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Antenna
Also known as a dish, the antenna is in the shape of a parabola, which has a special reflective property that enables it to reflect and converge electromagnetic waves to a single point, called the focal point.
Antenna Gain
The Gain is defined as the ability of the antenna to amplify the very weak incoming signal from the satellite.
Asymmetrical connection
See symmetrical connection
Azimuth angle
The Azimuth is the angle of the path to the satellite in the earth’s horizontal plane,measured clockwise from North. Therefore an object due East is said to have an azimuth of 90 degrees or one due South an azimuth of 180 degrees.
Backhaul Connection
This is the connection between the Hub and either the public network such as the Public Switched Telephone Network (PSTN), or the Internet.
Bandwidth
Bandwidth is the difference between the upper and lower frequencies of an analogue channel. Bandwidth is measured Hz. The capacity or data-rate that a channel can provide is measured in bps. There is a known but different relationship between bandwidth and data rate for all digital systems. Bandwidth and capacity are used interchangeably in most literature.
Bit
One bit of data. A bit is either a "1" or a "0". Measure in b, kb, Mb or Gb. (Note, the "b" is always in lower case without exception!
bps
Bits per second. A measure of capacity of a transmission channel used for digital systems. See Hertz and bandwidth.
BUC
Block Up Converter – The BUC can be thought of the “transmitter”, and its actions are effectively the direct opposite to the LNB. The BUC consists of the Up Converter and HPA.
Bursting
Bursting refers to the ability of a VSAT system to utilize bandwidth above and beyond its normal allocation. Bursting is only possible when several VSATs share a common pool of bandwidth. It only occurs when there is ‘free’ or available bandwidth in the pool.
Byte
One Byte is equal to 8 bits. The "B" is always a capital letter. Bytes are measured in B, kB, MB, GB.
Capacity
Capacity or Data Rate is a measure of the amount of information that a communications system can transfer from one place to another. It is common to refer to the transfer medium as a channel. A simplistic but helpful analogy to use is a water pipe. The pipe is equivalent to the channel, the water it carries equivalent to information and the size or capacity of the pipe the Data rate or capacity. The larger the volume of water (information) the pipe (channel) can carry (transmit or receive), the larger is the capacity of the pipe. Capacity is measured in bps, kbps, Mbps, Gbps.Capacity or data-rate is always measured in the amount of data transmitted per second. See bandwidth for note on the terminology usage.
Note also, 1 Mbps is equal to an actual 1024 bps. This is because we are dealing with numbers to the power of 2 and not to numbers to the power of 10.
Carrier signal
A signal that is used to carry useable information to and from the satellite. The information is combined with the carrier signal through a process called modulation.
C Band
The range of communication frequencies from 3 to 7 MHz used for satellite communications.
CIR
Committed Information Rate. CIR is the dedicated or guaranteed bandwidth that your service provider will make available. This CIR bandwidth is not shared with anybody else.
Decibel (dB)
The Bel is named after Alexander Graham Bell who first used it to define the relativedifferences in the loudness of sound. A decibel is one tenth of a Bel, as the larger unit is unusable for normal applications. Decibels (dB) are a logarithmic measure used to compare the differences in power of two signals. If one signal is assumed to be one Watt, then it gives a measure of relative gain or loss of power and is typically expressed as dBW. The advantage of using Decibels is that it enables one to manipulate input and output powers, their gain and loss with simple addition and subtraction arithmetic. 1 Bel = 10 dB
DSL
Digital Subscriber Line, a form of broadband or high speed terrestrial internet access service.
DVB
DVB is an open, non-proprietary standard developed for the transmission of digital video over satellite and terrestrial systems such as cable. It is especially efficient and effective at “transporting” digital data. Since Internet data is digital in nature, it also lends itself to effective transportation using DVB. The DVB-S standard describes how digital signals are transmitted over satellite links. Similar standards exist for cable (DVB-C) and terrestrial (DVB-T) radio systems.
