1 IntroductionThe telecommunications industry isconstantly developing to meet thechanging needs of the users.

Through new technologies telecom-munications service providers can:

- Expand the network coverage tonew areas

- Improve the quality of basic commu-nications services

- Reduce the costs of services to allowmore users

- Add new services to increase thevalue of telecommunications servi-ces to users.

An important part of the recent deve-lopment in telecommunications is theintroduction of VSAT systems.

Very small aperture terminal (VSAT)systems have developed rapidly overthe last years, and have been a majorpart of the recent development withinthe satellite communication industry.VSAT networks have been a successmainly because they address a topo-logy that appears to be ideally suitedto satellite communication - point-to-multipoint. Traditional terrestrial net-works always had trouble addressing

this requirement. Accompanied byvarious systems for mobile communi-cation, this development has madesatellites an attractive medium to alarge number of users based oncost/performance advantages.

VSAT networks are characterised by alarge population of small and inexpen-sive earth stations (VSATs) at the cus-tomer’s premises. They communicatethrough relatively small antennas witha central large earth station called thehub station (figure 1). The hub stationincludes a Network Management Sys-tem (NMS) which is responsible forthe monitoring and control of remoteVSATs. The communication with theterrestrial network is also via the hubnode.

The VSATs operate as part of a satel-lite network used for the distributionand/or exchange of data betweenusers. It is difficult to give a precisedefinition of a VSAT system becauseof the lack of standardisation. A VSATis usually defined as a terminal with anantenna with diameter 2.4 m or less,which is likely to provide digital servi-ces of 2 Mbps or less (1). Such servi-ces are data distribution, data network-ing, voice services and digitally com-pressed videoconferencing services.

2 Specifications

2.1 VSATIn order to get a more precise defini-tion of VSAT systems the EuropeanTelecommunication Standards Insti-tute (ETSI) has proposed the follo-wing specifications of the transmit andreceive terminals (2):

- Operating in the exclusive part ofthe Ku-band allocated to the FixedSatellite Services (FSS), 14.00 to14.25 GHz (earth-to-space), 12.50 to12.75 GHz (space-to-earth), and inthe shared parts of the Ku-band,allocated to the FSS and FS (FixedServices), 14.25 to 14.50 GHz (earth-to-space) and 10.70 to 11.70 GHz(space-to-earth)

- In these frequency bands linearpolarisation is normally used andthe system operates through satelli-tes with 3 degree spacing

- Designed for unattended operation

- Limited to reception and transmis-sion of baseband digital signals

- The information bitrate transmittedtowards the satellite shall be limitedto 2.048 Mbps

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Very Small Aperture Terminal (VSAT) systems - basic principles and designB Y L I V O D D R U N V O L L A N D G U N N K R I S T I N K L U N G S Ø Y R

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Figure 1 Example of a VSAT network in a Baking Environment

- Antenna diameter not exceeding 3.8m or equivalent corresponding aper-ture.

The equipment characterised compri-ses both the “outdoor unit” and the“indoor unit”. The outdoor unit is usu-ally composed of the antenna subsys-tem and the associated power ampli-fier and Low Noise Converter (LNC).The indoor unit is composed of theremaining part of the communicationchain, including the cable between theindoor and outdoor units.

This standard does not contain theVSAT network hub station.

2.2 Hub stationThe entire network is organised bythe hub station via the network mana-gement system (NMS). The operatorof the network management system isresponsible for the following essentialfunctions:

- Monitoring and controlling the net-work

- Configuring the network

- Troubleshooting the network

- Charging.

Communication between the NMSand network components is continu-ally maintained. The NMS regularlypolls the nodes of the network toobtain normal activity statistics, infor-mation about system failures and errorrecovery.

The VSAT systems present two kindsof topologies: star topology and meshtopology (figure 2).

The star topology is the traditionalVSAT network topology. The commu-nication links are between the hub andthe remote terminals. This topology iswell suited for data broadcasting ordata collection. The only way to com-municate between the remote termi-nals is via the hub station (doublehop). This makes it impossible to offerspeech services between the termi-nals, because the time delay in thedouble hop (500 ms) is too severe.

