00904508

Broad-Band Satellite NetworksThe Global ITBridgeABBAS JAMALIPOUR, SENIOR MEMBER, IEEE

Invited Paper

Next-generation broad-band satellite networks are being devel-oped to carry bursty Internet and multimedia traffic in addition tothe traditional circuit-switched traffic (mainly voice) on a globalbasis. These satellites provide direct network access for personal ap-plications as well as interconnectivity to the terrestrial remote net-work segments. The main requirement in success of these networksis that they should be able to transmit high data rate traffic with pre-scribed quality of service (QoS). Thus, the broad-band satellite net-work has no choice other than the use of ATM technology and tobe optimized for Internet-based traffic. ATM is the promising tech-nology for supporting high-speed data transfer potentially suitablefor all varieties of private and public telecommunications networks.IP, on the other hand, is the fast-growing Internet layer protocol thatis applicable over any data link layer. Internet-based applicationsare the emerging source of traffic in the future wireless networksand broad-band satellite networks should consider Internet as theprimary service. In this paper, we will discuss the traditional ATMand wireless ATM networks and explain the characteristics of thewireless IP networks. The paper then uses those concepts in definingthe criteria for the broad-band satellite networks such as the QoSand traffic management. Application of the broad-band satellite net-works will be also proposed.

KeywordsBroad-band satellites, IP networks, mobile net-works, QoS, routing, teletraffic, wireless ATM.

I. INTRODUCTION

Broad-band satellite network is a relatively new conceptin the satellite communications family. The main goal ofthe broad-band satellite networks is to provide a ubiquitousmeans of communications for multimedia and high-data rateInternet-based applications. These networks can be consid-ered as the new generation of satellite networks for personaltelecommunication services. The most important contributorin this generation change was the change in service require-ments from the simple low bit rate and voice applicationstoward the Internet and multimedia services. Satellites on

Manuscript received March 2, 2000; revised September 2, 2000.The author is with the School of Electrical and Information Engi-

neering, The University of Sydney, Sydney NSW 2006, Australia (e-mail:[email protected]).

Publisher Item Identifier S 0018-9219(01)00454-6.

nongeostationary orbits (or mobile satellites), however, hada significant role among the proposal of the past generationof personal satellite networks.

The idea of establishment personal telecommunicationservices via satellites on nongeostationary constellations forcommercial purposes has been proposed in the early 1990s[1][8]. The proposal suggested that with satellites on lowearth orbit (LEO) or medium earth orbit (MEO), it is possibleto get rid of the highly restrictive long propagation delay andpower loss characteristics of the traditional geostationaryearth orbit (GEO) satellites. Long propagation delay hasalways been a strict parameter in establishing long-distancereal-time communications such as voice and video telephonyvia satellites. Long propagation loss, on the other hand, hasalways put a lower bound on the size of mobile earth terminalsdirectly connecting to satellites. This is mainly because ofthe requirement of large battery capacity for transmittingsignal on the uplinks. By using satellites on lower orbits it ispossible to reduce the transmission delay and the power ofthe transmitters so that satellite handheld terminals become areality. These small satellite mobile phones also could provideusers to have a unique and international network accessidentification number (NAIN) regardless of their location onthe globe and the availability of the terrestrial telecommunica-tions infrastructures. These characteristics were so importantand attractive for modern telecommunications era that severalsatellite systems of this type were proposed one after anotherin a short period of time [1], [3], [9]. Although the majority ofthese systems have been proposed by U.S. companies, theywere highly supported internationally soon after so that thefirst system of this type started its service in 1998. Because ofimportance of the voice communications during the develop-ment of these satellite networks, only voice, fax, and low bitrate data applications were considered, thus taking the title ofnarrow-band satellite networks.

Mobile satellite systems for commercial purposes were de-veloped in parallel with the development of the second gen-eration of the terrestrial cellular systems. Both systems look

00189219/01$10.00 2001 IEEE

88 PROCEEDINGS OF THE IEEE, VOL. 89, NO. 1, JANUARY 2001

Fig. 1. Hierarchical scenario of different mobile communication systems.

somehow to the same goal; that is, achieving the issue of ter-minal mobility in telecommunications services. Some of thesecond generation terrestrial cellular systems such as globalsystem for mobile communications (GSM), however, wentfurther to provide personal mobility in addition. The differ-entiation between these two comes from the way the movingobject is defined. In the first proposals of commercial mo-bile satellite systems, the main purpose was to provide basictelecommunication services such as voice, telemessage, andpaging regardless of the location of the user, specifically inremote areas. The user in such a system can buy a specificterminal and subscribe to specific service(s) available withinthat terminal and the subscribed network. In order to sub-scribe to other network services, however, the user needs tobuy a different terminal compatible with the new services.In GSM system, on the other hand, the mobility is givento a user regardless of his terminal, and the user is free topurchase and use any GSM-compatible terminal and sub-scribe to new services after inserting his personalized sub-scriber identity module (SIM) card in the new terminal. Inboth systems, however, the main idea of the mobility, thatis, the ability of accessing telecommunication services fromdifferent locations by the terminal and capacity of networkto identify and locate the terminal is kept.

Another difference in the two developing mobile systemsis in the range of mobility. Mobile satellite systems providethe mobility in a broader range compared to the terrestrialsystems. For example, if we consider the mobility in cov-erage area of a single base station (BS), it would be in theorder of a few kilometers in radius for a cellular system andseveral hundreds to a few thousands of kilometers for an LEOsatellite (based on the altitude of the satellite). Erection of aBS tower in a terrestrial system will also be limited to areaswhere the network service provider (NSP) expects to havesome manhood population, such as cities, towns, and majorroads. This limitation is completely removed in the case of amobile satellite system that covers anywhere on the globe in-cluding areas with no population. Therefore, the coverage ofa satellite system is based on geographical coverage and noton population coverage as in terrestrial cellular system and

could be global. In this sense, we may consider the relationof satellite mobile systems and different terrestrial wirelesssystems in a hierarchical order, as shown in Fig. 1.

The mobile object also needs to be defined clearly whencomparing terrestrial cellular systems and mobile satellitesystems. The mobile object in a terrestrial system is the sub-scriber terminal, usually called a mobile station (MS), witha linear speed in the order of zero to a few hundred kilome-ters per hour. In a mobile satellite system, the moving objector the mobile is the satellite with much higher speed; for ex-ample in an LEO satellite system with a typical satellite alti-tude of 1500 km, the speed of the mobile comes to around 7.1km/s or 25 200 km/h. The change in the mobility characteris-tics, both the moving object and speed, asks for complicatedmobility management issues in mobile satellites compared toterrestrial mobile systems.

