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ATM Networking with VSATs

Tolga ÖRS, Zhili SUN and S.P.W. JONESCentre 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 bandwidthof 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, canpotentially 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) andbursty 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 fastdeployment, global reach, very flexible bandwidth-on-demand capabilities. The satellite networkconfiguration 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 ATMbased B-ISDN. However two main roles can be identified:

• The introduction phase when satellites will compensate the lack of sufficient terrestrial high bit ratelinks mainly by interconnecting a few regional or national distributed broadband networks, usuallycalled ’Broadband Islands’.

• The maturation phase when the terrestrial broadband infrastructure will have reached some degreeof maturity. In this phase, satellites are expected to provide broadcast service and also cost effectivelinks to rural areas complementing the terrestrial network. At this phase satellite networks willprovide broadband links to a large number of end users through a User Network Interface (UNI) foraccessing the ATM B-ISDN. They are also ideal for interconnecting mobile sites. Furthermore theyprovide 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

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relatively small number of large earth stations which have a relatively large average bit rate. The cost andthe 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 satellitetechnology with ATM and the terrestrial B-ISDN. The equipment developed during the CATALYST projecthas been able to interconnect ATM testbeds as well as the existing networks such as DQDB, FDDI andEthernet networks, all using ATM. A detailed explanation of the system design and performance is providedin [POLE94], [LOUV94], [HADJ94], [SUN95].

In the second scenario the satellite system is located at the border of the B-ISDN and provides access linksto a large number of users. This scenario is characterized by a large number of earth stations whose averageand 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. Furthermorethe 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 objectiveof this paper is to develop a system that uses on-board processing (OBP) satellites and VSATs in order tomake B-ISDN access affordable for a large number of users by lowering earth-station cost and providingbandwidth 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 todaynumerous global networks, using a mixture of terrestrial and satellite links. However, there is littleexperience concerning the interconnection of broadband networks using high-speed satellite links. Theproblem is made difficult from the fact that such a network must cope efficiently with both the hightransmission 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 isthe requirement to provide such a service over a wide geographical area, which is not possible without asubstantial initial investment. The actual demand and traffic characteristics of potential network users areyet unknown, and future customers are likely to be skeptical about joining a network at a very high initialpremium. Communication satellites, as a possible way of offering access to the broadband network appearto 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 forwidely dispersed users.

• The broadcast nature of satellites can be used where the same message has to be send to a largenumber 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 customerpremises (cost of earth station will have an important impact). Thus possible network enlargementis not a significant planning problem.

The network architecture consists of a large number of VSAT earth stations which are interconnected in amesh 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 theboundary between the terrestrial system and the satellite network. The arriving cells are controlled by theUPC for their conformance with the traffic contract. Only the peak rate is controlled for CBR, ABR andUBR traffic. However for VBR sources also the mean rate is controlled using a dual LB configuration shownin Figure 2. The UPC parameters for the ABR type traffic are adaptive (dynamic GCRA) and changedaccording to the explicit rate feedback signal from the satellite. The Cell Scheduling and Processing Moduleconsists of the cell scheduling and cell processing parts. The cell scheduling part is responsible forscheduling CBR, VBR, ABR and UBR traffic. CBR and VBR connections have priority and maximum burstutilization is achieved by detecting the silence periods of VBR voice connections. During these periods ABRand UBR traffic is transmitted. ABR traffic has higher priority than UBR but is limited by a certain ratewhich 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 cellsare interleaved in blocks of two. Each packet called Frame Unit (FU) shown in Figure-3 contains two cellsdestined for the same downlink beam. This simplifies the on-board processing. Then VPs are mapped intosatellite packet addresses for on-board routing and FU headers are assigned. The packets are than scrambledbefore 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 capabilitiesfor satellite ATM networking. Several concepts for on-board switching have been proposed. However, thesesystems have been mainly targeted at applications supporting a predominance of voice traffic, and thereforeemploy 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 theintroduction of new services (such as providing access to the information super-highway), the question ariseshow 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 maximumbandwidth utilization. On the uplink the satellite is accessed via 8 Uplink Groups (UG). Each UG comprisesof 16 carriers with a transmission rate of 2.048 Mbit/s. On board the satellite the 16 carriers are firstdemodulated and the FUs processed to extract the cells. Then all cells from the 16 carriers are multiplexeddiscarding empty cells. The output of the multiplexing buffer is 16 Mbit/s, the cell switch rate of one portof the OBP cell switch. In this scenario maximum utilization of the burst slots is not required (consideringthat VSATs don’t generate much traffic) for maximum bandwidth utilization of the satellite, by concentratingthe 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 essentiallychanges 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 theinput 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 iscalled 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 dueto 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 importantdrawbacks: long set-up time (about 500ms because of two hops assuming a negligible processing time atthe control station) and limited channel utility. Both are due to the long propagation delay of the satellitelink. Both disadvantages can be removed by using an on-board module providing fast channel set-up

