satellite rural communications: telephony and narrowband networks

INTERNATIONAL JOURNAL OF SATELLITE COMMUNICATIONS AND NETWORKINGInt. J. Satell. Commun. Network. 2005; 23:307–321Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/sat.819

Satellite rural communications: telephony andnarrowband networks

Roberto Conten,y

Centro de Investigacion Cientıfica y de Educacion Superior de Ensenada, Departmento de Electronica y

Telecomunicaciones, Km 107 Carretera Tijuana-Ensenada, Ensenada, BC 22860, Mexico

SUMMARY

Rural communications are important for large and developing countries, and telecommunications systemshave been implemented depending upon the available technology at the time. Rural users do not generatethe same amount of revenue as urban users do, thus lowering incentives for rural telecommunicationsinvestment with service to those regions delayed as long as possible. Voice and data communications areessential to the economic development of a region, and it has been shown that traffic increases rapidly assoon as the service is available. Satellite-based digital networks provide efficient long-distance service torural communities at lower cost than similar land-based wired networks with acceptable quality. Smallearth stations along with Wireless Local Loops can provide both local and long-distance service efficientlyand at low cost, offering digital multimedia services on a global scale. This paper focuses on the descriptionof different narrowband technologies used to service rural communities, namely basic telephone and low-bit-rate data (564 kbps) applications through the use of satellite and terrestrial wireless systems. A basicnetwork economic planning description is presented, and important parameters such as satellite networksize, topology and multiple access are identified in order to improve the process of effective and cost-efficient rural communications network design. Copyright # 2005 John Wiley & Sons, Ltd.

KEY WORDS: rural telephony; WLL; satellite network design and planning

INTRODUCTION

Local and long-distance rural communications

Remote and rural communities in large or developing countries have historically been left withpoor or non-existent communications due to a number of factors, although telecommunicationsservice has repeatedly been considered important for regional growth. Wireline networks areoften not an economic option due to its high initial investment and low financial returns,especially in small communities and isolated locations. Most current rural telecommunications

Received 28 June 2004Accepted 1 August 2005Copyright # 2005 John Wiley & Sons, Ltd.

yE-mail: [email protected]

Contract/grant sponsor: National Council for Science and Technology (CONACyT)

nCorrespondence to: Roberto Conte, Centro de Investigacion Cientıfica y de Educacion Superior de Ensenada,Departmento de Electronica y Telecomunicaciones, Km 107 Carretera Tijuana-Ensenada, Ensenada, BC 22860,Mexico.

networks exist due to government requirements for TELCO service providers to coverlow-density and small, remote locations, where maximum limits on tariffs are often imposed, sothe economic survival of most rural networks has been achieved mainly through governmentsubsidies or urban users’ fees [1].

Although analogue multiple-access radio was initially used in rural telephony applications,new digital wireless networks have become very attractive for several applications due to theirbetter service quality, increased capacity and cost-effective performance. Wireless local loop(WLL) systems have helped bring local telephone communications to remote locations, asinitially mentioned in Reference [2]. Since most wireline service operators may not serve remotelocations, WLLs can provide a wireless local ‘last-mile’ digital network, but it still requires along-distance access to the Public Switched Telephone Network (PSTN) or to the Internet. Thiscan be achieved through a digital satellite terminal, which has ubiquitous presence under thesatellite’s footprint. For this reason satellites are considered either as part of a hybrid (terrestrialWLL+satellite) system or as part of an integrated (cellular–satellite) system, as reported inReferences [3,4].

Rural communities without telecommunications service have two different problems: they canneither contact their neighbours (local calls) nor the outside world (long-distance calls). If arural village has at least one single long-distance telephone line placed at the local store,authority or health facility, then local people can at least communicate with remote relatives,authorities or other government offices. The calls may be for personal, emergency or officialgovernment messages, even if users have to walk to this single telephone line. Although manycountries force their national telecommunications companies to provide long-distance service tocommunities above certain size, these mandates are often ignored or delayed because ofeconomic factors. A rural village will hardly have local telephone service if they do not havelong-distance service already, which has higher priority, and data transmission may probably beunheard of. Local telephone service will usually be implemented in villages that reach a certainminimum size and economic conditions, and only after long-distance service has beenoperational and people are familiar with its use.

Communications network planning basics

A local telephone network not connected to the PSTN is called a private network. The localnetwork’s transmission media between the local telephone switch and the local user’s premises iscalled the last-mile technology, and it may consist of wired or wireless communication links.Local calls are handled by a local switch office, which connects calls from one villager to anotherrural subscriber according to the dialled number. If a long-distance number is dialled, the localrural office will switch the call to the long-distance switch or gateway, which will connect the callto the long-distance transmission system and carry the call to another gateway connected to thePSTN. Thus, the long-distance gateways are the most important elements of the telephonenetwork regarding rural telephony, and satellite systems can provide quality rural telephoneservice with a digital communications network. Special attention is drawn to the remote (rural)and gateway (urban) earth station elements as well as overall satellite network technology thatprovides long-distance telephone service using very small aperture terminal (VSAT) technologywhen following a cost-efficient design methodology.

