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Glossary: Satellite and Computer Technology
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ACTS (ADVANCED COMMUNICATIONS
TECHNOLOGY SATELLITE): The NASA experimental satellite project
which demonstrated the use of the Ka Band (30/20
GHz) services; on-board processing with multiple beams, electronic hopping
antenna beams, and dynamic adjuctment of power to cope with rain attenuation.
The experimental network supported low to medium rate mobile services
to high data rate HDTV fixed services.
ALGORITHM: A procedure or formula for solving a mathematical problem in a finite number of steps. Extremely complex algorithms or sets of algorithms are used to simplify, modify or predict data in the digital manipulation of information. Often, algorithms which are developed for digital applications are proprietary and are an important aspect of defining significant differences among the various digital compression and codec (coding/decoding) standards.
AM (AMPLITUDE MODULATION): Modulation
which is accomplished by varying the amplitude (height of the carrier
wave) of the carrier signal in accordance with the information to be transmitted.
ANALOG: Representing something in the real world. Analog is often used in comparison with digital, for example comparing analog and digital computers or analog or digital electrical signals. Analog computers actually measure, analyze and compute using real physical measures and numbers. Analog electrical signals (such as the radio frequencies used to transmit telecommunications information) are directly generated by physical stimuli in the form of light or sound waves.
APERTURE: In image scanning, the size of the sensitive spot that moves over the image. In an antenna, an imaginary cylinder in space surrounding the radiating elements. (Inglis & Luther, 1996)
APOGEE: The point in an elliptical satellite orbit which is farthest from the center of the earth (the opposite of perigee).
ARTIFACT: Any anomaly, distortion
or unnatural component in an image -- usually associated with digital
ATM (ASYNCHRONOUS TRANSFER MODE): A high-speed (155 and 622 Mb/s), fast-packet switching technique employing short, fixed length cells that are statistically multiplexed over virtual connections. Each cell consists of a 5-octet header and a 48-octet payload of user information (data, voice, or video). (Carne, 1995)
ATSC (ADVANCED TELEVISION SYSTEMS COMMITTEE): An international organization of approximately 200 members that is establishing voluntary technical standards for advanced television systems. ATSC Digital TV Standards include digital HDTV (High Definition Television), SDTV (Standard Definition Television), data broadcasting, multichannel surround-sound audio, and Satellite direct-to-home broadcasting. (ATSC Home Page, http://www.atsc.org/, Oct 06, 1999)
ATTENUATION: The loss of power of electomagnetic signals between transmission and reception points.(Pelton, 1995)
AZIMUTH: The definition from Webster's New Collegiate Dictionary states, "an arc of the horizon measured between a fixed point such as true north and the vertical circle passing through the center of an object." Azimuth is an important consideration in locating a satellite for transmission or reception of RF signals. The azimuth expressed in degrees of a circle will be the horizontal angle of rotation that the ground antenna must be rotated though to point at the specific satellite. Azimuth angles for any satellite may be calculated given the latitude and longitude of the ground station and the location of the satellite in geosynchronous orbit relative to true north.
BACKHAUL: A process where a television signal is transmitted from a remote site to a central site where the backhaul signal is integrated into a program which is being broadcast from the central site.
BANDwidth: A measure of radio frequency (RF) use or capacity. A terrestrial broadcast television channel, for example occupies a RF bandwidth of 6 MHz or six million cycles per second while a telephone voice transmission requires a RF bandwidth of only 3 KHz or 3,000 cycles per second.
BASEBAND: The bandwidth of the
Modulating (message) signal.
BAUD: A description of the rate of data transmission usually expressed in bit rate measurements of thousand bits per second (Kb/s). A 14.4 Baud modem for example will accommodate a bit rate of 14,400 bits per second.
BIG LEO: (See LE0: LOW EARTH ORBIT)
B-ISDN (BROADBAND ISDN): An ISDN (Integrated Services Digital Network) offering broadband capabilities. B-ISDN is a CCITT-proposed service that may(a) include interfaces operating at data rates from 150 to 600 Mb/s, (b) use ATM (Asynchronous Transfer Mode) to carry all services over a single, integrated, high-speed packet-switched network, (c) have LAN interconnection capability, (d) provide access to a remote, shared disk server, (e) provide voice/video/data teleconferencing, (f) provide transport for programming services such as cable TV, (g) provide single-user controlled access to remote video sources, (h) handle voice/video telephone calls and (i)access shop-at-home and other information services. (GSA, 1996)
BIT: (Abbreviation for "binary digit") The smallest increment of digital information. Often referred to as a 1 or 0 in the binary system. Also referred to as an 'on' or 'off' when referring to the mechanical operation of bits in the computer.
