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 (sraghava@vt.edu)

24 APR 2000

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This paper discusses in depth the issues in using satellite links as the physical media for ATM internetworking and uses it to build a design guide for implementation of ATM over satellite networks. The challenge here is to provide ATM, a connection-oriented protocol developed specifically for a reliable high-bandwidth wired infrastructure, along with its QoS guarantees, for mobile networks, which are characterized 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. ATM was originally designed for links with low BER like fiber. In the case of satellite links, the error rate is orders of magnitude higher. The bursty nature of the error in satellite links also poses a big problem. This paper will systematically deal with such major issues of Satellite ATMs (SATATM) and their implementation. A set of motivation examples or scenarios for SATATM networks will be discussed. This will be used to compile a host of design issues and the various options available for the same and hence can be used as a theoretical design guide for future ATM over satellite implementations.

The paper is arranged as follows. After a brief introduction to ATM, Satellite communications and Wireless ATM (WATM), motivating network architectures, which has a great diversity of requirements, are presented. The need for SATATMs in those particular 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 how satellite communications should be optimized to provide other requirements and ATM specific behaviors like service guarantees (ABR, CBR etc.,), congestion control, and AAL issues. The section also deals with routing in SATATMs, MAC protocols for satellite communications, optimizations needed to use TCP over SATATMs and IPV6 over SATATMs. All the above issues would be analyzed and correlated with the motivating architectures given in the preceding sections. The section will also explain some practical SATATM products available in the market and their features. The paper will end with a section on conclusions, summarizing all the ideas presented and comments about the whole concept of SATATMs. The conclusions section would summarize the ideas presented and will present design solutions for the implementation of the motivation scenarios and also will discuss related issues and tradeoffs for the solutions. The section following the motivation scenario presents SATATM solutions for the scenarios. The conclusion section justifies and endorses the same.

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Table of Contents Pg. No 1. Introduction 4

1.1. Satellite Communications 1.1.1. Architecture and purpose 1.1.2. Terminology, characteristics, advantages and disadvantages

1.2. ATM and WATM 5

1.2.1. ATM architecture 1.2.2. ATM internals and physical layer issues 1.2.3. WATM and its features

2. Motivating Scenarios 7

2.1. Description of the architectures 3. SATATM details 10 4. SATATM design specifics 11

4.1. Constellation of the satellite 4.2. Handovers and re-routing 4.3. Presence of inter-satellite links 4.4. Presence of OBP/OBS 4.5. MAC layer protocols, scheduling and ATM services mapping for QoS 4.6. Power management 4.7. Error correction scenarios 4.8. Traffic control and congestion control 4.9. Upper layer considerations

4.9.1. TCP changes for ATM UBR 4.9.2. TCP changes for ATM ABR 4.9.3. TCP changes for satellite communications 4.9.4. IPV6 over ATM over satellite communications

4.10 Attenuation considerations 4.11 ATM layer changes for satellite considerations 4.12 Link budget scenario 4.13 Elevation angles 4.14 Cell transport methods 4.15 Encryption of traffic

4.16 Related Information 4.16.1 HALE systems 4.16.2 Commercial SATATM products (from COMSAT) 4.16.3 Focus on NASA-ACTS 4.16.4 Commercial satellite design guide 4.16.6 Rule-based practical design approach for building commercial satellites 4.16.7 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 that provides a microwave communication link between two geographically remote sites. Due to its high altitude, satellite transmissions can cover a wide area over the surface of the earth. Each satellite is equipped with various “transponders” consisting of a transceiver and an antenna tuned to a certain part of the allocated spectrum. The incoming signal is amplified and then rebroadcast on a different frequency. Most satellites simply broadcast whatever they receive, and are referred to as “bent pipes”. The traditional applications were TV broadcasts and voice telephony. Satellite communications for packet data transmissions is being considered. The applications like mobile services, direct broadcast, 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 carrier frequencies 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 on the orbital radius, satellites fall into one of the following categories. They are Low Earth Orbit (LEO), Medium Earth Orbit (MEO), Geostationary Earth Orbit (GEO) and Highly Elliptic Orbit (HEO). The constellations are described below and their relative merits are tabulated.

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 miles 6000-12000 miles

22,282 miles Variable due to elliptical orbit

Time in LOS 15 min 2-4 hrs 24 hrs Variable

Merits

Lower launch costs, very short round trip delays, small path loss

Moderate launch costs, small round trip delays.

