10.1.1.22.5606_asynchronous transfer mode (atm) over satellite

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ECPE 6504: Wireless Networks and Mobile Computing

Individual Project Report

An In-Depth Design Guide to Asynchronous Transfer Mode (ATM) over Satellite

Communication Networks

Srihari Raghavan([email protected])

24 APR 2000


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This paper discusses in depth the issues in using satellite links as the physicalmedia for ATM internetworking and uses it to build a design guide for implementation ofATM over satellite networks. The challenge here is to provide ATM, a connection-oriented protocol developed specifically for a reliable high-bandwidth wiredinfrastructure, along with its QoS guarantees, for mobile networks, which arecharacterized by frequent breaks and makes of connections over a shared, unreliable

and limited-bandwidth wireless medium. A successful implementation of ATM inter-networks depends upon the Bit Error Rate (BER) of the underlying physical layer. ATMwas originally designed for links with low BER like fiber. In the case of satellite links, theerror rate is orders of magnitude higher. The bursty nature of the error in satellite linksalso poses a big problem. This paper will systematically deal with such major issues ofSatellite ATMs (SATATM) and their implementation. A set of motivation examples orscenarios for SATATM networks will be discussed. This will be used to compile a host ofdesign issues and the various options available for the same and hence can be used asa theoretical design guide for future ATM over satellite implementations.

The paper is arranged as follows. After a brief introduction to ATM, Satellitecommunications and Wireless ATM (WATM), motivating network architectures, which

has a great diversity of requirements, are presented. The need for SATATMs in thoseparticular situations is emphasized. After this, requirements for SATATMs like handoff(inter-satellite, inter-beam), error control mechanisms, architectural options, cost-performance tradeoffs are discussed. The section following this would handle howsatellite communications should be optimized to provide other requirements and ATMspecific behaviors like service guarantees (ABR, CBR etc.,), congestion control, andAAL issues. The section also deals with routing in SATATMs, MAC protocols for satellitecommunications, optimizations needed to use TCP over SATATMs and IPV6 overSATATMs. All the above issues would be analyzed and correlated with the motivatingarchitectures given in the preceding sections. The section will also explain somepractical SATATM products available in the market and their features. The paper will endwith a section on conclusions, summarizing all the ideas presented and comments about

the whole concept of SATATMs. The conclusions section would summarize the ideaspresented and will present design solutions for the implementation of the motivationscenarios and also will discuss related issues and tradeoffs for the solutions. The sectionfollowing the motivation scenario presents SATATM solutions for the scenarios. Theconclusion section justifies and endorses the same.


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Table of ContentsPg. No

1. Introduction 41.1. Satellite Communications

1.1.1. Architecture and purpose1.1.2. Terminology, characteristics, advantages and disadvantages

1.2. ATM and WATM 51.2.1. ATM architecture1.2.2. ATM internals and physical layer issues1.2.3. WATM and its features

2. Motivating Scenarios 72.1. Description of the architectures

3. SATATM details 10

4. SATATM design specifics 114.1. Constellation of the satellite4.2. Handovers and re-routing4.3. Presence of inter-satellite links4.4. Presence of OBP/OBS4.5. MAC layer protocols, scheduling and ATM services mapping for QoS4.6. Power management4.7. Error correction scenarios4.8. Traffic control and congestion control4.9. Upper layer considerations

4.9.1. TCP changes for ATM UBR

4.9.2. TCP changes for ATM ABR4.9.3. TCP changes for satellite communications4.9.4. IPV6 over ATM over satellite communications

4.10 Attenuation considerations4.11 ATM layer changes for satellite considerations4.12 Link budget scenario4.13 Elevation angles4.14 Cell transport methods4.15 Encryption of traffic4.16 Related Information

4.16.1 HALE systems4.16.2 Commercial SATATM products (from COMSAT)

4.16.3 Focus on NASA-ACTS4.16.4 Commercial satellite design guide4.16.6 Rule-based practical design approach for building commercial satellites4.16.7 VSAT terminals

5. Conclusions 29

6. References 31


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1. Introduction

1.1 Satellite Communications

1.1.1 Architecture and purpose

A communication satellite functions as an overhead wireless repeater station thatprovides a microwave communication link between two geographically remote sites. Dueto its high altitude, satellite transmissions can cover a wide area over the surface of theearth. Each satellite is equipped with various transponders consisting of a transceiverand an antenna tuned to a certain part of the allocated spectrum. The incoming signal isamplified and then rebroadcast on a different frequency. Most satellites simply broadcastwhatever they receive, and are referred to as bent pipes. The traditional applicationswere TV broadcasts and voice telephony. Satellite communications for packet datatransmissions is being considered. The applications like mobile services, directbroadcast, private networks and high-speed hybrid networks in which services would be

carried via integrated satellite-fiber networks are being considered [39].

Satellite links can operate in different frequency bands and use separate carrierfrequencies for the up-link and downlink. There are some common frequency bands.They are listed in the table below.

Table 1: Frequency spectrum allocation for some common bands [1]

BAND UP-LINK (GHz) DOWN-LINK (GHz) ISSUES

C 4 (3.7-4.2) 6 (5.925-6.425) Interference with ground links

Ku 11 (11.7-12.2) 14 (14.0-14.5) Attenuation due to rain

Ka 20 (17.7-21.7) 30 (27.5-30.5) High Equipment cost

L/S 1.6 (1.610-1.625) 2.4 (2.483-2.500) Interference with ISM band

1.1.2 Terminology, characteristics, advantages and disadvantages

Satellites can be positioned in orbits with different heights and shapes. Based onthe orbital radius, satellites fall into one of the following categories. They are Low EarthOrbit (LEO), Medium Earth Orbit (MEO), Geostationary Earth Orbit (GEO) and HighlyElliptic Orbit (HEO). The constellations are described below and their relative merits aretabulated.

Fig.1 GEO, LEO, MEO and HEO (Left-Right) constellations [41]


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The comparisons table between the different constellations is given below. [41][1]

Table 2: Salient features of different satellite constellations

Type LEO MEO GEO HEO

Height 100-300 miles6000-12000miles

22,282 milesVariable due to ellipticalorbit

Time inLOS

15 min 2-4 hrs 24 hrs Variable

Merits

Lower launchcosts, very shortround trip delays,small path loss

Moderatelaunch costs,small round tripdelays.

Covers42.2% of theearthssurface,constant view

Maximizes timespent overpopulated areas,superior line of sight,fewer satellites

Demerits

Very short

lifetime (1-3months),encountersradiation belts

Larger

delays,greater pathloss

Very largeround trip

delays,expensiveEarthStations.

Not a completecoverage.

There are several merits to satellite communications as a whole as they can giveglobal coverage to remote areas not connected by terrestrial network, chance to act asan alternate mode of communication in military applications and disaster recoveryscenarios, support for multipoint communications due to inherent broadcastingcapability, bandwidth on demand capabilities, ease of network expansion, flexibility ofstation organization etc., There are also several demerits associated with satellitecommunications such as their bursty error conditions, high BER characteristics, longdelay and the enormous cost associated with user terminals, earth stations and thesatellites as a whole. Also, the dependence of solar power for recharging also poses aproblem. The limited transmission power of both the ground terminals and satellite isalso a problem.

1.2 Asynchronous Transfer Mode (ATM) and Wireless ATM (WATM)1.2.1 ATM Architecture

Asynchronous Transfer Mode (ATM) is an International TelecommunicationUnion- Telecommunication Standardization Sector (ITU-T) standard for cell relaywherein information for multiple service types, such as voice, video, or data, is conveyed

in small, fixed-size cells. ATM networks are connection oriented. It is a cell-switching andmultiplexing technology that combines the benefits of circuit switching (guaranteedcapacity and constant transmission delay) with those of packet switching (flexibility andefficiency for intermittent traffic). It provides scalable bandwidth from a few megabitspersecond (Mbps) to many gigabits per second (Gbps). Because of its asynchronousnature, ATM is more efficient than synchronous technologies, such as time-divisionmultiplexing(TDM). With TDM, each user is assigned to a time slot, and no other stationcan send in that time slot. If a station has a lot of data to send, it can send only when itstime slot comes up, even if all other time slots are empty. If, however, a station has


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nothing to transmit when its time slot comes up, the time slot is sent empty and iswasted. Because ATM is asynchronous, time slots are available on demand withinformation identifying the source of the transmission contained in the header of eachATM cell.

ATM transfers information in fixed-size units called cells. Each cell consists of 53

octets, or bytes. The first 5 bytes contain cell-header information, and the remaining 48contain the "payload" (user information). Small fixed-length cells are well suited totransferring voice and video traffic because such traffic is intolerant of delays that resultfrom having to wait for a large data packet to download, among other things [42].

1.2.2 ATM internals and physical layer issues

An ATM network consists of set of ATM switches interconnected by point-to-pointATM links and there are two interfaces or links. They are User Network Interface (UNI)and Network-to-Network Interface (NNI). Then there are Virtual Connection Identifiers(VCI) and Virtual Path Identifiers (VPI), which are used to identify the next destination ofa cell as it passes through a series of ATM switches to reach the ultimate destination.