BIT RATE: The amount of data being transported, measured relative to quantity over time in bits per second (thousand bits per second or Kb/s, million bits per second or Mb/s, billion bits per second or Gb/s and trillion bits per second or Tb/s).
BIT STREAM: (see DIGITAL BIT STREAM)
BROADBAND: (See WIDEBAND)
BSS (BROADCAST SATELLITE SERVICE): The segment of Ku-band satellite service established by the FCC which is provided by high power satellites (or clusters of satellites) which must be separated in orbit by 9 degrees. The frequencies of RF signals transmitted to and received from the BSS satellites are higher than those permitted for FSS satellites. The BSS transmission frequency from high-power satellites to satellite receivers has been established by the FCC between 12.2 - 12.7 GHz.
BYTE: (Abbreviation for "binary term The simplest combination of bits producing recognizable information such as a number or word. For example, the number 50 is represented by a byte made up of the eight bits 0,0,1,1,0,0,1 and 0.
CACHE: A small fast memory which holds recently accessed data, designed to speed up subsequent access to the same data. Most often applied to processor-memory access but also used for a local copy of data accessible over a network, etc. (Howe, 1999)
C-BAND: A portion of the Radio Frequency (RF) spectrum located between 4 GHz and 8 GHz, a part of which is dedicated to satellite communications. Satellite downlink frequencies are located between 3.7 GHz and 4.2 GHz and uplink frequencies are located between 5.925 GHz and 6.425 GHz.
CDMA (CODE DIVISION MULTIPLE ACCESS): See (MULTIPLE ACCESS)
CLARKE ORBIT: (See GEO)
CLIFF EFFECT: A characteristic of the digital transmission of RF signals where there is a radical change in reception quality which results from a small change in reception power. By comparison, when an analog RF signal approaches the fringes of acceptable reception power, the television picture begins to experience gradual degradation with increasing sparkles or snow. As a digital RF signal reaches the fringes of acceptable reception power, there is no discernible degradation of picture quality until the level of reduced power reaches a threshold. At that point, picture quality changes from perfect to no picture.
CLIPPER CHIP: An asymmetric encryption chip proposed for use by the United States government so that the pervasive encryption of communication channels shall not protect both the law-abiding and law-breakers. Designated MYK-78, a secret key is created at the time of manufacture and placed in escrow. With court approval, government agencies can obtain this key and employ it to decipher encrypted messages obtained through wiretaps. (Carne, 1995)
COLOR DIFFERENCE: In a color video system, signals that represent the difference between specified colors and the luminance component. They have the property that they go to zero for monochrome images. (Inglis & Luther, 1996)
COMMON CARRIER: An entity which provides communication transmission facilities for use by other entities or which carries other entity's communications signals. Examples of common carriers include the telephone or telegraph companies and the companies which own communications satellites. Common carriers are subject to tariff regulation and must file rates for specific services with appropriate regulatory agencies such as the FCC or state regulatory agencies. As a rule, common carriers are not permitted to control content.
DARPA (DEFENSE ADVANCED RESEARCH PROJECTS AGENCY): An agency of the USDoD (United States Department of Defense) responsible for the development of new technology for use by the military. DARPA, originally ARPA, was established in 1958. The agency is independent from other more conventional military R&D operations and focusses on short (two to four-year) projects run by small, purpose-built teams. ARPA was responsible for funding development of ARPANET (which grew into the Internet), as well as the Berkeley version of Unix and TCP/IP. (Howe, 1999)
dB (DECIBEL): An analog unit of measure of signal strength, volume, or signal loss due to resistance as expressed in logarithmic form. (Satellite Industry News, Jan, 1999)
DCII (DIGICIPHER II): An MPEG II digital standard developed by General Instruments Corporation which uses the MPEG II video encoding standards and the ATSC (Advanced Television Systems Committee) Dolby AC-3 digital audio standard. This standard has been adopted for standard and high definition television (SDTV & HDTV) transmission in North America, South Korea and Taiwan (see also DVB).
DIGITAL: The conversion of information into a binary format (1's and 0's or on's and off's), the smallest or simplest unit of information being a bit. These bits of information may be transmitted using radio frequency techniques such as terrestrial broadcasting, satellite and microwave transmission or over coaxial or fiber optic cable and copper wire. Digital information may be manipulated at extremely high speeds.
