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atrexx' Satellite Glossary

atrexx' Satellite Glossary is a glossary of common Satellite, Internet via Satellite and Broadcast Satellite terminology including associated subjects such as Satellite Standards, VSAT - Corporate Satellite Solution and Satellite Network Architectures.

A B C D E F G H I J K L M N O P Q R S T U V W

 


 

A

 

AAL

ATM Adaptation Layer

ACSS

Access Control Sub-System

ADC

Analogue-to-Digital Conversion. Process of converting analogue signals to a digital representation. DAC = Digital-to-Analogue Conversion represents the reverse translation.

ADSL

Asymmetric Digital Subscriber Line
 

 

AM

Amplitude Modulation. The base-band signal is caused to vary the amplitude or height of the carrier wave to create the desired information content.

Amplifier

A device used to boost the strength of an electronic signal.

Analogue

Form of transmitting information characterized by continuously variable quantities, as opposed to digital transmission, which is characterized by discrete bits of information in numerical steps.

Anomalistic Period

The elapsed time between two successive perigees of a satellite.

Antenna

A device that concentrates a beam of electromagnetic waves to a focal point so as to send/receive radio signals. Depending on their use and operating frequency, antennas can take the form of a single piece of wire, a di-pole a grid such as a yagi array, a horn, a helix, a sophisticated parabolic-shaped dish, or a phase array of active electronic elements of virtually any flat or convoluted surface.

AOS

Acquisition of Signal. The time at which a radio signal is first heard from a satellite, usually just after it rises above the horizon.

Aperture

The effective cross sectional area of the antenna which is exposed to the satellite signal. The larger the aperture the stronger the signal the antenna receives or transmits.

Apogee

The point in an elliptical satellite orbit which is farthest from the surface of the earth. Geo-synchronous satellites which maintain circular orbits around the earth are first launched into highly elliptical orbits with apogees of 22,237 miles. When the communication satellite reaches the appropriate apogee, a rocket motor is fired to place the satellite into its permanent circular orbit at this distance.

Argument of Perigee

The polar angle that locates the perigee point of a satellite in the orbital plane; drawn between the ascending node, geo-centre, and perigee; and measured from the ascending node in the direction of satellite motion.

Ascending Node

The point on the ground track of a satellite orbit where the sub-satellite point(SSP) crosses the equator from the Southern Hemisphere into the Northern Hemisphere.

ATM

Asynchronous Transfer Mode

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.

Audio Subcarrier

The carrier between 5 MHz and 8 MHz containing audio (or voice) information inside of a video carrier. Some satellite transponders carry as many as four special audio or data sub-carriers whose signals may or may not be related to the main programming.

Autonomous System (AS)

A range of public IP addresses that are under full control of an Internet service provider (ISP). By means of peering agreement with other ISPs IP packets can by routed on direct ways between the connected ASs.

AZ/EL Mount

Antenna mount that requires two separate adjustments (Azimuth & Elevation) to move from one satellite to another.

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.
 


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B

 

Backbone

The central part of a large network that links two or more subnetworks and is the primary path for data transmission for a large business or corporation. A network can have a wired backbone or a wireless backbone

Backhaul

A terrestrial communications channel linking an earth station to a local switching network or population centre.

Backup network

Alternative secondary emergency network in case the main primary network fails. For example, when a terrestrial network fails the traffic is automatically switched to the VSAT backup network.

Bandwidth

The range of frequencies expressed in Hertz that can pass over a given transmission channel. This determines the rate at which information can be transmitted through that channel. For instance, a voice transmission by telephone requires a bandwidth of about 3000 cycles per second (3KHz). A TV channel occupies a bandwidth of 6 million cycles per second (6 MHz) in terrestrial Systems.

Baud

Unit of the signal rate of data transmission expressed in the number of signal elements or symbols transmitted per second. Equivalent: sym / s

Beacon

Low-power carrier transmitted by a satellite which supplies the controlling engineers on the ground with a means of monitoring telemetry data, tracking the satellite, or conducting propagation experiments. This tracking beacon often uses a horn or omni-directional antenna.

Beamwidth

The angular width of the main lobe of an antenna radiation pattern as measured between predetermined points on the lobe, usually the half-power (-3 dB) points. Large antennas have narrower beamwidths and can pinpoint satellites in space or dense traffic areas on the earth more precisely. Tighter beamwidths thus deliver higher levels of power and thus greater communications performance.

BER

see: Bit Error Rate.

Bird

Slang for a communications satellite located in geo-synchronous orbit.

Bit Error Rate (BER)

The fraction of a sequence of message bits that are in error. A bit error rate of 10-6 means that there is an average of one error per million bits.