There are certain interesting fields in ATM header like Congestion Loss Priority(CLP) and a Header Error Control (HEC). The former will allow the ATM switch to dropthe cells with CLP set, when there is congestion at the switch. The latter is used for errorcontrol. The ATM layers and the ATM Adaptation layer (AAL) are roughly analogous tothe data-link layer in the OSI model. The ATM layer is responsible for establishingconnections and passing cells through the ATM network. The AAL is used for isolatinghigher-layer protocols from the details of the ATM layer. The higher layers residingabove AAL will accept user data, arrange it into packets and hand it to AAL [42]. Thereare different AALs like AAL1, AAL3/4 and AAL5 for different types of data and voice andvideo packets.

ATM connections can be point-to-point and point-to-multipoint. ATM supportsQoS guarantee composed of traffic contract, traffic shaping and traffic policing. The firstclass called Constant Bit Rate (CBR) emulates fixed-bandwidth circuit switching. It hasPeak Cell Rate (PCR) as the traffic descriptor. Variable Bit Rate (VBR) allowsconnections to share network resources and the traffic descriptors are PCR, SustainableCell Rate (SCR) and Maximum Burst Size (MBS), The Available Bit Rate (ABR) isdependent on the network flow control, which assigns it a value, called Allowed CellRate (ACR), which is in-between traffic descriptors for this service like PCR andMinimum Cell Rate (MCR). Unspecified Bit Rate (UBR) has no traffic descriptors and noQoS guarantees.

ATM connection establishment process uses the one-pass method, just like thetelephone network. An ATM connection setup proceeds with a connection-signalingrequest from source end system and connections are set up throughout the network,allocating buffer spaces according to QoS guarantees and reaches the final destination,which either accepts or rejects the connection request. On acceptance, data transfer canbegin. The teardown is also done in the similar way. ATM networks can emulate aphysical LAN. LAN Emulation (LANE) is a standard defined by the ATM forum toemulate a LAN on top of an ATM network. It provides a service interface for higher-layers that is identical to that of existing LANs.


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1.2.3 Wireless ATM (WATM) and its features

WATM is ATM with physical layer being wireless medium. This gives a host ofchoices for the physical layer. The benefits of a wireless ATM access technology shouldbe observed by a user as improved service and improved accessibility. By preservingthe essential characteristics of ATM transmission, wireless ATM offers the promise of

improved performance and quality of service, not attainable by other wirelesscommunications systems like cellular systems, cordless networks or wireless LANs. Inaddition, wireless ATM access provides location independence that removes a majorlimiting factor in the use of computers and powerful telecom equipment over wirednetworks. The architecture proposed for wireless ATM is composed of a large number ofsmall transmission cells called pico cells. A base station serves each pico cell. All thebase stations in the network are connected via the wired ATM network. The use of ATMswitching for intercellular traffic also avoids the crucial problem of developing a newbackbone network with sufficient throughput to support intercommunication among largenumber of small cells. To avoid hard boundaries between pico-cells, the base stationscan operate on the same frequency.

2. Motivating Scenarios

There are different application scenarios, which are motivating factors behindSATATM networks. The following sections deal with a set of architectures for whichSATATMs can provide a good quality solution. The scenarios are discussed and theconnectivity requirements are studied and then, SATATM concept will be applied to thescenarios and its deployment requirements would be studied in the consequent sections.

2.1 Description of the scenarios

2.1.1 Geographically distributed computing

Geographically distributed computing allows more effective resource sharing andimproved utilization of computing resources. Major components of this scenario areinter-process communication and remote file I/O systems [37]. The main factor involvedthis scenario is the distance of separation between communicating nodes and ways toresolve them. The other factor involved is the necessity of broadband communicationswith QoS guarantees. Satellite communications can solve the distance factor and ATMcan solve the requirements of QoS guarantees. The other factors are a bigorganizations nature of having geographically dispersed supercomputers andworkstations in branch offices and the need to interconnect them. The pre-requisite isthe successful interconnection of terrestrial networks in a seamless way.

2.1.1.1 Requirements

The requirements here are QoS guarantee, fast user response, stableconnections, reachability etc.,

2.1.2 Mobility architecture in ATM and WATM networks

In ATM networks, there are different scenarios based on interconnection of ATMnetworks (which may be mobile) between themselves and the need to interconnect ATMend nodes, which may be geographically distributed. This motivation is on the basis ofthe following scenarios.


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High-speed network access by ATM end-nodes, which may be portable (hencemobile).

A class of applications, with respect to WATM deals with the mobility of the ATMswitch itself. Here pieces of ATM network, each consisting of ATM switches,could be in motion with respect to the fixed portion of the network. Applicationscenarios would involve mobile platforms with number of users on board. This

scenario is pertinent to airplanes, which provides communication andentertainment services to passengers. Here the ATM end nodes are not inmotion. Another scenario could be that ships (military and civil) having ATMnetworks want to communicate among them and with the land network. Themilitary networks would also entail security features for intruder-freecommunication.

2.1.2.1 RequirementsThe requirements here are maintaining quality connections, safeguard QoS

guarantees, smooth handoffs, secure communications etc.,

2.1.3 Distance learning and next-generation education

Distance learning and computer aided instructions are very important and could be Broadcast type communications characterized by one-way information flow Interactive communications characterized by full-duplex information flow and

Self-learning, in which students can retrieve learning materials remotely [28].These scenarios require multimedia communications of very high quality and the mainhindrance is the distance factor. Institutions in the developed countries can educatepeople in developing and under-developed countries if quality multimedia connection isachieved over a large distance. ATM is the de-facto standard for multimediacommunications due to its capacity to guarantee QoS and support for voice, video anddata simultaneously.

2.1.3.1 RequirementsThe main requirements are QoS guarantees, voice-video synchronization, large

bandwidth, bandwidth on demand, quality multimedia services etc.,

2.1.4 Multimedia and multi-service applications

Multimedia applications like video-conferencing and multi-service applications(interconnection of circuit-switched and packet-switched networks) scenarios are classicexamples of bandwidth guarantees and bandwidth on demand scenarios respectively.They also require synchronization over a great distance. By default, distance is a factorin these application scenarios. Multimedia communications is also driven by the

backbone concept, assumed to be provided by fiber cables. In many parts, these may beunviable, uneconomic or take too long to establish. Multi-service communications alsoentail interconnection of the mobile devices carried by company representatives.

2.1.4.1 RequirementsQoS guarantees, bandwidth on demand, large bandwidth, synchronization, and

backbone dependability are demanded by multimedia applications. Seamless andefficient integration schemes are needed by multi-service applications. Interactive


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computing and bulk transfers with high bandwidth requirements, informationdissemination including stock market data etc., and video broadcasts with low delayrequirements are some other multimedia applications to be taken care of [26].

2.1.5 Secure broadband communications

Secure communications are needed by military and sometimes, for bigcompanies, financial institutions and banks, having distributed branches. The main factoris that secure communications are needed over a geographically separated scenario inwhich distance is the main consideration.

2.1.5.1 RequirementsSecurity, encryption and interconnection between geographically diverse locations arethe main issues here.

2.1.6. Applicability of SATATM

SATATMs are most suitable in all the above scenarios because Satellites can eliminate the distance factor. ATM is the industry choice for QoS guarantees and multimedia

communications. Satellites can provide reachability. Satellites can provide reachability in cases

where geographical complexity precludes terrestrial network and in caseswhere the terrestrial network is made unusable due to natural or artificialdisasters.

In the past, fast user response may not be possible with SATATMs due to theinherent propagation delay associated with satellite communications.Recently, gigabit satellite networks made possible using NASAs AdvancedCommunication Technology Satellite (ACTS) [12][18].

Satellites can provide bandwidth on demand and provide error-tolerantconnections [9,13,15, 18,19,24,30,45].

They can also do encrypted communications [35].

Satellite communications can also guarantee QoS to all the servicecategories of ATM like CBR, VBR etc [10,14,30,43,44]. ConnectionAdmission Control (CAC) mechanisms have been devised for SATATMs [38].

They can also provide multi-service on demand [21]. There are handover protocols being devised for smooth handoffs and have

been found to be effective [11].Taking into consideration, all the above factors, SATATMs can be taken as the preferredchoice for the above scenarios. The following sections will show how SATATM satisfiesthe above requirements. SATATMs can be used in similar scenarios, which exhibit orneed QoS guarantees and high bandwidth requirements in the face of distance, beingthe overriding concern.

There are many issues to be addressed before SATATMs can be chosen as thepreferred solution. These are discussed in the next section. Particularly, there is a greatnumber of SATATM solutions and architectures available and these should be chosencarefully to particular application scenarios for optimum performance and meeting ofrequirements. These are addressed in the following sections. The paper proceeds by a


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discussion of a generic architecture and issues behind SATATMs and a design guide inthe following sections.

3. Satellite ATM details

In order to explain the SATATM network details and other issues, the following model

will be considered.