The mobile satellite systems has the ability to establisha mobile telecommunications network with or without theirterrestrial counterpart. In the regions with no terrestrial wire-less infrastructure, because of either economical or technicalreasons, mobile satellites can provide almost full range oftelecommunications services. In the regions with developedwireless facilities, such as capital cities, the satellite can com-plement the service or assist the terrestrial network in hotspotteletraffic handling.

As the new generation of the satellite network for personalcommunications, broad-band satellite systems are proposedto complement the new generation wireless networks such asUniversal Mobile Telecommunications System (UMTS) andInternational Mobile Telecommunications (IMT-2000) [10].Although the new features considered in the third-generationwireless networks such as roaming1 and support of multi-media and Internet services have covered some of the advan-tages of the narrow-band mobile satellite networks, the newbroad-band satellite networks can provide global Internet ac-cess, which is inaccessible by any terrestrial-based wirelessnetwork. In this regard, broad-band satellite networks are

1Roaming is an internetwork service in which a user or terminal who issubscribed to a particular network can ask to use temporarily a differentnetwork with the same standard that is not his home network (HN).

JAMALIPOUR: BROAD-BAND SATELLITE NETWORKS 89

designed to provide services that are desired in the futuretelecommunication industry and thus have a better positionin success than their narrow-band predecessors.

In order to discuss the broad-band satellite networks, weneed to consider two important technologies involved; i.e.,the asynchronous transfer mode (ATM) and the Internet pro-tocol (IP). In Section II, we will explain the ATM networksoriginally developed for wired networks as the most signifi-cant contribution to B-ISDN (broad-band integrated servicesdigital network) and how the ATM has been involved in wire-less environment, namely the wireless ATM. Section III givesan overview on IP networks and how they also came intothe wireless world after the invention of mobile IP (MIP) in1996. In Section IV, we will first review the characteristicsof the satellite networks with emphasis on mobile satelliteconstellations and the ATM-based satellite networks. Afterthat, we will discuss specific issues related on the way of in-tegration of ATM and IP networks with wireless and partic-ularly with the satellite systems. The most important issuesnecessary to be considered are the quality of service (QoS)and traffic characteristics. These issues are important in thesense that if considered carefully and then sophisticated tech-niques are designed and implemented, it would be possible toprovide better services to the users and hence achieve highernumber of satellite mobile users. QoS has been vastly con-sidered for wired network but needs to be redefined whenmobile and wireless channels are being involved. Differenttypes of traffic and their management techniques are also re-quired when considering multimedia and broad-band appli-cations over the wireless link. Perspective applications forbroad-band satellite networks will be provided then. Finally,we will conclude our results and discussions in Section V.

II. ATM NETWORKS

In this section, we will briefly explain the concept ofATM-based networks and how asynchronous mode oftransfer provides extremely high data rates in broad-banddigital communication networks. We will then develop therelatively new topic of wireless ATM and discuss the newelements added to the traditional ATM protocol stack. Thewireless ATM will be then extended in the networks em-ploying satellites in order to let those satellites be practicablefor the quality transmission of multimedia and broad-bandtraffic.

A. Traditional ATM NetworksWith the introduction of modern digital and high-speed

telecommunications with relatively low bit error rates, therequirements of long overheads on the packets of the tradi-tional packet switching networks (PSNs) became unneces-sary. Since overheads contain no user information, reductionin the amount of overhead bits could result in more efficientutilization of the channel capacity and higher data rates thanwhat can be achieved in traditional PSNs. Frame relay net-works make use of this fact to increase the data rate from 64kb/s of the PSN to up to 2 Mb/s. ATM networks, on the otherhand, reduce the overheads further by employing fixed-size

Fig. 2. ATM cell structure.

packets, or cells, and increase the data rate to tens and hun-dreds of megabits per second. As an analogy to frame relay,the ATM service cab be referred to as cell relay.

ATM is a connection-oriented method that had the mostsignificant contribution in standardization of B-ISDN [11],[12]. In ATM, the user information is split into 53-bytefixed-sized cells, as shown in Fig. 2, and then switchedusing fast hardware-based cell switching. Cell header, a5-byte label, carries the minimum of overhead to supportmultiplexing and switching of the ATM cells. ATM leavesmost of the error detection and error correction and alsoout-of-sequence cell detection tasks to the higher layers ofthe network protocol stack, above the ATM layer and theATM adaptation layer (AAL). The asynchronous featureof the ATM may seem conflicting to the periodic nature ofthe existing traffic from analog sources, such as voice orvideo. However, the apparent periodicity is a property of thechannel coding process and not of the information sourcesthemselves. With powerful source coding mechanismsavailable now, it is possible to exploit the ability of ATM toabsorb the essential burstiness that characterizes the analogsources. ATM has the ability to multiplex and switch datafrom various sources with varying rates and informationstatistics.

In ATM, logical connections are referred to as virtualchannel connections (VCCs). A VCC is the basic unitof switching in B-ISDN and is set up between end userpairs through the network. A variable-rate, full-duplex flowof ATM cells is exchanged over the connections. Theseconnections are also used for control signaling betweenuser and network and for network management and routingbetween one network and another. All VCCs with the sameendpoints are bundled in a virtual path connection (VPC)and switched along same route. By grouping connectionssharing common paths through the network into a singleunit, it is possible to control the cost of high-speed networkssignificantly. Relation between the above connections isshown in Fig. 3. Virtual path level and virtual channel levelform two sublayers of the ATM layer.

The cell header consists of 5 bytes. The format of the ATMcell header for the user-network interface (UNI) is shown inFig. 4. For the network-network interface (NNI), there is nogeneric flow control (GFC) field and the virtual path iden-tifier (VPI) field fills the whole first byte of the header. An

-bit label will support separate channels in the aggregatecell stream, and as we will see in Section IV, it is completelysufficient to support ISL routing in mobile ATM satellite sys-tems.

ATM is intended to transfer many different types of trafficsimultaneously, including real-time flows such as voice,video, and bursty TCP (transmission control protocol) flowsof the Internet. Traffic management techniques have beendeveloped for ATM in order to handle these different types oftraffic in an efficient manner based on the characteristics of


Fig. 3. Different connections in ATM networks.

Fig. 4. Cell header format in ATM networks (for user-networkinterface).

the traffic flow and the requirements of the applications. Allthese issues are important in development and operability ofa network, wireless or wired, designed to handle multimediatraffic and broad-band applications.

B. Wireless ATM NetworksATM indeed can be considered as the main standard

technology for broad-band communications in wireline in-frastructure. Recent development in wireless networks thatsupports the mobility of users and the strong requirement ofsupporting multimedia and specifically the Internet-basedapplications have opened new researches toward the in-tegration of ATM with the wireless, namely the wirelessATM (WATM). ATM has the advantages of high efficiencyand QoS support for users, and if integrates with wirelessnetworks, can provide mobility-supported high-efficiencymultimedia services.