Covers 42.2% of the earth’s surface, constant view

Maximizes time spent over populated areas, superior line of sight, fewer satellites

Demerits

Very short lifetime (1-3 months), encounters radiation belts

Larger delays, greater path loss

Very large round trip delays, expensive Earth Stations.

Not a complete coverage.

There are several merits to satellite communications as a whole as they can give global coverage to remote areas not connected by terrestrial network, chance to act as an alternate mode of communication in military applications and disaster recovery scenarios, support for multipoint communications due to inherent broadcasting capability, bandwidth on demand capabilities, ease of network expansion, flexibility of station organization etc., There are also several demerits associated with satellite communications such as their bursty error conditions, high BER characteristics, long delay and the enormous cost associated with user terminals, earth stations and the satellites as a whole. Also, the dependence of solar power for recharging also poses a problem. The limited transmission power of both the ground terminals and satellite is also a problem.

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

Asynchronous Transfer Mode (ATM) is an International Telecommunication Union- Telecommunication Standardization Sector (ITU-T) standard for cell relay wherein 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 and multiplexing technology that combines the benefits of circuit switching (guaranteed capacity and constant transmission delay) with those of packet switching (flexibility and efficiency for intermittent traffic). It provides scalable bandwidth from a few megabits per second (Mbps) to many gigabits per second (Gbps). Because of its asynchronous nature, ATM is more efficient than synchronous technologies, such as time-division multiplexing (TDM). With TDM, each user is assigned to a time slot, and no other station can send in that time slot. If a station has a lot of data to send, it can send only when its time 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 is wasted. Because ATM is asynchronous, time slots are available on demand with information identifying the source of the transmission contained in the header of each ATM 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 48 contain the "payload" (user information). Small fixed-length cells are well suited to transferring voice and video traffic because such traffic is intolerant of delays that result from 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-point ATM 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 of a 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 drop the cells with CLP set, when there is congestion at the switch. The latter is used for error control. The ATM layers and the ATM Adaptation layer (AAL) are roughly analogous to the data-link layer in the OSI model. The ATM layer is responsible for establishing connections and passing cells through the ATM network. The AAL is used for isolating higher-layer protocols from the details of the ATM layer. The higher layers residing above AAL will accept user data, arrange it into packets and hand it to AAL [42]. There are different AALs like AAL1, AAL3/4 and AAL5 for different types of data and voice and video packets.

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

ATM connection establishment process uses the one-pass method, just like the telephone network. An ATM connection setup proceeds with a connection-signaling request 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 can begin. The teardown is also done in the similar way. ATM networks can emulate a physical LAN. LAN Emulation (LANE) is a standard defined by the ATM forum to emulate 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 of choices for the physical layer. The benefits of a wireless ATM access technology should be observed by a user as improved service and improved accessibility. By preserving the essential characteristics of ATM transmission, wireless ATM offers the promise of improved performance and quality of service, not attainable by other wireless communications systems like cellular systems, cordless networks or wireless LANs. In addition, wireless ATM access provides location independence that removes a major limiting factor in the use of computers and powerful telecom equipment over wired networks. The architecture proposed for wireless ATM is composed of a large number of small transmission cells called pico cells. A base station serves each pico cell. All the base stations in the network are connected via the wired ATM network. The use of ATM switching for intercellular traffic also avoids the crucial problem of developing a new backbone network with sufficient throughput to support intercommunication among large number of small cells. To avoid hard boundaries between pico-cells, the base stations can operate on the same frequency.

2. Motivating Scenarios

There are different application scenarios, which are motivating factors behind SATATM networks. The following sections deal with a set of architectures for which SATATMs can provide a good quality solution. The scenarios are discussed and the connectivity requirements are studied and then, SATATM concept will be applied to the scenarios 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 and improved utilization of computing resources. Major components of this scenario are inter-process communication and remote file I/O systems [37]. The main factor involved this scenario is the distance of separation between communicating nodes and ways to resolve them. The other factor involved is the necessity of broadband communications with QoS guarantees. Satellite communications can solve the distance factor and ATM can solve the requirements of QoS guarantees. The other factors are a big organization’s nature of having geographically dispersed supercomputers and workstations in branch offices and the need to interconnect them. The pre-requisite is the successful interconnection of terrestrial networks in a seamless way.

