28-09-2013, 03:34 PM
Satellite Communications
Satellite Communications.pdf (Size: 99.18 KB / Downloads: 68)
Introduction
In 1962, the American telecommunications giant AT&T launched the world's first true communications
satellite, called Telstar. Since then, countless communications satellites have been placed into earth orbit, and
the technology being applied to them is forever growing in sophistication.
Basic Elements
Satellite communications are comprised of 2 main components:
The satellite itself is also known as the space segment, and is composed of three separate units, namely
the fuel system, the satellite and telemetry controls, and the transponder. The transponder includes the
receiving antenna to pick-up signals from the ground station, a broad band receiver, an input
multiplexer, and a frequency converter which is used to reroute the received signals through a high
powered amplifier for downlink. The primary role of a satellite is to reflect electronic signals. In the
case of a telecom satellite, the primary task is to receive signals from a ground station and send them
down to another ground station located a considerable distance away from the first. This relay action
can be two-way, as in the case of a long distance phone call. Another use of the satellite is when, as is
the case with television broadcasts, the ground station's uplink is then downlinked over a wide region,
so that it may be received by many different customers possessing compatible equipment. Still another
use for satellites is observation, wherein the satellite is equipped with cameras or various sensors, and
it merely downlinks any information it picks up from its vantagepoint.
The Ground Station.
This is the earth segment. The ground station's job is two-fold. In the case of an uplink, or transmitting
station, terrestrial data in the form of baseband signals, is passed through a baseband processor, an up
converter, a high powered amplifier, and through a parabolic dish antenna up to an orbiting satellite. In
the case of a downlink, or receiving station, works in the reverse fashion as the uplink, ultimately
converting signals received through the parabolic antenna to base band signal.
Television Signals
Satellites have been used for since the 1960's to transmit broadcast television signals between the
network hubs of television companies and their network affiliates. In some cases, an entire series of
programming is transmitted at once and recorded at the affiliate, with each segment then being
broadcast at appropriate times to the local viewing populace. In the 1970's, it became possible for
private individuals to download the same signal that the networks and cable companies were
transmitting, using c-band reception dishes. This free viewing of corporate content by individuals led
to scrambling and subsequent resale of the descrambling codes to individual customers, which started
the direct-to-home industry. The direct-to-home industry has gathered even greater momentum since
the introduction of digital direct broadcast service.
DBS
DBS (Direct Broadcast Satellite) -The transmission of audio and video signals via satellite direct
to the end user. More than four million households in the United States enjoy C-Band DBS.
Medium-power Ku-Band DBS surfaced in the late 1990s with high power Ku-Band DBS
launched in 1994.
Hybrid Networks
In today's global networking landscape, there are many ways to transmit data from one place to
another. It is desirable to be able to incorporate any type of data transmission media into a network,
especially in networks that encompass large areas. A hybrid network is one that allows data to flow
across a network, using many types of media, either satellite, wireless or terrestrial, transparently.
Since each type of media will have different characteristics, it is necessary to implement a standard
transmission protocol. One that is normally used in hybrid networks is TCP/IP. In addition, much work
is being done to use TCP/IP over ATM for the satellite segments of hybrid networks, about which
more will be discussed later.
One way to get around the need in ARR for the receiver to have to request retransmit via an expensive
and slow satellite link is to use a form of hybrid network. In one form of hybrid network, the reciever
transmits its requests back to the sender via a terrestrial link. Terrestrial link allows for quicker, more
economical and less error prone transmission from the reciever, and the costs associated with the
receivers hardware are greatly reduced when compared to the costs involved if it had to transmit back
over the satellite link. There are products on the market today that allow a home user to get intenet
access at around 400MB via digital satellite, while its retransmit signals are sent via an inexpensive
modem or ISDN line.
ATM Over Satellite
Two qualitites of Asynchronous Transfer Mode (ATM) made it highly desirable for the
implementation of satellite links within hybrid networks. The first is the ATM's asynchrony and the
second is its ability to use variable transfer rates. In addition, ATM fits well into existing networks
with its wide range of upper-layer services and its ability to operate in a wide range of environments.
There are problems, however. ATM's relatively large propagation delays can significantly increase the
latency of feedback mechanisms essential for congestion control. acquisition time
SATIN - Satellite Integrated Terrestrial Network:
The goal of SATIN is to create a fully integrated hybrid network in which the method of
communication, which can incorporate networks of local, metropolitan and wide area scope,
Broadband ISDN, Integrated Network Management, AIN (Advanced Intelligent Networks) and
PCS (Personal Communications Services), in addition to ATM (Asynchronous Transfer Mode)
over satellite, is totally transparent to the user. The difficulties inherent in this are obvious.
Differences in latency, noise, bandwidth and reliability must be equalized in all the media that
will encompass the network.