The connectivity on the space seg-ment is provided by digital carriers inboth directions, organised with vari-ous access schemes. The access tech-niques used in a star network can beboth FDMA (frequency division multi-ple access), TDMA (time division

multiple access), and CDMA (codedivision multiple access), but TDMAis the most common. The inboundchannel (remote VSAT to hub) oftenuse slotted Aloha which is a form ofRandom Access (RA).

In mesh topology there is direct com-munication between the remote VSATterminals. This minimises the timedelay which is critical concerningspeech services. The internal signal-ling network will have a star topology,because the signalling processor islocated in the central node, which isoften referred to as the DAMA(demand assignment multiple access).The access method used in a meshnetwork is typically Frequency Divi-sion Multiple Access (FDMA).

3 EvolutionSince their introduction VSATs of thiskind have followed an evolutionthrough which three distinct genera-tions can be identified. The first gene-ration of VSATs demonstrated, in thelate 70s and early 80s, the feasibility oftransmit and receive data communica-tion systems. The second generationintroduced the reduction of antennasize due to higher EIRP (effective iso-tropically radiated power) Ku-bandsatellite channels, and the advent ofbasic network management systems.VSATs of the third generation (deve-loped since 1987) are characterised bydesigns taking into account the needfor open and standard architectures.Many of these VSATs operate in swit-ched networks based on architecturescorresponding to the standards of thetelecommunications industry such asX.25.

The earliest VSAT systems had aSTAR topology and they started in theUSA during the late 70s, largely in pri-vate corporate networks containingthousands of sites. Some 85 per centof the world’s VSATs are located in theUSA, and about 90 per cent of USVSATs are in private dedicated hubnetworks operated for only one corpo-rate user (3). In Europe this privateVSAT solution will not be the rulesince regulation and other issues willdrive most customers to utilise a sha-red VSAT hub operated by a VSATService Provider. The average numberof VSATs per hub might, in the USA,approach 800, but internationally thenumber is closer to 100 sites per hub.

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Figure 2 VSAT system topologies: star topology and meshtopology. Mesh topology offers direct connecti-vity betwen the remote VSAT terminals

For some years also meshed VSATsystems have become available. Anapplication is for example telephoneservices in areas with insufficient ter-restrial networks. Circuits are design-ed on demand, allow-ing for an effici-ent use of the space capacity. Morerecently, high rate (typically 2 Mbps)VSAT services were introduced (forexample the NORSAT-B system).What these systems have in commonis that they require more powerfulVSAT stations and a high down-linkpower. Consequently only a smallnumber of carriers (channels) can besupported by a satellite transponderand transmission costs are correspon-dingly high.

The available access techniques allowfor inherent flexibility. TDMA (timedivision multiple access) gives thebest flexibility, but at the price of highearth station costs. When usingFDMA/SCPC (single carrier per chan-nel) flexibility is limited and earth sta-tion costs are lowered but remain stillon the high side. In other words, highrate meshed VSAT communicationsare currently handicapped by the needto operate powerful earth stations andby relatively high transmission costs.

4 VSAT systems in NorwayAs examples of VSAT systems two ofthe VSAT systems in Norway are de-scribed: NORSAT-B and NORSATPLUS. NORSAT-B, which is a wellestablished system, is given a fairlythorough description. NORSAT PLUS,which is a conventional VSAT system,is just briefly introduced.

4.1 NORSAT-BIn 1976 Norway became the first coun-try in Europe to use satellites in itsdomestic telecommunications net-work. This first system, NORSAT-A,was originally established to handlethe telecommunications traffic to theoil installations on the Norwegian con-tinental shelf and the Arctic islands ofSvalbard.

As the next step NORSAT-B was deve-loped in Norway by EB Nera in co-ope-ration with Norwegian Telecom. Thissatellite system became operative in1990. In addition to the areas servedby NORSAT-A, NORSAT-B was plan-ned to provide business communica-tion on the Norwegian mainland.Lately further expansion to a completeEuropean market has been considered.