WATM can be considered as an extension of a wiredbackbone network with the flexibility of wireless accessand mobility support [13][17]. The standardization ofthe WATM has been started within the ATM Forum andEuropean Telecommunications Standards Institute (ETSI)with contributions from other standardization institutes suchas the Internet Engineering Task Force (IETF). The firstdraft specification related to the WATM has been releasedin December 1998 [13].

The WATM network has the traditional wired ATM net-work as its backbone. Therefore, we may consider a WATMnetwork as a modified version of a wired ATM networkwith new wireless links and equipment. To include mobility,the traditional ATM switches are now complemented withmobility-supporting ATM switches connecting through en-hanced public/private network node interface (PNNI). Thesenew switches are connecting the wireless access points(APs) or BSs to the wired network. Mobile terminals (MTs),e.g., laptop or palmtop computers and mobile phones, areconnecting via new wireless UNI using radio channels.Connectivity between mobile and fixed hosts in the networkthus will be through wireless UNI, mobility supporting ATM

switches, traditional ATM switches, and traditional wiredUNI. The BS in this configuration is sometimes called theradio access unit (RAU), which contains all the link layerfunctional elements, including radio resource managementand medium access control functions, necessary to operateover a shared radio frequency (RF) medium. Fig. 5 shows ageneric configuration of such a WATM network.

Protocol architecture of the WATM also needs modifica-tion for mobility support [14]. Fig. 6 shows such a modifiedprotocol in which new mobility-related layers are shown ingray color. As seen in the figure, both user plane and controlplane should be modified to support the mobility in the net-work. Much attention should be given to inclusion of properMAC and wireless control protocols. More details on this ar-chitecture can be found in [13] and [14].

In the case of mobile networks, including WATM, the mainmobility functions are location management and handovermanagement. Since in wireless networks users have not com-mitted to be in any specific location, there is the requirementof finding actual location of the MT from time to time andalso specifying its nearest point of attachment to the wirednetwork. This issue will be required during the process ofrouting the information packets from an MT to another MTor to a fixed host and vice versa. An efficient, reliable andquick handover technique is also necessary in order to main-tain and reroute an ongoing session while the MT movesfrom the coverage area of a BS to the next one. The loca-tion management and handover management are referred toas mobility management in mobile networks.

III. IP NETWORKS

In this section, we will overview the traditional IP net-works in order to briefly explain the new concept of wirelessIP networks. Mobile IP [18], for example, is a solution inproviding macromobility in wireless IP networks, and IEEE802.11 wireless LAN [19] and Bluetooth [20] are providingwireless connectivity within the boundaries of smaller net-works. These concepts, though originally developed basedon terrestrial wireless infrastructure, could have no logicalobjection to be integrated in broad-band satellite networks.This integration will be discussed shortly in Section IV.

A. Conventional IP NetworksInternet can be defined as a connection of nodes on a

global network use a DARPA-defined (Defense ResearchProjects Agency) Internet address. The protocol suite thatconsists of a large collection of protocols that have beenissued as Internet standards, is referred to as transmissioncontrol protocol/Internet protocol (TCP/IP) [21]. TCP/IPwas a result of protocol research and development conductedon the experimental packet-switched network ARPANETfunded by DARPA. In contrast to the open system intercon-nection (OSI) reference model, which was developed bythe International Organization for Standardization (ISO),TCP/IP has no official protocol model, but can be organizedinto five layers of application, transport, Internet, networkaccess, and physical. The network access layer can further


Fig. 5. Wireless ATM network configuration.

Fig. 6. WATM protocol architecture.

be divided into two sublayers called logical link control(LLC) and medium access control (MAC). Information dataprocessed in each application on a host computer should gothrough all these layers until it can be transmitted throughthe physical media on a local area network (LAN) andthrough intermediate routing and switching facilities onthe wide area networks (WANs) and the Internet. Fig. 7illustrates the connections and the required protocol stack ina simple TCP/IP-based network.

Two main components of the Internet, which are shown inFig. 7, are the hosts and the routers. Hosts include any typeof computer such as PCs and workstations. Routers forwarddatagram packets between hosts and other routers when thereis no same link (e.g., a bus) connecting them. A router oper-ates at the network layer of the OSI model to route packetsbetween potentially different networks. Another componentthat could be considered here is a bridge, which operates atdata link layer and acts as a relay of frames between similarnetworks.

In order the routers perform their task, they use specialprocedures called routing protocols. Routing tables are builtusing these procedures and then a router can select a pathfor any given packet from a source host to a destination host.In the case of several routers between a source and a desti-nation, routing will be performed on a hop-by-hop basis, inwhich each router finds the next node (router) for sending a

given packet until the packet is being reached at its requesteddestination.

IP is the most widely used internetworking protocol at theInternet layer. An IP datagram includes a header and the pay-load. Payload of the IP packet contains all the higher layerheaders such as TCP in addition to the application layer data.The header for the IPv4 (i.e., the currently deployed versionof IP) contains 20 bytes in addition to a variable size op-tions field requested by the sending host, as shown in Fig. 8.

The most important parts of the header are the sourceaddress and the destination address. These are 32-b IPaddresses, as shown in Fig. 9, assigned to each networkinterface of a node. A node with multiple interfaces, suchas routers, then has more than one IP address. Each IPaddress has a network-prefix portion and a host portion. Anetwork-prefix is identical for all nodes attached to the samelink whereas the host portion is unique for each node on thesame link. In the next-generation IP, i.e., IPv6, address fieldsare extended into 128 bits, which increases available numberof hosts in the network. Moreover, in IPv6 options areplaced in separate optional headers that are located betweenthe IPv6 header and the transport-layer header. This willspeed up router processing of datagrams. In addition, otherenhancements such as address autoconfiguration, increasedaddressing flexibility for scalable multicast routing, andresource allocation that allows labeling of packets belong toa particular traffic flow for special handling, are included inthe new version of IP.

The most important task to be performed by the IP layeris routing. Whenever a packet is received by a node, a host,or a router, for which the node is not its final destination(i.e., having different destination IP address as the receivingnode), the node must find where the packet should be routein order to be closer to its final destination. Therefore, in theprocess of routing a packet, a forwarding decision must bemade by each node. This decision can be made using an IProuting table, which is maintained in each node.

Each row of the routing table usually has four components,namely, target, prefix length, next-hop, and interface. When-ever a node has a packet to forward, it checks for matchingbetween the packets IP destination address field and theleft-most prefix-length bits of the target field within the rows


Fig. 7. Internet connectivity through TCP/IP.

Fig. 8. IPv4 header format.