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capabilities.

The major issues in developing an OBP cell switch include: (i) the traffic model, (ii) traffic and congestioncontrol, (iii) testing and evaluation. Section 3 explains the used traffic model and traffic/congestion controlmechanism in detail whereas Section 4 provides simulation and theoretical results of the systemperformance.

3. TRAFFIC AND CONGESTION CONTROL

Providing the desired QoS for various traffic categories in ATM networks is not an easy task. The designof a suitable ATM traffic and congestion control is the most important challenge for the success of an ATMbased 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 twocategories: reactive control and preventive controls. Preventive control techniques attempt to preventcongestion by taking appropriate actions before they actually occur. A preventive flow control mechanismconsists of the connection admission control (call level) and usage parameter control (cell level). Reactivecontrol (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 usedconnection 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 Constant Bit Rate• rt-VBR Real-Time Variable Bit Rate• nrt-VBR Non-Real-Time Variable Bit Rate• UBR Unspecified Bit Rate• ABR Available Bit Rate

Service categories are distinguished as being either real-time (rt) or non-real-time (nrt). CBR and rt-VBRare 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 theconnection (or the silent periods are also transmitted at the peak rate). This service category is intended forreal-time applications, i.e. those requiring tightly constrained delay and delay variation, as would beappropriate 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 bediscarded.

3.1.2 Variable Bit Rate (VBR) Services

The traffic generated by a typical VBR source, in general, either alternates between the active and silentperiods and/or has a varying bit rate generated continuously. The peak-to-average bit rate (burstiness) of aVBR source is often much greater than one. VBR services can be described by different sets of trafficdescriptors. 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 trafficcharacteristics and not so stringent delay characteristics. A bound on the mean transfer delay is however

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provided. For both rt- and nrt-VBR services the required low cell loss ratio (CLR) is guaranteed by thenetwork.

3.1.3 Unspecified Bit Rate (UBR) Services

The UBR service category is intended for non-real-time applications. This category does not specify trafficrelated service guarantees. Specifically, UBR does not include the notion of a per-connection negotiatedbandwidth. No numerical commitments are made with respect to the cell loss rate experienced by a UBRconnection, 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 thenetwork may be changed subsequent to connection establishment. A flow control mechanism is specifiedwhich supports several types of feedback to control the source rate in response to changing ATM layertransfer characteristics. If the end-system adapts its traffic in accordance with the feedback it is guaranteeda certain CLR and it will obtain a fair-share of the available bandwidth according to a network specificallocation 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. Thebandwidth 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 real-time and non-real time traffic flows. CBR traffic and UBR traffic can easily be integrated in the trafficmanagement scheme. The bandwidth assignment to CBR services is the peak rate and traffic control of thepeak rate is not difficult. Thus CBR traffic will only have the effect of background traffic. Handling UBRtraffic 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 accordingto the traffic contract which is negotiated between the user and the network. Parameters which form thiscontract 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 throughthe whole network at its required QoS and maintain the agreed QoS of existing calls. This also applies tore-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 eliminatecell level congestion, it goes against the philosophy of ATM, which offers the potential for improvedbandwidth 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 thestatistical 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. TheConnection Admission Control (CAC) has to consider the CTD in order to allocate the necessary networkresources for the connection. The Usage Parameter Control (UPC) then monitors the conformance of cellsto the negotiated CTD. This ensures that the unintentional or malicious behaviour of some users will notresult 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 andCell Delay Variation (CDV) tolerance has been standardized by the ITU-T. Some other widely used traffic