Bit Rate

The speed of a digital transmission, measured in bits per second. Better: Data Rate

Block Down Converter

A electronic device used to convert the received signal from a satellite (e.g. in Ku-Band, C-Band) down to L-Band frequencies to allow for Inter Facility Link (IFL) by coaxial cable. See: LNB

Broad beam

A single large circular satellite beam that covers a large geographic area

Broadcast

The sending of one transmission to multiple users in a defined group (compare to unicast).  Transmissions through space utilizing pre-assigned radio frequencies typically for television radio or data.

BUC

Block Up-Converter. A BUC is an electronic device mounted near the feed of the antenna. It converts the modulated signals originating from from the IDU from L-Band to Ku-band or C-Band as needed and amplifies them. The signals are then transmitted from the VSAT to the satellite. The counterpart of a BUC is an LNB for reception of satellite signals.
 


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C

 

C-Band

Frequency band between 4 and 8 GHz with the 4 and 6 GHz  frequencies being used for satellite communications: 3.7 to 4.2 GHz as downlink frequencies in tandem with 5.925 to 6,425 GHz as the uplink. C-Band is less susceptible to rain and weather than Ku-band. However, C-Band is susceptible to terrestrial microwave interference which is a particular problem in urban areas.

C/N

Carrier to Noise Ratio. The ratio of the received carrier power and the noise power in a given bandwidth, expressed in dB. This figure is directly related to S/N. In a video signal the higher the C/N, the better the received picture.

Capacity

 Maximum amount of traffic that a circuit or circuit group can handle. For a bandwidth it is measured in Hertz (Hz), for a datrate in bit/s (bps)

Capacity & Facility Exchange

atrexx' trading platform provides a neutral and independent arena for buyers and sellers. Subscribers can post offers of spare capacity and other resources, such as production facilities, or ask for such resources.  The market exchange finally brings the two parties together in a well defined framework. 

Carrier

The basic radio, television, or telephony centre of frequency transmit signal. The carrier in an analogue signal is modulated by manipulating its amplitude (making it louder or softer) or its frequency (shifting it up or down) in relation to the incoming signal. Satellite carriers operating in the analogue mode are usually frequency modulated.

Carrier Frequency

The main frequency on which a voice, data, or video signal is sent. Microwave and satellite communications transmitters operate in the band from 1 to 14 GHz (a GHz is one billion cycles per second).

Cassegrain Antenna

The antenna principle that utilizes a convex sub-reflector at the focal point which reflects energy to or from a feed located at the apex of the main reflector.

CDMA

Code division multiple access. Refers to a multiple-access scheme where stations use spread-spectrum modulations and orthogonal codes to avoid interfering with one another.

CF-DAMA

Combined Free Demand Assignment Multiple Access. An access scheme for transmitting signals from a VSAT to the satellite.

Channel

In the satellite industry the term channel is often used interchangeably with the word transponder However, channel refers more accurately to the absolute frequency range over which the transponder operates (e.g. 11 GHz +/- 18 MHz). A related meaning of channel is TV or Radio Channel, which characterizes the creation and distribution of broadcast content.

CIR (Committed Information Rate)

The minimum Frame-Relay-network bandwidth guaranteed between two sets of terminal equipment connected via a PVC (Permanent Virtual Circuit).

Circular Polarization

A special method of radio energy emission where the electric and magnetic field vectors rotate about the central axis of radiation. As viewed along the radiation path, the rotation directions are considered to be right-hand-circular (RHCP) if the rotation is clockwise and left-hand-circular (LHCP) if the rotation is counter-clockwise. Both right-hand rotating and left-hand rotating signals can be transmitted simultaneously on the same frequency; thereby doubling the capacity of the satellite to carry communications channels.

Clarke Orbit

Circular orbit in space 22,237 miles from the surface of the equator at which geo-stationary satellites are placed. This orbit was first postulated by the science fiction writer Arthur C. Clarke in Wireless World magazine in 1945. Satellites placed in these orbits, although travelling around the earth at thousands of miles an hour, appear to be stationary when viewed from a point on the earth, since the earth is rotating upon its axis at the same angular rate that the satellite is travelling around the earth. It is also referred to as "geo-stationary orbit".

Co-Location

Ability of multiple satellites to share the same approximate geo-stationary orbital position. Different frequency bands on each satellite allow to increase the aggregate capacity which can be transmitted from the same orbital position.

Connection

Each link or telecommunication capacity that is used to connect sites to a Backbone node determined by a VPN Provider or to another node specifically requested by the Customer.

Coverage

Coverage or Footprint is an area on the earth's surface where effective transmission/reception to/from the satellite is possible. The contours of the footprint are usually specified by points at half the maximum power.

CRA

Collision Reduction Application (a proprietary optimisation of a VSAT inbound access scheme) or Continuous Rate Assignment.