There are many issues to be defined when designing rural telecommunication networks, all ofthem are of varying importance, but they all fall into three main areas: geopolitical, technical

Copyright # 2005 John Wiley & Sons, Ltd. Int. J. Satell. Commun. Network. 2005; 23:307–321

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and economic [5]. Geopolitical issues deal with the rural region’s development, based on itscultural, social, political, educational and economic history, which are important to define butwill not be covered in this work. Technical issues usually refer to available technologies andnetworking aspects, such as rural communities’ size and expected traffic, needed type ofcommunication services and desired applications, national regulatory framework and availabletechnical workforce. Economic issues deal with the definition of the required investment todeploy communications equipment in remote locations, its financing, competitive, operative andmaintenance costs, service pricing, expected revenue and user’s economic potential, so the ruraltelephone network is a healthy business, either self-supported financially or by keeping subsidiesto a minimum. The latter analysis must include both the capital and operational costs over time,the initial and future expected traffic and the performance metrics and technical standardsrequired by the national telecommunications regulators. It is in these two areas, technical andeconomic, where the present work is situated.

The design and specification of technical elements in narrowband communications networksfall almost entirely into the telecommunications engineering area, while the economic analysis ofthe network is mainly a business and financing problem. A common problem is that mostengineers are unaware of the economic planning part and most business people are unaware ofthe technical issues. The joint process for proper network design for future economic successis called communications network planning and it depends heavily on another engineeringbranch called systems engineering while supported by a technical, economic and financialdocument called the business plan.

Wireless local loops (WLL) for rural local communications

Currently, wireless technology offers different networking options for local loop telephoneapplications, not considering point-to-point or point-to-multipoint radiotelephones. Digitalwireless telephone technologies are also defined asWLL, based on their coverage area, technologyplatform, offered services and technical characteristics, and are called either cellular telephonesystems (CTS) or fixed wireless telephony (FWT). Since WLL work at ultra-high frequencies(UHF, 300–3000MHz) rain is not a problem, and for rural, scattered, semi-fixed users, multipathis not as big a problem as it is in urban applications [2,6]. The area coverage of WLL is usually afew kilometers per cell, depending upon the geography and topography of the terrain.

It is important to make here certain distinctions on similar concepts: both CTS and FWTnetworks may be based on either basic cellular or personal communications systems (PCS)technologies, but with specific differences based mainly on carrier frequency, traffic capacity anduser mobility aspects. PCS is a term often applied to basic CTS, although PCS and cellulartelephony are not exactly the same, as will be briefly explained next.

WIRELESS TELEPHONE TECHNOLOGIES

Basic CTS, also known as first-generation (1G, analogue) and second-generation (2G, digital)cellular systems, allowed only for voice and low-bit-rate (9600 bps) data services. 1G and 2Gcellular systems were assigned spectrum around the 400 or 800MHz frequency bands,depending upon the country and technology being used. On the other hand, PCS refers toadvanced second-generation (2.5G, narrowband) and third-generation (3G, broadband) digital


SATELLITE RURAL COMMUNICATIONS 309

systems, with frequency spectrum assigned around 1900MHz. This band allows broaderbandwidth channels to be used on higher bit-rate applications, thus making 2.5G (up to384 kbps) and 3G (up to 2Mbps) systems multimedia-capable, Internet-ready digital telephonenetworks, with user mobility advantages included. High-mobility (up to 100 km/h) users can beserved with both technologies, cellular and PCS, which might not necessarily be the case onrural applications. As far as the user is concerned, it makes no difference which frequency bandis being used, so the term ‘PCS’ has stuck as a single way to refer to all wireless telephony, andthe broader bandwidth PCS services are seen only by the common user as newer features of thesame old cellular system.

On the other hand, FWT networks are similarly based on cellular-like architectures, but itsusers usually use fixed radio telephone systems at basically the same frequencies as CTSnetworks, attached to their homes by a wireless link from the base station. This is an optionpopular on urban and suburban settings where it may be difficult or expensive to place new2-wire cabling, use existing posting, or where quick, inexpensive, basic telephone service isneeded. It is also an attractive option to use in emergency situations where the existinginfrastructure has been rendered unusable, either by natural disasters, sabotage or by civilian ormilitary conflicts. It is also a very efficient technology to be used on remote, rural settingswithout telephone service, where rural population might be dispersed over a large area with lowsubscriber density. Due to its fixed user location nature, the wireless coverage area may be largerthan that of cellular or PCS systems, since the base station may use higher gain directionalantennas in order to reach known user sites (homes, barns, public facilities, etc). Low usermobility allows for higher bit-rate applications to be offered on FWT networks, and multipathis usually not a problem.