Fig.2 Generic Satellite network model and its related issues

Modern satellites have Inter-Satellite Links (ISL), On Board Switching/Processing(OBS/OBP), data buffering and signal processing. They solve the main stumbling pointfor universal access for data services, namely distance. They are often equipped withmultiple transponders. The area of the earths surface covered by a satellitestransmission beam is referred to as the footprint of the satellite transponders. The up-link is highly directional, point to point link using a high gain dish antenna at the groundstation. The down-link can have a large footprint providing coverage for a substantialarea or a spot beam can be used to focus high power on a small region, thus requiringcheaper and smaller ground stations. Some satellites can dynamically change theircoverage area [40]. The main aspects of the satellite network with respect to Fig.2 are:

Network management in the multipoint implementation, a network control

center (NCC) is responsible for monitoring, controlling the synchronization of allterrestrial stations. It is also responsible for performance management,configuration management, resource planning and billing [10].

Traffic reconfiguration routing and traffic rate belong to this category.Bandwidth (BW) allocation scheme is necessary to maintain the appropriate QoSguarantee of any network and especially ATM network.

Data Transmissionit requires usually very high link integrity. ARQ methods areused on the uplink channel, which is multi-access channel with multiple usersaiming to access the network and downlink, which is a multicast channel.


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Burst Time PlanA BTP is required too Set up space segment (consisting of satellites) based on the previous

negotiation with the network userso Provide additional BW if a specific service asks for ito Incorporate new activated users to the network

Burst synchronizationwith the high rate digital transmission used in the satellite

link, this is needed. The satellite movement will affect the delay and loss ofsynchronization will lead to serious degradation. Guard times are used for thispurpose.

With respect to the figure, s1, s2 and s3 are three positions (at different times) ofthe same ship, s. The ships with networks (could be ATM) onboard represents amobile network and is shown in different positions so that, they are in differentspot-beams of the same satellite (s1 and s2), necessitating inter-beamhandovers and between different satellite footprints (necessitating inter-satellitehandovers).

OBS and OBP represent onboard switching and onboard processing capablesatellites and will be described in detail in the later sections.

4. SATATM design specifics

The design of SATATM networks will require a number of design issues and relatedparameters to be considered and analyzed. It is done in the following sections. Thefollowing sections are organized as follows. The design parameters would be given andwould be discussed and the advances in each of the parameters would be discussedand then a design guide would be provided based on these.

4.1 Constellation of the satellite

The orbital radius of the satellite greatly affects its capabilities and design. Thefollowing diagram shows the effects of the constellations for GEO and LEO

constellations.

Fig.3 Some of the effects of GEO and LEO constellationsTable 2 should be referred for more information or design decisions about the differentconstellations. Fig.3 shows the effects of LEO and GEO constellations on parameterslike Coverage, Received signal strength etc., These could be used to select theconstellation. There are many simulation models based on LEO constellation. An


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important measure of efficiency that affects SATATM is end-to-end delay. A model usesNumber of orbit planes, Number of satellites per orbit plane, Satellite altitude, Orbit planeinclination angle and Ground terminal coordinates to calculate the total propagationdelay from a source to destination through a LEO network. The end-to-end delay is thesum of transmission delay, uplink delay, downlink delay, ISL propagation delay,OBS/OBP delay and buffering delay. The propagation delay is characterized by downlink

delay, uplink delay and ISL propagation delay. In this model, delay variation caused byorbital dynamics, buffering, adaptive routing and OBP are not taken into account. LEOpropagation delay is of the order of 83.45 ms for a sample propagation delay calculationfrom Los Angeles to London with seven satellites in path [10]. GEO propagation delayfor ground terminals farther away from the equator is of the order of 275ms through asingle satellite.

Though LEO networks have relatively smaller propagation delays, the delayvariance is higher than GEO. This variation is due to handovers, satellite motion, OBSand adaptive routing. These should be considered while selecting the constellation.

Thus, when considering constellation of a satellite, the parameters to be takeninto account are launching cost (less for LEO), propagation delay (less for LEO), delayvariance (more for LEO, hence bad), coverage (more for GEO, change continuously for

LEO), altitude (low for LEO and hence small end-end delays, low power requirements)etc.,

4.2 Handovers and re-routing

The orbital revolution of satellites causes satellites to change position with respect toground terminals. As a result, the Network Control Center (NCC) in fig.2 must handoverconnections to another satellite whose footprint is relevant. In other cases, LEOsystems are not stationary. Hence, caller and called terminals do not remain in the samefootprint of the initial source and initial destination satellites. Thus the satellites need totransfer the ground caller and called terminals to others. This is called a handover. Thereare intra-orbit and inter-orbit handovers. GEO systems do not have too many handovers

due to its large distance from Earth and due to its high coverage area. Handovers forLEO satellites are estimated to occur on an average 8 to 11 minutes [10]. There is anamount of delay variance in LEO constellation due to these handovers. There aredifferent handover protocols being considered and Footprint Handover ReroutingProtocol (FHRP) is one of them [11]. LEO systems with multi-hop inter-satellite linksneed handover and rerouting protocols. This protocol has the following advantages [11].

Maintains optimality of initial route even after satellite handovers Handles the inter-orbit handover problem

Demands easy processing, signaling and storage costs Maintains cell order upon delivery for ATM

Relative performance of FHRP is not affected by heterogeneous traffic pattern.Possible after effects of handovers are listed below.

A new satellite may be added to existing connection route The existing connection route should be updated A new route/connection must be set up.

Addition of a new node could cause sub-optimal route and hence re-routing isnecessary. This causes additional signaling and processing overhead. The assumptionof FHRP is that all handovers are caused by the mobility of the LEO satellite instead ofthe ground terminal. Previous algorithms considered only intra-orbit handovers or inter-


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orbit handovers without multi-hop handover or handover re-routing problem. Thisalgorithm improves upon them.

4.3 Presence of Inter-satellite links

Inter-satellite links are links between satellites, which form a sub-network in

space. A major benefit of a developed ISL network is transporting long distance trafficover reliable and high capacity connections and with minimal terrestrial resources. Oldersatellite networks did not employ ISLs. Modern satellites employ ISLs due to theadvancement in OBS/OBP designs. Another motivation is that, since ATM switchingimplies low delay at each satellite node on the ISL route, the advantage gained from lowpropagation delay on the LEO/MEO up and downlink can be retained [16].

This algorithm uses a virtual topology approach and the search for available end-to-end routes is done within the ISL network by means of a modified Dijkstras SPFalgorithm, capable of coping with time-variant topology. ISL routing deals only withdeterministic and periodic orbits and hence is predictable. Hence the presence of ISLs is

justified.The inter-satellite link is also a part of propagation delay. ISLs may be in-planeor

cross-planelinks. In-plane links connect satellites within the same orbit plane and cross-plane links connect satellites in different orbit planes. In GEO systems, ISL delays canbe assumed to be constant, while in LEO systems ISL delays depend on the orbitalradius, the number of satellites-per-orbit and inter-orbital distance. The ISL delay in LEOsystems change frequently due to satellite movement and adaptive routing techniques.Thus LEO systems can exhibit a high variation in ISL delay [10]. There are someimprovements needed to this routing protocol as suggested in [16] and should beconsulted before usage.

Hence, the usage of ISLs is very much in vogue and recommended and routingstrategies to minimize average number of route changes without increase in path delayshould be considered before usage. The jitter due to ISLs is also reduced by usage ofthe routing protocol. Following is a sample of ISL delay for a GEO satellite constellation.

Table 3 : GEO Inter Satellite Link Delays

Number ofSatellites (N)

Inter-SatelliteLinkDistance (km)

Inter-Satellite LinkDelay (ms)

3 73,030 243

4 59,629 199

5 49,567 165

12 21,826 73

There are also millimeter-wave inter-satellite links and optical inter-satellite links [31].The link budgets of these ISLs are also given.

4.4 Presence of OBP/OBS


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Traditionally, the satellites have always been used as bent pipes with no otherprocessing at the satellite, except for reflecting transmitted waves. The alternative is toallow on board switching and processing. The requirement for satellite switching resultsfrom the need of small, inexpensive earth terminals. This could be supplied by multiplebeams [33]. However, multiple beams need switching between beams or inter-beamswitching and hence satellite switching must be considered. Satellites with no OBS limits

the applicability of satellites for internetworking to simply links connecting two terrestrialstations. With OBS, earth terminals with differing QoS requirements can share the uplinkchannel.

There are on-board switching architectures that implement the adaptation of real-timeand non-real time services to the satellite communication link, while achieving significantstatistical advantage on communication links, uplinks and downlinks [14]. This model isbased on the GEO constellation. It exploits the burstiness of real-time traffic, thisarchitecture achieves high system throughput. In this particular architecture the onboardswitch does demodulation, detection and correction of transmission errors, afterreceiving the signal and time-multiplexed into digital baseband streams. For thisparticular scheme, the traffic is divided into two types. The CBR and rt-VBR traffic

belong to one high priority category and the nrt-VBR, UBR and ABR class traffic belongto the second low priority category. The switch architecture includes

Input de-multiplexer for separation of the high and low priority traffic

A packet switch to route these traffic Output queuing packet switch, producing one queue per downlink satellite RF

carrier, allowing for doing congestion control on the low-priority traffic. Output interleavers, which insert low-priority cells into unused high-priority

spaces.Thus, the high-priority traffic is handled according to a circuit emulation mode whereasan ATM-like packet switch handles the low-priority traffic. The main advantages broughtabout by OBP are [14]

Significant increase in system throughput Offered a natural flexibility of a packet-oriented transfer mode Achieves true packet switching and statistical advantages for large capacity ATM

networks Achieves the required data rates with multimedia traffic from small terminals,

together with meshed networking. Regenerative switching and multi-beamonboard processing payload satellites can achieve this.

