SATELLITE SYSTEMS

Satellite Communications

• Based on microwave transmission

• Satellite communication systems consist of ground-based or earth stations (i.e.parabolic antennas) and orbiting transponders.

• The transponder receives a microwave signal from the ground unit (the uplink) amplifies it and then transmits it back to earth (the downlink).

Satellite Communication, cont.

• Two general configuration for satellite communication are:– Point-to-point– Broadcast

Point-to-Point Link via Microwave

Broadcast Link

Multiple receivers

Transmitter

Communication satellites can be grouped into 2 categories:– Geostationary (GEO)– Low Earth Orbit (LEO)

• Geostationary

these are placed in an orbit that is exactly synchronized with the earth (~ 20,000 miles above the earth).

• Low Earth Orbit

these orbit a few hundred miles above the earth.

Latency of Satellite Systems• GEO satellite systems have a high latency.• Consider a transmission between two remote

nodes. The sending station transmits a message to the satellite (uplink). The satellite transmits this message to the destination node (the downlink). The destination node sends an acknowledgement (ACK) to the satellite. The satellite then transmits the ACK to the sender.

• The total distance involved is 88,000 miles.• Dividing by the speed of light, we get a total

propagation delay of 470 milliseconds.

• The longer the latency, the less bandwidth the system can support.

• The bandwidth capability of satellite systems is a function of the frequency at which the satellites transmit.

• Four common frequencies are:– C-band 6GHz uplink and 4 Ghz downlink– Ka-band 28 Ghz uplink, 18 Ghz downlink– Ku-band 14 Ghz uplink, 12 Ghz downlink– V-band above 30 Ghz

Satellite Communication, cont.

• Two satellites using the same frequency will interfere with each other, if they are placed close together.

• To avoid this, current standards require a 4-degree spacing (angular displacement) at the same altitude.

• A consequence of this requirement is that only 90 satellites can be placed at the same altitude.

Applications of Satellite

• Television distribution.• Long-distance telephone transmission• Private commercial networks

The development of VSAT systems (very small aperture terminals) provide a low-costsolution for small enterprises.

• The biggest advantage of satellites is that they can reach geographically remote areas.

Trends in Satellite Networks

• Design of modern satellite networks (the so-called 3rd generation satellites) is highly influenced by the global trend of user instead of network oriented services.

• Due to the rapid growth in the cellular market, the telecommunication industry is making large investments in Mobile Satellite Services (MSS).

Trends in Satellite Networks, cont.• In order to meet the increasing demand for

real time traffic, channel access and link layer protocols will have to be optimized to ensure smooth operation over the satellite channel. TDMA and CDMA appear to be two of the strongest candidates for the MAC protocol.

Advantages of Satellites• covers very large areas• reaches geographical isolated areas

Disadvantages:• expensive• large propagation delay (high latency)• not very secure; signals can be easily intercepted• affected by atmospheric conditions

RECAP

Types of Transmission Media

• Guided Media:– Copper Wire

• Twisted Pair

• Coaxial Cable

– Optical Fibre

• Wireless Media:– Radio Waves

– Microwave

– Infrared waves

– Satellites

• Advantages of fiber over wire:– not susceptible to electrical interference– can carry signals much farther (up to 100 km)– can carry more information (up to 10 Gbps).

Ideal for transmitting multimedia information.– no need for circuits; only a single fiber is

needed.– Works better under harsh environmental

conditions than copper wire. More resistance to corrosion.

– Can withstand higher temperatures than copper.

Disadvantages of fiber:– expensive– requires specialized equipment– difficult to locate a broken fiber– difficult to join a broken fiber

• Radio Waves– Electromagnetic radio waves can be used to transmit

computer data to facilitate wireless networking.

– Each computer on the network attaches to an antenna which transmits and receives RF signals.

Advantages of radio waves– can travel long distances

– pass through obstacles well at low frequencies

Disadvantages:– subject to interference from motors and other electrical

equipment

– absorbed by rain.

• Microwave– this is a radio beam that uses very high

frequencies to send and receive data. – Transmission is aimed in a single direction to

prevent others from intercepting the signal. Stations are placed ~30 km apart.

– Carries more information than low-frequency RF transmissions.

– Microwaves cannot penetrate metal structures. Stations must be visible to each other.

– Many long-distance telephone companies use microwave as their transmission medium.

• Infrared– Infrared transmission uses low frequency light

waves to carry data through the air– used in wireless remote controls – limited to a small area (e.g. a single room)– requires line-of-sight. Transmitter must be

pointed towards the receiver– some computer networks use infrared

technology– more convenient for small portable computers

within a room– cannot be used outdoors.

Which Transmission Medium is Best?Several factors are important in selecting a transmission medium:• The type of network- some media are used only for WANs, whereas others are used for LANs. Fibre can be used for either.• Cost - twisted pair is generally cheapest. The cost of wireless media is driven more by distance. For short distances (several hundred metres) radio and infrared are cheapest; for several hundred miles, microwave is cheapest; for longer distances, satellite is cheapest.

• Transmission distance is a related factor. Twisted pair, coax, infrared and radio are all subjected to attenuation over short distances.

• Security - guided media is generally more secure than wireless. Fibre is the most secure.

• Error rates - wireless media are most susceptible to interference and thus have the highest error rates. For guided media, fibre provides the lowest error rates; coax is the next best and twisted pair the worst.

• Transmission speeds - vary greatly among different media.

Transmission Speeds for Various Media Types

Generally,• Twisted pair and coax provide up to 100 Mbps • Fibre: 100 Mbps up to 10 Gbps• Radio and infrared: 1 to 4 Mbps• Microwave and satellite: 20 to 50 Mbps

THE END