The openness and non-proprietary nature of the standard, and the fact that it could handle both video and data efficiently, made it attractive to equipment manufacturers and service providers leading to the development of cheap mass market receive devices. It has thus established itself as one of the major technical standards for internet offerings using VSATs. Combined with a return channel over satellite, a DVB-RCS (Return Channel via Satellite) open standard is now available for interactive VSAT networks.
EDGE
Enhanced Data rate for GSM Evolution
Effective Isotropic Radiated Power (EIRP)
EIRP is a measure of the satellite power or strength of the satellite signal that can bereceived at a given point on the earth, or transmitted towards a satellite from a dish. It is measured in decibel-Watt (dBW) when compared to a reference one-watt theoretical radiator. EIRP values are published by the satellite provider, usually in the form of contour maps. VSAT network designers must calculate the uplink EIRP value for a VSAT.
Elevation Angle
The angle of elevation is the angle of the straight line path to the satellite as measured above the horizon and can range from 0 degrees to 90 degrees.
Feed horn
The feed horn is a conically shaped device placed at the focal point of the dish. It responsible for collecting the incoming receive signal from the satellite after it is reflected from the antenna and directing the outgoing transmit signal to the antenna for reflection into space.
Foot print
This is the area on earth covered by a satellite beam.
Frequency
The number of signal cycles per second given in Hertz.
Frequency Band
The range of frequencies within which an electromagnetic wave may be transmitted or received. Three main frequency bands are used for satellite communications: C band, Ku band and Ka band. These bands, designation letters and their associated frequencies are determined and allocated by the ITU.
Gbps
Gigabits per second. 1 Gbps equals one billion bps
Geostationary Orbit
A special form of geosynchronous orbit, aligned in the same plane as the earth’s equator. See Satellite Orbit.
Geosynchronous Orbit
See Satellite Orbit
GPRS
General Packet Radio Service
Ground Segment
In a VSAT system, the dish and electronic equipment are said to form the “ground segment” of a satellite communications network. The ITU defines all "ground stations" as "earth stations".
GSM
Global System for Mobile Communication, a standard for mobile phone communication mainly used in Europe, Africa and Asia.
Hertz
Hertz is a measure of the frequency of a radio wave. 1 Hertz equals 1 cycle per second. Analogue bandwidth is also measured in Hz, kHz, MHz. (See bps and bandwidth).
Hub
This is a special earth station that acts as the “heart” of a star VSAT network, through which all communications flow. Individual VSAT stations are connected to the hub like a wheel’s hub and spokes. The Hub controls which VSAT can communicate in the network and how much bandwidth each VSAT is entitled to.
HPA
High Power Amplifier – a device used to amplify the signal to be transmitted, enablingit to traverse the thousands of miles to the satellite. The capacity or rating of the HPA is given in Watts (W). It follows generally that the higher the HPA rating, the larger the signal (and therefore amount of information) a particular VSAT system will be able to transmit. HPA ratings for VSATs usually vary from 1 to 40 W. HPAs, BUCs and SSPAs are only found on interactive systems.
HSCSD
High Speed Circuit Switched Data
ICT
Information and Communications Technology
Indoor Unit (IDU)
The electronic equipment used for processing information is referred to as the “Indoor Unit” because it is installed and kept indoors.
Inter Facility Link
The IFL is one or more co-axial cables connecting the ODU (specifically the LNB and BUC) to the IDU equipment (the receiver and modem). Typically, there are separate cables for the transmit and receive signals. Receive-only systems only have one cable in the IFL.
Inbound
Inbound connections refer to information flow from the user to the Internet. For VSATs, the inbound connection is also referred to as the transmit link or signal.
IP
Internet Protocol
ISDN
Integrated Services Digital Network
ISP
Internet Service Provider
ITU
International Telecommunications Union, a United Nations agency that oversees telecommunications standards, policies and frequencies used by the international community for telecommunication services.
kB
kilo Byte
kbps
Kilobits per second. 1 kbps equals one thousand bps.