2.1.1.1 Requirements

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

2.1.2 Mobility architecture in ATM and WATM networks

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

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

• A class of applications, with respect to WATM deals with the mobility of the ATM switch itself. Here pieces of ATM network, each consisting of ATM switches, could be in motion with respect to the fixed portion of the network. Application scenarios would involve mobile platforms with number of users on board. This scenario is pertinent to airplanes, which provides communication and entertainment services to passengers. Here the ATM end nodes are not in motion. Another scenario could be that ships (military and civil) having ATM networks want to communicate among them and with the land network. The military networks would also entail security features for intruder-free communication.

2.1.2.1 Requirements

The 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 main hindrance is the distance factor. Institutions in the developed countries can educate people in developing and under-developed countries if quality multimedia connection is achieved over a large distance. ATM is the de-facto standard for multimedia communications due to its capacity to guarantee QoS and support for voice, video and data simultaneously. 2.1.3.1 Requirements

The 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 classic examples of bandwidth guarantees and bandwidth on demand scenarios respectively. They also require synchronization over a great distance. By default, distance is a factor in these application scenarios. Multimedia communications is also driven by the backbone concept, assumed to be provided by fiber cables. In many parts, these may be unviable, uneconomic or take too long to establish. Multi-service communications also entail interconnection of the mobile devices carried by company representatives.

2.1.4.1 Requirements

QoS guarantees, bandwidth on demand, large bandwidth, synchronization, and backbone dependability are demanded by multimedia applications. Seamless and efficient integration schemes are needed by multi-service applications. Interactive

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computing and bulk transfers with high bandwidth requirements, information dissemination including stock market data etc., and video broadcasts with low delay requirements 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 big companies, financial institutions and banks, having distributed branches. The main factor is that secure communications are needed over a geographically separated scenario in which distance is the main consideration. 2.1.5.1 Requirements Security, encryption and interconnection between geographically diverse locations are the 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 cases where the terrestrial network is made unusable due to natural or artificial disasters.

• In the past, fast user response may not be possible with SATATMs due to the inherent propagation delay associated with satellite communications. Recently, gigabit satellite networks made possible using NASA’s Advanced Communication Technology Satellite (ACTS) [12][18].

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

• They can also do encrypted communications [35]. • Satellite communications can also guarantee QoS to all the service

categories of ATM like CBR, VBR etc [10,14,30,43,44]. Connection Admission 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 preferred choice for the above scenarios. The following sections will show how SATATM satisfies the above requirements. SATATMs can be used in similar scenarios, which exhibit or need QoS guarantees and high bandwidth requirements in the face of distance, being the overriding concern.

There are many issues to be addressed before SATATMs can be chosen as the preferred solution. These are discussed in the next section. Particularly, there is a great number of SATATM solutions and architectures available and these should be chosen carefully to particular application scenarios for optimum performance and meeting of requirements. 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 in the 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 point for universal access for data services, namely distance. They are often equipped with multiple transponders. The area of the earth’s surface covered by a satellite’s transmission 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 ground station. The down-link can have a large footprint providing coverage for a substantial area or a “spot beam” can be used to focus high power on a small region, thus requiring cheaper and smaller ground stations. Some satellites can dynamically change their coverage 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 all terrestrial 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 QoS guarantee of any network and especially ATM network.

• Data Transmission – it requires usually very high link integrity. ARQ methods are used on the uplink channel, which is multi-access channel with multiple users aiming to access the network and downlink, which is a multicast channel.

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• Burst Time Plan – A BTP is required to o Set up space segment (consisting of satellites) based on the previous

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

• Burst synchronization – with the high rate digital transmission used in the satellite link, this is needed. The satellite movement will affect the delay and loss of synchronization will lead to serious degradation. Guard times are used for this purpose.

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

• OBS and OBP represent onboard switching and onboard processing capable satellites 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 related parameters to be considered and analyzed. It is done in the following sections. The following sections are organized as follows. The design parameters would be given and would be discussed and the advances in each of the parameters would be discussed and 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. The following diagram shows the effects of the constellations for GEO and LEO constellations.

