NETWORK ARCHITECTURES, TECHNOLOGIES AND PROTOCOLS

Introduction

In this section, emerging technology trends in satellite networks are presented. In contrast with the earlier 1992/1993 NASA/NSF study, which found satellite network activities only in research programs, this panel observed considerable activity toward satellite networks deployment, manufacturing, applications, concept and technology development, standards, as well as research. The need to provide voice communications in the immediate future, and data and video services in the near future, to anyone, anywhere, anytime, in the growing global economy has provided an impetus to the rapid development of satellite networks. The increasing usage of the Internet is another major driving force. In the United States, with the launch of the Iridium satellite network, which will provide global voice and low data rate services, and the Globalstar system, satellite networks are becoming a reality. Both systems have extensive ground network systems. Network technologies are also being developed for the proposed Ka-band systems. As the awareness of network requirements to provide global data services develop, R&D activities are also increasing.

In this section, global trends in emerging satellite network architectures, infrastructure technologies, and protocols are briefly discussed. Issues identified during the site visits pertaining to the seamless interoperation of satellite and terrestrial networks are presented in a summary manner, and current research in these areas is discussed. Finally, research and experimental work in Europe, Japan, Canada and the United States is presented. In the WTEC panelists’ opinion, the work in the U.S.A. is oriented towards basic research coupled with experimental programs, whereas in Europe, Japan, and Canada, the work is more experimental. The typical approach is to install a testbed to conduct the experiments. The testbed facilities are described in this section.

System Architectures

The overall evolving communications network architecture observed during the WTEC visits is shown in Figure 4.15. in which current and planned satellite networks are being integrated with terrestrial networks (wireless and wireline) to provide end-to-end voice, video, and data services to users at various data rates. In this architecture, satellite networks are capable of interfacing with terrestrial networks at high data rates and also provide networking access to a variety of users directly. There is an increasing demand to support a variety of multimedia services, where large bandwidth video data is reduced to a few megabits per second and transmitted in combination with different signals to form multimedia data.

Within satellite networks, the current technology trend is to use Internet protocols and asynchronous transfer mode (ATM) to carry voice, video and data. The architecture is shown in Figure 4.16. The application of ATM technologies in satellite networks is expected to offer these multimedia services inexpensively on a global scale. Since satellite networks can easily access information resources located anywhere on the globe and then broadcast the information, they are very attractive for the provision of multimedia services. These services will then play a critical role in the global economy.



Fig. 4.15. Communication network architecture—top level.



Fig. 4.16. Satellite ATM network architecture.

The growth of the Internet has promoted the use of existing satellites to provide network services. The emerging hybrid service architectures are shown in Figure 4.17. These services are being provided from the United States to other parts of the world by Orion Network, PANAMSAT and INTELSAT. Loral Orion uses Frame Relay for transport control protocol/Internet protocol (TCP/IP) transport to ISPs located in Europe, Asia and Latin America. It is ideally suited for such an application due to its efficient use of bandwidth.



Fig. 4.17. Hybrid (satellite/wireline) Internet service architecture.

Emerging Applications

The panel observed significant application experiments and developments taking place at government and industrial research laboratories that are addressing emerging information infrastructure market needs world-wide. It is well known that satellites offer a variety of applications in broadcasting, mobile, and fixed services.

Emerging satellite networks and related technologies can provide a wide variety of applications. The panel gathered data about application and development activities via on-site demonstrations or briefings. Table 4.5 summarizes progress on 20 of those applications around the world. This panel surveyed activity in the following regions or countries: Canada, Europe, Japan, the United States, China, India, Israel, Korea, and Russia. Preceding the table are definitions of status stages.

The panel observed that the Internet/Intranet is a rapidly growing market for satellite communication network providers worldwide.

The Internet is based on open horizontal layer architecture that enables a large number of applications. In the case of Ka-band subscribers, particularly in developed markets, the prospects are for many of these applications to be delivered as part of a combined multimedia Internet package. In developing markets, Ka-band terminals will be more commonly deployed in support of specific applications, like telephony or corporate networks, which is comparable to the way VSATs are used today.

Table 4.5
Applications of Emerging Satellite Networks

APPLICATION

COUNTRY

 

Canada

Europe

Japan

U.S.