DIGITAL ARTIFACTS: Errors in a digital transmission of data which result from digital compression applications which remove information from a data inventory in an amount sufficient to cause noticeable anomalies.
DIGITAL BIT STREAM: A continuously changing flow of digital information which is being transported over some transmission medium such as cable or on a satellite microwave frequency.
DIGITAL COMPRESSION: A process by which complex sets of algorithms are employed to rearrange, reassemble and eliminate digital information from a data inventory in such a way that the amount of data (bits) to be stored or transported is reduced without a resulting loss of apparent resolution.
DSL(DIGITAL SUBSCRIBER LINE): In ISDN (Integrated Services Digital Networks), equipment that provides full-duplex service on a single twisted metallic pair at a rate sufficient to support ISDN basic access and additional framing, timing recovery, and operational functions. The physical termination of the DSL at the network end is the line termination; the physical termination at the customer end is the network termination. (GSA, 1996)
DVB (DIGITAL VIDEO BROADCAST): A suite of digital video standards formulated by the European Launch Group (ELG). The standard is based on MPEG II video. MPEG II/DVB uses the Musicam audio standard (see also DCII).
ELECTROMAGNETIC SPECTRUM: The entire range of frequencies of electromagnetic radiation from zero to infinity which measure electromagnetic energy. The Radio spectrum (less than 300 GHz bandwidth) is divided into subsets of various bandwidths which are identified by their frequency characteristics. For example, the Ku-band subset includes those frequencies of the radio frequency (RF) spectrum between 10.7 GHz and 17.8 GHz while the C-band subset includes those frequencies between 3600 MHz and 7075 MHz.
ENCAPSULATOR: An IP gateway at a satellite uplink or cable/wireless headend which is a router taking traffic from the terrestrial network and routing packets to the satellite WAN (Wide Area Network). The encapsulator offers streaming capabilities that encapsulate content into datagrams and then route it to subscriber subnets. (Satellite Communications, Oct., 1999)
ENCRYPTION: The action of disguising information so that it can be recovered relatively easily by persons who have the key, but is highly resistant to recovery by persons who do not have the key. Encryption is accomplished by scrambling the bits, characters, works or phrases in the original message. (Carne, 1995)
EQUIVALENT NOISE TEMPERATURE: The temperature, usually expressed in Kelvins, of a hypothetical matched resistance at the input of an assumed noiseless device, such as a noiseless amplifier, that would account for the measured output noise. (GSA, 1996)
ETHERNET: A standard protocol (IEEE 802.3) for a 10 Mb/s baseband LAN (local area network) bus using carrier-sense multiple access with collision detection (CSMA/CD) as the access method, implemented at the Physical Layer in the ISO Open Systems Interconnection-Reference Model, establishing the physical characteristics of the CSMA/CD network. (GSA, 1996)
ETHERNET CABLE: Cable transmission medium for Ethernet classified as XbaseY, where X is the data rate in Mb/s, "base" means baseband (as opposed to radio frequency) and Y is the category of cabling. The original cable was 10base5. Common XbaseY Ethernet Cable Classifications include:
FCC (FEDERAL COMMUNICATIONS COMMISSION): This is the national regulatory body for interstate telecommunications in the United States. The commission consists of five members all nominated to a specific term by the President of the United States and confirmed by the Senate. The current chairperson is William Kennard. The authority for the commission is contained within the Communications Act of 1934 as amended (most notably by the Telecommunications Act of 1996).
FDMA (FREQUENCY DIVISION MULTIPLE ACCESS): See (MULTIPLE ACCESS).
FEC (FORWARD ERROR CORRECTION): A technique which employs special codes that allow the receiver to detect and correct a limited number of errors without referring to the transmitter. (Carne, 1995)
FM (FREQUENCY MODULATION): Modulation in which the instantaneous frequency of a sine wave carrier is caused to depart from the center frequency by a amount proportional to the instantaneous value of the modulating signal. In FM, the carrier frequency is called the center frequency. In optical communications, even if the electrical baseband signal is used to frequency-modulate an electrical carrier, it is still the intensity of the lightwave that is varied (modulated) by the electrical FM carrier. The lightwave is varied in intensity at an instantaneous rate corresponding to the instantaneous frequency of the electrical carrier. (GSA, 1996)
FSS (FIXED SATELLITE SERVICE): The segment of Ku-band satellite service established by the FCC to be provided from medium power satellites. These satellites are separated in orbit by at least 2 degrees. RF signals are transmitted to FSS satellites in the 14 GHz to 14.5 GHz range and received from the satellite in the 11.7 GHz to 12.2 GHz range.