Fig. 9. IP address structure.

of the table. If such a match is found, the packet will be for-warded to the node identified by the next-hop field via thelink specified in the interface field in that row. In the caseof more than one matching, the packet will be forwarded tothe one that has the largest prefix length. This will ensurethat the next node is the closest node to the final destination.An entry in the routing table might be a host-specific route,with the prefix length of 32, which can match with only oneIP destination address; a network-prefix route, with a prefixlength between 1 and 31 bits, which match all destination IPaddresses with the same network-prefix; or a default route,with a prefix length of zero. This last route will match all IPaddresses but will be used only when no other matching isfound.

The routing tables might be created statically (manually)or dynamically. Usually, these routing tables are producedusing one of common shortest-path or least-cost algorithmssuch as Dijkstra or BellmanFord algorithms [21], widelyused in other packet-switched networks. Because the In-ternet routing is based upon the network-prefix portion ofthe packet destination address, it is greatly improves thescalability of the Internet.

B. Wireless IP NetworksWireless IP networks provide access to the Internet when

the user (or the terminal) is on the move and does not nec-essarily have a fixed point of attachment. A good example

to understand the concept of wireless IP networks is the mo-bile IP (MIP). MIP is an extension to the currently deployed(fixed) Internet protocol in order to provide wireless accessto the Internet users [18], [22][24]. MIP is described in arequest for comments (RFC) published by IETF first in Oc-tober 1996 [18]. The most important barrier in developingmobile internetworking is the way IP operates. ConventionalIP supports interconnection of multiple networking technolo-gies into a single logical internetwork and is the most widelyused internetworking protocol. An IP address is used to iden-tify a host and contains information used to route the packets.

Generally, in a mobile Internet environment the two lowerlayers, i.e., the physical and data link, are provided by cel-lular networks. However, the next upper layer protocols, i.e.,network and transport layers, should be modified in order toenable them to route and deliver a packet correctly to a mo-bile user. As explained in the previous section, an IP addressis assigned uniquely to each host in the network and is usedby the network layer to route the datagrams. The concept ofnetwork-prefix as a part of IP address, however, is contradic-tory with the mobility issue. This is because of the fact thatin the case of movement of a terminal in the network, it is notpossible to maintain a single point of attachment for the ter-minal to the network; i.e., no logical network-prefix wouldbe available. Thus, any solution for supporting mobility inthe Internet is constrained by the requirement of the existingIP function and networking applications. As a mobile user is


Fig. 10. Packet flow in a mobile IP network.

Fig. 11. IP-within-IP encapsulation method in MIP tunneling.

roaming between foreign networks, it will acquire a new IPaddress causing the established connection of the node to belost. In addition, any extension of IP to mobile network hasto take the compatibility of the network requirement to thewired Internet into account.

MIP provides a means of delivering packets addressed tothe mobile node. By defining special entities, home agents(HAs) and foreign agents (FAs), a mobile node (MN) is ableto cooperate in moving without changing its IP address. In-versely, it provides a means for MIP to deliver packets ad-dressed to a particular MN in the network. Furthermore, thesolution can be appropriately expanded to accommodate anincreasing population of mobile users (i.e., supporting thescalability).

In MIP, each mobile node is given a virtual home network.This remains unchanged and is used to assign the mobilenode a constant IP address in the same manner that a standardIP address is given to a stationary host. On the home network,a location information database is maintained for each of itsattached MNs (which are currently visiting other networks).The accuracy of this information become vital when routersare to deliver any MN-addressed datagrams.

The core operations involved in MIP include agent dis-covery, registration, and packets tunneling. This is exactly

what mobility management is defined to be; i.e., to detectMNs change of location, register the new location with HA(either directly or via FA), and finally to perform handoveras MN moves to a new network.

Upon detection of a change in location, the roaming MNacquires a new IP address, a care-of address (CoA), eitherfrom the received foreign agent advertisement (FACoA)or from an external dynamic host configuration protocol(DHCP) server, or a co-located CoA (CCoA). MN thennotifies HA of the new location through the process ofregistration. Fig. 10 shows how mobile IP works.

Data packets from a corresponding node (CN) are gener-ally routed by default to the MNs home address. HA attractspackets destined for nodes that are away from their homenetwork and redelivers them according to the correspondingCoAs being registered by each roaming node.

When the registration with the HA is completed, the mo-bility management protocols should secure a way for packetsto be routed to the current point of attachment, namely thetunneling. The method used to forward data to the roamingMN is known as encapsulation. Though MIP assumes anIP-within-IP encapsulation methodology, shown in Fig. 11,other encapsulation mechanisms are applicable upon agree-ment made between relevant networks.


Table 1Advantages of the Mobile IP Protocol

In general, MIP provides a good framework for handlingusers mobility in a way that was never possible within theconventional IP networks. There are many benefits associ-ating with this particular mobility management technique.Table 1 briefly summaries some of those characteristics.

Despite the advantages of the MIP in providing a mobilecomputing environment, there are a few concerns about itsefficiency. Basically, the inefficiencies in MIP can be clas-sified into three main categories according to each step ofthe mobility management process, say location, routing, andhandover management. Regarding the location management,a serious inefficiency is widely evident because a registra-tion process with HA is required at every handover whenchanging either the network or the link within the same net-work. This results in wasting resources that are associatedwith the frequent location updates arising from every singleMNs movement.

One of the biggest concern in routing management for MIPis the inefficiency associated with the way packets are deliv-ered to roaming MNs, namely the triangle routing; an asym-metric routing with respect to topology. Specific concernsin this aspect include packet losses during handovers, highdata latency, and inefficient use of the network resources dueto tunneling. Route optimization techniques are being de-veloped to cope with this issue. The handover managementalso would be necessary to develop in order to control largenumber of handovers by MNs as the size of cells in cellularsystem becomes smaller.

MIP, which uses the terrestrial cellular infrastructure,could be a good start point for implementation of IP ser-vices over the broad-band satellite link. The inefficienciesdiscussed above, however, should be carefully considered inlong-delay satellite links.

IV. BROAD-BAND SATELLITE NETWORKS

Broad-band satellite networks are the new generation ofsatellite networks in which the Internet-based applicationsand services will be provided to users regardless of their de-gree of geographical mobility. The main difference from thetraditional satellite networks, thus, will be the support of highdata rates and broad-band services. As the Internet is themost rapidly spreading technology and many new applica-tions such as e-commerce find their way through the Internet,it is not surprising that the broad-band satellite networks takethe Internet-based applications as their primary service goals.Nevertheless, still voice and low bit rate applications are in-cluded in the list of network services.

In order to achieve the ultimate goals of the broad-bandsatellite networks, we need to consider the integration of theATM and IP technologies into a satellite link. This will be thesubject of this section. Although some of broad-band satellitesystem proposals consider the use of satellites in the geosta-tionary orbit, because of unavoidable long propagation delayin those systems and the importance of a delay constraint inIP applications, mobile satellites will be the most promisingcandidates. Therefore, in this section we will mainly focuson characteristics of mobile satellites.