As it may be assumed, each WLL technology has its own specific characteristics regardingtraffic capacity and system performance depending upon its network size, desired services andapplications such as e-mail and the Internet as well as number of subscribers, as will be brieflydescribed next.

Cellular and PCS WLL

A WLL is basically a cellular or PCS system that provides a wireless connection from the user’sterminal (portable, mobile or semi-fixed) to the PSTN through a radio channel. The three mostimportant 2G digital cellular standards are known as GSM, CDMA and TDMA. The first,global system for mobile or GSM, is a European standard which is currently the most widelyused digital cellular standard in the world, also used as a PCS standard on its DCS 1800 version.The second standard, CDMA/IS-95, uses code division multiple access (CDMA) and is an U.S.standard introduced by the company Qualcomm. The third standard, USDC/IS-136 or TDMA,also an U.S. standard, uses time division multiple access (TDMA) and is the digital evolution ofthe first-generation AMPS analogue system. The main technical parameters of the 2G WLLsystems mentioned above are presented in Table I. Since third-generation (3G) systems areaimed at broadband communications, they will not be mentioned here.

Either mobile or fixed WLL technology could provide adequate local telephone service in aremote rural application for a private network, although the technical and economicimplications between standards vary a great deal. In order to connect the rural WLL privatenetwork to the PSTN gateway a wired or wireless long-distance link to the PSTN central office(CO) switch is needed for remote site applications. Satellite systems have major advantages over


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all the other media when away from a long-distance access point, with the long propagationtime delay as their only, although serious, disadvantage. A brief description of rural satellitenetworking is explained next.

SATELLITE COMMUNICATIONS NETWORKING

Since the early 1960s the improvement of satellite communications has been impressive in termsof spacecraft technology and traffic capacity as well as in network performance. Earth stationtechnology has also greatly improved in this time, both in size and efficiency. Mostcommunications satellites have been placed in geostationary earth orbit (GEO) positionsaround the earth’s equator, and recent global satellite systems are placing a large number of lowearth orbit (LEO) satellites in several inclined orbital planes in order to provide serviceanywhere in the world. This arrangement is called a satellite constellation and it presents severaladvantages over GEO satellites, mainly regarding full-earth coverage as well as lower time delayand propagation losses.

Any combination of satellites and earth stations may form a public or private satellitecommunications network. All communications networks interconnect a number of userterminals that provide or receive the same or similar type of information between all availablenodes (user terminals). The most important elements of a communications network are its size(number of nodes), location and functional capabilities of its nodes as well as the expected trafficbetween nodes. A satellite network provides service to multiple simultaneous users over largecoverage areas through the transmission of radio frequency (RF) carriers between nodes (earthstations) over the satellite. Proper network design must consider both the service applicationrequirements as well as the satellite’s features and limitations, and two of the most importantconsiderations on satellite network design are its topology and its multiple access.

A topology is the physical or logical configuration of the network regarding theinterconnection of its nodes, that is, the different routes the communications link can possiblytake between communicating nodes. A network may be connected into a number of differenttopologies, depending upon its main application and technical requirements. The most commonsatellite topologies are star and meshed, since other topologies such as ring or bus architectureshave no real significant use. Still, the choice of star or mesh topologies for a satellite network hassignificant technical and economic implications in the network’s performance.

Multiple access is the way in which all interested users must get organized in order to haveaccess to the satellite, that is, the way the common resource (satellite bandwidth) is shared.

Table I. Second-generation cellular telephone systems used as WLL networks.

Cellular standard IS-54/136 GSM IS-95 DCS 1800

Multiple access TDMA/FDMA TDMA/FDMA CDMA/FDMA TDMA/FDMAFreq. bands Fwd 869–894MHz 935–960MHz 869–894MHz 1805–1880MHz

Rev 824–849MHz 890–915MHz 824–849MHz 1710–1785MHzModulation p/4 DQPSK GMSK BPSK/QPSK GMSKRF channel 30 kHz 200 kHz 1250 kHz 200 kHzCarriers per channel 3 8 Variable 8Channel bit rate 48.6 kbps 270.833 kbps 1.2288Mchip/s 270.833 kbps



When more than one signal is sent to the satellite, an organized way to accommodate eachcarrier is needed, so channels are divided and used according to a pre-defined multiple-accessscheme, which heavily influences the satellite network’s bandwidth efficiency and throughput.Multiple-access protocols are classified by Peyravi [5] into several types: fixed assignment,demand assignment, random access, hybrid random access and reservation, and adaptiveprotocols. Fixed assignment protocols are good on high traffic, small (few nodes) networks.Demand assignment protocols are better suited to low traffic, larger networks, such as satelliterural telephony. Random access protocols are mainly used to transmit low-to-medium trafficdata in packet networks, while hybrid random and reservation protocols work best at mediumtraffic data packet networks with occasional higher traffic by just one or a few of the nodes atany time. Finally, adaptive protocols allow completely random access for low traffic and changedynamically to reservation for higher traffic loads.