Fig.3 Some of the effects of GEO and LEO constellations

Table 2 should be referred for more information or design decisions about the different constellations. Fig.3 shows the effects of LEO and GEO constellations on parameters like Coverage, Received signal strength etc., These could be used to select the constellation. 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 uses Number of orbit planes, Number of satellites per orbit plane, Satellite altitude, Orbit plane inclination angle and Ground terminal coordinates to calculate the total propagation delay from a source to destination through a LEO network. The end-to-end delay is the sum 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 by orbital dynamics, buffering, adaptive routing and OBP are not taken into account. LEO propagation delay is of the order of 83.45 ms for a sample propagation delay calculation from Los Angeles to London with seven satellites in path [10]. GEO propagation delay for ground terminals farther away from the equator is of the order of 275ms through a single satellite. Though LEO networks have relatively smaller propagation delays, the delay variance is higher than GEO. This variation is due to handovers, satellite motion, OBS and adaptive routing. These should be considered while selecting the constellation. Thus, when considering constellation of a satellite, the parameters to be taken into account are launching cost (less for LEO), propagation delay (less for LEO), delay variance (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 to ground terminals. As a result, the Network Control Center (NCC) in fig.2 must handover connections to another satellite whose footprint is relevant. In other cases, LEO systems are not stationary. Hence, caller and called terminals do not remain in the same footprint of the initial source and initial destination satellites. Thus the satellites need to transfer the ground caller and called terminals to others. This is called a handover. There are 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 for LEO satellites are estimated to occur on an average 8 to 11 minutes [10]. There is an amount of delay variance in LEO constellation due to these handovers. There are different handover protocols being considered and Footprint Handover Rerouting Protocol (FHRP) is one of them [11]. LEO systems with multi-hop inter-satellite links need 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 is necessary. This causes additional signaling and processing overhead. The assumption of FHRP is that all handovers are caused by the mobility of the LEO satellite instead of the 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. This algorithm 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 traffic over reliable and high capacity connections and with minimal terrestrial resources. Older satellite networks did not employ ISLs. Modern satellites employ ISLs due to the advancement in OBS/OBP designs. Another motivation is that, since ATM switching implies low delay at each satellite node on the ISL route, the advantage gained from low propagation 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 Dijkstra’s SPF algorithm, capable of coping with time-variant topology. ISL routing deals only with deterministic 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-plane or cross-plane links. 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 can be assumed to be constant, while in LEO systems ISL delays depend on the orbital radius, the number of satellites-per-orbit and inter-orbital distance. The ISL delay in LEO systems change frequently due to satellite movement and adaptive routing techniques. Thus LEO systems can exhibit a high variation in ISL delay [10]. There are some improvements needed to this routing protocol as suggested in [16] and should be consulted before usage.

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

Table 3 : GEO Inter Satellite Link Delays

Number of Satellites (N)

Inter-Satellite LinkDistance (km)

Inter-Satellite Link Delay (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 other processing at the satellite, except for reflecting transmitted waves. The alternative is to allow on board switching and processing. The requirement for satellite switching results from the need of small, inexpensive earth terminals. This could be supplied by multiple beams [33]. However, multiple beams need switching between beams or inter-beam switching and hence satellite switching must be considered. Satellites with no OBS limits the applicability of satellites for internetworking to simply links connecting two terrestrial stations. With OBS, earth terminals with differing QoS requirements can share the uplink channel. There are on-board switching architectures that implement the adaptation of real-time and non-real time services to the satellite communication link, while achieving significant statistical advantage on communication links, uplinks and downlinks [14]. This model is based on the GEO constellation. It exploits the burstiness of real-time traffic, this architecture achieves high system throughput. In this particular architecture the onboard switch does demodulation, detection and correction of transmission errors, after receiving the signal and time-multiplexed into digital baseband streams. For this particular 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 belong to 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 whereas an ATM-like packet switch handles the low-priority traffic. The main advantages brought about 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-beam onboard processing payload satellites can achieve this.

• The inherent broadcast function. Every subscriber located within the same downlink spot beam as the called subscriber can, receive a message forwarded to this user station. The normal mode of operation is user specific. An extension of the broadcast nature along with return link provides the necessary interactivity required 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 be addressed, 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 weather conditions and interference.

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

• 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 size should be carefully chosen. Simulation studies for different types of ATM traffic are done and should be used [14] before choosing the value for this parameter.

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

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

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

• Due to restricted lifetime of satellites, fault tolerance should be added by introducing fault detection and redundancy, both internal and external to the switch [33].

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

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

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

• Power consumption and power dissipation are other significant factors to be considered.

• CLRs should be in the range of 10^-10 to meet the QoS of high-performance traffic 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 virtual buffer based. There are issues in choosing fully output buffered switch. After sorting through the issues, a fully interconnected fabric with output port concentrators similar to the knockout switch is being proposed. The high CLR of these types of switches should also be taken into consideration [33].

