loss and talker activity model for TASI and for packetswitched speech", IEEE Trans. on Communications, V.COM-26, pp.1253-1257, 1979 [WILT94] WILTS R., WITTERS J., and PETE G.H., "Throughput Analysis of a Usage Parameter Control Function Monitoring Misbehaving Constant Bit Rate Sources", Second IFIP Workshop on Performance Modelling and Evaluation of ATM Networks, 1994 [WRIG92] WRIGHT D. and BALOMBIN J., "ACTS System Capabilities and Performance", pp.1135-1146, AIAA’92
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