4.1.1 Network configuration

NORSAT-B consists of one main earthstation (MS) and a network of userstations (US) sited at the users’ premi-ses. The main station is located at Eikearth station outside Stavanger. Allestablishment of connections, monito-ring and control of the network andthe transponder is done from this sta-tion. It also takes care of all charginginformation.

From a signalling point of view, it is astar network. That is, signalling be-tween two user stations will always govia the main station. Concerning traf-fic, it is a meshed network with directUS to US connectivity. That is, the traf-fic itself is conveyed directly betweenuser stations. In this way single-hoptraffic, which minimises time delay, isoffered.

Signalling information between themain station and the user stations isexchanged by using dedicated signal-ling channels. The main station iscontinuously broadcasting on a Broad-casting Signalling Channel, while theuser stations share the capacity of aCommon Signalling Channel.

Transponder capacity for user datatraffic is shared among user stations inFDMA (Frequency Division MultipleAccess).

NORSAT-B is designed for using a Ku-band transponder, that is 14 GHz up-link and 11/12 GHz down-link. Todaya transponder in INTELSAT VA (359° E)is used. This transponder covers Nor-thern and Central Europe (figure 3).Satellites from EUTELSAT and thenext generation of INTELSAT satelli-tes (INTELSAT VII, 1994) give betterEuropean coverage. Most likely NOR-SAT-B will be transferred to INTEL-SAT VII when this satellite is ready foruse.

4.1.2 Available bitrates and connection types

Today the available bitrates are N * 64kbps, where N is 1, 6, 12, or 32. Thatis, NORSAT-B offers digital connec-tions from 64 kbps to 2 Mbps. Thenext generation of NORSAT-B termi-nals will probably include N = 2, 3, 4,5, 10, 15, and 20 as well.

The type of connections possible inNORSAT-B are point-to-point, point-to-

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-3dB -5dB

Figure 3 Coverage of the Intelsat VA transponder used for NORSAT-B

multipoint and multipoint-to-multipoint(conference). Both duplex and sim-plex connections can be offered, andtransmit and receive bitrates can bechosen independently.

4.1.3 Establishment of connections

NORSAT-B allows connections to beestablished in three different ways:

- Fixed circuits: This corresponds toleasing fixed lines. The capacity inthe satellite is permanently reserv-ed, and cannot be used by others,irrespective of whether or not onechooses to transmit information allthe time.

- Prebooked circuits: Some time inadvance the customer makes anorder for a circuit to be set up be-tween specified user stations. Thisorder is fed into the main station,which then establishes the circuit atthe desired time.

- Switched circuits: The circuit is con-nected and disconnected on de-mand, at request from the user. Thenecessary information concerningthe circuit can be pre-stored in theuser station, or fed into it from amanual keyboard.

Prebooked and switched connectionswill compete for the same networkresources. If establishment of a pre-

booked connection is impossible dueto network congestion, an alarm willbe given.

4.1.4 Charging

For fixed connections you pay a fixedprice a year. This price depends onbandwidth used. For both switchedand prebooked connections costs arecharged only for the call duration.Price per minute depends on band-width used.

If specified, charging information isavailable on a per call basis. This willbe transmitted after disconnection.

4.1.5 User stations

Two standard types of user stationsare defined, referred to as Standard Aand Standard B. Any user stationwhich cannot be classified accordingto these standards is assigned to athird group called Standard S (speci-ally built stations).

The Standard A station uses an offsettype antenna of 3.3 metres diameter.The station offers all available bitrates:today they are 64 kbps, 384 kbps, 768kbps and 2.048 Mbps.

The Standard B station is the smallestone. The antenna diameter is 1.8metres. The station offers only 64kbps connections.

The Standard S stations are “customerbuilt”. They can be adapted to theneeds of the individual customer. Pos-sible variants may be stations withduplicated equipment to fulfil string-ent requirements on communicationsreliability, stations with non-standardsizes of antenna or transmitters, or sta-tions with additional channel units.