Generally, integration of IP in ATM networks involvesconsideration of both service and performance issues. Thisconsideration becomes even more important when we applythe two protocols in a mobile and wireless environment. Inprinciple, QoS is the major issue supported in ATM net-works, and applicability of IP over any data link layer is themain characteristic of IP networks. Integration of these twoprotocols aims to take the advantages of both and to optimizethe integrated network. Another important topic that requiresmore investigation here is the different type of traffic to betransmitted over the network and traffic management poli-cies. Therefore, in this section we look over the issues of QoSand traffic management and then discuss the perspectives andapplications of the integration of IP and ATM in wireless andsatellite environments.

A. Narrow-Band Mobile Satellite NetworksMobile satellite networks refer to systems in which the

telecommunications satellites are on orbits other than thegeostationary orbit. According to the Keplers third law, thegeostationary orbit is a unique equatorial orbit at a distancearound 35 800 km from the earths surface [8]. A satelliteon the geostationary orbit can cover almost one third of theearth, and hence, three satellites would be sufficient to coveralmost all parts of the globe. This coverage excludes polarzones and relatively high latitudinal areas. Since the satellitesare stationary with regards to the movement of the earth, an-tenna tracking would be minimal and the earth gateways toterrestrial public switching telephone networks (PSTNs) canalways be faced to satellites for maximum signal reception.These type of satellites, then, can be easily used for long-dis-tance telecommunications and broadcasting purposes. Be-cause the length of the satellite link is independent of the ac-tual land distance of any given pair of the hosts on the earth,the long-distance communications cost will only depend onwhether or not a satellite link is used.

GEO satellite systems were successful in providingcommercial services, both in telecommunications and


broadcasting, since establishment of the first system, theINTELSAT, in 1965. The key characteristic of GEO satellitesystems was that they could be considered as a part of thefixed public switching network. In 1982, INMARSAT, an-other key pioneer in satellite systems especially for mobilepurposes, introduced its mobile satellite services (MSS)using GEO satellites in order to provide telecommunicationsservices to ships and other large mobile vehicles. This canbe considered as the start point of mobile communicationsvia satellites. However, there always was the problem ofthe round-trip distance between earth and a GEO satellitethat makes it impractical to have small-size terminals otherthan vehicle-mounted. This restrictive issue has becomemore visible when people started to think about personalcommunications services (PCS). It was clear that withGEO satellites it is difficult, if it not impossible, to providepersonal communications with small handheld terminalsand phones.

Requirements of lower propagation delay and propagationloss together with the coverage of high-latitude regions forpersonal communication services have started a vast researchon employment of satellites on lower orbits, which in naturewill have nongeostationary characteristics. Due to the exis-tence of the two Van Allen radiation belts, these mobile satel-lites were categorized into low earth orbit with an altitude of5002000 km and medium earth orbit at around 10 000-kmheight. Generally, the lower the orbit, the lower the propa-gation delay and loss and the higher the number of satellites(and the orbital planes) to cover the entire globe is resulted.Fig. 12 shows the relationship between the altitude of satel-lites and the number of satellites and the number of orbits,respectively [8]. Besides, the figure spots the actual constel-lation of some PCS nongeostationary satellite systems.

As discussed in Section I, the idea behind these mobilesatellite systems that could provide a single and worldwideaccess number was so attractive that in a short period of time,many of these systems have been proposed and found multi-national support in the 1990s [3]. Table 2 summarizes someof these (first-generation or narrow-band) PCS-based mobilesatellite systems. Most of these systems are proposed and de-signed to provide narrow-band services (fax, paging, low bitrate data, and in particular voice communications) ubiqui-tously due to the dominant role of the voice in the early 1990smobile communications.

Among the systems shown in Table 2, Iridium [9], thefirst completed LEO satellite PCS system, has a unique de-sign to achieve essential coverage with minimal requirementsof land-based gateways that connect to the PSTN. This isachieved by employing links between satellites, or intersatel-lite links (ISLs) working at 23 GHz, which enable the systemto route the traffic from one satellite to another, forming a net-work in the space. A general overview of the Iridium systemis shown in Fig. 13.

Each Iridium satellite has powerful onboard processingand routing facilities. Traffic arrived in a time division mul-tiple access (TDMA) timeslot, will be processed by the satel-lite and the routing decision will be made. The next desti-nation could be a ground gateway station via 20-GHz links

(a)

(b)Fig. 12. Required number of satellites (a) and orbital planes(b) versus the height of satellites.

or one of the four nearest satellites via ISLs. This type ofuser-satellite-gateway connectivity is shown in Fig. 14. TheIridium system employs circular polar orbits (86.5 inclina-tion), which guarantee the service coverage to high latituderegions. The footprint of each Iridium satellite is divided into48 cells via three -band antennas forming a total of 3168cells on the earth surfacea cellular-type satellite system.From those cells however, only 2150 cells would be enoughfor a global coverage.

The next Big-LEO satellite system is Globalstar. Thissystem does not claim offering a global coverage; instead itwill provide coverage to its partners in different countrieswith sufficient population. This fact, together with higheraltitude of the satellites, results in fewer satellites thanthe Iridium system. Since the satellite orbits have a 52inclination, little or no coverage is provided beyondlatitude. At most times, two or more Globalstar satellites willbe visible from the designated areas on the earth. Anotherdifference between the Iridium and the Globalstar is that thelatter does not employ ISLs, and as a result, a subscriber


Table 2Global Satellite System Proposals for PCS at Low and Medium Earth Orbits

Fig. 13. General overview of the Iridium system and services.

can access to the system on a bent-pipe fashion through agateway station, as shown in Fig. 15. For a typical servicearea of about 1600 km around a gateway station, globalcoverage requires more than 200 earth stations, which arenot planned in the system. Therefore, Globalstar will likelyserve national roamers in general. A satellite that is workingas a repeater is sometimes referred to as a transparentsatellite.

The above discussion and explanation on mobile satellitenetwork proposals should make it clear that these systemswill have a significant (if not dominant) role in the next-gen-eration wireless communications. The recent financial failureof the Iridium, however, does not change this role. On thecontrary, it states the fact that the future trend in wirelesscommunications is the Internet and broad-band services andany system optimized for voice-only communications sub-jects to failure, regardless whether it is terrestrial or satellite.In the following sections, we will explore the ATM and IPnetworks and the potential integrity of the mobile satelliteswith these networks. This will be the most important issue

for the mobile satellite systems in order to compete or com-plement the next-generation terrestrial cellular networks.