The inherent broadcast function. Every subscriber located within the samedownlink spot beam as the called subscriber can, receive a message forwardedto this user station. The normal mode of operation is user specific. An extensionof the broadcast nature along with return link provides the necessary interactivityrequired by multimedia services [31].

Flexibility of the switch to act both in circuit-switched and packet switched modes.

The large capacity achieved. Improved connectivity

Processing gain, coding gain and optimized link designs[3].

Hence the use of OBP/OBS is very much recommended and the issues to beaddressed, before the selection of OBP/OBS are

Space environment considerations and associated delays (e.g., GEO systems)


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Satellite limitations like long transmission delay, link noise, local weatherconditions and interference.

Cost of operation of satellite and launch costs. The costs associated withlaunching satellites with OBS/OBP are high compared to that of bent pipesatellites.

Lifetime of the satellite. Generally the satellites last for an average of ten years. Onboard buffer size. This is a very important issue, since the real estate or

memory requirements onboard the satellite are scarce and hence the buffer sizeshould be carefully chosen. Simulation studies for different types of ATM trafficare done and should be used [14] before choosing the value for this parameter.

Capacity and port rate are other important parameters in addition toimplementation considerations. These are addressed in [33].

While terrestrial switches should be modular to cater to a broad range ofcapacities, OBS could be a lot simpler and tailored to satellite communications.

Due to restrictions on payload size and costs, there should be distribution ofATM-layer functions between onboard switch, NCC and ground terminals.

Due to restricted lifetime of satellites, fault tolerance should be added byintroducing fault detection and redundancy, both internal and external to theswitch [33].

Because of switching delay in the satellites and also to prevent retransmissionsin a long-delay path, the onboard buffers should be larger than the terrestrialswitches to limit onboard congestion.

Due to hostile radiation environment, particularly in GEO constellations, theswitch ASICs and memory chips for buffers should be suitably safeguarded. Therad-hard technology is advised [33].

Switch architectures with a large number of components may be unsuitable dueto satellite limitations in terms of size, mass and power.

Power consumption and power dissipation are other significant factors to beconsidered.

CLRs should be in the range of 10^-10 to meet the QoS of high-performancetraffic and avoid costly retransmissions [33]. To get good throughput/delay performance, output or shared queuing should be

used. The output queuing mechanism could be physical buffer based or virtualbuffer based. There are issues in choosing fully output buffered switch. Aftersorting through the issues, a fully interconnected fabric with output portconcentrators similar to the knockout switch is being proposed. The high CLR ofthese types of switches should also be taken into consideration [33].

Functions that could be considered for OBS/OBP are switching, queuing, flowcontrol and scheduling. Connection admission control and resource allocationshould be handled at NCC preferably. All delay-tolerant functions should be kepton the ground.

4.5 MAC layer protocols, scheduling and ATM services mapping for QoS

The key difference between a SATATM network and the terrestrial network is the factthat the SATATM network uses multiple access in the uplink. The choice of multipleaccess schemes has a great impact on the SATATM network. The primary goal in theassignment process is

Satisfy the users QoS in the form of maximum cell transfer delay (maxCTD),peak cell delay variation (peakCDV) and cell loss rate (CLR).


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Maximize the utilization of the uplink

Cell delivery in a timely manner and with minimal collisions [26].Satellite networks present unique challenges in system design related to QoSprovisioning. MAC protocols are behind the delivery of QoS contract. MAC protocolshould achieve QoS provisioning, efficiency and service interoperability [26].

Satellite environments affect MAC protocols with the long propagation delay, physicalchanges to the controllers in space, dynamic nature of satellite links and limited buffermemory.

The traditional CSMA/CD schemes cannot be used with satellite channels, since it is notpossible for earth stations to do carrier sense on the up-link due to the point-to-pointnature of the link. A carrier sense at the downlink informs the earth stations aboutpotential collisions that may have occurred 270ms ago. Such delays are not practical [1].Most SATATM schemes use dedicated channels in time and/or frequency for each user.ALOHA, Frequency Division Multiple Access (FDMA), Time Division Multiple Access(TDMA) and Code Division Multiple Access (CDMA) are such schemes. The ability touse OBP and multiple spot beams will enable future satellite to reuse the frequencies

many times more than todays system. Demand Assigned Multiple Access (DAMA)systems allow the number of channels at any time is less than the number of potentialusers. Satellite connections are established or dropped only when traffic demands them.Protocols like Packet Reservation Multiple Access (PRMA), an improved form of TDMAwith techniques from S-ALOHA, could also be used. Its application will depend on

Round trip delay (higher is bad for PRMA) Application and required QoS

BER of link (high BER is bad) [1]

CDMA is another preferred method. It uses a type of spread spectrum communicationand its inherent advantages like distributed coordination, chipping code method ofauthentication, high security and reuse of same frequencies has made it a good method

to use for satellite communications. The main disadvantage is the increase in BER withthe increase in the number of users.

MF-TDMA is another protocol for consideration. MF-TDMA is a

Preamble-less TDMA. It gives bandwidth-on-demand capacity allocation and saves uplink transmission

power. MF-TDMA is divided into two areas, each of which has fixed-size slots. The

signaling and synchronization area allow the terminal to request and receive thetiming information necessary for its synchronization, as well as the sending ofATM and satellite signaling for connection establishment and initial entry. Thedata area of the uplink frame is where ATM cells are transmitted. The slots canalso hold Forward Error Correction (FEC) and in-band signaling. The ATM cellpayload capacity on each frequency in the data area is 2Mbps [8].

MF-TDMA is the preferred MAC protocol for some commercial SATATM products.There are five specific uplink access schemes [8,25] to support the connections and theyare

Random Access Fixed Assignment

Fixed-rate demand assignment


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Variable-rate demand assignment and

Free assignment. Adaptive protocols

Hybrid protocols

The merits of these schemes are discussed in [46]. When a cell arrives at a queue,signaling messages are sent to the satellite notifying it of its arrival. When the satellitereceives this information, it dynamically assigns slots to the connection. The drawback isthe delay for the signaling message sent to the satellite. Thus there is a minimum delay(~0.5s) to be taken into consideration, irrespective of the other conditions. On thedownlink, transmission is multicast and the suitable protocol is Time DivisionMultiplexing (TDM). In order to achieve a greater efficiency in SATATM networks, theDAMA scheme can be employed with other access schemes like MF-TDMA and SCPC[30].3.5.1 Design considerations

There are design considerations based on the mode of usage of satellites and theresulting source traffic at the satellite network level [26]. The following diagram shows

two satellite system network scenarios [26], which can help decide which MAC protocolwould be better for different scenario.

Fig.4 Satellite network scenarios based on traffic aggregation

Demand Assigned Multiple Access MAC protocol can be used, when Burstiness of traffic is high

Low bit-rates are to be supported BW conservation

Delay requirements not critical.Based on the above diagram, DAMA can be readily applied to the wireless cell scenarioand not for the Internet backbone case. For this case, fixed bandwidth allocation couldbe used. There are different DAMA techniques in use and research has been done onthe various DAMA protocols [26]. The most preferred mode of usage of DAMA is fornrtVBR, whose requirements is low packet loss and for ABR, DAMA and hybrids of AMAare attractive solutions [26].


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An in depth study on MAC protocols for Mars Regional Network [47] could be consultedfor more information. In another extensive study [25], a set of performance objectivesare identified and different MAC protocols are analyzed. The performance objectives are

High channel throughput Low transmission delay Channel stability Protocol scalability

Channel reconfigurability Broadband applicability

Low ComplexityFollowing tables from [25] should be used to differentiate among the plethora of MACprotocols.

Table 4: Relation between traffic models and MAC choices

Traffic Model MAC class choice

Non-bursty traffic Fixed Assignment

Bursty traffic Random access

Bursty traffic, long messages, large

number of users

Reservation protocols with contention

Bursty traffic, long messages, smallnumber of users

Reservation protocols with fixedTDMA reservation channel.