Cross Modulation

A form of signal distortion in which modulation from one or more RF carrier(s) is imposed on another carrier.

CUG

Closed User Group The limited number of users whose members are able to interchange information via a VPN service but who cannot exchange information with third-party sites that are not part of the group.
 


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D

 

DA

Dedicated Access. An inbound transmission mode used when a remote VSAT requires a higher level of throughput for a continuous volume of traffic, such as in a file transfer.

DAC

Digital-to-Analogue Conversion. Process of converting digital signals to analogue representation. ADC = Analogue-to-Digital Conversion is the reverse translation.

DAMA

Demand-Assigned Multiple Access.  Access scheme for transmitting signals from a VSAT to the satellite. DAMA is a highly efficient means of instantaneously assigning telephony channels in a transponder according to traffic as it allows many users to access the same channel on demand.

dBi

The power in Decibel (dB) relative to an isotropic source.

DBS

Direct Broadcast Satellite. Refers to service that uses satellites to broadcast multiple channels of television programming directly to home mounted small-dish antennas.

dBW

The ratio of the power to one Watt expressed in decibels.

Decibel (dB)

The standard unit used to express the ratio of two power levels P1 and P2: 10*log (P1/P2). It is used in communications to express either a gain or loss in power between the input and output devices.

Declination

The offset angle of an antenna from the axis of its polar mount as measured in the meridian plane between the equatorial plane and the antenna main beam.

Delay

The time it takes for a signal to go from the sending station through the satellite to the receiving station. This transmission delay for a single hop satellite connection is very close on one-quarter of a second.

Demodulator

A satellite receiver circuit which extracts or "demodulates" the "wanted "signals from the received carrier.

Descending Node

The point on the ground track of the satellite orbit where the sub-satellite point (SSP) crosses the equator from the Northern Hemisphere into the Southern Hemisphere.

Deviation

The modulation level of an FM signal determined by the amount of frequency shift from the frequency of the main carrier.

DHCP

Dynamic Host Configuration Protocol. A means of automatically assigning an IP address to a device (eg, PC) on a network as it comes alive.

Digital

Conversion of information into bits (represented by 1 or 0) of data for transmission through wire, fiber optic cable, satellite, or over the air techniques. Digital transmission allows simultaneous communication of voice, data or video.

Dish

Common name for satellite antenna as they are often concave shaped like a dish.

Double hop

Transmission of data from terminal to terminal in two stages, for example, from one VSAT to another via a satellite link trough a central hub station. This is the only form of communication between terminals in a star network.

Downlink

1. In satellite TV a downlink consists of a small receiving dish and a receiver which is connected to a television monitor. The dish must have a clear view to the satellite's position. The exact angle of view from the dish to the satellite depends on the location of the venue .
2. The transmission path from the satellite to the receiving earth station, as opposed to the uplink.

Dual Spin

Spacecraft design whereby the main body of the satellite is spun to provide altitude stabilization, and the antenna assembly is de-spun by means of a motor and bearing system in order to continually direct the antenna earthward. This dual-spin configuration thus serves to create a spin-stabilized satellite.
 

Duplex Transmission

Capability for simultaneous data transmission between a sending station and a receiving station.

DVB

Digital Video Broadcasting. The DVB system combines the MPEG-2 standards with technologies enabling digital data to be broadcast via satellite (DVB-S) and cable(DVB-C). DVB-S is DVB’s baseline specification for digital television, sound and data services in the satellite frequency range.DVB is an ETSI standard.

DVB-RCS

Digital Video Broadcast - Return Channel via Satellite, an ETSI standard, forms the specification for the provision of the interaction channel for satellite interactive networks with fixed return channel satellite terminals (RCST).

 

DVB-S

Digital Video Broadcast - Satellite. The DVB standard used in many parts of the world for digital TV over Satellite and for the outbound digital signal transmission of  VSATs and multicast over satellite applications.
 



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E

 

Earth Station

Combination of antenna, low-noise amplifier (LNA), down-converter, and receiver electronics used to receive a signal transmitted by a satellite. Earth Station antennas vary in size from the 65 cm to 3.7 m diameter size used for TV reception to as large as 30 meters in diameter sometimes used for international communications.

Eccentricity

The orbital parameter used to describe the geometric shape of an elliptical orbit; eccentricity values vary from (e = 0) to (e = 1), where (0) describes a circle and (1) describes a straight line

Echo Effect

A time-delayed electronic reflection of a speaker's voice. Echo is largely eliminated by modern digital echo cancellers.

Edge of Coverage

Limit of a satellite's defined service area. In many cases, the EOC is defined as being 3 dB down from the signal level at beam centre. However, reception may still be possible beyond the -3dB point.