Other

I. A1. Internet access

Emerging

Nascent: D

Nascent: D

Emerging

China: Emerging

India: Emerging

Israel: Emerging

A2. Multicasting

Nascent: C

Nascent: C

Nascent: C

Nascent: D

No data

A3. Backbone

Emerging

Nascent: D

Nascent: D

Growth

No data

B. Multimedia

Nascent: D

Nascent: D

Emerging

No data

No data

C. Global Telephony

Emerging

Emerging

Emerging

Emerging

Emerging in the rest of the world

II. A. Telemedicine

Nascent: D

Nascent: D

Nascent: D

Nascent: D

Russia: Nascent: D

B. Teleeducation

Emerging

Nascent: D

Nascent: D

Emerging

Korea: Nascent: D

C. Library, museum

No data

No data

Nascent: R

Nascent: R

No data

D. News gathering service

No data

No data

Nascent: D

Nascent: D

Korea: Nascent: D

E1. Data broadcasting

No data

No data

Nascent: D

Nascent: D

No data

E2. Digital broadcasting

Growth

Growth

Growth

Growth

Korea: Growth

F. Government

Nascent: D

No data

Emerging

No data

China: Emerging

India: Emerging

G. Telecontrol

No data

No data

Emerging

No data

No data

H. Teleconferencing

Nascent: D

Nascent: D

Nascent: D

Emerging

No data

I. Telecommuting

No data

No data

No data

No data

No data

J. Electronic commerce

Emerging

Emerging

Emerging

Emerging

Emerging in the rest of the world

K. High data-rate transfer

Nascent: R

Nascent: R

Nascent: R

Nascent: R

No data

L. Distributed computing

Nascent: R

Nascent: R

Nascent: R

Nascent: R

No data

M. Disaster recovery

Nascent: R

Nascent: R

Nascent: R

Nascent: R

No data

N. Aeronautical

No data

Nascent: D

No data

Nascent: D

No data

Other: China, India, Israel, Korea, Russia

1. Nascent: (C)oncept, (D)evelopment, (R)esearch

2. Emerging market

3. Growth market

4. Mature market



A broad consensus exists that there are between 60 and 70 million Internet subscribers today worldwide. There has been more variety in estimates of the future—with some forecasts saying subscribers will reach around ten times this number by 2000.1 The growth has been led by the United States— with perhaps up to 50 million people on-line at the start of 1997. It is now the Far East and Europe that are increasingly providing the highest growth rates. Internet activity—particularly in Japan—has been substantial over the last eighteen months. Forecasts suggesting that the Asia/Pacific region will have around 20 million users by 2000 are not unrealistic. In Europe Internet subscription is considered to be around four or five years behind the US. Ovum’s "European Telematics profile (Ovum 1997) and data from the ITU (ITU 1997) suggest that there are currently 10-13 million European Internet subscribers.

Network Wizards, one of the most quoted sources, suggest that globally the number of domains (roughly equivalent to organizations or individuals) is doubling every nine or so months—a slightly diminishing but still dramatic growth rate (previously domains were recorded as doubling every six months) (www.nw.com/).

In this market scenario, the development of advance satellite network technologies is self-evident. The section below describes the state of art in ATM over satellites. This is followed by discussion of Internet protocols over satellite systems, including descriptions of various experimental programs around the globe.

ATM Over Satellite Technology

Asynchronous transfer mode is a packet communications scheme in which all packets are of equal length, and consist of an address field and a "payload." This approach was chosen to permit high-speed switching by fast hardware, with the bits in the address field selecting the path of the packet through the switch. ATM has additional features (e.g., quality of service provisions) that make it an attractive candidate for the operation of future data-centric backbone networks, though some observers speculate that the emergence of very high-speed routers, together with dense wave division multiplexing (the use of many different colors of light on the same fiber), could allow the use of IP over synchronous optical network (SONET) fiber links to become the standard.

Owing to their inherently higher noise levels, satellite links have higher bit error rates (BER) than fiber optic links. (A typical satellite link might operate with a BER of 1 in 106 while a fiber link may achieve 1 in 109 or 1010 BER.) The ATM frame carries enough information to correct one bit error in the address field. Thus to avoid packets being dropped because of incorrect addresses, satellite systems can best support ATM through the use of specially conditioned links. Commercial devices ("Link Accelerator," "Link Enhancer") are available that provide this conditioning and are indicated conceptually in Figure 4.18 as an ATM satellite interworking unit (ASIU).