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(1)(1)

(2)(2)

parameters have also been proposed: mean rate and mean burst length [VAKI91], sustainable cell rate andburst tolerance [ATMF94]. In this paper we will use the peak rate, CDV, mean cell rate and mean burstduration 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 interactivedata services [ONVU94]. Using the on-off model the ATM cell stream from a single source is modelled asa sequence of alternating burst periods and silence periods as shown in Figure-4. a and b are the transitionrates between the on and off states. The duration of each burst is exponentially distributed with mean1/ 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/ bms follows. If N identical on-off sources are multiplexed, [WEIN78] found that the number of active sourcesi can be modelled very well by a continous-time birth-death (b-d) proccess, shown in Figure-5. Theinterrarival time of cells in state i is 1/(i PCR). This b-d model greatly simplifies the simulation by reducingthe simulation time since for N sources 2N states are required for the on-off model and N states for the b-dmodel. 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 termsof 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 buffersize to burst length ratio, for the source types considered there, was close to zero. The maximum numberof identical sources in the on-state without cell loss is equal to the capacity C =link rate/PCR. As the numberof 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 Pb is given by the binomial distributioni

Dividing the rate of lost ATM cells with the maximum number of generated cells N 1 PCR, the cell loss rate(CLR) is obtained:

where 1 =a /(a +b ).-1 -1 -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 networkresources (in particular the satellite link capacity) from misbehaviour that could affect the QoS of other

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established connections.

The Leaky Bucket (LB) is generally agreed to achieve the best performance compromise of the mechanismsstudied 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 tokento enter the network. Tokens are generated at constant rate r . The size of the bucket imposes an upper boundon the burst length and determines the number of cells that can be transmitted back to back, controlling theburst length. Provided that the burst is short, the bucket will not empty and no action will be taken againstthe cell stream. However, if a long burst of higher-rate cells arrives, the bucket will empty and the UPCfunction will take actions against cells in that burst. The tolerance allowed for the connection depends onthe 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 deterministicservice time. It is also obvious that the LB enforces the rate r and allows temporary bursts above the rater depending on the bucket size (M). The implementation requires a simple up/down counter to reflect thecontents of the token bucket.

A number of variations of the basic LB are possible. Instead of discarding or marking cells when the tokenbucket is full, arriving cells can be allowed to queue in a data buffer (B ). The input or data buffer smoothsD

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 cellrate and allowing some margin for the CDV. Previous work [WILT94] has analyzed the CDV caused byvarious topologies in the access network and has given some typical values that could be used as default forthe 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. Thenthe dimensioning of the LB is very straightforward. However if only the mean burst duration is known thanthe mean cell rate is not so easy to control. This is due to the fact that a long observation time is requiredbefore detecting any non- conforming cells. A large token buffer has therefore to be chosen in order tominimize the probability of dropping/marking cells which conform to the negotiated traffic parameters. Thishowever causes a long reaction time to increases in the mean cell rate whereas fast detection of violationsis required. This problem can be solved by introducing a data buffer (B ). In order to distribute the bufferD

to obtain the best performance, in terms of reaction time and queuing delay, the buffered LB was analyzedin [ORS95].