EIRP

Effective Isotropic Radiated Power - This term describes the strength of the signal leaving the satellite antenna or the transmitting earth station antenna, and is used in determining the C/N and S/N. The transmit power value in units of dBW is expressed by the product of the transponder output power and the gain of the satellite transmit antenna.

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 is tilted to a point directly overhead, the satellite antenna has an elevation of 90 degrees. A 90 ° elevation happens only at the equator, when the site of the earth station is directly below the satellite - at the sub-satellite point.

Elliptical Orbit

Those orbits in which the satellite path describes an ellipse with the Earth at one focus.

Epoch

The reference time at which a particular set of parameters describing satellite motion (Keplerian elements) are defined.

Equatorial Orbit

An orbit with a plane parallel to the earth's equator.

Extranet

A VPN linking together the sites that form part of the Customer's VPN network or linking together one or more other VPN networks belonging to the same or a different Customer.
 


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F

 

F/D

Ratio of antenna focal length to antenna diameter. A higher ratio means a shallower dish, lower F/D ratio a deeper dish.

Faraday Rotation

Rotation of the plane of polarization of an electromagnetic wave when travelling through a magnetic field. In space communications, this effect occurs when signals traverse the ionosphere.

FCA

Free Capacity Assignment. A capacity optimisation access scheme used on the inbound from a VSAT back to the hub.

FDMA

Frequency Division Multiple Access. Refers to the use of multiple carriers within the same transponder where each uplink has been assigned a frequency slot and a bandwidth. FDMA is one access scheme for the inbound channel of VSAT networks, as opposed to TDMA (Time Division Multiple Access) 

FEC

Forward Error Correction. Method to increase the realibility of a link. At the source unique bit streams are added to the transmission signal which allow the reciver to detect and to correct errors occured during transmission to a certain extend. An FEC rate of 2/3 translates into one error correction bit  being added to two data bits.

Feed

A device (feed horn) mounted at the focal point of antenna that emits signals into an antenna. It is also used to describe the feed system of an antenna.

FLSS

Forward Link Sub-System. A system of components within a hub.

FM

Frequency Modulation. A modulation method whereby the base-band signal varies the frequency of the carrier wave.

Focal Length

Distance from the focal point (feed) to the center of the parabolic antenna.

Focal Point

The area toward which the primary reflector directs and concentrates the signal received.

Footprint

A map of the signal strength showing the EIRP contours of equal signal strengths as they cover the earth's surface. Different satellite transponders on the same satellite will often have different footprints of the signal strength. The actual EIRP levels of the satellite tend to decrease slowly as the spacecraft ages.
 

Frequency

The number of times that an alternating current goes through its complete cycle in one second. One cycle per second is also referred to as one hertz (Hz); 1000 cycles per second, one kilohertz (kHz); 1,000,000 cycles per second, one megahertz (MHz) and 1,000,000,000 cycles per second, one gigahertz (GHz).

FTDMA

Frequency and Time Division Multiple Access.
 


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G

 

G/T

Figure of merit of the receiving part of an earth station expressed in dB . "G" is the net gain of the system (Antenna Gain) and "T" the noise temperature of the system which relates to the low noise amplifier. The higher the G/T number, the better the receiving performance of the system.

Gain

A measure of amplification expressed in dB.

Gateway

A device linking the VPN network to another type of network.

Geo-stationary

Refers to a geo-synchronous satellite angle with zero inclination. A geo-stationary satellite appears to hover over one spot on the earth's equator. See also: Clarke Orbit.

Gigahertz (GHz)

One billion cycles per second. Signals operating above 3 Gigahertz are known as microwaves. above 30 GHz they are know as millimetre waves. As one moves above the millimetre waves signals begin to take on the characteristics of light-waves.

Gregorian

A dual-reflector antenna system employing a parabolic main reflector and a concave ellipsoidal sub-reflector.

Ground Station

A radio station, on or near the surface of the Earth, designed to transmit or receive to/from a spacecraft.

Ground-track

The imaginary line traced on the surface of the Earth by the sub-satellite point (SSP).

Guard Channel

High frequency channels are separated in the frequency spectrum by spacing them several megahertz apart. This unused space serves to prevent the adjacent channels from interfering with each other.
 

 
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H

 

Hertz (Hz)

The unit of frequency measurement. An electromagnetic wave completing a full oscillation from its positive to its negative pole and back again in what is known as a cycle. A single Hertz is thus equal to one cycle per second.

HPA

High Power Amplifier.

HSP

Hub Satellite Processor.

Hub

The satellite central earth station through which all traffic is routed in a star configuration VSAT network. The hub is the master station through which all communications to, from and between VSATs must flow.
 


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I

 

IB

Inbound. Refers to the path the signal takes as it is transmitted from the remote VSAT via the satellite to the hub. Inbound from the point of view of the hub.