Fig. 4.18. ATM satellite interworking unit (ASIU).

The ASIU is responsible for management and control of system resources and overall system administrative functions. The key functions of the ASIU include real-time bandwidth allocation, network access control, system timing and synchronization control, call monitoring, error control, and traffic control. The protocol stack from the satellite networks based on ATM switching is shown in Figure 4.19.

Figure 4.20, shows the detailed interface between the ASIU and other modules, and the internal architecture of the ASIU, respectively. To accommodate ATM networks seamlessly, the ASIU needs to support the existing ATM cell transport methods such as SONET (synchronous optical network)/SDH (synchronous digital hierarchy), PDH (plesiochronous digital hierarchy), and PLCP (physical layer convergence protocol). As shown in Figure 4.20, when SONET frames conveying ATM cells arrive at an ASIU, ATM cells are extracted from the frames. Extracted ATM cell streams are classified according to the traffic classes, and each classified cell stream is placed into a buffer with associated priority before transmission into the satellite channel.



Fig. 4.19. Protocol stack for the satellite network based on ATM switching.



Fig. 4.20. Internal architecture of the ATM satellite interworking group.

An effective error correction coding scheme should be employed in the ASIU because satellite networks often introduce multiple bit errors. Furthermore, in order to operate with the existing high-speed ATM networks that use optical fiber as a transmission medium, the BER of satellite links should be comparable to the BER of optical fiber links. The coding scheme can be applied over ATM cells after they are extracted from the received frames. The usual approach to error control is to employ forward error correction (FEC) (e.g., a Viterbi convolutional code that adds redundant bits, thereby allowing errored bits to be recognized and corrected). The power of these schemes increases in proportion to the number of redundant bits that are added, with typical choices being 1 in 7 (i.e., rate 7/8), 1 in 3 (rate ) or 1 in 2 (rate ). However, these schemes fail whenever more bits are in error than can be corrected unambiguously, and there is then a "burst" of errors. To remedy this, a second outer block code (e.g., Reed Solomon) may be employed with a trellis buffer. That is, the data are read into the columns of a buffer memory and read out of the rows. This spreads out the errored bits allowing the second outer coder to correct them.

Thus one approach to error correction in an ASIU is to employ increased levels of FEC and RS coding in a concatenated fashion as the link performance degrades. More sophisticated approaches entail (a) compressing the header containing the address, (b) compressing the payload, and (c) reformatting both of these into a new frame that has a link error dependent amount of Reed Solomon outer coding applied and allows for the synchronization between transmitter and receiver. The advantage of this approach is that it is less bandwidth intensive. The TIA TR34.1 Committee is currently considering recommendations for the approach that should be followed.

Since satellite bandwidth is a limited resource and should be shared between earth stations fairly, a flexible and efficient bandwidth management scheme is required in the ASIU. In other words, it is important to assign the bandwidth dynamically and efficiently based on the various user requirements.

The ASIU also needs to support an appropriate satellite link access scheme to send data into the satellite channel. Link access schemes should be chosen to provide high efficiency utilization of satellite bandwidth. The demand assignment multiple access (DAMA) scheme is preferred because it allows each earth station to request only the bandwidth that will actually be used.

Another important factor that should be considered for overall performance of satellite ATM networks is the quality of satellite links. The willingness of ATM users to adopt satellite communications will largely depend on providing high quality, cost-effective satellite links. The elements of a satellite link which can affect network performance include link budget process, satellite equipment latency, data rate, modem type, buffer management scheme, coding/modulation, throughput, interface, and satellite type. They should be chosen properly according to user and network requirements.


1 See for example Matrix Information and Directory Services: World User Figures, 18th February 1997. MIDS believe there were 57 million Internet users as of January 1997 with 71 million having email access. Their projections for 2000 suggest 707 million Internet subscribers with 827 million having email (see http://www.mids.org/press/pr9701.html). Even as far back as 1992 the president of the Internet Society was quoted as making the prediction that "...by the year 2000 the Internet will consist of some 100 million hosts, 3 million networks, and one billion users"
Published: December 1998; WTEC Hyper- Librarian