4.1.6 Applications

NORSAT-B was one of the first highspeed switched systems available.High speed connections (2 Mbps) areused for bulk data transfer. Examplesare transmission of pictures from theERS-1 satellite from Tromsø SatelliteStation to FFI at Kjeller and transmis-sion of environmental data from Finn-marksvidda to Kjeller (NORSAR).This represents large amounts of datato be transferred (figure 4).

2 Mbps connections are also used forremote printing of newspapers. In thisway the newspaper can be printedsimultaneously at several places, redu-cing both transportation costs and dis-tribution time.

Another application is video conferen-cing. The conferencing is not limitedto two parts only (max five parts). Thevideo conference market has beenexpected to grow rapidly for years, butthe growth is still slow.

NORSAT-B is also used as back-up forterrestrial circuits when a high degreeof reliability is imperative. This increa-ses reliability because NORSAT-B ter-minals are installed at the users’ pre-mises and the network is independentof other telecommunications networks(figure 5).

Since NORSAT-B offers single hoptraffic (transmission delay of 250 ms),speech transmission is of acceptablequality. In addition to fax and otherlower rate data transmissions, it cantherefore be used for ordinary telep-hone connections.

NORSAT-B’s advantage is its flexibi-lity. The user is allowed to set up avariety of different network topologiesranging from simplex point-to-point tofull duplex multipoint-to-multipoint.Bitrates can be chosen from 64 kbpsto 2 Mbps independently for transmit-ting and receiving. This can be utilisedin companies spread over a large geo-graphical area with need for a widerange of communication facilities. The

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Figure 4 Examples of NORSAT-B applications

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Figure 6 NORSAT PLUS Network

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Figure 5 NORSAT-B used as back-up for terrestrial circuits

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same system can be used for datatransmission of different bitrates,video conferences, distance education,telephone and fax services, etc.

Another example of integration of ser-vices can be in telemedicine, whereboth video conferences, distance edu-cation, transmission of data and pictu-res with different resolutions are need-ed.

NORSAT-B’s main drawbacks are thehigh price level and the limited cove-rage area (see figure 3). The coveragearea problem will probably be reducedwhen Intelsat VII comes into operationin late 1994.

The price level can be reduced whennew and cheaper user stations areavailable. Such stations are underdevelopment. A large part of the totalcosts is due to the satellite transpon-der, and can therefore be reduced ifthe number of users increases.

4.2 NORSAT PLUSNORSAT PLUS is a two-way VSATstar-type network which will be putinto operation autumn 1992. It consistsof a hub station, multiple remote sites,and a network management systemwith colour graphics user interfaces(figure 6). The hub is located at Nitte-dal Earth Station and is operated at a24 hours/day basis. The system offersdata transmissions up to 64 kbps andsupports IBM SNA and X.25 protocolsin standard configuration. The systemis manufactured by GTE Spacenet. Itis operating in Ku-band with 1.2 metreantennas and will make use of INTEL-SAT V space segment. NORSAT PLUSis dedicated to business data commu-nications and primary application isexpected to be typically transactionoriented database enquiry and selec-tive data broadcasting from a centraldatabase to groups of users.

4.2.1 Hub Station

The hub station resides at the centralnetwork facility to provide host con-nectivity into the network. Hub stationcomponents include:

- Hub Radio Frequency (RF) Equip-ment

- Satellite Access Controllers (SAC) -SACs provide logic for processingtransmission and receipt of data viasatellite

- Data Concentrators (DC) - DCs pro-vide device (host ports, modems,printers, etc.) connectivity in 8 portsbuilding blocks into the network.DCs operate protocols at user selec-table port speeds up to 64 kbps. DCssupport protocols, including IBMSNA (System Network Architec-ture) and X.25. With the unique“plug and play” architecture for pro-tocol support, additional protocolscan be easily implemented

- Network Management System(NMS) - the NMS provides com-plete control and monitoring facili-ties for network operation.

The hub station components are con-nected via a high-speed LAN.