• Functions that could be considered for OBS/OBP are switching, queuing, flow control and scheduling. Connection admission control and resource allocation should be handled at NCC preferably. All delay-tolerant functions should be kept on 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 fact that the SATATM network uses multiple access in the uplink. The choice of multiple access schemes has a great impact on the SATATM network. The primary goal in the assignment process is

• Satisfy the user’s 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 QoS provisioning. MAC protocols are behind the delivery of QoS contract. MAC protocol should achieve QoS provisioning, efficiency and service interoperability [26]. Satellite environments affect MAC protocols with the long propagation delay, physical changes to the controllers in space, dynamic nature of satellite links and limited buffer memory. The traditional CSMA/CD schemes cannot be used with satellite channels, since it is not possible for earth stations to do carrier sense on the up-link due to the point-to-point nature of the link. A carrier sense at the downlink informs the earth stations about potential 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 to use OBP and multiple spot beams will enable future satellite to reuse the frequencies many times more than today’s system. Demand Assigned Multiple Access (DAMA) systems allow the number of channels at any time is less than the number of potential users. Satellite connections are established or dropped only when traffic demands them. Protocols like Packet Reservation Multiple Access (PRMA), an improved form of TDMA with 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 communication and its inherent advantages like distributed coordination, chipping code method of authentication, 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 with the 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 the timing information necessary for its synchronization, as well as the sending of ATM and satellite signaling for connection establishment and initial entry. The data area of the uplink frame is where ATM cells are transmitted. The slots can also hold Forward Error Correction (FEC) and in-band signaling. The ATM cell payload 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 they are

• 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 satellite receives this information, it dynamically assigns slots to the connection. The drawback is the 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 the downlink, transmission is multicast and the suitable protocol is Time Division Multiplexing (TDM). In order to achieve a greater efficiency in SATATM networks, the DAMA 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 the resulting source traffic at the satellite network level [26]. The following diagram shows two satellite system network scenarios [26], which can help decide which MAC protocol would 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 scenario and not for the Internet backbone case. For this case, fixed bandwidth allocation could be used. There are different DAMA techniques in use and research has been done on the various DAMA protocols [26]. The most preferred mode of usage of DAMA is for nrtVBR, whose requirements is low packet loss and for ABR, DAMA and hybrids of AMA are attractive solutions [26].

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An in depth study on MAC protocols for Mars Regional Network [47] could be consulted for more information. In another extensive study [25], a set of performance objectives are 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 Complexity

Following tables from [25] should be used to differentiate among the plethora of MAC protocols.

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, small number of users

Reservation protocols with fixed TDMA reservation channel.

Table 5: Performance comparison Protocol Average

th’put Mean delay

Stability Scalability Recon-figur-ability

B’band apps

Cost-Comp-lexity

Fixed assignment B-TDMA G-TDMA

Low High

Low/Med Low

Med/High High

No No

No No

Yes Yes

Med Med

Demand Assignment MSAP

Med/high

Med/high

Med/high

No

No

Yes

Med-high

Random Access S-Aloha

Low

Very Low

Low

Yes

Yes

No

Low

Reservation R-Aloha

High

Very Low

Med

Yes

Yes

No

Low

Hybrid Aloha-R RRR

High High

Low-Med Low-Med

Med Med

Yes Yes

Yes Yes

Yes Yes

Med Med

Adaptive SRUC MDMA

High High

VeryLow Low-Med

High Low

Yes Yes

Yes Yes

Yes No

High High

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. Interested readers are referred to [3, 25] for further information on these protocols and other variations of the same. Some of the conclusions are that the hybrid protocols that take advantage of both random access and reservation protocols have better throughput versus delay characteristics. The basic assumptions behind the study are bursty traffic and asymmetric satellite links. Another study [7] on MAC protocols compares them in a different plane and is given below.