B. ATM-Based Satellite NetworksIn the discussion on WATM given in Section II, we have

not specified any type of physical channel used for trans-mitting radio signals. Consequently, in general it is possibleto consider any type of wireless media including satellitechannels. Indeed, this is the idea behind the new generationof mobile satellite systems based on the ATM architecture astheir mode of transfer [25][29]. The satellites in these sys-tems are usually multispot-beam with onboard processingcapabilities. These systems will provide services at high datarates in the order of 2 Mb/s or higher usually at Ka-band(30/20 GHz up/down) where the required bandwidth isavailable. Table 3 summarizes some of the satellite systemproposals for broad-band applications [26]. Among thesesystems, SkyBridge is the only one that will use Ku-band(14/11 GHz up/down). This band has already been used bythe fixed satellite service (FSS).


Fig. 14. Connectivity in the Iridium system.

Fig. 15. Connectivity in the Globalstar system.

Among the satellite systems listed in Table 3, Teledesicis the most supporting network with global coverage, as amultimedia and Internet-optimized satellite system using

-band. The system is planned to have data and Internetservices at high data rates and in its original proposal 840LEO satellites were considered. With a compromise on theasymmetric data rate actually required for the subscribersand the integration with the Internet service providers (ISPs),Teledesic has now changed its design to a higher orbit heightat 1400 km, which reduces the total number of satellites into288 and may change further. Teledesic will use 1-Gb/s linksand 13.3-Gb/s capacity satellites, which state the potentialapplications of the system to be broad-band multimedia.

Both transparent satellite networks and systems withonboard processing satellites can be integrated with ATMnetworks. In the former satellite ATM network, all protocolprocessing is performed on the ground at the user terminal,gateway stations, and the network control center (NCC),since there is no such onboard processing facilities in thesatellite to perform the required processing at the ATMlayer or above. These systems, however, can provide a quickdeployment of ATM connectivity using exiting satellitesand, hence, providing high-speed network access by userterminals and high-speed interconnection of remote ATMnetworks. We will not discuss this type of satellite ATMnetworks here, but a detailed discussion on the networkarchitecture of these systems can be found in [25].

In networks with onboard processing satellites, controlfunctions perform proportionally in onboard ATM switch

and the NCC on ground. ATM interfaces between thepayload switch and ground terminals can be either a UNIor an NNI [25]. If satellite links are low speed, then theywill be used to connect remote ATM hosts to a terrestrialnetwork. Here, the interface between the ATM hosts andonboard switch is a UNI, and the one between the onboardswitch and the terrestrial ATM network is an NNI. Withhigh-speed satellite links, the onboard satellite will functionas an ATM node and the interfaces will be NNI type. In asatellite system employing ISLs, each satellite in the spacenetwork acts as an ATM node and the network providesboth network access and network interconnectivity. Here theinterfaces between satellites are the NNI type.

Fig. 16 shows simple end-to-end communications be-tween two satellite mobile terminals, and , andbetween a mobile terminal and a fixed terminal con-nected to the PSTN, , respectively. In this figure, it isassumed that the mobile terminals have direct access to LEOsatellites and that the satellites have mutual connectionsvia ISLs. What this simple figure illustrates is that in amobile satellite system with ISL networking, it is possibleto achieve high data rate long-distance communicationsdirectly between terminals, both mobility-supported andfixed ones. The directly connectable terminal in such asystem contains a satellite adaptation unit that performs allnecessary user terminal protocol adaptations to the satelliteprotocol platform. This unit also includes all physical layerfunctionality such as channel coding, modulation/demod-ulation, the radio frequency, and the antenna parts. Thesatellite contains onboard signal regeneration and performsmultiplexing/demultiplexing, channel coding/decoding, andATM switching.

In the communication path between the mobile terminaland the fixed terminal, there is a gateway station thatprovides connectivity between the satellite and the groundsegments. An interworking unit (IWU) included in thegateway station performs all necessary translations betweenthe satellite segment and other ground-based networks.The ground networks include PSTN, narrow-band andbroad-band types of ISDN, frame relay networks, theInternet, and private and public ATM networks. A fixed userterminal equipment could belong and be connected to any ofthese networks. A network control center might be requiredfor an overall control of the satellite network resourcesand operations. This includes allocation of radio resourcesto the gateway stations, call routing and call managementfunctions such as location update, handover, authentication,registration, deregistration, and billing. In a complete LEOsatellite system employing ISLs, however, all these taskscould be distributed among the satellites, providing a morereliable control and then no NCC will be required. Anillustrative architecture for a global ATM connectivity usingmobile satellites is shown in Fig. 17.

Another point that can relate the mobile satellite systemsemploying ISLs with ATM networks is that we can considereach satellite as an ATM node, each ISL as a single VCC, andthe routing path of a connection as a VPC of an ATM net-work. Therefore, we can build a complete high-speed ATM


Table 3Broad-Band Satellite System Proposals

Fig. 16. Simple end-to-end communication via ISLs in a LEO satellite system.

network in the space using LEO satellites as its nodes andthen apply similar ATM-based algorithms in that network.Specifically, applying the VPC and VCC concepts in a mo-bile satellite system will benefit us by utilizing many advan-tages of the ATM routing and transmitting schemes. It wouldalso be much more convenient for mobile satellite networksto access fixed terrestrial ATM networks. As explained inSection II, an ATM cell header in the NNI format contains 12bits for VPI. This allows a maximum of VPs foreach single ISL step. This number is more than adequate fora mobile satellite system since the number of ISLs for eachsatellite nodes in system is only between two and four (twolinks to satellites in same orbital plane and two to satellite inthe first neighboring planes). One VPC has impact on onlyone ISL if the node is at the terminal point and on two if thenode is a middle transient one. Thus, the maximum numberof simultaneous VPCs equals the total number of the all pairnodes, which can be defined as , where is thetotal number of the satellites in the system. More discussionon applying ATM routing concepts for mobile satellite sys-tems can be found in [27][32].

In conclusion, satellite ATM networks can be used to pro-vide broad-band access to remote areas and also to serveas an alternative to wired backbone networks. These satel-lite networks can effectively provide both real-time and non-real-time communication services in a global basis to remoteareas and other regions where land-based facilities are notsufficient or not available.

C. QoS RequirementsIn general, bandwidth, throughput, timeliness (including

jitter), reliability, perceived quality, and cost are considered

as QoS metrics in telecommunication networks [33][36].Management of the system components becomes more com-plicated as we move from simple voice or data services intomultimedia and broad-band applications and from fixed tomobile networks. In this regard, because of certain limita-tions in portable computers, such as restriction of battery life,screen size, and connection cost, management of deliveringthe required QoS in a mobile environment becomes morecomplicated.

We may also define different QoS characteristics ac-cording to the applications. For example, in transferring animage file, the picture quality and the response time couldbe considered as appropriate factors. In general, the maintechnology-based QoS parameters are [33]:

1) Timeliness, including several parameters such as:a) delay (transmission time for a message);b) response time (time between the transmission of

a request and receiving a reply);c) jitter (variation in delay time).