LAN
Local Area Network
Latency
This is the delay between transmitting a signal and receiving it at the other end. An example of latency is the delay on a telephone line that one experiences when making long-distance telephone calls or the out of sync voice and video in a video conference (the participants lips move but the sound arrives a few seconds later), all of which can be quite unnerving. See satellite hop.
Link Budget
The link budget is a consideration of the signal’s power gains and losses from the transmitter to the receiver (much like a monetary budget is about financial losses or expenses and gains or income). As the signal leaves the transmitter, traverses space, and arrives at the receiver, it experiences gains and losses in its power or signal strength.
When engineers calculate link budgets, they are making sure that the system is capable of and optimized to receive a signal of acceptable strength from the transmitter. This involves varying several parameters including the size of receive and transmit antenna, bandwidth of the signal and the power of the transmitter. In the case of VSATs, link budgets are used to arrive at the optimum combination of the dish size, transmit BUC and receive LNB rating for given link bandwidth.
LNB – Low Noise Block
A device used to amplify or boost the weak received signal without amplifying the noise signals (hence the “low noise” part of LNB) and to convert the high frequencies of the signal into lower frequencies, a process called down converting, for conveyance to the indoor equipment (demodulator) for processing.
MB
Mega Byte
Mbps
Megabits per second. 1 Mbps equals one million bps
MTBF
Mean Time Between Failures. A measure of the reliability of a piece of equipment or system.
Modem
A modulator and demodulator. The modulator is the transmit device, whilst the demodulator is the receive device. See modulator, and receiver.
Modulator
A modulator is used only for transmission and therefore is found in only interactive systems. Modulators mix the useful information e.g. internet data, another signal, called a carrier signal, in a process called modulation.
MRTG
Multi Router Traffic Grapher, a web based application used to monitor and graphicallydisplay traffic and usage of an internet connection.
MS Messenger
Microsoft Messenger
OMT
The OMT is only found on interactive systems and is installed right behind the feed horn. It is responsible for separating transmitted and received signals and directing them appropriately to the BUC and LNB respectively.
ODU
The antenna and receive/transmit assembly is collectively referred to as the “Outdoor Unit” or ODU because it is installed outdoors.
Outbound
Outbound connections refer to information flow from the Internet to the user. For a VSAT system, the outbound connection is also called the receive link or receive signal.
PC
Personal Computer
Receiver
This is an electronic device that processes the received signal to obtain the useable data from the received signal by processing it through a complex array of electronic devices. Receivers are also variously referred to as “set top boxes” or “decoders”.
Remote Terminal
The remote terminal is the earth station or VSAT.
Satellite
In its loosest form, a satellite is any object or body that moves in circular-like orbit or path around a planet. The moon, for example, is a satellite of the earth. The term has however become synonymous with communications satellites, which are manmade objects that revolve around the earth and provide communications links among two or more points on earth. There are other types of man-made satellites such as those used to track weather conditions and for the global positioning system (GPS). The first man-made communications satellite, named Sputnik, was launched into space in 1957 by the Soviet Union.
Satellite Beam
This is a pattern of electromagnetic waves transmitted by the satellite. Just as a torch shining onto the floor in a dark room has a defined beam or collection of rays of light, the transmission from a satellite also has a defined pattern. The beam can be wide or narrow covering a large or small area on earth. Using a system of varying frequencies and alignment of antennas onboard the satellite, each satellite can have several beams within which all or most of the satellite’s power is concentrated. There are four common types of satellite beams:
- Global beams: covering almost 1/3 of the earth's surface.
- Hemi beams: covering almost 1/6 of the earth's surface, shaped to cover specific regions and usually covering an entire continent.
- Zone beam: covering a large landmass, usually consisting of a few countries.
- Spot beam: covering a specific geographic area usually a specific service area. Spot beams are provided by "steerable antennas" on the satellite.
