Multimedia Satellite Networks and TCP/IP Traffic Transport

ABSTRACT

To meet an increasing demand for multimedia services and electronic connectivity across the world, satellite networks will play an indispensable role in the deployment of global networks. A number of satellite communication systems have been proposed using geosynchronous (GEO) satellites, medium earth orbit (MEO) and low earth orbit (LEO) constellations operating in the Ka-band and above. At these frequencies satellite networks are able to provide broadband services requiring wider bandwidth than the current services at C or Ku-band. Most of the next generation broadband satellite systems will use ATM or “ATM like” switching with onboard processing to provide full two-way services to and from earth stations. The new services gaining momentum include mobile services, private intranets and high data rate internet access carried over integrated satellite-fiber networks. Several performance issues need to be addressed before a transport layer protocol, like TCP can satisfactorily work over satellite ATM for large delay-bandwidth networks. In this paper, we review the proposed satellite systems and discuss challenges such as, traffic management and QoS requirements for broadband satellite ATM networks. The performance results of TCP enhancements for Unspecified Bit Rate over ATM (ATM-UBR+) for large bandwidth-delay environments with various end system policies and drop policies for several buffer sizes are presented.

1 INTRODUCTION

The rapid globalization of the telecommunications industry and the exponential growth of the Internet is placing severe demands on global telecommunications. This demand is further increased by the convergence of computing and communications and by the increasing new applications such as Web surfing, desktop and video conferencing. Satisfying this requirement is one of the greatest challenges before telecommunications industry in 21st century. Satellite communication networks can be an integral part of the newly emerging national and global information infrastructures (NII and GII).

1.1 Motivation

Satellite communication offers a number of advantages over traditional terrestrial point-to-point networks. These include:

· Wide geographic coverage including interconnection of remote terrestrial networks ("islands”)

· Bandwidth on demand, or Demand Assignment Multiple Access (DAMA) capabilities

· An alternative to damaged fiber-optic networks for disaster recovery options

· Multipoint-to-multipoint communications facilitated by the Internet and broadcasting capability of satellites

During the next millennium, wireless satellite systems will play a significant role in meeting telecommunication needs. The next generation satellite systems, often termed “broadband satellite networks” or “multimedia satellite networks,” are being developed to provide global, broadband communication services including high data rate Internet access, private Intranets, and TV broadcasting. Some of these systems will offer data communication services at Ka-band and digital broadcasting at Ku-band. Satellite communication networks can interoperate with the current major technology developments, e.g., Internet Protocol (IP) and Asynchronous Transfer Mode (ATM).[1]

1.2 Why Ka-band?

Until recently, Ka-band was used for experimental satellite programs in the U.S., Japan, Italy, and Germany. In the U.S, the Advanced Communications Technology Satellite (ACTS) is being used to demonstrate advanced technologies such as onboard processing and scanning spot beams. A number of applications were tested including: distance learning, telemedicine, credit card financial transactions, high data rate computer interconnections, video conferencing and HDTV. The growing congestion of the C and Ku bands and the success of the ACTS program increased the interest of satellite system developers in the Ka-band satellite communications network for exponentially growing Internet access applications. A rapid convergence of technical, regulatory, and business factors has increased the interest of system developers in Ka-band frequencies. Several factors influence the development of multimedia satellite networks at Ka-band frequencies:

· Adaptive Power Control and Adaptive Coding: Adaptive power control and adaptive coding technologies have been developed for improved performance, mitigating propagation error impacts on system performance at Ka-band.

· High Data Rate: A large bandwidth allocation to geosynchronous fixed satellite services (GSO FSS) and non-geosynchronous fixed satellite services (NGSO FSS) makes high data rate services feasible over Ka-band systems.

· Advanced Technology: Development of low noise transistors operating in the 20 GHz band and high power transistors operating in the 30 GHz band have influenced the development of low cost earth terminals. Space qualified higher efficiency traveling-wave tubes (TWTAs) and ASICs development have improved the processing power. Improved satellite bus designs with efficient solar arrays and higher efficiency electric propulsion methods resulted in cost effective launch vehicles.

· Regulatory Issue: The orbital congestion at C- and Ku-bands has necessitated the move to Ka-band.