Table 5: Performance comparisonProtocol Average

thputMeandelay

Stability Scalability Recon-figur-ability

Bbandapps

Cost-Comp-lexity

FixedassignmentB-TDMA

G-TDMA

Low

High

Low/Med

Low

Med/High

High

No

No

No

No

Yes

Yes

Med

Med

DemandAssignmentMSAP Med/high Med/high Med/high No No Yes Med-highRandomAccessS-Aloha Low Very

LowLow Yes Yes No Low

ReservationR-Aloha High Very

LowMed Yes Yes No Low

HybridAloha-RRRR

HighHigh

Low-MedLow-Med

MedMed

YesYes

YesYes

YesYes

MedMed

AdaptiveSRUCMDMA

HighHigh

VeryLowLow-Med

HighLow

YesYes

YesYes

YesNo

HighHigh

Here, Mini-Slotted Alternating Priorities Protocols (MSAP), Slotted-Aloha (S-Aloha),Round Robin Reservation (RRR) protocol, Split Reservation Upon Collision (SRUC),


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Minimum Delay Multi-Access protocol, Generalized TDMA are being used. Interestedreaders are referred to [3, 25] for further information on these protocols and othervariations of the same. Some of the conclusions are that the hybrid protocols that takeadvantage of both random access and reservation protocols have better throughputversus delay characteristics. The basic assumptions behind the study are bursty trafficand asymmetric satellite links.

Another study [7] on MAC protocols compares them in a different plane and is givenbelow.

Table 6: Performance comparisonAccessprotocol

Efficiency Delay Stability Robustness Complexity

S-ALOHA .37 Low Low High Low

Tree CRA .43-.49 Medium Medium Poor MediumDAMA(reservation)

.6-.8 High High High Medium

Hybrid(reservation-random)

0.6-0.8 Variable Medium High medium

Here Tree Contention Resolution Access (Tree CRA) and others are being used.

3.5.2 MAC on the uplink for ATM traffic a case studyMedium access protocols on the uplink for ATM traffic should be made suitable fordifferent kinds of ATM data connections and service classes. A sample assignmentscheme is discussed in [8]. The concentration is on CBR and VBR traffic. This schemeassumes Multiple Frequency-TDMA (MF-TDMA) as the MAC protocol.

The following tables and explanation gives the overview of a sample assignmentstrategy for ATM traffic [8]. Let Aii cells/frame be the bandwidth (BW) allocated for fixed-rate demand assignment to connection ii in a particular uplink beam. Let Bii cells/framebe the same connections variable-rate demand assignment allocation. Let Cii be thetotal BW allocated for connection ii. The terms PCR refers to Peak Cell Rate, MCR toMinimum Cell Rate and SCR refers to Sustained Cell Rate.

Table 7: Resource allocation for ATM service classes [8]

ATM class Cii Aii Bii

CBR PCR PCR -

VBR SCR to PCR QoS dependent QoS dependent

ABR MCR to PCR - MCR to PCR

UBR 0 - -


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Statistical multiplexing is one of the key benefits of ATM. The most common method ofexploiting stat-mux is to merge multiple VBR streams with similar statistical propertiesinto a common FIFO queue, which may be given some constant rate of service. Thesecould be intra-terminal statistical multiplexing or inter-terminal statistical multiplexing.These also could be taken care of in satellite framing [8]. There is a Hierarchical RoundRobin scheduler discussed in [8] which can schedule the uplink access. The advantages

reported are its simplicity, fine BW granularity and avoidance or large delay jitter. Themain pre-condition is the presence of OBS/OBP in the satellite.

In another study [44], some simple rules for ABR service on SATATM networks werefound and studied. This relates to the count of missing resource management cells(Crm) parameter of the ABR source behavior. Based on the study, the size of theTransient Buffer Exposure (TBE) parameter was set to 24 bits, and no size was enforcedfor the Crm parameter. According to the study, this simple change improved thethroughput over OC-3 satellite links from 45Mbps to 140 Mbps. It was also found thatlarge values are needed for Crm parameter for long delay links or high-speed links.

3.6. Power management

One of the major challenges in the design of a satellite network is the limitedtransmission power of both the ground terminals and the satellite. Transmissions in thenetwork should be such that the user terminals at different geographical areas are givenaccess in the most power efficient manner [8]. Multi-beam satellites are proposed forthis. Multi-beam systems need OBS/OBP. Hence when doing power management, theissues regarding OBP/OBS should also be taken into consideration. To further save onuplink transmission power, MAC protocols like MF-TDMA can be used as the data-linkprotocol.

4.7 Error Correction ScenariosIn satellite channels under consideration, transmission bit errors occur in bursts due tolink attenuation and use of convolution coding to compensate for channel noise.

Because ATM was designed to be robust with respect to bit errors randomly distributed,burst errors introduce cell loss (CL). For a BER of 10^-7, the CL ratio can be as high as10^-6. Though AAL5 has a 32-bit CRC, it is not used due to the high cell discard rate atthe physical level [30]. There are several schemes for error correction like

Interleaving mechanism Error recovery algorithms And efficient coding schemes, for improving error performance.

It has been shown that when interleaving is done, the ATM cell discard probability (CDP)and probability of undetected errors are less. Interleaving the ATM cell tends todistribute or spread the bit errors at the cost of increased delay. The interleavingalgorithm can be applied differently according to the AAL types. There is a chance that

errors can occur in the interleaved cells. Another problem is that the interleaving depthfor optimal error performance is still not evident [30].Error recovery algorithms like automatic repeat request (ARQ) could be used to lowererror ratio for loss-sensitive, delay-insensitive scenarios. There are stop-and-wait, Go-Back-N and Selective-repeat algorithms. See [40] for more details in error recoveryalgorithms. Go-Back-N and Selective-repeat are better than stop-and-wait algorithms.Coding scheme can be used for error correction or prevention. Currently, convolutioncode with viterbi decoding is used to achieve 10^-3 to 10^5 BER [30]. This is not fit forSATATM networks because of the loss-sensitive ATM traffic. Hence concatenated


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coding with outer coding as Reed-Solomon (RS) coding with Forward Error Correction(FEC) as the internal convolution code is being currently used and is a good performer inthis area [30]. Here also, optimal interleaving depth for SATATM networks should still bedetermined.An in-depth study of the impact of transmission error characteristics on SATATMs isstudied in [18]. The ATM cell performance measures are Cell acquisition time (CAT),

Cell in-synch time (CIT) and cell discard probability (CDP). Satellite links that operate athigh rates employ error correction schemes for providing acceptable BER. Burst errorsare generated by these error correction schemes. The ATM HEC is capable of correctingonly single-bit errors. A method called ATM link enhancement (ALE) was developed,which incorporates a selective interleaving technique allowing it to be transparentlyintroduced into the satellite link. More information is given in the section underCommercial SATATM Products in this paper. Studies confirming its validity are shown in[18]. AAL1 uses a 3-bit CRC, AAL3/4 uses a 10-bit CRC and AAL5 uses 32-bit CRC forerror detection and error correction. All the codes used for AALs are sensitive to bursterrors, hence the need for better error control algorithms.

In a related experiment [47], an error correction scheme using side information is

proposed to improve the throughput of ATM transmission over Rayleigh fading channellike a satellite link using binary phase shift keying (BPSK) modulation. The methodcombines the ARQ protocol and the error correction scheme with side information (a bit-marking technique is employed to get an idea of erroneous bits) to improve thethroughput.

In another experiment [24], a shorter error correction model called Bose-Chaudhuri-Hocquenghem (BCH) code could be used. A more ATM oriented solution is alsodiscussed, which is called the Partial Packet Discard (PPD), which on detection oferroneous cells at the satellite switch, these and consecutive ones are dropped andhence reduce the traffic. This suffers from the retransmission problems (increase incongestion) due to obvious reasons. The study goes on to explain implementations for

the different AAL layers for ATM. A comparison of PPD approach with a LLC layermechanism is also carried out.

In another related study [9], a solution is proposed for the error control mechanisms toadapt to the satellite channel by moving the error recovery and detection to a higherlayer of the ATM. This is based on the ability of the ATM to determine the service of theretransmission and to base recovery on that service. The study also shows simulationresults to confirm a significant increase in raw data throughput and that in ATM transferefficiency 7.5%. The results also show that it is possible to guarantee data services withno loss of data under certain conditions. The author does this by changing the currentATM adaptation layer with a proposed Convergence sub-layer AAL. It is also proposedthat differentiation based on the service during recovery and re-transmissions is

necessary.

The relation between BER and CLR has been studied and documented in [15]. TheCLR-vs-BER performance is quite linear. The effects and graphs are to be studiedbefore implementation.

4.8 Traffic Control and Congestion Control


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Traffic Control is a measure that takes actions to avoid congestion conditions.Congestion control acts after congestion is set. Traffic Control is congestion avoidance.This is very important, since the satellite links are bandwidth limited [30]. The algorithmsshould act faster and more efficiently due to the long delay. The basic QoS parametersare Cell Loss Ratio (CLR), maximum and mean cell transfer delay (CTD) and cell delayvariation (CDV) and the extended QoS parameters are cell error ratio (CER), severely

errored cell block ratio (SECBR) and cell mis-insertion ratio (CMR) are alsorecommended. The impact of satellite delay on some basic services is tabulatedhere[30].

Table 8: Effect of satellite link delay on applicationsApplication and properties Sensitivity to satellite link delay

Video and voice service- generatesbursty traffic.