GAIN: The ratio of output current, voltage or power to input current, voltage or power, respectively. Gain is usually expressed in dB. If the ratio is less than unity, the gain expressed in dB, will be negative, in which case there is a loss between input and output. (GSA, 1996)
GEO (GEOSYNCHRONOUS EARTH ORBIT): This is the orbital altitude of 35,580 km (22,237 miles) above the earth's surface where a satellite's velocity matches with the rotation of the earth. A satellite which is in a GEO position above the earth's equator (geostationary) will appear from the earth to be occupying a stationary position. The geosynchronous earth orbit is also referred to as the Clarke Orbit (named in honor of Arthur C. Clarke, a science fiction writer who first postulated the characteristics of this orbit in 1945).
GMT (GREENWICH MERIDIAN TIME): The time zone which includes Greenwich, England, and which is bisected by 0 Degree Longitude. This is the time notation which is used for booking international satellite time. (also referred to as ZULU Time)
GROUND SEGMENT: A term which describes that portion of the total communications satellite system which is situated on the earth such as transmitting and receiving antenna, RF (radio frequency) signal generating facilities, RF receivers and digital encoding equipment and decoders.
G/T (GAIN TO NOISE TEMPERATURE): In the characterization of antenna performance, a figure of merit, where G is the antenna gain in dB (decibels) at the receive frequency, and T is the equivalent noise temperature of the receiving system in Kelvin. (GSA, 1996)
HDTV (HIGH DEFINITION TELEVISION): The term refers to the new format of digital signal to be seen on the television screen. The HDTV format will be delivered by the ATV (Advanced Television) digital system developed by the members of the Grand Alliance (AT&T, David Sarnoff Research Center, General Instrument, Massachusetts Institute of Technology, North American Philips, Thomson Consumer Electronics and Zenith Electronics). This new system will provide a television picture with about 1000 scanlines per picture (compared to 525 for existing standard) and a picture aspect ratio of 16:9 (compared to 4:3 for existing standard). This format will be broadcast over the UHF spectrum.
KILOHERTZ (KHz): one Thousand cycles/second (10p3)An electromagnetic wave oscillates from positive to negative poles. An oscillation from positive to negative and back to positive again is one complete cycle.
HIGH-POWER SATELLITE: A satellite with greater than 100 Watts of transponder radio frequency (RF) transmitting power.
IEEE (INSTITUTE OF ELECTRICAL AND ELECTRONIC ENGINEERS, INC.): A USA based international technical/professional society founded in 1884. The IEEE promotes standardization in technical disciplines which include aerospace, biomedical technology, computers and communications, consumer electronics and electrical power. The institute conducts educational programs, sponsors international technical conferences and symposia and publishes nearly 25% of the world's technical papers in electronics and computer engineering and computer science.
IEEE 802.2: Defines Logical Link Control (LLC, the upper portion of the data link layer) for local area networks.(Howe, 1999)
INCLINED ORBIT: A condition in which a satellite is unable to maintain a geostationary position above the earth's equator. Almost all satellites generate electrical power to operate their transponders by converting the sun's energy to electricity. Energy used for station keeping, that is keeping the satellite within a very narrow range of movement north or south of the equator, is stored on board the spacecraft and over time is dissipated, usually after passage of the design life of between 8-15 years. When this fuel runs out, the satellite is no longer able to keep station above the equator. Since satellite uplink and downlink antenna are aimed at a particular point above the equator, a satellite in inclined orbit will move in and out of the antenna's "range of vision" as the satellite's orbit fluctuates north and south of the equator.
INTERLACED SCANNING: The television picture in the NTSC system is made up of 525 scan lines. One-half of the scan lines which make up the video picture are traced on the screen approximately 30 times a second alternating with the other one-half of the scan lines which are traced on the screen approximately 30 times a second thus providing 60 interlaced line scanning events each second. (see also PROGRESSIVE SCANNING)
INTERNET: The Internet is the largest interconnection of networks in the world. It is a three level hierarchy composed of backbone networks such as ARPAnet, NSFnet and MILNET, mid-level networks, and stub networks. These include among others commercial (.com or .co), university and educational (.edu or .ac), other research networks (.org or .net), governmental (.gov) and military (.mil) networks and span may different networks around the world with various protocols including the Internet Protocol. (Howe, 1999)
IP (INTERNET PROTOCOL): See TCP/IP.
