SATELLITE DATA NETWORK COMMUNICATIONS

OVERVIEW• History of Satellite

Network• A Communication

satellite functions as an overhead wireless repeater station

• It provides a microwave communication link between two geographically remote sites

History of Satellite NetworkIn 1962, the world's first active communications satellite, Telstar 1, was launched. This satellite was built by Telesat's predecessors at AT&T and Bell Laboratories. During its seven months in operation, Telstar 1 dazzled the world with live images of sports, entertainment and news. It was a simple single-transponder low-earth-orbit (LEO) satellite, but its technology of receiving radio signals from the ground, and then amplifying and retransmitting them over a large portion of the earth's surface, set the standard for all communications satellites that followed.

History of Satellite NetworkDuring the 1960s and 1970s, advances in satellite performance came quickly and a global industry began to develop. Satellites were mainly used at first for international and long-haul telephone traffic and distribution of select television programming, both internationally and domestically. In 1973 the Canadian Broadcasting Corporation began distributing its video programming to Canadian customers using Telesat’s Anik A satellite. Then in 1975 HBO began distributing its video programming to US customers by satellite. The commercial and technical success of these ventures led to a greater use and acceptance of satellite broadcasting. By the 1990s, satellite communications would be the primary means of distributing TV programs around the world.

TRANSPONDERS• Each Satellite is

equipped with various “transponders” consisting of a transceiver and an antenna tuned to the part of the allocated spectrum

• Most satellites simply broadcast whatever they receive and they are called as “bent pipes”

DATA TRANSMISSION• Use of satellites in

packet data transmission has been on the rise

• They are typically used in WAN networks which acts as the backbone links to geographically dispersed LAN’s and MAN’s

SATELLITE COMMUNICATION CHANNELS

• Wide Area coverage of the earth's surface.

• Long transmission delays.

• Broadcast transmission.

• Large Channel Bandwidth.

• Transmission costs independent of Distance.

MICROWAVE POWER• The received microwave

power involved in satellite links is typically very small (of the order of a few 100picowatts).

• Specially designed earth stations that keep C/N (carrier to noise ratio) to a minimum are used to transmit/receive satellite communications.

SATELLITE LINKS• Satellite links can

operate in different frequency bands and use separate carrier frequencies for the up-link and down-link or "spot beam"

• The area of the earth's surface covered by a satellite's transmission beam is referred to as the "footprint" of the satellite transponders.

EARTH SATELLITE COMMUNICATION

SATELLITE CONSTELLATIONS• Satellites can be

positioned in orbits with different heights and shapes

• Depending on the shapes they are of 3 types: LEO: Low Earth Orbit MEO: Medium Earth

Orbit GEO: Geostationary

Earth Orbit

SATELLITE CONSTELLATIONS• Satellites are also

classified in terms of their payload.

• Satellites that weigh in the range of 800-1000 kg fall in the "Small" class

• The heavier class is named as "Big" satellites. GEO satellites are typically "Big" satellites

• LEO satellites can fall in either class

DIFFERENT SATELLITE CONSTELLATIONS

MAC PROTOCOLS FOR SATELLITE LINKS

• Satellite channels require special considerations at the DLC (Data Link Control) layer of the OSI model they are referred to as “Long Fat Pipes”

• Implementing CSMA/CD (Carrier Sense Multiple Access/Collision Detection) protocol is not possible

• Delay occurs in the channel in which they cause potential or multiple collisions in the satellite link

MAC SCHEMES • Most satellite MAC schemes usually assign dedicated

channels in time and/ or frequency for each user • ALOHA

Pure Aloha allows every competing stations to transmit anytime

Its uses S-ALOHA (Slotted ALOHA) to increase its efficiency

• FDMA (Frequency Division Multiple Access) The available satellite channel bandwidth is

broken into frequency bands for different earth stations

ES controls the microwave power in the band for not spilling into other channels

MAC SCHEMES • TDMA (Time Division Multiple Access)

Channels are time multiplexed in a sequential fashion.

Time synchronization is required between the Earth Station

• CDMA (Code Division Multiple Access) Uses a hybrid of time/frequency

multiplexing Form of spread spectrum modulationNo Time synchronization

: How Satellites are used• Service Types

Fixed Service Satellites (FSS)• Example: Point to Point

Communication Broadcast Service Satellites

(BSS)• Example: Satellite

Television/Radio• Also called Direct Broadcast

Service (DBS). Mobile Service Satellites (MSS)

• Example: Satellite Phones