Table 6: Performance comparison Access protocol

Efficiency Delay Stability Robustness Complexity

S-ALOHA .37 Low Low High Low Tree CRA .43-.49 Medium Medium Poor Medium DAMA (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 study Medium access protocols on the uplink for ATM traffic should be made suitable for different kinds of ATM data connections and service classes. A sample assignment scheme is discussed in [8]. The concentration is on CBR and VBR traffic. This scheme assumes Multiple Frequency-TDMA (MF-TDMA) as the MAC protocol. The following tables and explanation gives the overview of a sample assignment strategy 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/frame be the same connection’s variable-rate demand assignment allocation. Let Cii be the total BW allocated for connection ‘ii’. The terms PCR refers to Peak Cell Rate, MCR to Minimum 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 of exploiting stat-mux is to merge multiple VBR streams with similar statistical properties into a common FIFO queue, which may be given some constant rate of service. These could 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 Round Robin 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. The main pre-condition is the presence of OBS/OBP in the satellite. In another study [44], some simple rules for ABR service on SATATM networks were found 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 the Transient Buffer Exposure (TBE) parameter was set to 24 bits, and no size was enforced for the Crm parameter. According to the study, this simple change improved the throughput over OC-3 satellite links from 45Mbps to 140 Mbps. It was also found that large 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 limited transmission power of both the ground terminals and the satellite. Transmissions in the network should be such that the user terminals at different geographical areas are given access in the most power efficient manner [8]. Multi-beam satellites are proposed for this. Multi-beam systems need OBS/OBP. Hence when doing power management, the issues regarding OBP/OBS should also be taken into consideration. To further save on uplink transmission power, MAC protocols like MF-TDMA can be used as the data-link protocol. 4.7 Error Correction Scenarios In satellite channels under consideration, transmission bit errors occur in bursts due to link 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 as 10^-6. Though AAL5 has a 32-bit CRC, it is not used due to the high cell discard rate at the 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 to “distribute” or spread the bit errors at the cost of increased delay. The interleaving algorithm 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 depth for optimal error performance is still not evident [30]. Error recovery algorithms like automatic repeat request (ARQ) could be used to lower error 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 recovery algorithms. Go-Back-N and Selective-repeat are better than stop-and-wait algorithms. Coding scheme can be used for error correction or prevention. Currently, convolution code with viterbi decoding is used to achieve 10^-3 to 10^5 BER [30]. This is not fit for SATATM 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 in this area [30]. Here also, optimal interleaving depth for SATATM networks should still be determined. An in-depth study of the impact of transmission error characteristics on SATATMs is studied 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 at high rates employ error correction schemes for providing acceptable BER. Burst errors are generated by these error correction schemes. The ATM HEC is capable of correcting only single-bit errors. A method called ATM link enhancement (ALE) was developed, which incorporates a selective interleaving technique allowing it to be transparently introduced into the satellite link. More information is given in the section under Commercial 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 for error detection and error correction. All the codes used for AALs are sensitive to burst errors, 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 channel like a satellite link using binary phase shift keying (BPSK) modulation. The method combines 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 the throughput. In another experiment [24], a shorter error correction model called Bose-Chaudhuri-Hocquenghem (BCH) code could be used. A more ATM oriented solution is also discussed, which is called the Partial Packet Discard (PPD), which on detection of erroneous cells at the satellite switch, these and consecutive ones are dropped and hence reduce the traffic. This suffers from the retransmission problems (increase in congestion) 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 layer mechanism is also carried out. In another related study [9], a solution is proposed for the error control mechanisms to adapt to the satellite channel by moving the error recovery and detection to a higher layer of the ATM. This is based on the ability of the ATM to determine the service of the retransmission and to base recovery on that service. The study also shows simulation results to confirm a significant increase in raw data throughput and that in ATM transfer efficiency 7.5%. The results also show that it is possible to guarantee data services with no loss of data under certain conditions. The author does this by changing the current ATM adaptation layer with a proposed Convergence sub-layer AAL. It is also proposed that 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]. The CLR-vs-BER performance is quite linear. The effects and graphs are to be studied before 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 algorithms should act faster and more efficiently due to the long delay. The basic QoS parameters are Cell Loss Ratio (CLR), maximum and mean cell transfer delay (CTD) and cell delay variation (CDV) and the extended QoS parameters are cell error ratio (CER), severely errored cell block ratio (SECBR) and cell mis-insertion ratio (CMR) are also recommended. The impact of satellite delay on some basic services is tabulated here[30].

Table 8: Effect of satellite link delay on applications Application and properties Sensitivity to satellite link delay Video and voice service- generates bursty traffic.

Very sensitive- real time services, good as long as delay variation is kept very small

Text or data service – needs reliability Not sensitive Video telephony Not sensitive, future video telephony

may be sensitive Computer Supported Cooperative Work (CSCW)

Not sensitive, but delay on TCP/IP due to satellite delay degrades entire performance. [see section on Upper Layer Concerns in this paper]

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

• Traffic Shaping o Mechanism to change the traffic characteristics of a cell stream to