IRD (INTEGRATED RECEIVER/DECODER): Satellite ground segment equipment which receives and decodes digital information transmitted via satellite RF signals. Manufactures have until now incorporated proprietary algorithms into digital transmission systems which result in the lack of interoperability among the differing systems. An IRD will only operate successfully within the particular proprietary system of which it is a part.
ISDN (INTEGRATED SERVICES DIGITAL NETWORK): A standard for the integrated transmission of voice, video and data developed by the Consultative Committee on International Telephony and Telegraphy (referred to as the CCITT now combined with the CCIR, Consultative Committee on International Radio to form the TSS, Telecommunications Standardization Sector). The ISDN bandwidths include the Basic Rate Interface or BRI at 144 Kb/s and the Primary Rate Interface or PRI at 1.544 and 2.048 MB/s (see T-l LINE).
ISL (INTER-SATELLITE LINK): A satellite architecture whereby two or more satellites are configured in such a way that they may communicate directly with one another. This architecture is used in the TDRESS system of satellites and is a component of the design of Big LEO communications satellite systems.
ISO (INTERNATIONAL STANDARDS ORGANIZATION): An international organization which developed the MPEG and JPEG standards and which is closely allied with the CCITT (part of the Telecommunications Standardization Sector, TSS)
ITU (INTERNATIONAL TELECOMMUNICATIONS UNION): A United Nations treaty organization(originally created in 1865 as the International Telegraph Union with a membership of twenty nations...current name adopted in 1932) which supports procedures for the international allocation of the radio frequency (RF) spectrum and provides the platform for the World Radio Conference (WRC), a biannual meeting of world communication leaders. The WRC publishes 'International Radio Regulations' for the RF spectrum. The ITU conducts ongoing policy and study group sessions.
ITU-T MULTIMEDIA TELECONFERENCING STANDARDS: Standards developed and ratified by the Telecommunication Standardization Sector of the International Telecommunications Union which define the core technologies for multimedia teleconferencing. Standards are developed to promote interoperability among different proprietary systems. These core standards include:
T.120: Data protocols for Real Time Data Conferencing (Audiographics). These standards cover document conferencing and application sharing.
KA-BAND: A portion of the RF spectrum located between 18 GHz and 31 GHz. Downlink frequencies for satellite communications are located in the 20 GHz range and uplink frequencies are located in the 30 GHz range.
KU-BAND: A portion of the RF spectrum located between 10.9 GHz and 17 GHz, a part of which is dedicated to satellite communications. Satellite downlink frequencies are located between 11.7 GHz and 12.2 GHz and uplink frequencies are located between 14 GHz and 14.5 GHz.
LAN (LOCAL AREA NETWORK): A bus or ring connected, limited distance network that serves the data communication needs of users within a building or several buildings in proximity to each other.(Carne, 1995) Ethernet is an example of a standard Local Area Network.
LATA (LOCAL ACCESS AND TRANSPORT AREA): A grouping of exchange areas served by a single Local Exchange Carrier (LEC). Traffic between LATAs must be carried by an Inter-Exchange Carrier (IXC). (Carne, 1995)
LCD (LIQUID-CRYSTAL DISPLAY): An electronic display panel that is based on the properties of a liquid-crystal material sandwiched between two transparent plates and illuminated from behind. The transmittance of the liquid-crystal material changes with the applied electric field. (Inglis & Luther, 1996)
LEC (LOCAL EXCHANGE CARRIER): Provides public, switched telephone services within limited areas called Local Access and Transport Areas (LATA). The LEC provides connections between users located within the same exchange area, between users located in different exchange areas that are in the same LATA, and delivers traffic directed to an exchange area outside the LATA to Inter- Exchange Carriers (IXC). (Carne, 1995)
LEO (LOW EARTH ORBIT): Until recently, a distinction was made between LEO and MEO (Medium Earth Orbit) orbital classifications. LEO was classified as an earth orbit with an altitude of between 200-2400 kms. The MEO designation classified orbits between 2,400 and 10,000 km. Current literature frequently refers to any orbital distances from earth of less than that for geostationary orbit as being LEO. Almost all of the Big LEO and Little LEO systems which are currently planned will operate in orbits ranging between 640 km and 2,500 km above the earth. Satellites which are placed in LEO orbits move rather swiftly in relation to the earth, generally from a westerly to an easterly direction. Some of the newly designed LEO communications satellite networks will launch some of the satellites in the network constellation into polar earth orbits. The relative motion of the satellite in relation to the earth slows as the altitude of the satellite increases. The rapid relative movement in low earth orbit enables data gathering and communication satellites to cover large areas of the earth's surface in short periods of time. The space shuttle is injected into a low earth orbit, for example.
