Glossary
AM Amplitude Modulation
Analog data Data represented by a physical quantity that is considered to be continuously variable and whose magnitude is made directly proportional to the data or to a suitable function of the data.
Asynchronous transmission A method of data transmission which allows characters to be sent at irregular intervals by preceding each character with a start bit, and following it with a stop bit.
Availability The ratio, expressed as a percentage, of the total time a functional unit or service is capable of being used or is available to be used during a given interval to the length of the interval; e.g., if the unit is not capable of being used for 20 minutes in a week, the availability is 99.8 percent (10080 - 20 minutes/10080 minutes X 100).
Backhaul Point-to-point transmission from a remote site back to a central site for further distribution.
Bandwidth The bandwidth of a device is the difference between the limiting frequencies within which performance with respect to some characteristic falls within specified limits.
Baud A unit measuring the rate of information flow, with five baud roughly equivalent to one alphanumeric character.
Binary Phase Shift Keying ( BPSK) A digital modulation scheme used in transmission communications.
Broadcasting Using the electronic media to reach a wide-area audience.
Byte A sequence of eight adjacent binary digits usually treated as a unit.
C-band A band of frequencies used for satellite and terrestrial communications. The range of frequencies from 4 to 6 gigaHertz (billion cycles per second) is used by most communications satellites.
Call (1) Any demand to set up a connection. (2) A unit of traffic measurement.
Carrier The basic radio signal that transfers the information signal. It occupies a single radio frequency (see also Common Carrier).
Circuit mode A circuit switched operational mode for transferring (transporting and switching) user information through a network. Contrast with packet switching mode.
Compression A technique to reduce the amount of data to be transmitted that reduces the amount of bandwidth needed to transmit video or audio, thus increasing the capacity of a satellite transponder.
Data Representation of facts, concepts, or instructions in a formalized manner suitable for communication, interpretation, or processing by humans or by automatic means.
Data Encryption Standard (DES) A cryptographic algorithm for the protection of unclassified computer data, issued as Federal Information Processing Standard Publication 46-1.
dB Decibel; an analog unit of measure of signal strength, volume, or signal loss due to resistance as expressed in logarithmic form.
Demand Assigned Multiple Access (DAMA) Refers to contention access schemes that allow multiple communications users to share a discrete portion of the bandwidth.
Demodulation The process for retrieving an information signal that has been modulated onto a carrier.
Digital data Data represented by discrete values or conditions (i.e., "0" or "1"), as opposed to analog data.
Digital voice Analog voice signals that have been converted into digital signals.
Direct Broadcast Satellite (DBS) Transmits TV signals directly to dishes at viewer's homes. Usually a high-powered satellite that requires only small dishes.
Downlink Transmissions from a satellite to a ground station; also, the dish used for reception.
Echo suppression Refers to the cancellation of any reflected signal in a communications channel.
EIRP Effective Isotropic Radiated Power
Electronic access The capability to access information via on-line access (dedicated or dial-up), E-Mail, and FAX.
Electronic Data Interchange (EDI) The exchange of routine business transactions in a computer-processable format, covering such traditional applications as inquiries, planning, purchasing, acknowledgments, pricing, order status, scheduling, test results, shipping and receiving, invoices, payments, and financial reporting.
Encrypt To convert plain text into an unintelligible form by means of a cryptosystem.
FM Frequency Modulation
Full-duplex operation A mode of operation in which simultaneous communication in both directions may occur between two terminals. Contrast with half duplex or simplex operation in which communications occur in only one direction at a time.
Gateway In a communication network, one of the network nodes equipped for interfacing with a network using different protocols. Note 1: A gateway may contain devices such as protocol translators, impedance matching devices, rate converters, fault isolation, or signal translators as necessary to provide system interoperability.
Geosynchronous Orbit (GSO) Satellite in geosynchronous orbit travel around the Earth in an area approximately 22,300 miles from the surface at the same rate that the Earth turns, therefore completing one revolution every 24 hours.
gHz GigaHertz, equals one billion hertz
Half-duplex operation That mode of operation in which communication between two terminals occurs in either direction but in only one direction at a time. Contrast with duplex or simplex operation. Note: Half-duplex operation may occur on half-duplex circuits or on duplex circuits, but it may not occur on simplex circuits.
Hertz (Hz) Cycle per second; a measure of electromagnetic frequency that represents the number of complete electrical waves in a second. One kiloHertz (kHz) is one thousand cycles per second; one megaHertz (MHz) is one million; one gigaHertz (gHz) is one billion.
High Power Amplifier (HPA) Equipment used to amplify signals.
Hub A network operations center for very small aperture terminal (VSAT) operation (see also Shared Hub).
Hybrid satellite A satellite which carries two or more different communications payloads (i.e., C-band and Ku-band).
