VSAT Glossary and Terminology
(VSAT: Very Small Aperture Terminal)
Acceleration:
As related to satellite technology, it's the term describing speciific techniques (both hardware and software) used to optimize satellite service or transfer performance.
Aperture:
A cross sectional area of the antenna which is exposed to the satellite signal.
Attenuation:
The loss in power of electromagnetic signals between transmission and reception points.
Attitude Control:
The orientation of the satellite in relationship to the earth and the sun.
Asymmetrical:
Devoting more bandwidth to downstream (downloads) traffic than upstream (uploads).
Azimuth:
The angle of rotation (horizontal) that a ground based parabolic antenna must be rotated through to point to a specific satellite in a geosynchronous orbit. The azimuth angle for any particular satellite can be determined for any point on the surface of the earth giver the latitude and longitude of that point. It is defined with respect to due north as a matter of easy convenience.
Bandwidth:
Metaphorically, used to describe channel capacity or throughput, the rate at which bits may be transmitted through the system - typically expressed bits per second, eg., Kbps, Mbps.
BUC:
Block up-converter used in the transmission (uplink) of satellite signals. It converts a band (or "block") of frequencies from a lower frequency to a higher frequency.
C Band:
A VSAT frequency allocation between 3 to 7 GHz, having a large area footprint and minimal signal degradation related to "rain fade".
CIR:
"Committed Information Rate". The average bandwidth for a virtual circuit guaranteed by an ISP to work under normal conditions. At any given time the bandwidth should not fall below this committed figure. It is generally expressed in units as, kilobits per second (kbps).
Contingency Ratio:
The number of users or subscribers sharing the allotted bandwidth channel expressed as a ratio, eg, 10:1, 25:1, 60:1. In contrast to a dedicated channel which is not shared and can be expressed as having a contingency ratio of 1:1, you are sharing the allotted bandwidth with yourself only. The higher the ratio results in an increased likelihood of network congestion/competition (shared bandwidth) resulting in diminished network performance.
Conus:
Contiguous United States. In short, all the states in the U.S. except Hawaii and Alaska.
Cross-polarization (Cross-Pol):
Clarke Belt:
Science-fiction writer Arthur C. Clarke invented the geo-synchrous orbit, upon which satellite communications is based, more than a half century ago. The mathematical question was how high would a satellite have to be in orbit to appear to be stationary over the same spot on the Earth at all time. The answer, in rough terms, was 23,000 miles high and flying west-to-east.
3DES:
Sometimes called Triple DES. A method of triple-securing data (encrypt-decrypt-encrypt) for ensuring privacy in satellite VSAT transmissions. Security experts went to three-way encryption to foil what are known as meet-in-the-middle cipher attacks.
Downstream:
Elevation:
The upward tilt to a satellite antenna measured in degrees required to aim the antenna at the communications satellite. When. aimed at the horizon, the elevation angle is zero. If it were tilted to a point directly overhead, the satellite antenna would have an elevation of 90 degrees.
Feed:
This term has at least two key meanings within the field of satellite communications. It is used to describe the transmission of video programming from a distribution center. It is also used to describe the feed system of an antenna. The feed system may consist of a subreflector plus a feedhorn or a feedhorn only.
Forward Error Correction (FEC):
Adds unique codes to the digital signal at the source so errors can be detected and corrected at the receiver.
Gigahertz (GHz):
One billion cycles per second. Signals operating above 3 Gigahertz are known as microwaves. above 30 GHz they are know as millimeter waves. As one moves above the millimeter waves signals begin to take on the characteristics of Iightwaves.
Geostationary:
Geosynchronous:
Inclination:
The angle between the orbital plane of a satellite and the equatorial plane of the earth.
IPSec:
Internet Protocol Security. IPsec is a framework for a set of protocols for security at the network or at the packet processing layer of the network communication. Used in virtual private networks, one advantage of IPsec is that security arrangements can be handled without requiring changes to individual computers.
Isolation:
Ka Band:
A VSAT frequency allocation range from 18 to 31 GHz, having a small area footprint and severe signal susceptability related to "rain fade".
Ku Band:
A VSAT frequency allocation range from 10.9 to 17 GHz, having a medium area footprint and moderate signal susceptability related to "rain fade".
Link Budget:
The term for correctly sizing uplink and downlink paths. Factors include antenna size, satellite transmission power and potential atmospheric effects (from weather to sunspots).
Latency:
Sometimes called "Ping Time," it refers to the number of milliseconds (ms) it takes to send a data packet and receive an acknowledgement. Latency across the Internet is generally 100ms or less. In the satellite world, even with transmission speeds approaching the speed of light, the round-trip off the satellite can create roughly 700ms - 900ms of round-trip latency. Satellite service providers use advanced techniques to minimize latency, but some applications -- most notably interactive gaming -- will perform poorly.
LNB:
Low Noise Block Converter. The role of the LNB is to take a wide block (or band ) of relatively high frequencies, amplify and convert them to similar signals carried at a much lower frequency (called intermediate frequency or IF). These lower frequencies travel through cables with much less attenuation of the signal, so there is much more signal left on the satellite receiver end of the cable.
Parabolic Antenna:
The most frequently found satellite TV antenna, it takes its name from the shape of the dish described mathematically as a parabola. The function of the parabolic shape is to focus the weak microwave signal hitting the surface of the dish into a single focal point in front of the dish. It is at this point that the feedhorn is usually located.
Rain Fade:
Rain fade refers to the absorption of a microwave Radio Frequency (RF) signal by rain or snow, and is especially prevalent in frequencies above 11 GHz . It also refers to the degradation of a signal caused by the electromagnetic interference of the leading edge of a storm front. Rain fade can be caused by rain or snow at the uplink or downlink location. It does not need to be raining at a location for it to be affected by rain fade. The signal may pass through rain or snow many miles away, especially if the satellite dish has a low look angle.
Satellite Footprint:
A satellite Internet footprint diagram will show a picture of the signal level received at a particular location in terms of the Effective Isotropic Radiated Power (EIRP) received from the satellite. Areas of common reception are linked by lines that look like isobars on a weather map; when numbers are included on the banded lines, the higher the number, the greater the signal strength.
Skew:
TCP/IP:
Transmission Control Protocol / Internet Protocol. The backbone of the Internet. The IP standard controls how packets of information travel on the Internet and TCP enables two hosts to establish a connection and exchange streams of data.
TDMA:
Time Division Multiple Access. Refers to a form of multiple access where a single carrier is the shared by many users. Signals from earth stations reaching the satellite consecutively are processed in time segments without overlapping.
UDP:
User Datagram Protocol. Used primarily for broadcasting messages over local networks. It is a connectionless protocol and therefore cannot be accelerated for use on satellite Internet connections.
Upstream:
VSAT: " Very Small Aperture Terminal ". Term describes a ground unit designed for two-way communication (to receive and send data) via satellite. Generally consists of an ODU (Outdoor Unit) and IDU (Indoor Unit). ODU components include an antenna equipped with a feed system capable of receiving and transmitting, a microwave radio known as a BUC (Block-Up-Converter) and an LNB (low noise block converter) used to convert the signal gathered by the feed. The IDU is typically a modem and it converts the data, video, or voice generated by the customer application for transmission over satellite. The power of a BUC is measured in watts of transmitting power.
|