Very sensitive- real time services, goodas long as delay variation is kept verysmall

Text or data serviceneeds reliability Not sensitive

Video telephony Not sensitive, future video telephonymay be sensitive

Computer Supported Cooperative Work(CSCW)

Not sensitive, but delay on TCP/IP dueto satellite delay degrades entireperformance. [see section on UpperLayer Concerns in this paper]

ITU-T and ATM Forum have specified traffic control functions, which manage and controltraffic to avoid congestion in ATM networks. These functions should be considered.There are different traffic control procedures described. They are

Traffic Shapingo Mechanism to change the traffic characteristics of a cell stream to

achieve desired characteristicso Should maintain call sequence integrityo Are peak cell rate reduction, burst length limiting and CDV reductiono Cannot be used when network is congested

Priority control and selective cell discard mechanismo CLP bit is manipulated as a means of traffic control to discard the ATM

cells with lower priority.o Not efficient in ensuring data deliveryo Can aggravate congestion due to retransmissions

Connection admission control (CAC)o For occasional congestiono Is the set of actions taken by a network to establish whether an ATM

connection can be accepted or rejectedo Useful only in the call-setup phase for SATATM networks.

Congestion control mechanisms are of many types [30]. A frequently used scheme isselective cell discard. It has advantages and disadvantages as briefed above. Anothermethod is Explicit Forward Congestion Indication (EFCI) incorporated with a feedbackmechanism. EFCI is used to convey congestion notification to the source. Thedestination protocol is required to notify the source of congestion. This whole process isForward Explicit Congestion Notification (FECN). In SATATM, this is not very wellmatched due to the minimum delay of one-way propagation for the notification.Backward Explicit Congestion Notification (BECN) is a mechanism, which could be used


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to send a notification in the reverse direction of the congested path. Buffering and VCprioritization can also be used in congestion control. The satellite on-board buffer couldalso be considered. This could introduce jitter, if not properly done. A related mechanismis VC prioritization. Other congestion control mechanisms are discussed in [49], althoughthese should be changed for SATATM considerations. Thus the criteria of choosing thealgorithms should be that, these should not affect the delay-sensitive traffic for

SATATMs.

A study of a CAC scheme that exploits statistical multiplexing of radio resources in anintegrated ATM-satellite environment has been done [38]. The proposed CAC strategyeffectively exploits the satellite BW and provides QoS to both real-time and non-real-timeVBR sources, while permitting contemporary access to the resources to a great numberof users.

4.9 Upper layer concerns

There is a service specific convergence sub-layer (SSCS) in AAL. This SSCS is dividedinto service co-ordination function (SSCF) and SSCOP. The service specific connection

oriented protocol (SSCOP) can run on all protocol stacks. Its main function is to provideassured delivery of PDUs and use error-recovery procedures if necessary. The followingfeatures [18] are very favorable to SATATM networks. They are the selectiveretransmissions, nearly infinite window size definition capability, superior flow control,optimized support for high-speed and long-delay networks and the protocol is designedto be insensitive to network delay. SSCOP has been proposed by some people as apossible replacement for TCP as a wide-area transport protocol, however some doubtshave been expressed as to its efficiency in the face of errors, congestion, variabledelays. A thorough investigation of SSCOP, including simulation to determine itsperformance in terms of throughput etc., in a typical error/congestion/delay environmentshould be carried out.

TCP is the de-facto standard for the Internet transport protocol. Considerations for usingTCP over ATM over satellite communications have been studied in sufficient depth[5,6,7,20]. The considerations and findings are explained in this section. A thoroughstudy [7] gives the TCP performance and buffer requirements over the satellite-ATM-UBR service and provides guidelines on improving TCP performance in such situations.

4.9.1 TCP changes for ATM UBR

The ATM UBR service category is expected to be used by a wide range of applications.Buffer requirements increase with increasing delay-bandwidth product. The efficiency ofTCP over UBR is measured byEfficiency = (Sum of TCP throughputs)/(Maximum possible TCP throughput

Fairness Index = (xi )^2 / (N * xi^2 )Where xi = throughput of the ith TCP source and N = number of TCP sources.The buffer requirements are as follows.

For very small buffer sizes, the resulting TCP throughput suffers. TCP performance increases with increase in the buffer sizes TCP performance over UBR for sufficiently large buffer sizes is scaleable with

respect to the number of TCP sources. A buffer size of 0.5*RTT to 1*RTT is sufficient to provide over 98% throughput.


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Fairness is high for a large number of sources. This shows that TCP sources with agood per-VC buffer allocation policy like selective drop, can effectively share the BW.Providing a guaranteed rate (GR) to UBR traffic has been discussed as a possiblecandidate to improve TCP performance over UBR service. Guaranteed Frame Rate(GFR) is also being discussed as an enhancement to the UBR category. For TCP over

4.9.2 TCP changes for ATM ABR

For this case, virtual source and virtual destination can be used to isolate long delaysegments from terrestrial segments [5], which help in efficiently sizing buffers in routersand ATM switches. Therefore, terrestrial switches only need to have buffers proportionalto the BW-delay product. Employing feedback is also a mechanism for giving feedbackto the sources.

4.9.3 TCP changes for satellite communications

These issues are important and must be taken into consideration before choosing on a

Transport layer protocol. An RFC [20] published recently, does an in-depth study onTCP over satellite communications and has come up with the followingrecommendations and hence could be followed.

Maximum window size remains a hindrance to the SATATM networks. Anincrease in the window to 2 ^30 is being proposed [6]. Also, larger initial windowsize has been recommended

TCP for transactions could be used due to the lesser number of handshakes Slow start wastes network capacity and are also inefficient for transfers that are

shorter in size. To counter delayed ACK caused delay in the sender side to increase the window,

byte counting approach is being studied. Otherwise, delayed ACKs must be

used only after the slow start phase. The Fast Recovery method should take into account, information provided by

SACKs sent by the receiver. The Forward Acknowledgement algorithm was developed to improve TCP

congestion control during loss recovery. Explicit congestion notifications should be used

Differentiating between congestion and corruption is a difficult problem for TCP.Doing it would be of great use to TCP over SATATM networks. This is handled in[20].

During congestion avoidance, in the absence of loss, the TCP sender addsapproximately one segment to its congestion window during each RTT. Thisleads to unfairness and hence fair queuing and TCP-friendly buffer managementin network routers is being considered.

The use of multiple data connections for transferring a file in a SATATM networkimpacts the network and should be used after careful review.

Rate-based pacing (RBP) is being considered to counter the slow windowopening during slow-start and could be used.

TCP header compression is a viable alternative for bandwidth-sensitive SATnetworks.


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Sharing TCP state among similar connections could be used to overcomelimitations in the configuration of the initial state.

In highly asymmetric networks like satellite links, a low-speed return link cancause performance drop due to congestion in the acks returning to the sender.Hence Ack Congestion Control (ACC) must be done.

Ack filtering can be done in the previous case to limit the number of acks in the

return direction. This could be done taking into advantage, the cumulativeacknowledgement scheme of TCP.

These are some of the TCP improvements to be made for supporting satellite networksin general and will apply to SATATMs as well.

4.9.4 IPv6 over ATM over satellite communications

IPV6 projects are being undertaken over ATM over satellite communications [49, 50, 51].IPv6 will support hierarchical addressing, routing, Quality of Services, mobility, security,multi-peer communications. IPv6 coupled to Asynchronous Transfer Mode (ATM) andGEO/LEO satellites technologies is being investigated as a solution to meet the Air

Traffic Management and passengers applications requirements for Air-Air, Air-Groundand Ground-Ground segments with multimedia high-bandwidth [50]. The issues relatedto the management of the QoS over an aggregation of ATM and Satellite networks fallinto several general classes [51]

how to map the Internet IntServ model to the ATM QoS model, how to make RSVP, the Internet signaling protocol, run over ATM and Satellite, how to handle the ATM VCs to be able to provide the requested QoS and to

optimize the network resources, how to aggregate IP flows, how to handle the many-to-many connectionless features of IPv6 and RSVP, how to map efficiently the routing algorithms with the switching mechanisms, how and when to use satellite to dynamically set up shortcut route between

nodes, which time-critical data should be routed over a satellite overlay network on top

of a terrestrial network, how to balance the load between satellite and terrestrial links, how and where to monitor the achieved QoS performance, which measures to prevent misuse/unauthorized use of network resource, how to optimize the use of network resource to fulfill the required QoS.

The research work on using IPv6 over SATATM is still going on and many results areawaited.

4.10 Attenuation considerations

Path loss can occur in satellite transmissions due to the following conditions. Weatherconditions like rain, integrated water vapor concentrations and cloud liquid watercontents can affect the transmission. Attenuation due to rain is a major problem in theKa and Ku bands. The effect of airline traffic on satellite transmissions is also studied[45]. Global predictions of slant path attenuation are also being studied and should be


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taken into consideration. Information related to attenuations could be further studied at[54].

4.11 ATM layer changes for satellite considerations

There are changes proposed to the ATM layer and specifically in the Service Specific

Sub-layer of AAL to incorporate satellite communications as the physical layer transportof ATM. In one type of change, the CRC and the sequence numbers are moved to thehigher convergence sub-layer. This will entail larger blocks to put error detection andcorrection on. It is suggested here [9] that high-speed, long-delay satellite links need aunique AAL. That is answered in [24], where a separate layer called S-ATM layer isprovided for satellite communication scenarios. In another study [43], the groundsegment proposal is based on a new AAL called AAL2, which is considered to play amajor role in offering an efficient way to provide multimedia services over ATM networks.It allows easy encapsulation of the complete set of media component sessions, whichforms a multimedia transaction into a single ATM VC connection.