· Global Connectivity: Advanced network protocols and interfaces are being developed for seamless connectivity with terrestrial infrastructure.

· Efficient Routing: Onboard processing and fast packet or cell switching (e.g., ATM) makes multimedia services possible.

· Resource Allocation: Demand Assignment Multiple Access (DAMA) algorithms along with traffic management schemes provide capacity allocation on a demand basis.

· Small Terminals: Multimedia systems will use small and high gain antenna on the ground and on the satellites to overcome path loss and gain fades.

· Broadband Applications: Ka-band systems, combining traditional satellite strengths of geographic reach and high bandwidth, provide the operators a large subscriber base with scale of economics to develop consumer products.

2 PROPOSED MULTIMEDIA SATELLITE SYSTEMS – PARTIAL LIST

In the past three years, interest in Ka-band satellite systems has dramatically increased, with over 450 satellite applications filed with the ITU. In the U.S., there are currently 13 Geostationary Satellite Orbit (GSO) civilian Ka-band systems licensed by the Federal Communications Commission (FCC), compromising a total of 73 satellites. Two Non-Geostationary Orbit (NGSO) Ka-band systems, compromising another 351 satellites, have also been licensed. Eleven additional GSO, four NGSO, and one hybrid system Ka-band application for license and 16 Q/V-band applications have been filed with FCC. Table 1 provides a partial list of proposed satellite systems at Ka-band.[1]

Brief descriptions of these systems are based on FCC filings. However, all these systems are being redesigned to meet their business plans and dynamically changing market demands.

2.1 Astrolink

Lockheed Martin’s Astrolink system is composed of a space segment and a ground segment. The space segment is made up of an initial constellation of up to five GEO satellites, interconnected by inter-satellite links. This constellation will later be augmented to nine to meet the traffic demand. The ground segment is made of three principal elements: Subscriber terminals located at the customer premises; gateway earth stations that connect the Astrolink system to major customers and Public Switched Network; and Regional Network Control Center that performs subscriber verification, call set-up, and billing.

The Astrolink network architecture is based on the ATM technology to support the integrated voice, data, video, and multimedia services. The system supports 52,000 full duplex circuits per satellite at 64 kbps or 6.6 Gbps per satellite. The user terminal uplinks employ a hybrid multifrequency time division multiple access scheme.

The Astrolink antenna is a multibeam antenna composed of eight reflectors, four transmitters, and four receivers. Each antenna is equipped with a multitude of feed horns capable of multiple spot beams in one or both circular polarizations. Each of the four transmit antennas generates the spot beams at one of the four user uplink frequencies. Each of the four receive antennas generates the congruent receive spot beams.

2.2 Spaceway

Hughes has proposed Spaceway System comprising 20 GEO satellites in 15 orbital locations. Spaceway can support 230,000 users worldwide at data rates of 384 kbps. Communication services will be provided at rates of 161 bps to 1.544 Mbps via terminals with antennas in the range of 66 to 200 cm in diameter. Onboard processing and ATM-based switching is used to route the traffic.

2.3 GE*Star

GE American Communications, Inc., has proposed a system of nine GEO satellites occupying five orbit locations. GE American proposes to purchase satellites that each produce 44 spot beams for transmitting and receiving, operating in a fourfold frequency reuse pattern. GE*Star plans for a minimum inbound rate of 128 kbps and 24 Mbps information stream (40 Mbps raw data transmission) in the outbound direction. GE*Star strongly considered inter-satellite links.

2.4 PanAmSat

PanAmSat was the first private company to offer global services. It now has five operational satellites providing services over the Atlantic, Pacific, and Indian Oceans. Presently, it is capable of providing services to Latin America, Africa, and Central/Eastern Asia. This system does not have inter-satellite links. The first four satellites operate in C- and Ku-band.

2.5 Teledesic

Teledesic, originally proposed to consist of 840 LEO satellites, has been redesigned and the number of satellites is reduced to 288. Teledesic supports inter-satellite links. Teledesic has chosen a LEO system based on the argument that GEO propagation delays are a problem for video conferencing and internet access protocols. GEO systems have developed techniques such as “spoofing” to enhance the Internet protocol performance. In addition, the Internet Engineering Task Force (IETF) has developed selective acknowledgments (SACK) and New Reno versions of TCP to improve t