2) Bandwidth, which may be defined as:a) system level data rate (required or available

bandwidth in bits per second);b) application level data rate (application specific

bandwidth in its unit per second);c) transaction rate (processing rate or requested rate

of the operations).3) Reliability, which can be measured by:

a) mean time to failure;b) mean time to repair;c) mean time between failures;d) loss or corruption rate (due to network errors).


Fig. 17. Global connectivity in ATM networks using mobile satellites.

From a user level, the following QoS requirements mightbe considered:1) Critically, i.e., priority among different flows in multi-

media streama) perceived QoS, which, based on type of data trans-

mission application, can be defined by:i) picture detail (e.g., resolution);

ii) picture color accuracy;iii) video rate (frame per second);iv) video smoothness (frame rate jitter);v) audio quality (sampling rate);

vi) video/audio synchronization.2) Cost (a significant parameter considered by users) which

can be either of:a) per-use cost (connection establishment and/or re-

source access cost);b) per-unit cost (per second or per unit of data).

3) Security, required in most probable applications, in-cluding:

a) confidentiality;b) integrity;c) digital signatures;d) authentication.

Certain controls and supervision, namely QoS manage-ment techniques, are required to attain and sustain the

desired QoS [33]. These techniques are required not onlyat the initiation of an interaction (namely, the static func-tions) but also during that interaction (namely, the dynamicfunctions). Definition of QoS requirements, negotiation, ad-mission control, and resource reservation are some of thestatic functions, whereas measuring the actually providedQoS, policing, maintenance, renegotiation, adaptation, andsynchronization (e.g., combining speech and video streamswith temporal QoS) are examples of dynamic functions.In the case of ATM satellite networks with onboard pro-cessors in which multiple IP flows are aggregated onto asingle VC, a QoS manager classifies the flows of IP trafficsin order to utilize the bandwidth efficiently [37]. The QoSmanager uses IP source and destination address pairs. Themanager can further classify IP datagrams based on typeof service field (see Fig. 8) requested and available in theIP header.

In a mobile environment, mobility results in significantchanges in QoS and a mobile system has to be able to adaptsuch changes. For the first QoS metric, i.e., the bandwidth,we have to accept that for some time the wireless networkscan provide only bandwidth in an order much lower thanfixed networks. The freedom in mobility of terminals willalso be limited by the coverage area of wireless infrastruc-ture that a user is subscribed. Obviously, here another issueof QoS, i.e., the cost, will arise. As we move to a broader


Table 4Relationship Between the Bandwidth and Coverage in Wireless Networks

coverage and higher mobility during connection, e.g., fromwireless LAN into cellular networks and to satellite sys-tems, higher costs may be required though they may notprovide higher data rate supports proportionally.2 Table 4summarizes the relationship between coverage and band-width for several wireless networks. As it can be seenfrom this table, all these wireless networks provide muchlower data rates than typical Ethernet LAN networks of10 Mb/s1 Gb/s.

Nevertheless, in a mobile environment with data traffic ap-plications, QoS management requires much more sophisti-cated techniques than fixed networks, because:

1) a short loss of communication during a handover,which is usually acceptable in a voice application, isnot desirable in data applications;

2) new point of attachment after a handover requireshaving similar facilities and resources as the old one,thus, renegotiation procedures would be required;

3) blind spots where signal is very weak and has lowquality are unavoidable in wireless systems.

Certain specifications of portable terminal such as laptopcomputers may also affect the end-user QoS requirementsin mobile environment compared to fixed networks. Theselimitations include battery limits, processing power with lowpower consumption, screen size, and their screen resolution.

D. Traffic RequirementsAs explained in Section III, the main structure of an IP

application is based on TCP, and thus the performance ofTCP is crucial in running IP applications efficiently. In prin-ciple, TCP should work anywhere regardless of underlyingnetwork architecture, however, it is optimized for operatingin a wired network with relatively low bit error rates (BERs),say in the order of 10 or less [38]. Consequently, TCP as-sumes that the major cause of problems in packet handlingin the network is the congestion. In the case a wireless linkis used for transmission of packets, however, this assump-tion would be no longer valid as the main cause is the highBER of the wireless link. In the case a satellite link is used asthe wireless channel, the situation becomes even worse. ForGEO satellites the long delay and for LEO/MEO satellitesthe rapid delay variation causes the acknowledgment, and thetime-out-based TCP congestion control mechanism performs

2Note that here we consider real terminal mobility and not nomadicsystems, which can provide acceptable data rates at relatively low costby using dial-up connections.

weakly. This in turn results in a large number of retransmis-sions that degrade the performance of the TCP. Therefore,the relation of bandwidth-delay product and round-trip delayvariation to the performance of TCP requires development ofnew congestion control and traffic management mechanismsin the TCP layer [37], [38].

One issue in integration of IP traffic into an ATM mobilesatellite is to accommodate multiple IP traffic onto a singleVC. The primary reason for this to be important is that the IPtraffic must be transmitted within the ATM cells and throughATM VCs and that the number of these VCs is limited be-cause of the limitation of the earth stations and onboard satel-lites. The classification of a large number of IP datagramsinto a limited number of available VCs is performed by aQoS manager discussed in the previous section.

ATM is intended to carry different types of traffic simul-taneously including real-time flows such as voice and videostreams and bursty TCP flows [11]. Therefore, the ATMForum has defined real-time and nonreal-time service cat-egories to accommodate all applications that require eitherconstant or variable bit rates. In general, real-time servicesare concerned about the amount of delay and the variabilityof delay (jitter). These applications typically involve aflow of information to a user that is intended to reproducethat flow at a source (e.g., voice or audio transmission).On the other hand, nonreal-time services are intended forapplications that have bursty traffic characteristics and donot have tight constraints on delay and delay variation(more flexibility for the network to handle traffic and to usestatistical multiplexing).

The real-time services of the ATM include constant bitrate (CBR) and real-time variable bit rate (rt-VBR). Non-real-time services also include nonreal-time VBR (nrt-VBR),unspecified bit rate (UBR), and available bit rate (ABR).Among these services, UBR is suitable for applications thatcan tolerate variable delays and some cell losses (such asTCP-based traffic). Thus, no initial commitment is made to aUBR source and no feedback concerning congestion is pro-vided. This service is best suited for the IP applications inwhich a best-effort service (i.e., the primary service of IP)is sufficient. The ABR service has been defined to improveservice provided to bursty sources. In this service, a peak cellrate (PCR) and a minimum cell rate (MCR) are specified andthe network allocates at least MCR to an ABR source. Theleftover capacity or unused capacity is shared fairly amongall ABR and then UBR sources. A guaranteed frame rate(GFR) has recently been proposed by the ATM Forum that


Fig. 18. Applications of broad-band ATM satellite networks.

provides a minimum rate guarantee to VCs at the frame leveland could enhance the UBR service [39].