IDU

In-Door Unit. A satellite modem that is located at each remote VSAT site and connects the users PC or network to the hub via the satellite link.

IFL

Inter-Facility Link. The connection between the ODU (antenna, LNB and transmitter) and the IDU (satellite modem), usually realized with coaxial cables.

Inclination

The angle between the orbital plane of a satellite and the equatorial plane of the earth.

Increment

The change in longitude of the ascending node between two successive passes of a specified satellite, measured in degrees West per orbit.

Interference

Energy which tends to interfere with the reception of the desired signals, such as fading from airline flights, RF interference from adjacent channels, or ghosting from reflecting objects such as mountains and buildings.

Internet access

A service that comprises physical access means to the Backbone of the ISP plus the connectivity to the Internet, i.e. the capability to route IP traffic to and from public IP addresses. 
   

Internet via Satellite

Connection to the Internet Backbone through the use of an IP based satellite link and a  VSAT. With Internet via Satellite it no longer matters where the customers are, making it an ideal tool for ISPs in areas with poor landline infrastructure. The bandwidth of the link is scalable from speeds below 1 Mbit/s up to 45 Mbit/s per satellite transponder.
 

IPA

Internet Page Accelerator. The name given to a proprietary HTTP acceleration service.

IRD

An integrated receiver and decoder for reception of a transmission of voice, video and data.

Isotropic Antenna

A hypothetical omni-directional point-source antenna that serves as an engineering reference for the measurement of antenna gain.
 


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J

 

Jammer

An active electronic counter-measures (ECM) device designed to deny intelligence to unfriendly detectors or to disrupt communications.
 


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K

 

Ka-Band

The frequency range from 18 to 31 GHz.

Keplerian Elements

The classical set of six orbital element numbers used to define and compute satellite orbital motions. The set is comprised of inclination, Right Ascension of the Ascending Node (RAAN), eccentricity, argument of perigee, mean anomaly, and mean motion, all specified at a particular epoch or reference year, day, and time. Additionally, a decay rate or drag factor is usually included to refine the computation.

Klystron

A microwave tube which uses the interaction between an electron beam and the RF energy on microwave cavities to provide signal amplification. The klystron operates on principles of velocity modulation very similar to those in a Traveling Wave Tube (TWT) except that klystron interaction takes place at discrete locations along the electron beam. Common types of klystrons are the reflex klystron (an oscillator having only one cavity), two-cavity klystron amplifiers and oscillators, and multi-cavity klystron amplifiers.
 

Ku-Band

Ku-Band covers the frequency range from 10.9 to 17 GHz. The Ku band is primarily used for satellite communications, particularly for editing and broadcasting satellite television. This band is split into multiple segments broken down into geographical regions, as determined by the ITU (International Telecommunication Union).

The Ku band is a portion of the electromagnetic spectrum in the microwave range of frequencies ranging from 11.7 to 12.7GHz. (downlink frequencies) and 14 to 14.5GHz (uplink frequencies).

 

   
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L

 

LAN

Local Area Network. A system of connecting PCs and other devices within the same physical proximity for sharing resources such as an Internet connections, printers, files and drives. When Wi-Fi is used to connect the devices, the system is known as a wireless LAN or WLAN.

Layer-2 VPN 

There are logical links between all sites of the entire (corporate) network at Layer 2 in terms of the OSI layer model (data link layer). Typical protocols are Frame Relay and ATM. Precondition is a fixed connection to the nearest Point-of-Presence of the provider that requires Leased Lines as access method. Nowadays DSL access becomes a cost-effective alternative as fixed access method. Security and performance are the main advantages of Layer-2 VPNs.
 

Layer-3 VPN

There are no logical links between all sites of the entire (corporate) network at Layer 3 in terms of the OSI layer model (network layer). Internet Protocol IP is per definition a connectionless service. Scalability and flexibility are the main advantages of Layer-3 IP-VPNs.

L-Band

The frequency range from 0.5 to 2.0 GHz. Also used to refer to the 950 to 1450 MHz Intermediate Frequency (IF) range of satellite communications.

LHCP

Left Hand Circular Polarisation. See: Circular Polarisation

LNA

Low Noise Amplifier. This is the preamplifier between the antenna and the earth station receiver. For maximum effectiveness, it must be located as near the antenna as possible, and is usually attached directly to the antenna receive port. The LNA is especially designed to contribute the least amount of thermal noise to the received signal.
 

LNB

Low Noise Block Converter. An electronic device combining  a Low Noise Amplifier (LNA) and Block Down-Converter. The LNB is attached to the waveguide interface of the feed and is part of the Out-Door Unit (ODU).
 