Integrated Services Digital Network (ISDN) A network that provides end-to-end digital connectivity to support a wide range of services, including voice and non-voice services, to which users have access by a limited set of standard multi-purpose user network interfaces, as defined in the ITU-T1 series.
Internetworking The process of interconnecting a number of individual networks to provide a path from a terminal or a host on one network to a terminal or a host on another network. The networks involved may be of the same type, or they may be of different types. However, each network is distinct, with its own addresses, internal protocols, access methods, and administration.
IRD Integrated Receiver/Decoder
ISDN Integrated Services Digital Network
Ka-band A higher frequency than Ku-band, operating from 18 to 31 GHz.
k/bs Kilobit per second
Ku-band The range of frequencies between 11 and 14 GHz used increasingly by communications satellites.
LNA Low Noise Amplifier
Local Area Network (LAN) A data communications system that (a) lies within a limited spatial area, (b) has a specific user group, (c) has a specific topology, and (d) is not a public switched telecommunications network, but may be connected to one.
Modem Acronym for MOdulator-DEModulator. A device that modulates and demodulates signals. Note: Modems are primarily used for converting digital signals into quasi-analog signals for transmission over analog communication channels and for reconverting the quasi-analog signals into digital signals.
Modulation The process of superimposing an information signal onto a carrier for transmission.
MSS Mobile Satellite Service
Narrowcasting Using the electronic media to reach a specific audience
On-line Electronic availability on demand from a computer-based system without mounting removable media such as magnetic tape or disks.
Outage A telecommunication service condition wherein a user is deprived of service due to a malfunction of the communication system.
Packet In data communication, a grouping of a sequence of binary digits, including data and control signals, that is transmitted and switched as a composite whole. The data, control signals, and possibly error control information, are arranged in a specific format. The packet can be of either fixed or variable length.
Packet mode A packet switched operational mode for transferring (transporting and switching) user information through a network without establishing a connection. The packets do not necessarily arrive at their destination in the order they were sent, unlike the circuit mode of transmission. See packet switching.
Protocol Any set of standard procedures that permit devices to intercommunicate.
Public Switched Telephone Network (PSTN) Any common carrier network that provides circuit switching among public users. Note: The term is usually applied to the public switched telephone network, but it could be applied more generally to other switched networks, e.g., packet switched public data networks.
Quadrature Phase Shift Keying (QPSK) Digital modulation scheme used in transmission communications that allows increased sending capacity.
Radio Frequency Interference (RFI) A term used to denote apparent disturbance or distortion to satellite signals.
Shared hub A satellite communications operations center which is shared among a number of separate network users; often used for VSAT operations.
Simplex operation That mode of operation in which communication between two points occurs in only one direction at a time. Contrast with half duplex or duplex operation.
Solid State Power Amplifier (SSPA) A high power amplifier using semiconductors to amplify the signal.
Spot beams The focusing of energy from a satellite to the ground to concentrate the limited available effective radiated power (EIRP).
Synchronous transmission Digital transmission in which the time interval between any two similar significant instants in the overall bit stream is always an integral number of unit intervals. Note: Isochronous and anisochronous are characteristics, while synchronous and asynchronous are relationships.
T1 TDM digital channel carrier (1.544 MBPS)
Telemetry, Tracking and Control (TT&C) Means of accessing the control systems in a spacecraft in order to position and configure the spacecraft; or the facility which monitors and controls the health of a satellite.
Teleport Ground facility used to interface with a satellite during communications.
Television Receive Only (TVRO) A satellite receiving antenna, also known as a downlink or a backyard dish
Terrestrial In communications parlance, this refers to Earth, land or ground-based telecommunications systems.
Time Division Multiple Access (TDMA) A method of accessing a satellite using time separation; sending simultaneous data streams interleaved using a clock to separate the various data packets.
Transceiver A combination transmitter and receiver.
Transponder Satellite component that receives, amplifies, and retransmits a TV signal or perhaps many narrower-band data or audio channels.
Ultra High Frequency (UHF) The band in the 500 to 900 MHz range, including TV channels 14 through 83.
Uplink To transmit to a satellite for relay; also, the dish used to transmit.
Very High Frequency (VHF) The band in the 30 to 300 MHz range, including TV channels 2 through 13.
Very Small Aperture Terminal (VSAT) A means of narrowcast transmission of video, voice and data by SCPA means to a satellite, typically used in business applications.
Video Transmission Services (VTS) The transmission of compressed or wideband video signals.
Virtual Circuit (VC) A communication arrangement in which data from a source user may be passed to a destination user over various real circuit configurations during a single period of communication.
Virtual Private Network (VPN) A capability that allows users to create their own private information network.
WAN Wide Area Network
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