Similar to the WLL standards mentioned previously, fixed- and demand-assignment versionsof frequency division multiple access (FDMA), TDMA and CDMA are the most commonlyused multiple-access techniques in satellite communications for telephony applications.

Satellite systems used for rural telephony

Rural telephony by satellite has been widely studied and described since the birth of satellitecommunications, but in fact much of that work never became a reality until recent times forseveral reasons. VSAT manufacturing companies and telephone service providers are lookingfor more cost-efficient designs that may be attractive for all parts involved (manufacturers,operators and users).

Several satellite system technologies can be used in rural telephony, requiring a differentanalysis of their particular technical parameters. The satellite network’s technology heavilyinfluences the user terminal (small earth station) characteristics as well as its overall costs.Typical uses of small or personal satellite terminals are

* Serve remote areas without wireline or cellular service,* Replace telephone networks in disaster situations, or* As an auxiliary buffer when the wired or wireless communications capacity has been reached.

A remote user can access the PSTN via satellite in one of two ways: through a direct access tothe satellite or through an indirect link access: The direct access architecture allows the user totransmit from a mobile terminal directly to the satellite (integrated system, Figure 1). Theindirect access to the satellite (hybrid system, Figure 2) is made from a wired or WLL facility,through a small satellite terminal and a GEO satellite.

Typical integrated systems are satellite telephone systems such as Thuraya, Iridium orGlobalstar. Integrated systems may include both local and long-distance terminals on the samehand-held device, being able to operate at either cellular, PCS or satellite frequencies, dependingupon a set of previously defined priorities and local service availability through a mobiletelephone switching centre (MTSC). This includes the use of AMPS, GSM, TDMA and/orCDMA protocols for the local (terrestrial) segment as well as the proprietary satellite protocolsrequired in each case. Integrated systems are better suited to individual users rather thancommunity-based applications.

Typical examples of hybrid systems are those where a private branch exchange (PBX) or asmall central office is required for local area telephone and call switching. These systems require


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a small capacity, fixed earth station pointed towards a GEO satellite. Hybrid systems are bettersuited to provide service to clusters of small communities with dispersed users in each areathrough the use of WLL technology. The user may use a wireless phone for local calls, whilelong-distance traffic is routed through the satellite at the earth station location.

First rural satellite systems

The first telephone links via satellite were implemented during the 1960s for international long-distance links among very large (30m) earth stations in several countries, mainly to interconnecttheir national telephone networks, but rural telephony was not economically attractive at thatpoint. Rural satellite systems were eventually implemented later in satellite history in developingcountries such as Nigeria and Sudan through the support of the International Telecommunica-tion Satellite Organization (INTELSAT), which helped provide telephone service during the1970s. Most of the early work involved the use of the INTELSAT VISTA network, whichincluded the use of INTELSAT satellites and small earth terminals especially designed for ruraltelephony [7]. Although the VISTA network was technically good at the time and did fulfil itsgoal, it was an expensive and limited solution for most countries. Nevertheless, the old VISTAconcept has still been an important service at INTELSAT and rural telephone satellite systemshave been improving thanks to lower cost, better satellite and earth station technology [8].Countries such as the US (Alaska), Canada, Australia, Mexico, Brazil, India and Indonesia

House

Convenience Store

Church, School

Earth Station

PSTNGateway

PSTNMTSC

GEO/LEO Satellite

Figure 1. Direct satellite access through an integrated terrestrial/satellite system.



have since benefited from their domestic satellites for rural communications. Other countriessuch as South Africa, Chile and Peru have benefited from international (INTELSAT,Panamsat) or leased domestic and commercial satellite capacity for rural telephony. Thetechnology used in each case has been mainly through small earth stations with telephoneinterfaces that allow interconnection to the PSTN over a typical GEO satellite at either C or Kuband. Most rural satellite service is still subsidized at some point.

VSAT-based rural satellite systems

Very small aperture system (VSAT) satellite networks were originally developed during the early1980s for business data transmission between multiple remote sales offices or branches and itsmainframe computer or communications centre [9]. They have been heavily used for retailing,banking, financial and parts distribution in large data networks systems. A VSAT terminalusually includes a 0.6–2.4m dish antenna and a transceiver radio (outdoor unit}ODU) as wellas a set of baseband and IF subsystems (indoor units}IDU).