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M

 

Margin

The amount of signal in dB by which the satellite system exceeds the minimum levels required for operation.

MCD

Multi Carrier Demodulator

MCPC

Multi Channel per Carrier. Refers to the multiplexing a number of digital data channels into a common digital bit stream, which is then used to modulate a single carrier that conveys all of the services to the end user.  
 

Mean Anomaly

An angle that increases uniformly with time, starting at perigee. Used to indicate where a satellite is located along its orbit. Mean Anomaly is usually specified at the reference epoch time where the Keplerian Elements are defined.

Mean Motion

The Keplerian Element that indicates the complete number of orbits a satellite makes in a day.

Mesh network

A type of network where all points can directly communicate with each other in a single hop.

MF-TDMA

Multi-Frequency Time-Division Multiple Access. A variant on TDMA in which several parallel TDMA carriers are used for inbound traffic. This has the advantage that network capacity can be high but each terminal only requires a relatively low data rate transmit capability, hence minimizing cost of remote stations.

MGT

Multicast Group Table

Microwave

Line-of sight, point-to-point transmission of signals at high frequency. Microwaves are also used for data, voice, and indeed all types of information transmission.

Microwave Interference

Interference which occurs when an earth station aimed at a distant satellite picks up a second, often stronger signal, from a local telephone terrestrial microwave relay transmitter. Microwave interference can also be produced by nearby radar transmitters as well as the sun itself. Relocating the antenna by only several feet will often completely eliminate the microwave interference.

MMT

Multicast Mapping Table

Modem

Modulator/Demodulator - accepts data from a computer or terminal device in the form of a digital signal and transforms the data into a form suitable for transmission over a communication facility.

Modulation

The process of manipulating the frequency or amplitude of a carrier in relation to an incoming video, voice or data signal.

Modulator

A device which modulates a carrier. Modulators are found as components in broadcasting transmitters and in satellite transponders.

MPLS

Multi-Protocol Label Switching. In MPLS, data transmission occurs on label-switched paths (LSPs). Each data packet encapsulates and carries the labels during their journey from source to destination. MPLS addresses issues related to scalability and routing based on QoS and service quality metrics. 

Multicast

Multicast extends the broadcast concept of one to many by allowing the sending of one transmission to many users in a defined group, but not necessarily to all users in that group. Multicast is frequently used for data broadcast applications.

Multiple Access

Access to a transponder by more than one user. Transponders can be accessed in three ways, by frequency (FDMA), time (TDMA) and code (CDMA).

Multiplexing

A technique that combines many simultaneous circuits over a single link in order to accommodate multiple users or applications.
 


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N

 

NF

Noise Figure. A ratio, expressed in dB, comparing the noise generated by an electronic device, such as an LNA or receiver, with a perfect device.

NOC

Network Operation Center. The center at which the hub and satellite network is monitored, managed and controlled.

Nodal Period

The amount of time difference between two successive ascending nodes of a satellite orbit.

Noise

Any unwanted and un-modulated energy that is always present to some extent within any signal.
 


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O

 

OB

Outbound. Refers to the path the signal takes as it is transmitted from the hub via the satellite to the remote VSAT. Outbound from the point of view of the hub.

ODU

Outdoor Unit. Comprises all components of a VSAT that are stationed outdoors. Includes antenna, mount, feed-arm, transmitter (HPC, SSPA or BUC), receiver (LNB) and cabling (IFL).

Off-net solution

The entire VPN is built by several 'patches'. One possibility is to have one service provider that cares for the entire network or to deal with several (regional) providers. In both cases two VPN connections traverse networks both from local carriers as well as international Internet service providers.

Orbital Period

The time that it takes a satellite to complete one circumnavigation of its orbit.

Orbital Plane

An imaginary plane, extending throughout space, that contains the satellite orbit.

Orbital Position

Position of geo-stationary satellites
 

 
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P

 

Pass-band

The range of frequencies handled by a satellite translator or transponder.

PEP

Performance Enhancing Proxy. A PEP is used to optimize the TCP protocol in order to enhance its performance over the satellite link.

Perigee

The point in a satellite's orbit where it is closest to Earth.

Point-to-Multipoint Communication

Point-to-Multipoint refers to  staellite's ability of broadcasting and multicasting. This architecture provides a powerful platform for video broadcasting services and for data multicast applications. With regard to cost, satellites are unrivaled when it comes to broadcast TV signals to language regions or continents.

Polar Mount

Antenna mechanism permitting steering in both elevation and azimuth through rotation about a single axis. While an astronomer's polar mount has its axis parallel to that of the earth, satellite earth stations utilize a modified polar mount geometry that incorporates a declination offset.