Satellite hop
When a signal travels from one VSAT to another via a satellite, it is said to have made a single hop. For satellites in geostationary orbits, this “hop” takes about one quarter of a second. In a network with a Hub, any interactive communication will involve a double hop, i.e. information has to flow from one VSAT to the Hub and then out to the Internet (or to another VSAT) and back, which introduces high latency on the satellite link.
Satellite Link
The signal from the VSAT to the satellite or vice versa is said to form a “link”. The "uplink" is the link from a VSAT (earth station) to a satellite. The "downlink" is from a satellite to a VSAT (earth station).
Note: The ITU refers to all stations on the earth as "earth stations" or "ground stations" and all satellites as "space stations".
Satellite Orbit
This is the path of the satellite around the earth. There are three main types of orbitsdifferentiated by how far from the earth’s surface the satellite is located which also determines how many times in a day the satellite can circle the earth.
- Low Earth Orbit (LEO): 500-2,000 km above the earth
- Medium Earth Orbit (MEO): 8,000-20,000 km above the earth
- Geostationary Orbit (GEO): 35,786 km above the earth.
At both low and medium orbits, the satellite circles the earth several times in a day. This means that if you remain at a fixed point on earth, the satellite would pass overhead several times in a day. For continuous communication from a single point on earth in this case, for example when making a telephone call, multiple satellites are required with one satellite handing over communication to another before disappearing over the horizon in one long “relay”.
In a geosynchronous orbit, a satellite’s speed of orbiting the earth is the same as the earth’s rotation. In other words, the satellite takes about 24 hours to move around the earth. When the orbit is in the same plane as the equator, the satellite appears “stationary” relative to any fixed point on earth and is called geostationary.
Geostationary orbits are therefore a special form of geosynchronous orbits aligned in the equatorial plane.
Satellite Signal
A form of electromagnetic waves carrying information to or from the satellite.
Space Segment
This is the satellite portion of the network. Space segment is also used to refer to bandwidth of the satellite.
Symmetrical/ Asymmetrical connections
A symmetrical connection is one with the same amount of bandwidth for the outbound (receive) and inbound (transmit) connections. Conversely, an asymmetrical connection is one where the inbound and outbound connections have different bandwidths. For Internet access, the connections are almost always asymmetrical with the inbound usually set at ¼ or 1/3 of the outbound. This is based on the premise that the information you request, for example if you click a link on a website, is small while the information that comes back, the linked webpage, is large. If you plan to host your own website at the VSAT location, you will then have to adjust the inbound upwards to cater for the fact that you will be “serving” the larger information for people whose incoming requests are “small”.
Transponder
A transponder is a receive-amplify-transmit device onboard the satellite. Its purpose is to receive the signals transmitted from earth, amplify them, change (shift or translate) the frequency and transmit the amplified signal with the new frequency back to earth.
The frequency change is necessary to avoid interference between the received and transmitted signals. Transponders are analogue devices defined by their bandwidth, typically 27, 54 MHz or 36, 72 MHz. Transponder capacity on a modern satellite is defined in terms of the number of "36 MHz equivalent transponders" to allow meaningful comparisons to be made between satellites. Smaller satellites have about 15 transponders on board whilst the larger ones can have more than 90 transponders.
Up Converter
A device that forms part of the BUC. The Up Converter converts the low frequency signal from the indoor equipment carried along the co-axial cable to a higher frequency for transmission to the satellite.
UPS
Uninterruptible Power Supply
VOIP
Voice Over Internet Protocol
VPN
Virtual Private Network
VSAT
Very Small Aperture Terminal. It refers to a dish-like ensemble (or simply dish), usually less than 3.8 m in diameter, together with electronic equipment used to transmit and receive information (voice, video and data) via a communications satellite in space. The dish is also referred to as a satellite antenna, terminal or earth station.
WIFI
Wireless Fidelity
WIMAX
Worldwide Interoperability Microwave Access