BIG LEO: A space segment architecture which consists of a constellation of many satellites in Low Earth Orbit in a configuration which will permit the delivery of global mobile telephony and data services. The system is served by a network of ground stations which provides gateway access from terrestrial networks as well as management, control and orbital correction functions. This system is intended to provide telecommunication service to remote sites, high latitude geographic locations which are not accessible by GEO satellites and to omnidirectional antenna on handset transceivers. Examples of Big LEO systems include Teledesic and Iridium.
LMDS (LOCAL MULTIPOINT DISTRIBUTION SYSTEM): A fixed, broadband point-to-multipoint wireless service which can provide one-way and two-way high capacity voice, video and data service. Services are licensed and bandwidth auctioned by the FCC. Service rules were adopted by the FCC in March, 1997.
LOW-POWER SATELLITE: A satellite with less than 30 Watts of transponder radio frequency (RF) transmitting power.
LUMINANCE: In color video systems, a component signal that represents the brightness of the image. (Inglis & Luther, 1996)
MEO (MEDIUM EARTH ORBIT): See LEO (LOW EARTH ORBIT).
MID BAND: A portion of the VHF (Very High Frequency) RF spectrum located between television channels 6 and 7 (88 MHZ to 174 MHZ) which has been reserved by the FCC for air, maritime and land mobile units, FM radio and aeronautical and maritime navigation. The frequencies between 108 MHZ and 174 MHZ can be used to provide additional channels on cable television systems.
MODEM: This term is a contraction of "modulator" and "demodulator". A modem coverts digital data generated by and coming into a computer which are transmitted over telephony circuits or by using terrestrial or satellite RF (radio frequency) circuits.
MODULATION: The process of modifying the channel signal (often called the "carrier") to represent information to be transmitted. Typical modulation methods involve the frequency, amplitude or phase of the carrier. (Inglis & Luther, 1996)
MPEG (MOTION PICTURE EXPERTS GROUP): A working group of a committee of the ISO (International Standards Organization). They developed the MPEG worldwide standard for motion video compression and transport. (Inglis & Luther, 1996)
MTOPS (MILLIONS OF THEORETICAL OPERATIONS PER SECOND): A measure of computer processing speed used by the US Department of Commerce (USDoC) to define a classification system for export controls applicable to high-speed computers. For example, in February, 2000, the USDoC eliminated export controls on computers with speeds below 12,300 MTOPS to all countries with the exception of Iraq, Libya, North Korea,Cuba, Sudan and Syria.
CDMA (CODE DIVISION MULTIPLE ACCESS): A multiple access scheme whereby ground station uplinks access a satellite transponder using spread-spectrum modulations and orthogonal codes to avoid interfering with other transmissions using the same transponder. In contrast with the FDMA scheme which attempts to minimize the transmitted bandwidth, in this scheme all users transmit signals simultaneously across all of the dedicated multiple access channel. Receivers use a code corresponding to the transmission code to demodulate the signal or separate it from other signals on the channel.
MULTIPLEXING: A process in the transmission of RF (radio frequency) signals whereby a number of simultaneous transmissions can be accommodated on a single circuit.
NETWORK LAYER: (See OSI)
NOISE: 1. An undesired disturbance with the frequency band of interest; the summation of unwanted or disturbing energy introduced into a communication system form man-made and natural sources. 2. A disturbance that affects a signal and that may distort the information carried by the signal. 3. Random variations of one or more characteristics of any entity such as voltage, current or data. 4. A random signal of known statistical properties of amplitude, distribution and spectral density. 5. Loosely, any disturbance to interfere with the normal operation of a device or system. (GSA, 1996) (See EFFECTIVE NOISE TEMPERATURE)
NTSC (NATIONAL TELEVISION SYSTEMS COMMITTEE): The organization (RCA/NBC) which established the standards for the television broadcast system in the United States. A close approximation of this system was also adopted in Japan. The US and Japanese video systems are now referred to as ‘NTSC' systems. This video standard provides for a screen density of 525 scanlines per picture and operates at 60 cycles per second. This system was developed so that existing black and white (B/W) mono audio television receivers would be compatible with color stereo audio broadcasts.