Satellite Communications.pdf (Size: 99.18 KB / Downloads: 68)
Introduction
In 1962, the American telecommunications giant AT&T launched the world's first true communications
satellite, called Telstar. Since then, countless communications satellites have been placed into earth orbit, and
the technology being applied to them is forever growing in sophistication.
Basic Elements
Satellite communications are comprised of 2 main components:
The satellite itself is also known as the space segment, and is composed of three separate units, namely
the fuel system, the satellite and telemetry controls, and the transponder. The transponder includes the
receiving antenna to pick-up signals from the ground station, a broad band receiver, an input
multiplexer, and a frequency converter which is used to reroute the received signals through a high
powered amplifier for downlink. The primary role of a satellite is to reflect electronic signals. In the
case of a telecom satellite, the primary task is to receive signals from a ground station and send them
down to another ground station located a considerable distance away from the first. This relay action
can be two-way, as in the case of a long distance phone call. Another use of the satellite is when, as is
the case with television broadcasts, the ground station's uplink is then downlinked over a wide region,
so that it may be received by many different customers possessing compatible equipment. Still another
use for satellites is observation, wherein the satellite is equipped with cameras or various sensors, and
it merely downlinks any information it picks up from its vantagepoint.
The Ground Station.
This is the earth segment. The ground station's job is two-fold. In the case of an uplink, or transmitting
station, terrestrial data in the form of baseband signals, is passed through a baseband processor, an up
converter, a high powered amplifier, and through a parabolic dish antenna up to an orbiting satellite. In
the case of a downlink, or receiving station, works in the reverse fashion as the uplink, ultimately
converting signals received through the parabolic antenna to base band signal.
Television Signals
Satellites have been used for since the 1960's to transmit broadcast television signals between the
network hubs of television companies and their network affiliates. In some cases, an entire series of
programming is transmitted at once and recorded at the affiliate, with each segment then being
broadcast at appropriate times to the local viewing populace. In the 1970's, it became possible for
private individuals to download the same signal that the networks and cable companies were
transmitting, using c-band reception dishes. This free viewing of corporate content by individuals led
to scrambling and subsequent resale of the descrambling codes to individual customers, which started
the direct-to-home industry. The direct-to-home industry has gathered even greater momentum since
the introduction of digital direct broadcast service.
DBS
DBS (Direct Broadcast Satellite) -The transmission of audio and video signals via satellite direct
to the end user. More than four million households in the United States enjoy C-Band DBS.
Medium-power Ku-Band DBS surfaced in the late 1990s with high power Ku-Band DBS
launched in 1994.
Hybrid Networks
In today's global networking landscape, there are many ways to transmit data from one place to
another. It is desirable to be able to incorporate any type of data transmission media into a network,
especially in networks that encompass large areas. A hybrid network is one that allows data to flow
across a network, using many types of media, either satellite, wireless or terrestrial, transparently.
Since each type of media will have different characteristics, it is necessary to implement a standard
transmission protocol. One that is normally used in hybrid networks is TCP/IP. In addition, much work
is being done to use TCP/IP over ATM for the satellite segments of hybrid networks, about which
more will be discussed later.
One way to get around the need in ARR for the receiver to have to request retransmit via an expensive
and slow satellite link is to use a form of hybrid network. In one form of hybrid network, the reciever
transmits its requests back to the sender via a terrestrial link. Terrestrial link allows for quicker, more
economical and less error prone transmission from the reciever, and the costs associated with the
receivers hardware are greatly reduced when compared to the costs involved if it had to transmit back
over the satellite link. There are products on the market today that allow a home user to get intenet
access at around 400MB via digital satellite, while its retransmit signals are sent via an inexpensive
modem or ISDN line.
ATM Over Satellite
Two qualitites of Asynchronous Transfer Mode (ATM) made it highly desirable for the
implementation of satellite links within hybrid networks. The first is the ATM's asynchrony and the
second is its ability to use variable transfer rates. In addition, ATM fits well into existing networks
with its wide range of upper-layer services and its ability to operate in a wide range of environments.
There are problems, however. ATM's relatively large propagation delays can significantly increase the
latency of feedback mechanisms essential for congestion control. acquisition time
SATIN - Satellite Integrated Terrestrial Network:
The goal of SATIN is to create a fully integrated hybrid network in which the method of
communication, which can incorporate networks of local, metropolitan and wide area scope,
Broadband ISDN, Integrated Network Management, AIN (Advanced Intelligent Networks) and
PCS (Personal Communications Services), in addition to ATM (Asynchronous Transfer Mode)
over satellite, is totally transparent to the user. The difficulties inherent in this are obvious.
Differences in latency, noise, bandwidth and reliability must be equalized in all the media that
will encompass the network.