During the early 1990s a basic telephone interface and handset was included at the remoteterminals, along with a PSTN interface at the hub, in order to provide remote telephone access.Since they were designed for urban usage, VSATs initially required large amounts of electricalpower and some temperature-controlled and protected environment facilities. This usually

Base Station

House

Base Station

Church, School

Farm, Rural house

Convenience Store

Earth Station

PSTNGateway

PSTN

GEO Satellite

PBXVSAT

Figure 2. Indirect satellite access through a hybrid WLL+VSAT repeater system.


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implied an extra expense in some rural locations due to the need of large solar panels andbattery banks, as well as some kind of fixed shelter for harsh environments. In hybridarchitectures the VSAT is connected to a WLL base station through the local switchingexchange, so these requirements could be arranged for, but in most direct access systems,batteries were always the weakest part of the communications system.

Examples of VSAT-based rural telephony systems, some of them hybrid, were those offered byGillam (Belgium), Gilat (Israel/U.S.), and Titan, ViaSat, STM Wireless and Hughes NetworkSystems in the U.S. [10]. These systems provided long-distance telephone service through aVSAT terminal for remote access to the PSTN on a GEO satellite. The users could use either theVSAT-supplied telephone set, or connect its telephone interface to a common wireless phone orPBX, depending upon the service provider’s local network configuration. The satellite frequencyspectrum required for these systems might be that of any available C or Ku band transponderthat services the coverage area, but the local WLL frequency spectrum, cell size and distribution,as well as required radio power, must be defined and approved locally. Most systems may includesolar-cell-powered equipment for remote rural communities if needed.

Future broadband satellite systems

The Internet explosion has created a steadily growing global demand for broadband dataservices that cannot be met by current wireline terrestrial networks only. Most cellular and othercurrent terrestrial wireless systems do not have the capacity to provide wide area broadbandservices yet. Although new wireless technology, such as Wi-Fi (IEEE 802.11x) wireless LANs, orwireless metropolitan area networks (IEEE 802.16) are able to provide broadband services inlocal, urban and some rural communities, it is either still expensive (802.16), or it has limitedcoverage (802.11x). Satellite systems, on the other hand, have entered the wide area, broadbandcommunications arena through the use of direct to home/direct broadcast satellites (DTH/DBS), such as Hughes’s DirecWAY and DirectPC products. VSAT technology is also used forbroadband applications, mainly through specific systems offered by different companies such asGilat (Israel/U.S.), HNS, STM Wireless, ViaSat (U.S.), NEC (Japan) or Datel and Norsat(U.K.). Most of these VSAT-based systems already offer narrowband (64–386 kbps) satelliteconnection to the Internet through various hub gateways and operate mostly over a startopology at Ku band with antenna sizes between 0.8 and 2.4m. Thus, there is a naturalevolution to digital telephone service over broadband links, either through circuit (E1/DS1) orpacket (IP) switching networks. The most likely technology to provide packet-switchedtelephony, able to run through the Internet, is based on the voice-over-internet protocol (VoIP)application. Although still in its infancy, and currently facing serious economic and regulatoryhurdles, VoIP will probably be the telephony choice of the future, and its transmission oversatellite links will bring a new set of challenges and opportunities to satellite communicationsengineers.

Following upon the promise of mobile communication satellites with expanded bandwidthcapacity, a number of companies are developing broadband mobile satellite systems at bothLEO and GEO orbits in order to provide broadband multimedia-capable digital services at aglobal level. These networks will provide broadband integrated services digital network(B-ISDN) channel capacity and high-speed IP or ATM packet switching services to every cornerof the earth. The only part of the current RF spectrum with available global bandwidth for such



applications is Ka band (30/20GHz), so most broadband satellite systems are planning to usethat frequency band, although initial systems are also using Ku band frequencies.

TECHNICAL–ECONOMIC ISSUES FOR RURAL COMMUNICATIONS

The design of rural telecommunications networks involves two main areas: communicationsengineering and economic planning. Since both areas are important, the first part of the networkdesign process involves a joint collaboration between engineers and economists to try to definethe expected communications needs for a certain region. This will allow an intelligent choice oftechnology regarding system capacity and cost-effectiveness, in order to offer a technically andeconomically feasible solution. All components, systems and subsystems of the proposedcommunications network have an associated economic cost, and its continued operation willalso generate expenses throughout the system’s lifetime. On the other hand, the operation of allsystems involved will also generate revenue, allowing costs to be recovered due to network’susage during the system’s lifetime. Costs should be kept to a minimum while still providingquality service to the users, and revenue should be maximized while still charging a competitive,low usage cost, which then becomes an optimization problem.

