ECPE 6504: Wireless Networks and Mobile Computing. Individual Project Report

Transcription

1 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 24 APR 2000

2 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 connectionoriented 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 internetworks 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, costperformance 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. 2

3 Table of Contents Pg. No 1. Introduction Satellite Communications Architecture and purpose Terminology, characteristics, advantages and disadvantages 1.2. ATM and WATM ATM architecture ATM internals and physical layer issues WATM and its features 2. Motivating Scenarios Description of the architectures 3. SATATM details SATATM design specifics 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 TCP changes for ATM UBR TCP changes for ATM ABR TCP changes for satellite communications 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 HALE systems Commercial SATATM products (from COMSAT) Focus on NASA-ACTS Commercial satellite design guide Rule-based practical design approach for building commercial satellites VSAT terminals 5. Conclusions References 31 3

4 1. Introduction 1.1 Satellite Communications 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 ( ) 6 ( ) Interference with ground links Ku 11 ( ) 14 ( ) Attenuation due to rain Ka 20 ( ) 30 ( ) High Equipment cost L/S 1.6 ( ) 2.4 ( ) Interference with ISM band 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] 4

5 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 Time LOS Merits Demerits miles miles 22,282 miles in 15 min 2-4 hrs 24 hrs Variable Lower launch Moderate costs, very short launch costs, round trip delays, small round trip small path loss delays. Very short lifetime (1-3 months), encounters radiation belts Larger delays, greater path loss Covers 42.2% of the earth s surface, constant view Very large round trip delays, expensive Earth Stations. Variable due to elliptical orbit Maximizes time spent over populated areas, superior line of sight, fewer satellites 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) 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 5

6 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] 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 higherlayers that is identical to that of existing LANs. 6

7 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 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 Requirements The requirements here are QoS guarantee, fast user response, stable connections, reachability etc., 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. 7

8 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 Requirements The requirements here are maintaining quality connections, safeguard QoS guarantees, smooth handoffs, secure communications etc., 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 Requirements The main requirements are QoS guarantees, voice-video synchronization, large bandwidth, bandwidth on demand, quality multimedia services etc., 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 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 8

9 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] 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 Requirements Security, encryption and interconnection between geographically diverse locations are the main issues here 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 9

10 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 uplink 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. 10

11 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 11

12 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 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- 12

13 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 endto-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 crossplane 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 Satellites (N) of Inter-Satellite LinkDistance (km) Inter-Satellite Delay (ms) Link 3 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 13

14 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) 14

15 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). 15

16 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 16

17 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] 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]. 17

18 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 Reservation protocols with contention number of users Bursty traffic, long messages, small number of users Reservation protocols with fixed TDMA reservation channel. Protocol Fixed assignment B-TDMA G-TDMA Average th put Low High Table 5: Performance comparison Stability Mean delay Low/Med Low Med/High High No No No No B band apps Yes Yes Scalability Reconfigurability Cost- Complexity Med Med Demand Assignment MSAP Med/high Med/high Med/high No No Yes Med-high Random Access S-Aloha Low Very Low Yes Yes No Low Low Reservation R-Aloha High Very Low Med Yes Yes No Low Hybrid Aloha-R RRR Adaptive SRUC MDMA High High High High Low-Med Low-Med VeryLow Low-Med Med Med High Low Yes Yes Yes Yes Yes Yes Yes Yes Yes Yes Here, Mini-Slotted Alternating Priorities Protocols (MSAP), Slotted-Aloha (S-Aloha), Round Robin Reservation (RRR) protocol, Split Reservation Upon Collision (SRUC), Yes No Med Med High High 18

19 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 Medium Medium Poor Medium DAMA.6-.8 High High High Medium (reservation) Hybrid (reservationrandom) Variable Medium High medium Here Tree Contention Resolution Access (Tree CRA) and others are being used 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 fixedrate 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

20 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 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 20

21 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 bitmarking 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 21