13.5 A REPRESENTATIVE NETWORK

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Using any of the multiple access techniques discussed, a variety of applications can be supported on the satellite network. Some typical applications are:

Interconnecting the local area networks of different branch offices of an organization spread over large distances.
Interconnecting the PBXs of different branch offices of an organization.
Providing audio/video broadcasting from a central station with provision for audio/text interaction with users at the remote site.

A satellite-based network is the best choice to broadcast multimedia content to a large number of remote stations which are distributed geographically. From the hub, the information can be broadcast in TDM mode, and all the remotes will be in receive-mode. Low-speed channels are used from the remotes to the hub for interaction with the hub location.

Figure 13.8 shows the equipment configuration for providing distance education through satellite from a central location to a number of remotes located at different colleges/universities in a country. At the hub, there will be video transmission equipment. Additional infrastructure such as local area network, PBX, etc. also can be connected to the baseband interface equipment of the satellite hub. The baseband equipment provides the necessary interfaces to the video equipment, router connected to the LAN, PBX, etc. At each remote, there will be video reception equipment and also PBX and LAN. A lecture can be broadcast from the hub in the TDM mode. All the remotes can receive the lecture containing video, audio, and text. Whenever a user at a remote needs to interact with the professor at the hub, he can make a request, and a carrier is assigned to the remote in which he can send either text or voice.

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Figure 13.8: (a) Hub configuration for video conferencing and (b) remote configuration.

Systems with similar architecture also are used to provide telemedicine service to rural areas. The central location is connected to a hospital, and the remotes are located at remote locations. The patients can consult the doctor at the central location using video conferencing.

Note

To conserve radio spectrum, multimedia communication over satellite networks use Voice/ Video over IP protocols in which voice and video are encoded using low-bit rate coding techniques.

Summary

Ever since the first communication satellite was launched in 1962, satellite communication systems have been used for broadcasting and tele-communications as well as for providing location information. The attractive feature of satellite communication is its insensitivity to distance. Hence, satellite communication is a very cost-effective way of providing telecommunication facilities to rural and remote areas. Satellite communication systems operate in C, Ku, and Ka bands.

Satellite communication systems operate in star and mesh configurations. In star configuration, there will be a central station (hub) and a number of remotes. All the remotes communicate via the central station. In mesh configuration, remotes can talk to each other directly. Star configuration is more attractive than mesh configuration because in star configuration, small antennas called very small aperture terminals (VSATs) can be used to reduce the cost of the remote. In star configuration, the central station broadcasts in time division multiplex (TDM) mode to all the remotes, and remotes can transmit in time division multiple access (TDMA) mode. The three multiple access techniques with which mesh configuration can be obtained are: DAMA-SCPC (demand assigned multiple access-single channel per carrier), TDM-SCPC, and TDMA. With the availability of onboard processing on satellites, satellite communication systems are now being used to provide mobile communications as well. Satellite communications can be effectively used for applications such as distance education and telemedicine.

References

R. Horak. Communications Systems and Networks. Wiley-Dreamtech India Pvt. Ltd., 2002.
http://www.intelsat.int Web site of Intelsat, which provides satellite services.
http://www.isro.org Web site of Indian Space Research Organization.

Questions

List the various applications of satellites.
What are the problems associated with satellite communication?
What is the impact of high propagation delay on voice communication?
What is the impact of high propagation delay on data communication?
What are the frequency bands of operation for communication satellites?
Explain the various multiple access techniques used in satellite communication.

Exercises

1.	Prepare a technical report on direct broadcast satellites. Direct broadcast satellites are used for transmitting TV programs directly to homes. These satellites operate in the 17/12GHz band.
2.	Prepare a technical report on remote sensing satellites.
3.	For supporting voice services on satellite-based networks, study the various voice encoding techniques used.
4.	Carry out a paper design to develop a satellite-based video surveillance system. The system has to capture video data of a specific location and send it to an Earth station.
5.	In a satellite network, the roundtrip delay is about 0.5 second. If the stop-and-wait protocol is used on such a network for data communication, study how effective the satellite channel utilization is.

Answers

1.	Direct broadcast satellites transmit TV programs directly to homes. These systems operate in the frequency band 17/12GHz. However, due to the wide penetration of cable TV, direct broadcast satellite technology has not taken off well in India, though DBS is extensively used in North America and Europe.
2.	Remote sensing satellites are used for a variety of applications: to find out the areas in which natural resources are available, to determine the water table under the earth's surface, to analyze the fertility of lands, and so on. Remote sensing satellites have sensors that operate in the infrared and near- infrared bands. The satellite imagery is sent to the ground stations. The imagery is analyzed based on the application. Indian Space Research Organization (ISRO) launched the IRS (Indian Remote Sensing Satellite) series satellites exclusively for remote sensing.
3.	In satellite communication, efficient use of the bandwidth is very important, as bandwidth is much costlier than the terrestrial media bandwidth. Low bit rate coding of voice is done using ADPCM, LPC, and CELP techniques.
4.	In satellite-based video surveillance, there should be a system that captures the video and transmits it to the ground continuously. The video camera captures the scene and digitizes it using a standard coding technique such as MPEG2 or MPEG4 and transmits to the Earth station. The minimum data rate to be supported by the link is 1Mbps to obtain reasonably good quality video.
5.	In a satellite network, the round-trip delay is about 0.5 second. If stop-and- wait protocol is used on such a network for data communication, the satellite channel is not used effectively. After a packet is transmitted from one side, it takes 0.5 seconds to reach the destination (assuming that there is no other delay). Then the acknowledgement will be sent by the other station, and it will be received by the station after another 0.5 seconds. So, effectively one packet is transmitted every one second!

Projects

Work out the details of the commercial equipment required to implement a satellite-based distance education system for which the architecture is explained in this chapter. You can obtain the details of the commercial products from the Web sites of equipment vendors such as Hughes Network Systems, Paragea, Scientific Atlanta, etc.
Design and develop a communication system used in surveillance satellites. The video signal has to be encoded at 384kbps data rate using a commercially available video codec and transmitted to the Earth station.

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