OC-1: 51.84 Mb/s.See: STS-N (SYNCHRONOUS TRANSPORT SIGNAL -- LEVEL N) and SONET (SYNCHRONOUS OPTICAL NETWORK). (Carne, 1995)
OSA (OPEN SYSTEMS ARCHITECTURE): Permits transparent communication between cooperating machines. OSA is embodied in international standards referred to as the Open Systems Interconnection Reference Model (OSI).(Carne, 1995)
OSI (OPEN SYSTEMS INTERCONNECTION REFERENCE
MODEL): The reference model, also referred to as OSIRM, provides
a framework for communication that relies on standard non-proprietary
interconnection protocols (protocol stack). It consists of seven layers
that serve to decompose the complexity of information flow between cooperating
machines into consecutive steps that are substantially independent of
each other. (Carne, 1995) The seven OSI model layers
PACKET JITTER: (see PACKET LOSS)
PACKET LOSS: Packet Loss and Packet Jitter are types of degradation in the delivery stream of digital information which result from unfavorable network conditions such as extremely heavy or bursty traffic. Individual packets comprising the data stream can arrive early, late or out of sequence causing unstable video reception, pauses, the appearance of artifacts or the loss of the data altogether.
PAL (PHASE ALTERATION SYSTEM): A German developed color television standard which provides for a screen density of 625 scanlines per picture and which operates at 50 cycles per second. This system is common in the UK, much of Europe, Africa, Asia and Oceania and is incompatible with the North American and Japanese NTSC standards. The IntelSat satellite system often employs the PAL standard.
PERIGEE: The point in an elliptical satellite orbit where it is closest to the center of the earth (the opposite of APOGEE).
POP (POINT-OF-PRESENCE): In the field of telephony, POP refers to the facility interface between the Local Exchange Carrier (LEC) and the Inter-Exchange Carrier (IXC). A POP must be established on each trunk path that connects switches that belong to the local and long-distance carriers. On one side of the POP, the LEC is responsible for service; on the other side, the IXC is responsible (Carne, 1995)
PROGRESSIVE SCANNING: The method for tracing scan lines onto a computer monitor whereby all scanlines are presented in sequence 60 or more times per second. Unlike the NTSC video standard, there is no single standard for computer display. Most computer monitors will accept a wide range of screen resolutions and scanning rates utilizing multiscan technologies. (see also INTERLACED SCANNING)
QPSK (QUADRATURE PHASE SHIFT KEYING): Used to modulate digital information onto an RF carrier when satellite transponders are used to transmit MPEG 2 signals. Rather than using the amplitude or frequency of the carrier to convey the information, QPSK modulates the phase of the carrier signal. Depending on the data being modulated, the carrier is forced into one of four different phase states known as a symbol. The advantage of this method is that each symbol contains two data bits, thus doubling the potential amount of data that is transmitted over conventional amplitude or frequency modulation techniques. (Hewitt, 1999)
RAM (RANDOM ACCESS MEMORY): In computers, the main system memory, usually consisting of volatile memory (memory that loses its data when power is removed) solid-state chips. (Inglis & Luther, 1996)
RBOC (REGIONAL BELL OPERATING COMPANY): An independent common carrier that provides local exchange services within specific areas. The term originally referred to the local telephone service companies which were created as a result of the break-up of AT&T in 1984. The term now refers to all local exchange services. (Carne, 1995)
RF (RADIO FREQUENCY): Any frequency within the electromagnetic spectrum normally assiciated with radio wave propagation. Organizations such as the FCC and ITU have divided the radio frequency spectrum into subdivisions for management purposes. (GSA, 1996)
ROM (READ-ONLY MEMORY): In computers, nonvolatile memory (memory that retains its stored data when power is removed that stores permanent programs. ROM usually consists of solid-state chips. (Inglis & Luther, 1996)
SAR (SPECIFIC ABSORPTION RATE): Measures the maximum quantity of radiation absorbed by one Kilogram of tissue from a cell phone. Nokia, Motorola and Ericsson are working to establish international benchmark standards.
SDH (SYNCHRONOUS DIGITAL HIERARCHY): An international digital telecommunications network hierarchy which standardizes transmission around the bit rate of 51.84 megabits per second, which is also called STS-1. Multiples of this bit rate comprise higher bit rate streams. SONET is the American version of SDH. (Howe, 1999)
SECAM (SEQUENTIAL ENCODED COLOR AMPLITUDE MODULATION): A French developed color television standard which is common in Francophile countries and the former Soviet Union. The system operates with a screen density of 625 scanlines per picture and 50 cycles per second but inverts the signal making it incompatible with the PAL and NTSC video standards.