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

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

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

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

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

Congestion control mechanisms are of many types [30]. A frequently used scheme is selective cell discard. It has advantages and disadvantages as briefed above. Another method is Explicit Forward Congestion Indication (EFCI) incorporated with a feedback mechanism. EFCI is used to convey congestion notification to the source. The destination protocol is required to notify the source of congestion. This whole process is Forward Explicit Congestion Notification (FECN). In SATATM, this is not very well matched 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 VC prioritization can also be used in congestion control. The satellite on-board buffer could also be considered. This could introduce jitter, if not properly done. A related mechanism is VC prioritization. Other congestion control mechanisms are discussed in [49], although these should be changed for SATATM considerations. Thus the criteria of choosing the algorithms 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 an integrated ATM-satellite environment has been done [38]. The proposed CAC strategy effectively exploits the satellite BW and provides QoS to both real-time and non-real-time VBR sources, while permitting contemporary access to the resources to a great number of users. 4.9 Upper layer concerns There is a service specific convergence sub-layer (SSCS) in AAL. This SSCS is divided into 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 provide assured delivery of PDUs and use error-recovery procedures if necessary. The following features [18] are very favorable to SATATM networks. They are the selective retransmissions, nearly infinite window size definition capability, superior flow control, optimized support for high-speed and long-delay networks and the protocol is designed to be insensitive to network delay. SSCOP has been proposed by some people as a possible replacement for TCP as a wide-area transport protocol, however some doubts have been expressed as to its efficiency in the face of errors, congestion, variable delays. A thorough investigation of SSCOP, including simulation to determine its performance in terms of throughput etc., in a typical error/congestion/delay environment should be carried out. TCP is the de-facto standard for the Internet transport protocol. Considerations for using TCP 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 thorough study [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 of TCP over UBR is measured by Efficiency = (Sum of TCP throughputs)/(Maximum possible TCP throughput Fairness Index = (Σxi )^2 / (N * Σxi^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 a good 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 possible candidate 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 delay segments from terrestrial segments [5], which help in efficiently sizing buffers in routers and ATM switches. Therefore, terrestrial switches only need to have buffers proportional to the BW-delay product. Employing feedback is also a mechanism for giving feedback to 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 on TCP over satellite communications and has come up with the following recommendations and hence could be followed.

• Maximum window size remains a hindrance to the SATATM networks. An increase in the window to 2 ^30 is being proposed [6]. Also, larger initial window size 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 adds approximately one segment to its congestion window during each RTT. This leads to unfairness and hence fair queuing and TCP-friendly buffer management in network routers is being considered.

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

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

• TCP header compression is a viable alternative for bandwidth-sensitive SAT networks.

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

• In highly asymmetric networks like satellite links, a low-speed return link can cause 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 cumulative acknowledgement scheme of TCP.

These are some of the TCP improvements to be made for supporting satellite networks in 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) and GEO/LEO satellites technologies is being investigated as a solution to meet the Air Traffic Management and passenger’s applications requirements for Air-Air, Air-Ground and Ground-Ground segments with multimedia high-bandwidth [50]. The issues related to the management of the QoS over an aggregation of ATM and Satellite networks fall into 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 are awaited. 4.10 Attenuation considerations Path loss can occur in satellite transmissions due to the following conditions. Weather conditions like rain, integrated water vapor concentrations and cloud liquid water contents can affect the transmission. Attenuation due to rain is a major problem in the Ka 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

26

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 transport of ATM. In one type of change, the CRC and the sequence numbers are moved to the higher convergence sub-layer. This will entail larger blocks to put error detection and correction on. It is suggested here [9] that high-speed, long-delay satellite links need a unique AAL. That is answered in [24], where a separate layer called S-ATM layer is provided for satellite communication scenarios. In another study [43], the ground segment proposal is based on a new AAL called AAL2, which is considered to play a major 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, which forms 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 calculations designed to model the performance of a communications link. In a simplex satellite communication, 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 link budgets, as in [31]. The link budget parameters of an optical ISL could be Operational wavelength, Telescope diameter, Receiver type, Modulation, Coding, Distance, Antenna Gain, 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 of GEO 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 as MEO and LEO. By using MEO/LEO satellites and selecting an appropriate inclination angle, these orbits can offer much higher minimum elevation angles over high altitude regions. High elevation angles will lead to a very low probability of shadowing and therefore 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. There are some inefficiency regarding rerouting difficulty and redundant operations. SDH was developed to take care of the totally synchronized network. SDH is much preferred to PDH [30]. PLCP is another cell transport method and it is found to be not suitable in the burst 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 study was done on ACTS ATM Internetwork (AAI) platform with a prototype ATM encryption device [35]. One of the new generations of encryptors for unclassified ATM networks is called 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 lightweight airplanes carrying communications equipment that will act as very low earth orbit geo-synchronous satellites. High efficiency turbine engines or a combination of battery and solar power will power these crafts. At an altitude of only 70,000 feet, HALE platforms will offer transmission delays of less than .001 seconds and even better signal strength for very lightweight hand-held receiving devices. 4.16.2 Commercial SATATM products Commercial SATATM products are available in the markets. COMSAT is a company specializing in SATATM products and the ATM Link Enhancer (ALE) discussed before 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 at fractional T1 to DS3 rates. ALE-2000 is a networking device that allows customers to interconnect ATM networks over satellite and wireless links at DS3 and E3 rates. The advantages or properties of the product involve efficient bandwidth utilization, fiber-like link quality and significantly improves the performance of applications over satellite and WATM. The error-correction for the error-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 at fractional T1 to 8.448 Mb/s, symmetric and asymmetric data rates. It also supports rate adaptation, ATM cell header compression and cell payload loss less compression. Linkway 2000 product from ComSat can be used to cope with the heterogeneity of network protocols and interfaces and develop satellite network solutions that can accommodate these in a bandwidth efficient manner. Using this product, ComSat researchers have shown an overview of a network consisting of IP, ATM, FrameRelay, ISDN and SS7 services in a fully meshed mode 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 of –10 or better.