The engineering of rural communication networks involves the gathering of geographicaldata, considering the location, number and size of all potential communities, as well as lookingfor available technologies that could provide such service. A very important issue is to estimate,within some limits, the amount of expected local and long-distance traffic for the separate nodes,and narrow it down to a cautiously reasonable figure. If there is interest or the need to providedifferent types of communications services (voice, data, video, fax, Internet access), this shouldalso be considered in the traffic and system capacity analyses, along with the proper networkinterconnection standards. At this point all cabled, fibre optic, radio and satellite options mustbe evaluated in order to define the best technology that allows interconnecting all remote sites tothe PSTN or to any other desired public or private digital communications network. Theprevious network design experience of the engineers involved at this stage of the project is veryimportant, in order to eliminate obviously inappropriate or inefficient technologies according tothe specifications of the situation. A network topology must also be defined after determiningthe total number of nodes in order to provide the most efficient access to the desired local andlong-distance networks and their expected quality of service (QoS).

Traffic intensity and grade of service

Traffic intensity is one of the most important parameters of telecommunications system design,and its dimensioning is crucial for both technical design (satellite capacity) and economic design(business case) of the rural satellite communications network.

The traffic figure will define the expected number of minutes per year that each earth stationwill be operating, which, multiplied by the number of earth stations in the network and per-minute cost, will determine the network’s total yearly revenue. A common way to determine theexpected traffic intensity A on a telecommunications network has traditionally been by assigningan expected m average number of call requests per unit time with an H average duration of thesession, where A ¼ mH: This analysis provides a traffic usage estimate initially used todimension the required circuit capacity in switched telephone networks, now extended to


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determine the number of required satellite communication circuits (either frequency carriers ortime slots), through the use of the statistical Erlang B formula, expressed in Equation (1):

PrðbÞ ¼

AC

C!PC

k¼0Ak

k!

¼ GoS ð1Þ

where Pr(b) is the circuit’s blocking probability, A is the traffic intensity per satellite channel orcircuit in Erlangs, and C is the number of available satellite channels or circuits.

When using a demand assignment system, a number of available channels C must exist inorder to provide a certain grade of service (GoS) to the user. GoS indicates the probability that anew request for a channel will be blocked due to all channels being busy at the peak traffic hourof the day. Most PSTNs generally allow from 0.5 to 5% blocking probability, or GoS, butusually aim for a 1% GoS, that is, one blocked call every 100 attempts at peak time.

A low GoS increases the number of available channels, thus providing not only a betterservice to the user, but also increasing overall network cost. On the other hand, a high GoSreduces the number of available channels and overall network cost, which is good for the serviceprovider but at the expense of service quality to the user. A balance is usually found bycompromising service quality and overall network cost, aiming at providing a low GoS at areasonable network cost. Freeman [9] mentions that a GoS of 0.01 (1%) is a typical value formost PSTNs. That value is acceptable in a typical wired local area loop since the channel cost israther low, but in satellite networks the transponder channels are more expensive. A GoSbetween 1 and 2% is considered as good quality on most satellite applications, meaning that onecall between 50 and 100 will be blocked or lost during the peak busy hour.

Satellite networks use the same traffic and blocking probability analysis to define the desirednumber of circuits (frequency carriers or time slots) needed to provide narrowbandcommunications service on demand to multiple earth stations. One difference between wiredtelephone local loops and radio and satellite networks is that, due to frequency spectrumlimitations, the radio network’s chosen topology and multiple access exert a strong influence inthe traffic intensity analysis parameters, with an important impact in overall network cost.

Grade of service (GoS) and quality of service (QoS)

Although both terms sound similar, GoS and QoS are not the same. GoS is the blockingprobability of a call during peak time, a quantifiable and measurable parameter that helpsindicate the availability of lines or channels on a circuit-switched network. On the other hand,QoS is a rather intangible concept, which really means how happy (or unhappy) a subscribermight be with the received service over a packet-switched network. A number of parametersrelated to either time, bits or data packets over digital communication networks can help tobetter describe the QoS concept: its latency (end-to-end delay), steadiness (jitter, variations ondata flow) and integrity (bit-error rate}BER, and packet loss rate}PLR), as well as its datatransport effectiveness (throughput). These parameters are critical when designing any packet-switched communications network and are very sensitive to satellite applications, requiringextreme care when designing the satellite network.



Multiple-access techniques

There are several techniques that allow channel access to multiple users on radio and satellitenetworks, either fixed (FA), random (RA) or by demand (DAMA). All these technologies havea profound impact on the satellite network’s technical performance and its associated cost.During the engineering and planning of satellite networks, the choice of multiple-accesstechnology must be carefully evaluated, not only on its technical merits but also on its economicimpact during the network’s operation. The most important aspect might be related to thenumber of available satellite circuits n compared to the total number of earth stations N in thenetwork. The point here is to properly define how n satellite circuits should be assigned to Nearth stations in an efficient and cost-effective manner, depending upon the chosen multiple-access technology [11].