4.12 Link budget scenario

Link budget is a generic term used to describe a series of mathematical calculationsdesigned to model the performance of a communications link. In a simplex satellitecommunication, two link budgets are needed, one for uplink and one for downlink. See[52] for more information. For example, the ISL performance could be studied by linkbudgets, as in [31]. The link budget parameters of an optical ISL could be Operationalwavelength, Telescope diameter, Receiver type, Modulation, Coding, Distance, AntennaGain, Space loss, Total transmission, Receiver sensitivity and Required Transmit power[31]. These are important parameters to be considered before the design of the systems.

4.13 Elevation angles

Impact of elevation angles on SATATM network design has been studied in [13]. Use ofGEO satellites means lower elevation angles and large delays in high altitude regions.These problems can be solved by the use of satellites at much lower altitudes such asMEO and LEO. By using MEO/LEO satellites and selecting an appropriate inclinationangle, these orbits can offer much higher minimum elevation angles over high altituderegions. High elevation angles will lead to a very low probability of shadowing andtherefore offer a very high availability of service.

4.14 Cell transport methods

Various schemes are possible here. They are plesiochronous digital hierarchy (PDH),

SONET synchronous digital hierarchy (SDH), physical layer convergence protocol(PLPC) and no framing. Studies have been done about the differences between them[30]. PDH was developed to carry digitized voice efficiently in major urban areas. Thereare some inefficiency regarding rerouting difficulty and redundant operations. SDH wasdeveloped to take care of the totally synchronized network. SDH is much preferred toPDH [30]. PLCP is another cell transport method and it is found to be not suitable in theburst error environment [30]. Thus SDH is preferred.

4.15 Encryption of traffic


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There are tools in production for encryption of ATM traffic over satellite links. A studywas done on ACTS ATM Internetwork (AAI) platform with a prototype ATM encryptiondevice [35]. One of the new generations of encryptors for unclassified ATM networks iscalled FASTLANE. The study reported a successful encryption based experiment.Encryption of SATATM traffic is certainly possible and should be used when necessary.

4.16 Related Information

4.16.1 High Altitude Long Endurance (HALE) systems

Experimental HALE platforms are essentially highly efficient and lightweightairplanes carrying communications equipment that will act as very low earth orbit geo-synchronous satellites. High efficiency turbine engines or a combination of battery andsolar power will power these crafts. At an altitude of only 70,000 feet, HALE platformswill offer transmission delays of less than .001 seconds and even better signal strengthfor very lightweight hand-held receiving devices.

4.16.2 Commercial SATATM products

Commercial SATATM products are available in the markets. COMSAT is acompany specializing in SATATM products and the ATM Link Enhancer (ALE) discussedbefore is an innovation from COMSAT. More details are given below.

COMSAT Link Enhancer (ALE-2000) and Link Accelerator (CLA-2000/ATM)provide an essentially error-free satellite link in a bandwidth efficient manner atfractional T1 to DS3 rates. ALE-2000 is a networking device that allowscustomers to interconnect ATM networks over satellite and wireless links at DS3and E3 rates. The advantages or properties of the product involve efficientbandwidth utilization, fiber-like link quality and significantly improves theperformance of applications over satellite and WATM. The error-correction for theerror-prone satellite links, is taken care of by introducing Reed-Solomon forward

error correction into the data stream and introducing interleaving. The CLA-2000/ATM is designed for use over links (satellite or otherwise) operating atfractional T1 to 8.448 Mb/s, symmetric and asymmetric data rates. It alsosupports rate adaptation, ATM cell header compression and cell payload lossless compression. Linkway 2000 product from ComSat can be used to cope withthe heterogeneity of network protocols and interfaces and develop satellitenetwork solutions that can accommodate these in a bandwidth efficient manner.Using this product, ComSat researchers have shown an overview of a networkconsisting of IP, ATM, FrameRelay, ISDN and SS7 services in a fully meshedmode at data rates ranging from 64Kb/s to 32 Mb/s [21].

ALA-2000 also provides interconnection of standard ATM interface rates to non-standard satellite link rates. It is also compatible with standard ATM switches and

modems and provides cell error ratios of 10 to the power of10 or better.

4.16.3 NASA-ACTS

Operating in the Ka-band (20/30 GHz) where there is 2.5 GHz ofspectrum available (five times that available at lower frequency bands).

Very high-gain, multiple hopping beam antenna systems which permitsmaller aperture Earth stations.


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On-board baseband switching which permits interconnectivity betweenusers at the individual circuit level.

A microwave switch matrix, which enables gigabit per secondcommunication between users.

a Ultra-Small Aperture Terminal (USAT) which can support 9.6 kbps froma 35 cm antenna to a 1.2 m hub. In experiments, the USAT has beendemonstrated at up to 1.544 Mbps using a 60 cm antenna and the 4.7 mLET hub.

High Data Rate Earth Station. The ACTS High Data Rate terminal iscapable of transmit ting data at 622 Mbps using a 3.5 m antenna.Alternatively, up to 4 stations operating at 155 Mbps can be supportedsimultaneously in a satellite switched time division multiple access(SS/TDMA) mode. The Harris T1 (1.544 Mbps) Very Small ApertureTerminal (VSAT) - using the ACTS baseband processor (BBP) mode, theT1 VSAT supports up to 1.728 Mbps using a 1.2 m antenna. High Speed

VSAT. A modification of the T1 VSAT will allow it to operate with the BBPat rates up to 22.5 Mbps in small, limited networks. [49]

Gigabit satellite networks have been proposed and is operational [12]

4.16.4 Commercial satellite design guide

It gives a comprehensive overview of hardware and technical information onsatellite networks is present online at [52]. It is strongly suggested that the document bereviewed before design of SATATM networks. SATATM networks were used in oilindustry, which is being used to gather, process and exchange oil-exploration ideas [53].

4.16.5 Rule-based practical design approach for building commercial satellites

A practical design guide for large satellite networks, which gives a design technique,which employs a set of rules for satellite network design, in combination with extensivedatabases of satellite parameters, earth-station parameters and user trafficrequirements, to synthesize a network architecture. This is a very important step forpractical implementation of satellite networks [17].

4.16.6 VSAT terminals

Due to high performance requirements, the design of an earth station is quitecomplicated. This increases the costs and the need for maintenance. VSAT provides asolution to this problem. The key point in VSAT networks is that either the transmitter or

the receiver antenna on a satellite link must be larger. In order to simplify VSAT design,a lower performance microwave transceiver and lower gain dish antenna (smaller size)is used. They act as bi-directional earth stations that are small, simple and cheapenough to be installed in the end user's premises. VSAT networks are typically arrangedin a star based topology, where each remote user is supported by a VSAT. The Earthhub station acts as the central node and employs a large size dish antenna with a highquality transceiver. The satellite provides a broadcast medium acting as a commonconnection point for all the remote VSAT earth stations. VSAT networks are ideal for


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centralized networks with a central host and a number of geographically dispersedterminals.

The weaker signal from the remote ES is amplified at the satellite acting as a bent pipeand received by the hub ES. Thus, the lower gain at the uplink is compensated at thedownlink by the high performance hub ES. The down side of this arrangement is that

when two VSATs need to communicate, two satellite hops are required because allconnections must pass through the hub ES node. The data link supported from the hubto the VSAT is typically slower (19.2 kilobits (kbps)) than that in the reverse direction(512 kbps) [52]. DirecPC services from HNS, is one of the examples of VSAT systems.The main disadvantage is that TCP/IP is not well suited here and X.25 is the commonprotocol.

5. Conclusions

The paper discussed a comprehensive discussion about ATM over satellitecommunications, analyzed its issues, explained its tradeoffs, and went over scenarios,which warrant SATATM solutions. This paper is meant to be a good theoretical design

guide, for a group starting to achieve some thing in SATATMs. This paper is by nomeans a complete design guide and the references section is meant to consummate theideas and research presented in this paper. Interested users should use this paper as astarting material and go to each of the catalogued references to do further analysis.

The section following the motivating scenarios, mentioned SATATM solutions for thesame. Here, a particular motivating scenario will be dealt with, giving justifications forselection of the same. For the design solution, the mobility architecture would beconsidered (see sec. 2.1.2). Let fig.2 also be considered. A perfect example of mobilenetworks is the presence of networks in the ships and the ship wants to handlecommunications with other ships and also with the ground station. For discussion sake,let us consider that the ship needs encrypted communications for security and that the

distance between ground station or land-based ATM network and the ship-based ATMnetwork is pretty huge. The cost of implementation is not a factor. The communicationsare delay-sensitive. Multimedia traffic is assumed.