Considering unavoidable delay and delay variation inmobile satellite networks, UBR and ABR seem to be themost practical options for implementation of TCP/IP overATM satellites. In particular, UBR routers (connectingthrough a satellite ATM network) can make the use ofGFR service to establish VCs between one another. In thecase of ABR service, a network can maintain low cell lossratio by changing the ACR through the use of a rate-basedclosed-loop end-to-end feedback congestion control mech-anism. In the case of satellite systems that suffer from longround-trip delay, the control loop can be segmented usinga virtual source and virtual destination concept, whichresults in less buffer requirements. For more discussionson the teletraffic issues and operation of TCP over wirelesschannel, see [40][42].

Nevertheless, new algorithms are being developed in orderto evolve the best effort service of the Internet into a QoS-supported one. Among them, the resource reservation pro-tocol (RSVP) is enabling reservation of resources within IPnetwork [43]. This protocol provides a way for every senderto establish paths for identified IP flows. With such proto-cols, it would be possible to define guaranteed QoS servicesfor the delivery of IP datagrams within a fixed delay and noloss.

E. Applications

ATM and IP networks have several common structuralviewpoints so that the concept of classical IP over ATM hasbeen ignited in the IETF working group [44]. In particular, IPdatagrams, IP addresses, IP routing, IP QoS, and IP multicastcould be mapped onto the corresponding ATM cells, ATM

addresses, ATM VC switching, ATM QoS, and ATM point-multipoint features [45]. Thus, in this model the IP layer isentirely mapped onto the ATM layer in order to use the gen-eral applicability of the IP over any data link layer. Despitethe disadvantage of having many task duplications in the twolayers in this approach, this example shows that IP and ATMhave sufficient potentiality for integration. Such an integra-tion, however, could provide new services for IP networksother than the traditional best effort; e.g., the QoS-supportedservices. Nevertheless, some optimization is required for anefficient integration of IP and ATM.

Since ATM is based on cell switching and not the conven-tional circuit switching, network resources can be utilizedoptimally. The guaranteed QoS, the variable-rate support,and the low-cost ATM chips are also additional advantagesof ATM in implementation of high-speed broad-band wire-less pipes within the base station and advanced mobile ter-minals. By use of wireless ATM technologies, including sig-naling, access control, and resource management, it is pos-sible to achieve high data rate broad-band personal commu-nication services in order of 210 Mb/s or more. Transmis-sion of a number of IP flows on individual VCs accordingto their source and destination addresses for a better QoS hasopened research activity in the area of IP over ATM networks(e.g., see [46][48]).

Broad-band satellites with ATM switching would besuitable for some applications but less appropriate for others[49]. Some of potential applications are shown in Fig. 18.Telnet, or remote computer access, which is categorized ininteractive computing applications, is feasible in satellitesystems on low earth orbits. The LEO satellite can provide arelatively prompt response to a telnet connection. Multicas-ting and broadcasting of large data files can be efficientlysupported by the mobile satellite networks. The primary


reason for this is the global coverage and star topology ofsatellite networks. Video broadcasting is usually sensitive todelay variation and not delay itself. The reason is that fora QoS playback of video streams it is necessary that eachframe be equally spaced. Multicasting of image and videofiles through group mailing list and e-mail also could befeasible with the broad-band satellite networks. Included inmulticast applications is the transmission of geographicalposition information to be used in global positioning system(GPS).

Network conferencing and video conferencing arealso ideal point-to-point and multipoint applications forbroad-band satellite networks. Delay would be a problem intransmission of high-speed and high-quality video images,but the LEO satellite link can be comparable with otherlong-distance communication media. Applications that arenot delay sensitive are the most promising services of thesatellite networks. This includes bulk transfer of data. Inaddition to the above applications, low bit rate voice andimage transmissions, paging, and short message services areincluded in basic applications of the broad-band satellites.

V. CONCLUDING REMARKSNext-generation broad-band satellite networks are being

developed to carry bursty Internet and multimedia trafficin addition to the traditional circuit-switched traffic. Thesesatellites provide direct network access for personal appli-cations as well as interconnectivity to the terrestrial remotenetwork segments. In a data transmission environment, atraditional circuit switching method would be insufficient,as it cannot utilize the link capacity efficiently. ATM, onthe other hand, can provide high QoS support at a highdata rate and good channel utilization. Moreover, becauseof the existence of the real-time traffic such as voice andvideo transmission, satellites on nongeostationary orbitswith low-propagation delay have found much attention inthe development of broad-band satellite networks. In thisregard, the next-generation broad-band satellite networkswould have the concept of integration of mobile satellitesand ATM networks.

In this paper, we examined mobile satellite, ATM, and IPtechnologies as well as their mutual integration for providingwireless multimedia services. In addition, we discussed thenew concept of integration of the three technologies in orderto provide global mobility to multimedia terminals. Differenttypes of traffic to be handled through these networks and QoSrequirements have been explained. This integration could beconsidered as the main research topic for the next-genera-tion broad-band satellite networks that connect all terrestrialhigh-speed networks and provide the required communica-tions means of the future, i.e., the Internet.

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[47] J. Aracil, D. Morato, and M. Izal, Analysis of Internet services inIP over ATM networks, IEEE Commun. Mag., vol. 37, no. 12, pp.9297, 1999.

[48] M. A. Labrador and S. Banerjee, Packet dropping policies for ATMand IP networks, IEEE Commun. Surveys, 3rd Quarter, 1999.

[49] D. P. Connors, B. Ryu, and S. Dao, Modeling and simulation ofbroadband satellite networksPart I: Medium access control forQoS provisioning, IEEE Commun. Mag., vol. 37, no. 3, pp. 7279,1999.

Abbas Jamalipour (Senior Member, IEEE) re-ceived the Ph.D. degree in electrical engineeringfrom Nagoya University, Nagoya, Japan, in 1996.

He is currently with the School of Electricaland Information Engineering at the Universityof Sydney, Australia, where he is responsiblefor teaching and research in data communicationnetworks and satellite systems. He was anAssistant Professor at Nagoya University beforemoving to Sydney. His current areas of researchinclude data communication and ATM networks,

mobile IP networks, mobile and satellite wireless communications, trafficand congestion control, switching systems, and switch design. He is theauthor of the first technical book on LEO satellites, entitled Low EarthOrbital Satellites for Personal Communication Networks (Norwood, MA:Artech House, 1998) as well as the author of many papers in IEEE andIEICE Transactions and Journals and international conferences. He hasserved as the Registration Chair at the 1998 IEEE Global Telecommunica-tions Conference (GLOBECOM98) held in Sydney.

Dr. Jamalipour is an organizing committee member of the joint IEEENSW Communications and Signal Processing chapter. He is the recipientof a number of technology and paper awards.


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