Polar Orbit

An orbit with its plane aligned in parallel with the polar axis of the earth

Polarization

A technique which uses - in a linear cross polarization scheme-  electromagnetic waves with horizontal and vertical electric field vectors . It allows reusing the satellite transponder frequencies: half of the transponders beam their signals to earth in a vertically polarized mode; the other half horizontally polarize their down links. Because of geometric decoupling, both waves do not interfere with each other, although having the same frequency. The earth station must be fitted with a properly polarized feed-horn to select the vertically and / or horizontally polarized signals as desired.

Polarization Rotator

A device that can be manually or automatically adjusted to select one of two orthogonal polarizations.

POS

Point of Sale.

PPPoE

Point-to-Point Protocol over Ethernet.

PVC

Permanent Virtual Circuit The connection between two sites interlinked via the network. 
 


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Q

 

QoS

Quality of Service. Network traffic management concept for controlling throughput and adjusting priorities for specified applications, TCP / UDP ports and customers.

QPSK

Quadrature Phase Shift Keying. A digital modulation scheme where four phases of the radio carrier, each differing from each other by 90° degrees, are employed to map two bits of the data stream. QSPK is currently the most used modulation in digital satellite communication.
 


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R

 

Rain Outage

Loss of signal due to absorption and increased sky-noise temperature caused by heavy rainfall.

Receiver (Rx)

An electronic device which enables a particular satellite signal to be separated from all others being received by an earth station, and converts the signal format into a format for video, voice or data.

Receiver Sensitivity

Expressed in dBm this tells how much power the detector must receive to achieve a specific base-band performance, such as a specified bit error rate or signal to noise ratio.

Repeater

Equipment which receives, amplifies, and retransmits signals in order to maintain adequate signal strength and intelligence characteristics.

RF  

Radio Frequency.

RLSS

Return Link Subsystem. A system of components within a hub.

Round Trip Time

The time of a packet send forth and back between two Customer Premises Equipment (CPE) routers connected to the VPN. The RTT is expressed in milliseconds (ms).

Router

Network layer device that determines the optimal path along which network traffic should be forwarded. Routers forward packets from one network to another based on network layer information.
 


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S

 

Satellite

A sophisticated spacecraft incorporating an electronic communications relay station. A communications satellite receives radio signals from earth stations and retransmits them to other earth stations at distant locations.

Satellite access

Allows sharing of bandwidths among multiple scheme users. Access methods increase the satellite's information-carrying capacity to reduce satellite idle time and spread costs among multiple users. Among the most commonly used access methods: FDMA, TDMA, CDMA and sophisticated versions of ALOHA.

Satellite broadband

A wireless high-speed Internet connection provided by satellites. Some satellite broadband connections (VSATs) are two-way up and down. Others are one-way, with the satellite providing a high-speed downlink and then using a dial-up telephone connection or other land-based system for the uplink to the Internet.

Satellite delay

Time taken for transmissions to reach the satellite and return to earth.

Satellite hop

Transmission of information from an earth station to a satellite and retransmission back to another earth station.

Satellite Pass

Segment of orbit during which the satellite "passes" nearby and in range of a particular ground station.

SCPC

Single Channel per Carrier. A satellite transmission method which employs a separate carrier for each channel, as opposed to Frequency Division Multiplexing (FDM) that combines many channels on a single carrier.

SCR

Sustained Cell Rate. The minimum bandwidth guaranteed in an ATM network between two terminal devices connected via a PVC.

Sidelobe

Off-axis response of an antenna.

Sidereal Day

The amount of time required for the Earth to rotate exactly 360 degrees about its axis with respect to the "fixed" stars. The Sidereal Day contains 1436.07 minutes. See also: Solar Day.

S/N

Signal to Noise Ratio. The ratio of the signal power and noise power. A video S/N of 54 to 56 dB is considered to be an excellent S/N, that is, of broadcast quality.

Simplex Transmission

Capability for transmission in only one direction between sending station and receiving station.

Single hop

Transmission of information from terminal to terminal in one stage via a satellite link. For example, in a mesh network.

SIT

Satellite Interactive Terminal. Another name for an IDU or satellite modem.

 

Skew

An adjustment that compensates for slight variance in angle between identical senses of polarity generated by two or more satellites.

SLA

Service Level Agreement. Agreeement bewetween a service and a customer which guarantees certain quality parameters for the provision and performance of the contracted service at each site.

Slant Range

The length of the path between a communications satellite and an associated earth station.

Slot

That longitudinal position in the geo-synchronous orbit into which a communications satellite is "parked".

Solar Day

The Solar Day contains exactly 24 hours (1440 minutes). During the Solar Day, the Earth rotates slightly more than 360 degrees. See also: Sidereal Day.

Solar Outage

Solar outages occur when an antenna is looking at a satellite, and the sun passes behind or near the satellite and within the field of view of the antenna. Solar outages occur each year at the beginning of spring and fall and can be exactly predicted as to the timing for each site.