Prior to going into the solution, the design guides [17] and [52] should be reviewed.Assuming the above conditions, one way of designing the SATATM network would be asfollows. The satellites in the network is a LEO-based with multiple satellites connectedby optical ISLs (for performance). The attenuation factors for the communication shouldbe taken into account. The handover protocol is chosen as FHRP due to its superiorperformance in the face of delay-sensitivity and OBP/OBS is assumed to be present,due to the great advantages offered by it and DAMA is not used in this case (due todelay-sensitivity). MF-TDMA can be considered here. RS error code is used as the

external code and FEC as the internal code for error correction scenario. Suitableadjustments are made for rain and air traffic attenuation. More over, a CAC schemebased on [38] is used due to the superior performance of this scheme in the face ofmultimedia traffic. TCP/IP is assumed to be used, since TCP is a stable protocol and allthe feasible changes according to [20] are assumed to be handled. High elevationangles are assumed. The cell transport method can be SDH. The encryption is doneusing FASTLANE, due to its superior performance. There is S-ATM layer present in theAAL. This is done since, S-ATM is superior in the face of multimedia traffic and givesbetter QoS.


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This could be the modality of using this particular paper for design decisions. The futuredirections could be adding practical considerations like cost of the hardware andavailability could be added to this and the overall structure improved to handle moredesign choices. A software, could be designed taking in, the environment restrictionscould be taken in and the output of the software could be a high-level design solution.


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6. References

[1] R.M. Mir, "Satellite Data Networks," http://www.cis.ohio-state.edu/~jain/cis788-97/satellite_data/.[2] D. Hart, Satellite Communications, http://www.cis.ohio-state.edu/~jain/cis788-97/satellite_nets/index.htm.

[3] S. Fahmy et al., "A Survey of Congestion Control Techniques and Data LinkProtocols in Satellite Networks," submitted for publication to the Intl J. Satellite Comm.,1995, http://www.cis.ohio-state.edu/~jain/papers/sat_surv.htm.[4] Issues in Linking ATM Networks Via Satellite, http://www.erg.abdn.ac.uk/users/silas/issues.html.[5] R. Goyal et al., "Traffic Management for TCP/IP over Satellite-ATM Networks," IEEEComm. Magazine, Vol.37, No.3, pp.56 March 1999.[6] Higher Layer Protocols (TCP/IP) Over Satellites,http://itri.loyola.edu/satcom2/04_05.htm.[7] S. Kota et al., "Satellite ATM Network Architectural Considerations and TCP/IPPerformance", Proc. of the 3rd Ka Band Utilization Conference, Italy, September 15-18,1997, pp. 481-488.

[8] A. Hung et al., ATM via Satellite: A Framework and Implementation, WirelessNetworks, Vol. 4, Issue 2, 1998, pp. 141-153.[9] J. Murphy, Resource Allocation in ATM networks, doctoral dissertation, Dublin CityUniv., 1996.[10] R. Goyal et al., "Analysis and Simulation of Delay and Buffer Requirements ofSatellite-ATM Networks, submitted to IEEE J. Selected Areas in Comm., March 1998.[11] H. Uzunaliongcaron et al., A Connection Handover Protocol for LEO Satellite ATMNetworks, Proc. Of the Third Annual ACM/IEEE Intl Conf. On Mobile Computing andNetworking. Sept. 26-30, 1997, Budapest Hungary.[12] M.A. Bergamo et al., Gigabit Satellite Network Using NASA's AdvancedCommunications Technology Satellite (ACTS): Features, Capabilities, and Operations,17th Annual Pacific Telecommunications Conf., Jan. 22-26, 1995.

[13] B.G. Evans et al., Future Multimedia Communications Via Satellite, Second ka-band Utilization Conf. and Intl Workshop on SGCII Sep. 2426, 1996.[14] A. Baiocchi et al., An ATM-like System Architecture for Satellite CommunicationsIncluding On-board Switching, Intl J. Satellite Comm., Vol. 14, pp. 389-412, 1996.[15] Can ATM Technology Work on Satellites? Yes! It Can! ,http://www.telesat.ca/news/speeches/95-4.html .[16] M. Werner et al. "ATM-Based Routing in LEO/MEO Satellite Networks with Inter-Satellite Links", Intl J. on Selected Areas in Comm., vol. 15, No. 1 Jan 1997.[17] C. Cotner et al., An Architecture Design Approach for Large Satellite Networks,Intl J. Satellite Comm. Vol. 12, pp. 197-210 1994.[18] D.M. Chitre et al., "Asynchronous Transfer Mode (ATM) Operation via Satellite:Issues, Challenges, and Resolutions," Intl J. Satellite Communications, Vol. 12, pp. 211-

222,1994.[19] M.H. Hadjitheodosiou et al., Broadband Island Interconnection via Satellite-Performance Analysis for the RACE IICatalyst Project, Intl J. SatelliteCommunications, Vol. 12, pp. 223-238, 1994.[20] M. Allman et al., Ongoing TCP Research Related to Satellites, RFC 2760.[21] P. Chitre et al., Next-Generation Satellite Networks: Architectures andImplementations, IEEE Communications Magazine, Vol. 37, No. 3, pp.30-37, Mar 1999.[22] TIA/EIA Telecommunications Systems Bulletin 91 (TSB-91), Satellite ATMNetworks: Architectures and Guidelines.


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[23] S. Ray, Network Segment Mobility in ATM networks, IEEE CommunicationsMagazine, Vol.37, No.3, pp.38-45, Mar 1999[24] I. Mertzanis et al., Protocol Architectures for Satellite ATM Broadband Networks,IEEE Communications Magazine, Vol.37, No.3, pp.46, Mar 1999[25] H. Peyravi, Medium Access Control Protocols Performance in SatelliteCommunications, IEEE Communications Magazine, Vol.37, No.3, Mar 1999

[26] D.P. Connors et al., Modeling and Simulation of Broadband Satellite NetworksPart1: Medium Access Control for QoS provisioning, IEEE Communications Magazine,Vol. 37, No. 3, pp.72, Mar 1999[27] Y. Takefuji et al., ATM and Wireless Experiments for Remote Lectures, IEEECommunications Magazine, Vol. 37, No. 3, pp.97-101, Mar 1999[28] S. Yoshida et al., Interactive Multimedia Communication Systems for Next-Generation Education Using Asymmetrical Satellite and Terrestrial Networks, IEEECommunications Magazine, Vol.37, No.3 Mar 1999[29]W.R. Schmidt et al, Optimization of ATM and Legacy LAN for High Speed SatelliteCommunications, Transport Protocols for High-Speed Broadband Networks workshop,held at Globecom '96, November 22, 1996[30] I.F. Akyilidiz et al., Satellite ATM Networks: A Survey, IEEE Communications

Magazine, July 1997[31] M. Wittig et al., Large-Capacity Multimedia Satellite Systems IEEECommunications Magazine, July 1997[32] Satellite Communications: An Overview, http://www.doc.ic.ac.uk/~gmp1/article1/[33] J. Gilderson et al., Onboard Switching for ATM via Satellite, IEEECommunications Magazine, Vol. 35 No.7, pp.66-70, July 1997[34] On-board Processing, http://www.comsat.com/labs/network_tech/on-board.htm[35] M. Ehlrich et al., Encrypting ATM traffic over the ACTS ATM Internetwork, IEEECommunications Magazine, Aug. 1997[36] Supporting ATM on a Low-Earth Orbit Satellite System,http://www.isoquantic.com/pr/ATMsatellites-1.htm.[37] P.W. Dowd et al., Geographically Distributed Computing: ATM over the NASA

ACTS Satellite, Proc. IEEE MILCOM95, Oct 1995[38] Antonio et al., Integration of ATM and Satellite Networks: Traffic ManagementIssues, IEICE Trans. Comm., Vol. E83-B, No.2, Feb 2000[39] B.R. Elbert, The Satellite Communications Applications Handbook, Artech House,Inc. MA, 1997.[40] A.H. Tanenbaum, Computer Networks, 3rd Edition, PH, 1996.[41] Introduction to Global Satellite Systems,http://www.compassroseintl.com/Pubs/Intro_to_sats.html[42] Asynchronous Transfer Mode(ATM) Switching, http://alliancedatacom.com/cisco-atm-tutorial.htm[43] R. Mauger et al., QoS Guarantees for Multimedia Services on a TDMA-BasedSatellite Network, IEEE Communications Magazine, pp. 56-65, Jul 1997.

[44] S. Fahmy et al., On Source Rules for ABR service on ATM Networks with SatelliteLinks, Proc. First Intl Workshop on Satellite-based Information Services, Nov.1996.[45] H. Zhang et al., The Prediction of Attenuation Due to Aircraft's Flying across theEarth-Satellite Link at SHF, Electronic and Radio Applications, Vol.E81-B No.8 p.1687.[46] H.W. Lee et al., Combined Random/Reservation Access for Packet-switchedTransmission over a Satellite with On-board Processing-Part II: Multibeam Satellite,IEEE Tran. On Comm., Vol.32, No.8, pp.1093-1104, October 1984[47] H. Peyravi, A Survey of MAC Protocols for Satellite Communications,http://mars.mcs.kent.edu/~peyravi/MAC/mac95.ps


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