4.16.3 NASA-ACTS

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

• Very high-gain, multiple hopping beam antenna systems which permit smaller aperture Earth stations.

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• On-board baseband switching which permits interconnectivity between users at the individual circuit level.

• A microwave switch matrix, which enables gigabit per second communication between users.

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

• High Data Rate Earth Station. The ACTS High Data Rate terminal is capable of transmit ting data at 622 Mbps using a 3.5 m antenna. Alternatively, up to 4 stations operating at 155 Mbps can be supported simultaneously in a satellite switched time division multiple access (SS/TDMA) mode. The Harris T1 (1.544 Mbps) Very Small Aperture Terminal (VSAT) - using the ACTS baseband processor (BBP) mode, the T1 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 BBP at 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 on satellite networks is present online at [52]. It is strongly suggested that the document be reviewed before design of SATATM networks. SATATM networks were used in oil industry, 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 extensive databases of satellite parameters, earth-station parameters and user traffic requirements, to synthesize a network architecture. This is a very important step for practical implementation of satellite networks [17]. 4.16.6 VSAT terminals

Due to high performance requirements, the design of an earth station is quite complicated. This increases the costs and the need for maintenance. VSAT provides a solution 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 cheap enough to be installed in the end user's premises. VSAT networks are typically arranged in a star based topology, where each remote user is supported by a VSAT. The Earth hub station acts as the central node and employs a large size dish antenna with a high quality transceiver. The satellite provides a broadcast medium acting as a common connection point for all the remote VSAT earth stations. VSAT networks are ideal for

29

centralized networks with a central host and a number of geographically dispersed terminals.

The weaker signal from the remote ES is amplified at the satellite acting as a bent pipe and received by the hub ES. Thus, the lower gain at the uplink is compensated at the downlink 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 all connections must pass through the hub ES node. The data link supported from the hub to 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 common protocol.

5. Conclusions The paper discussed a comprehensive discussion about ATM over satellite communications, 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 no means a complete design guide and the references section is meant to consummate the ideas and research presented in this paper. Interested users should use this paper as a starting material and go to each of the catalogued references to do further analysis. The section following the motivating scenarios, mentioned SATATM solutions for the same. Here, a particular motivating scenario will be dealt with, giving justifications for selection of the same. For the design solution, the mobility architecture would be considered (see sec. 2.1.2). Let fig.2 also be considered. A perfect example of mobile networks is the presence of networks in the ships and the ship wants to handle communications 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 ATM network is pretty huge. The cost of implementation is not a factor. The communications are 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 as follows. The satellites in the network is a LEO-based with multiple satellites connected by optical ISLs (for performance). The attenuation factors for the communication should be taken into account. The handover protocol is chosen as FHRP due to its superior performance 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 to delay-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. Suitable adjustments are made for rain and air traffic attenuation. More over, a CAC scheme based on [38] is used due to the superior performance of this scheme in the face of multimedia traffic. TCP/IP is assumed to be used, since TCP is a stable protocol and all the feasible changes according to [20] are assumed to be handled. High elevation angles are assumed. The cell transport method can be SDH. The encryption is done using FASTLANE, due to its superior performance. There is S-ATM layer present in the AAL. This is done since, S-ATM is superior in the face of multimedia traffic and gives better QoS.

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

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