3.4 Reactive Congestion Control

Although preventive control tries to prevent congestion before it actually occurs the satellite system mayexperience congestion due to multiplexing buffer or switch output buffer overflow. In this case, where thenetwork relies only on the UPC and no feedback information is exchanged between the network and thesource, 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 networkrequires them to do so. Likewise, they may wish to increase their sending rate if there is extra bandwidthavailable within the network. This kind of applications are supported by the ABR service class. Thebandwidth 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 conveyedto the source through special control cells called Resource Management (RM) cells [ATMF94]. Rate

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infromation can be conveyed back to the source in two forms:

• Binary Congestion Notfication (BCN) using a single bit for marking thecongested and not congested states.• Explicit Rate (ER) indication, with which the network notifies the source of theexact 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 isparticularly appealing for a satellite-based network because of it’s inherent broadcasting capability, it maytake several round trips before the source will adjust to the right rate. This is unaccaptable for satellite linkswith long propagation delays. A better strategy is for the on-board switch to send RM information to thestations which send RM cells to the source containing the rate it should change to. Various algorithms havebeen proposed for the calculation of the fair bandwidth share per connection [JAIN95]. When the satelliteis sending RM information to one station, all the active ground stations covered by the same beam canobtain the same information. The source model used for the ABR service is the persistent source whichalways sends with the maximum permitted rate. Different simulations [BARN95] have shown that this modelimposes the heaviest constraints on the network and is therefore very appropriate for testing the throughputand cell loss of the ABR service.

The switch can determine congestion either by measuring the traffic arrival rate or by monitoring the bufferstatus. We will use the former method where the on-board switch measures the current load by counting thenumber of cells received during a fixed averaging interval. Based on the known capacity of the link, theswitch 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 thevoice source are PCR=32kbit/s, mean burst duration=0.352 ms, mean silence duration=0.65 ms. Thesimulation results using the on-off and b-d model are compared to theoretical results (1) showing that theN-state b-d model can be used instaed of N identical on-off sources obtaining obtaining a very similar cellrate distribution (see Figure-6). The only problem with the b-d model is to decide how long the processshould stay in a certain state. The logical solution would be to generate i exponential random numbers withmean a and b and chose the smaller number to determine the duration of state i. As the number of states-1 -1

increases for large N, the simulation time spend in generating random numbers also increaes. We thuslimited 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 relativefrequency histogram where Pb (Number of cells generated in state i / Total number of generated cells) isi

a function of i / N .

As Pb is proportional to the CLR for i > C +1, C has to be chosen so that Pb is very small. Figure-7 showsi i

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. Thusfor 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 notingthat the small modification resulted in more accurate results which were verified by simulations. Figure-8shows the normalized capacity (C / N ) for a certain CLR if 800 (maximum number of voice sources whichcan be supported by one UG with 16 carriers) or 400 voice sources are multiplexed. It can be seen that forN=800 an effective bandwidth of 0.4375 32 kbit/s=14 kbit/s is sufficient for a CLR of 1.45 10 . This is-9

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/orABR 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 6Kbit/s.

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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 thebinomial model slightly overestimates the cell loss, assigning too much bandwidth to a source. The MCRfor each ABR source was assumed zero so that the bandwidth assigned to each source was the same. Furthersimulations are required to obtain the CLR of ABR traffic on-board the satellite under different loadconditions.

5. CONCLUSIONS

As user demand is becoming more complex, VSAT satellite networks, which have so far been successfully used inproviding 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 VSATsare interconnected in a mesh configuration. In our proposal maximum utilization of the burst slots using complexDAMA schemes, which introduce high delay, is not required. Maximum bandwidth utilization of the satellite is achievedby statistical multiplexing the traffic on-board the satellite. Greater connectivity, improved spectral efficiency, improveduse of the switch capacity and greater e.i.r.p. per VSAT terminal are the main advantages of using a state-of-the-artsatellite 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 andcongestion 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 veryimportant to provide the required quality of service.

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Figure-1 VSAT Earth Station Functions

Figure-2 UPC to control mean rate

Figure-3 Frame Unit Format

Figure-4 On-Off Source State Model

Figure-5 Birth-Death Process Model

Figure-6 Cell rate dist. for 50 voice sources

Figure-7 Cell rate dist. for different N

Figure-8 CLR as function of C/N

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REFERENCES

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