Fixed access (FA) technologies such as FDMA, single channel per carrier (SCPC), or TDMA,do not need traffic analysis estimates since each earth station has been assigned a permanentindividual carrier frequency or time slot. In fixed multiple-access technologies n ¼ N; thereforefixed multiple-access technologies are typically used on high capacity, high-traffic applicationswhere the channel will be occupied most of the time.

Random access (RA) technologies such as ALOHA and slotted-ALOHA (S-ALOHA) aretypically used on highly interactive, short burst communications, where there is only oneavailable channel to be taken by all users based on different contention algorithms. Thesemultiple assignment technologies use a different methodology to define its access to the satellitechannel and, therefore, to define the resulting traffic capacity, but in all cases n ¼ 1; independentof the N network nodes.

As for demand assignment multiple-access (DAMA) networks, there are a number of navailable satellite channels, which are distributed among the N terminals based on demand.DAMA networks use the Erlang B equation to define the network’s traffic capacity since not allearth stations operate at the same time, which allows to define the number of n satellite circuitsavailable for simultaneous communication at any given time. Since n5N; only n circuits will beneeded to carry the network’s estimated long-distance traffic for a certain GoS. Thus, to find thecomplete network n-to-N relationship, each earth station ESi needs to estimate its individuallocal loop traffic AWLL(i) in order to define the number ni of wireless RF carriers for a specificGoS(i). On the other hand, the overall traffic A for the satellite network needs to be defined inorder to set the required N satellite circuits. The usage of the variables shown in the Erlang Bformula will be n for the number of satellite channels and/or WLL carriers (equivalent to C inEquation (1)), while A remains the same for traffic intensity in Erlangs.

Most rural satellite telephone communication networks use DAMA technology in order tooptimize resources, since rural communications usage and traffic behaviour places it on thissection, especially since subsidies are to be avoided while keeping the network financiallyprofitable.

Business plan

Once the technical design is accepted as achievable, a technical–economic analysis of the ruralcommunications network must clearly show its potential for business success. The work thatbusiness planners must do involves many economic aspects of network users in order to make apotentially profitable business case, otherwise the project will never take place. The first elementof data that must be estimated into the technical analysis is the expected rural communications


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traffic, which is the single most important factor for determining expected revenue and, thus,support the profitability of a network. There must be an evaluation of the potential market inorder to assess if the rural community has any interest or need for the communications service,as well as any other potential threat from competition or cultural barriers. Once again, theprevious design experience of all the people involved is crucial here, since there is no historicalcommunications usage information to compare against. A financial study is also needed toanalyse the rural community’s economic purchasing and spending capacity and determine ifeither the local (WLL) and/or long-distance (satellite) communications service are sustainable,especially if subsidies are to be avoided.

All these aspects are included into a single, very important document, needed to propose anycommunications network, which is known as the ‘Business plan’. There are three main areas thathelp define the viability of the project, and must be based on a set of formal technical,economical and user data evaluations, shown in Figure 3.

The elements shown in Figure 3 can be explained as follows.

(1) A technical feasibility analysis, which demonstrates that the proposed communicationsand networking technology is suitable, available and reliable for commercial usage. Allrequired link design, traffic capacity, network coverage, system performance, GoS orQos, required personnel and technical training, as well as any other (hardware, software,regulatory and operational) issues must be successfully defined here. This analysis isusually done by communications and systems engineers.

(2) A financial availability study, which shows the monetary issues to finance the project,from conception to operation (public or private investors, bank loans, venturecapitalists, etc.) A management plan must also be included here, in order to foreseethe system’s required administrative needs, operational expenses and planned growthduring a certain amount of operational time, usually between three and five years,sometimes up to ten years. That should include both the terrestrial and the spacesegment costs along with maintenance and other operational expenses. Assumingthat the estimated rural communications traffic per terminal (or node) generates acertain income by way of service charges, an estimated profit must be shown overtime for the company to subsist. There must be projections for bad- (break-even),

Business Plan

Technical Analysis

Financial Study

Market Research

Viable Concept

Financial Availability

Market Supportability

Technical Feasibility

Figure 3. Viability of a technical project and its resulting business plan.



expected- (profitable) and preferred- (highly profitable) business scenarios, which usuallydepend upon different expected traffic figures. This study is usually done by economists,business managers or financiers.

(3) A market supportability research, which shows the rural community’s current andexpected communications needs and requirements, along with its economic potential asservice consumers. Once the rural communications network has been financiallyapproved along with the engineers’ technical design, there must be a market analysis ofthe network’s impact on the rural community over its operational life. The marketanalysis must include present and potential market competition, describe their equivalentproducts, services and prices, and highlight its own competitive advantages over itscompetitors in terms of pricing, performance, quality or any other economic or technicalfeatures. This research is usually done by marketing firms and agencies, or by thecommunications company’s internal Marketing Department.