4.2.2 Remote Sites

Each network location is equippedwith a VSAT where all communicationdevices are located indoors with theexception of the satellite dish and theoutdoor unit (ODU). Each VSAT maycontain up to two Universal ProtocolCards (UPC). VSAT UPCs supportone or more protocols (up to four),including SNA and X.25, allowing con-nectivity for a wide variety of devices.

4.2.3 Satellite Access Methods

NORSAT PLUS uses the AdaptiveAssignment Time Division MultipleAccess (AA/TDMA) method and thePermanent Assignment Time DivisionMultiple Access (PA/TDMA) methodfor inbound (remote to hub) transmis-sion, and continuous time divisionmultiplexed (TDM) for outbound (hubto remote) transmission. Bothinbound and outbound carriers offerdata transmission at bitrates up to 64kbps.

5 Future trendsDr Golding at Hughes Network Sys-tems has stated that future trends inVSAT networks will be driven by thefollowing goals (4):

- Lowering costs of the VSAT termi-nals, hub stations and installation ofthese networks

- Providing a greater range of service,including voice and compressedvideo services

- Providing networks that are moreuser friendly and flexible in terms ofoperations, administration and main-tenance

- Integration of these networks with alarger variety of Customer PremisesEquipment (CPE), and more advan-ced terrestrial networks includingfibre optic networks, newer swit-ching equipment and ISDN.

Today, integration of the VSAT net-works with the terrestrial commoncarrier network is via gateways gene-rally located at the hub station. In thefuture, the VSAT networks will beinterfacing with the ISDN terrestrialnetwork and multiple gateway interfa-ces may become more important withgreater use of full mesh network archi-tecture. It is important that in theselection of link and network layerprotocol standards for ISDN, satellitenetworks will be considered withrespect to unique properties of thesenetworks, such as time delay andbroadcast capabilities.

In addition to future trends in theVSAT ground networks one canexpect new technology to be impor-tant in the space segment area. Newcommunication satellites will incorpo-rate the following features, which willhave a significant impact on futureVSAT networks:

- Amplifiers with higher output power

- Use of spot beams and scanningbeams

- On-board processing

- Intersatellite links.

These features will permit highercapacity VSAT networks with lowercost earth stations and greater flexibi-lity. The use of intersatellite links mayprovide direct connectivity and inte-gration into other networks withoutrequiring terrestrial connections.Direct integration with mobile net-works may also be possible by thismethod.

VSAT systems have not grown asrapidly in the rest of the world as inthe USA. This is mainly because of theregulatory environments. In order tomake the situation in Western Europeapproach the situation in the USA, theEuropean Community intends toextend the applications of the gene-rally agreed principles of Communitytelecommunications policy to satellitecommunications. This will implicateliberalisation of earth segment and ter-minals.

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After the recent development in Eas-tern Europe this has been consideredto be a new and very promising mar-ket for VSAT systems because of thelack of terrestrial networks. ESA(European Space Agency) has studiedthe market opportunities for VSATnetworks in Eastern Europe. Theyconclude that the market for tradi-tional business VSAT (star topology)is limited, but for the so-called “uncon-ventional” VSAT systems (mesh net-works), which offer telephony, themarket appears to be very promising(5).

References

1 Pelton, J N. International VSATApplications and ISDN. IEEE Com-munications Magazine, 60-61, May1989.

2 ETSI. Satellite Earth Stations(SES); Transmit/receive VSATsused for data communications ope-rating in the FSS 11/12/14-GHz-bands. (DE/SES-2002), ETS 300159, February 1992.

3 Mesch, R G. Shared VSAT HubExpectations. VSAT ’91 Conference& European Satellite Users Show,Luxembourg, 5-7 November 1991.

4 Golding, L S. Future Trends inVSAT Networks. IEEE Communi-cations Magazine, 58-59, May 1989.

5 Pinglier, A. Marked opportunitiesfor VSAT networks in EasternEurope. In: Proceedings of The 8thEuropean Satellite Communicati-ons Conference, London, December1991, 129-139.

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