Space segment

 Portion of the satellite bandwidth and transmission power assigned to the communication network.

Spectrum

The range of electromagnetic radio frequencies used in transmission of voice, data and television.

Spill-over

Satellite signal that falls on locations outside the beam pattern's defined edge of coverage.

Spin Modulation

Periodic amplitude variations resulting from the rotation of a satellite's antennas about its axis, rotating the antenna peaks-and-nulls.

Spin Stabilization

A form of satellite stabilization and attitude control which is achieved through spinning the exterior of the spacecraft about its axis at a fixed rate.

Splitter

A passive device (one with no active electronic components) which distributes a RF signals carried on a cable to two or more paths.

Spoofing

A common term to describe the action of the PEP when optimising the TCP data stream. Spoofing is an internal emulation of processes by acknowledging control and supervisory data frames locally rather than transmitting them over the satellite link.

Spot Beam

A focused antenna pattern sent to a limited geographical area.

Spread Spectrum

The transmission of a signal using a much wider bandwidth and power than would normally be required. Spread spectrum also involves the use of narrower signals that are frequency hopped through various parts of the transponder. Both techniques produce low levels of interference between the users. They also provide security in that the signals appear as though they were random noise to unauthorized earth stations. Both military and civil satellite applications have been developed for spread spectrum transmissions.

SSP

Sub-Satellite Point. The point on the surface of the Earth directly between the satellite and the geo-centre

SSPA

Solid State Power Amplifier. A VSLI solid state device that is replacing Travelling Wave Tubes in satellite communications systems because they are lighter weight and are more reliable.

Star network

VSAT network consisting of a central hub and many remote stations. All the terminals can communicate with each other only through the hub in a double hop.

Subcarrier

A second signal "piggybacked" onto a main signal to carry additional information. In satellite television transmission, the video picture is transmitted over the main carrier. The corresponding audio is sent via an FM sub-carrier. See also: Audio Subcarrier

SVC (Switched Virtual Circuit)

A temporary virtual circuit that is set up and used only as long as data is being transmitted.
 


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T

 

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.

Terminal Equipment

The equipment connected directly to the line termination. In most cases a router.

Terrestrial

Earth-based. Usually refers to typical telecommunications via copper.

TFSS

Timing/Frequency Subsystem

Transmission Capacity

 

In order to make more efficient use of transmission capacity, digital broadcast signals are reduced in size by digital compression. This has a minimum effect on the received signal quality but allows several compressed TV channels to be transmitted in the space required for one analogue channel.

Tx

Transmitter. An electronic device consisting of oscillator, modulator and other circuits which produce a electromagnetic wave signal for radiation into the atmosphere or uplinking to a satellite by an antenna.

Transponder

Transponders were originally electronic circuits that were attached to some item whose position or presence was to be determined. Another major category of transponders is the use of transponders in radio relay systems such as fixed/mobile radio networks and satellite transmissions.
 

 
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U

 

Unicast

A unicast application transmits a copy of every packet to every receiver.

UPC

Uplink Power Control. The mechanism to increase the transmission power in order to overcome rain fade.

Uplink

In order for a satellite broadcast to take place, the point of origin must transmit (uplink) a signal to a satellite that it then relays back to Earth. The word uplink also characterizes the infrstructure used for transmitting a signal to the satellite: audio and video control equipment, transmitters, parabolic antenna.

 


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V

 

VBDC

Volume Based Dynamic Capacity

VBR

Variable Bit Rate

VPN

Virtual Private Network. A corporate network using  public infrastructures, mostly the Internet, as its backbone for data transport. VPN need encryption and other security mechanisms to ensure that only authorized users can access the network and that the transmission cannot be intercepted. The concept of VPN extends to satellite networks.

VPN Provider

Two general groups of VPN providers can be defined: operators relying on public Internet and those using other international backbone networks with transport technologies such as a private IP backbone, Frame Relay, and ATM. The underlying infrastructure the VPN operates across is kept transparent to the customers.

VSAT

Very Small Aperture Terminal. Low-cost small earth station, which can be used for one-way and/or interactive communications in a star, mesh or point to point satellite network. ETSI define VSAT antenna size to being less than or equal to 3.8 m at Ku-Band and 7.8 m at C-Band. VSAT networks offer value-added satellite-based services capable of supporting the Internet, data, video, LAN, voice/fax communications.

 


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W

 

Waveguide

A metallic microwave conductor, typically rectangular in shape, used to carry microwave signals into and out of microwave antennas.

Window

Overlap region between acquisition circles of two ground stations referenced to a specific satellite. Communications between two stations is possible when the sub-satellite point is within the window.
 


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