The three documents defined above help creating the telephone network’s business plan, andguarantee up to a point the network’s viability and profitability over time. Viability is thendefined as the total cost of the telephone network over its operational lifetime, which thenbecomes the cost charged to the network’s users over that same lifetime period. If the pricescharged are competitive and there is an estimated surplus of income over expenses, then thecommunications network will generate profits. All the above work must be done prior to anyequipment capital investment and should provide guidance for a financially healthy business. Itsounds easy to do, but involves an extremely careful planning and design process, preferably byexperienced professionals in each area.

An important aspect not to miss during the satellite rural communications network planningprocess is the fact that, over time, all three areas will change: technology quickly evolves, marketinterests change and financiers tend to migrate to newer or better business opportunities.Therefore, the impact of technology in user cost must be continuously evaluated, without everlosing sight of the dynamic market and technology evolutions. The case for broadbandcommunications is even more complex, since solid business models are still being developed andtechnology is ever changing and quickly evolving into larger bandwidth multimedia applications.

CONCLUSIONS

Rural telephony service has historically been ignored or under-served mainly due to economicreasons, based on an estimated high initial investment cost and low revenues due to dispersedand low-income potential users with low expected traffic. Most rural telephone networks arecurrently subsidized in some form. Hybrid rural telephone networks based on terrestrial wirelesslocal loop and satellite technologies allow for efficient and cost-effective rural telephone networkoperation when paired for local (WLL) and long-distance (satellite) narrowband digitaltelecommunications service. Several commercial products already provide such service, but notall cases can be treated with the same technology nor provide cost-effective solutions for afinancially healthy business. Besides choosing the proper technology for technical networkdesign, economic and financial studies must also be evaluated in order to support a profitablebusiness based on rural narrowband communications, aiming for the minimum possibleeconomic subsidy. A number of factors have an important effect in such analysis, mainly theproposed terrestrial and satellite technologies and standards, along with the chosen multiple


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access and topologies for specific network sizes and desired Grade of Service. Nevertheless, it ishighly emphasized that the expected local and long-distance telephone traffic and per-minutecosts will be the definitive factors for a rural telephone network’s implementation and operationas a healthy and profitable business. A successful design of rural telephone networks mustinclude a serious technical–economical analysis for each specific situation, where a cost-benefitevaluation of the above-mentioned parameters must be performed in order to provide cost-effective and quality telephone service to rural communities based on hybrid terrestrial-satellitetechnologies.

ACKNOWLEDGEMENTS

The author wishes to acknowledge Professors Tim Pratt and George Morgan, from Virginia PolytechnicInstitute and State University (U.S.A.), for their insight and helpful comments to the subject matterpresented in this paper. This work was partially supported by Mexico’s National Council for Science andTechnology (CONACyT).

REFERENCES

1. Hudson HE. When Telephones Reach the Village: The Role of Telecommunications in Rural Development. AblexPublishing Corp.: Norwood, 1984.

2. Cox DC. Wireless loops: what are they. International Journal of Wireless Information Networks 1996; 3(3):125–138.3. Schwartz RE. Wireless Communications in Developing Countries: Cellular and Satellite Systems. Artech House:

Boston, 1996.4. Del Re E. The GSM procedures in an integrated cellular/satellite system. IEEE Journal on Selected Areas in

Communications 1995; 13(2):421–430.5. Peyravi H. Medium access control protocols performance in satellite communications. IEEE Communications

Magazine 1999; 37(3):62–71.6. Anvekar D et al. Fixed cellular rural networks in developing countries: a performance evaluation. Proceedings of the

IEEE-ICPWC’96, International Conference on Personal and Wireless Communications, India, 1996; 33–38.7. Rosa J. Rural telecommunications via satellite. Telecommunications 1981; (November):75–81.8. Albuquerque J et al. VSAT networks in the INTELSAT system. International Journal of Satellite Communications

1993; 11:229–240.9. Freeman RL. Telecommunications Systems Engineering (3rd edn). Wiley: New York, 1996.10. COMSYS. The 2001 VSAT Report. www.comsys.co.uk, 2004.11. Maral G. VSAT Networks. Wiley: Chichester, 1995.

AUTHOR’S BIOGRAPHY

Roberto Conte received his BS degree in Electronics and CommunicationsEngineering in 1986 from Universidad Autonoma de Nuevo Leon, Monterrey,Mexico, MS degree in Microwave Telecommunications in 1988 from the Center forScientific Research and Higher Education of Ensenada (CICESE), in Ensenada,Mexico, and PhD in Electrical Engineering in 2000 from Virginia PolytechnicInstitute and State University, U.S.A. He has been with CICESE since 1988,working on digital microwave and satellite technology, where he is currently aresearcher with the Wireless Communications Group. His current research interest isthe economic and technical analysis of terrestrial and space-based wirelessbroadband networks, oriented towards cost-effective network design and planning.



satellite rural communications: telephony and narrowband networks

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