SONET (SYNCHRONOUS OPTICAL NETWORK): A synchronous digital network that employs optical fibers (exclusively). Each facility conforms to standards that include electrical and optical speeds, and frame formats.(Carne, 1995) SONET carries circuit-switched data in frames at speeds in multiples of 51.84 megabits per second. SDH is the international term for SONET. (Howe, 1999)
SPECTRUM: A short-hand reference to the ELECTROMAGNETIC SPECTRUM.
STS-N (SYNCHRONOUS TRANSPORT SIGNAL -- LEVEL
N): In SONET, a frame of N x 6480 bits that
is transported at N x 51.84 Mb/s. STS-N signals are created by interleaving
N STS-1 signals, octet-by-octet. For various reasons, the values of N
+ 3, 12, 24, 48 and 96 are preferred. The optical equivalent OF STS Levels
are expressed as OC-N levels.
T-1 LINE: A transmission medium with a transmission bit rate of 1.544 million bits per second (Mb/s), equivalent to the ISDN Primary Rate Interface (PRI) for the United States. The European T-1 transmission bit rate is 2.048 Mb/s.
TCP/IP (TRANSMISSION CONTROL PROTOCOL over
INTERNET PROTOCOL): The de facto standard Ethernet
protocols incorporated into 4.2BSD (Berkeley System Distribution-Version
4.2) Unix. TCP/IP was developed by DARPA
(Defense Advanced Research Projects Agency) for internetworking and encompass
both network layer and transport
layer protocols. While TCP and IP specify two protocols
at specific protocol layers, TCP/IP suite is
often used to refer to the entire DoD (Department of Defense) protocol
suite based upon these, including telnet (Remote Terminal Protocol), FTP
(File Transfer Protocol) and UDP (User Datagram Protocol). (Howe,
TCP (Transmission Control Protocol): The most common transport layer protocol used on the Ethernet and the Internet. TCP is built on top of IP (Internet Protocol) and is nearly always seen in the combination TCP/IP. It adds reliable communication, flow control, multiplexing and connection-oriented communication. It provides full-duplex, process-to-process connections.
TCP/IP SUITE: Members of the Internet community share a family of protocols called TCP/IP. The name is shorthand for a suite of protocols whose major members are TCP (transmission control protocol), a transmission layer procedure, and IP (internet protocol), a network layer procedure.
The five Layers of the TCP/IP suite of protocols are:
TDMA (TIME DIVISION MULTIPLE ACCESS): See MULTIPLE ACCESS.
TOKEN-RING LOCAL AREA NETWORK: A
computer LAN arbitration scheme in which conflicts
in the transmission of messages are avoided by the granting of "tokens"
which give permission to send. A station keeps the token while transmitting
a message, if it has a message to transmit, and then passes it on to the
next station. Often, "Token Ring" is used to refer to the IEEE 802.5 token
ring standard which is the most common type of token ring. (Howe,
TRANSPONDER: A combination receiving and transmitting antenna on a communications satellite. A frequency converter is also including in the transmit/receive package which converts the uplinked signal frequency to a transmission or downlink frequency.
TRANSPORT LAYER: (See OSI)
UNIX: An interactive time-sharing operating system (OS) invented in 1969 by Ken Thompson of Bell Labs. Twenty-two years after its invention, Unix had become the most widely used multi-user general-purpose operating system in the world. It was the first source-portable OS. Unix is now offered by many manufactures and is the subject of an international standardization effort with the Unix trademark being owned by X/Open. A Unix reference desk may be found at http://www.geek-girl.com/unix.html (Howe, 1999)
VGA (VIDEO GRAPHICS ADAPTER): A video display standard for computer monitors which evolved from the CGA (computer graphics adapter) and EGA (enhanced graphics adapter) standards of the early 1980's. In 1987, IBM introduced the PC-AT computer which adopted a new display standard called VGA which allowed computer monitors to show integrated lifelike pictures using multiple shades of color. This standard uses progressive line scanning.
WIDEBAND: The property of any communications facility, equipment, channel or system in which the range of frequencies used for transmission is greater than 0.1% of the midband frequency. Wideband is often used to distinguish it from "narrowband", where both terms are subjectively defined relative to the implied context. The term also applies to the property of a circuit that has a bandwidth wider than normal for the type of circuit, frequency of operation or type of modulation. Synonymous with Broadband. (GSA, 1996)
WIRELESS CABLE: A multichannel broadcast service where the multiple channels are transmitted with frequency division multiplex (see FDMA: FREQUENCY DIVISION MULTIPLE ACCESS) in a manner similar to cable television. (Inglis & Luther, 1996)
ZULU TIME: (See GMT-Greenwich Meridian Time)
IMTC (1999) International Multimedia Teleconferencing
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