active antenna: a receiving antenna with a built-in preamplifier
aliasing: distortion caused by using too low a sampling rate
anechoic chamber: an enclosure lined with material that absorbs electromagnetic radiation
antenna: a device to radiate or receive electromagnetic radiation at radio frequencies
apogee: the point farthest from earth in a satellite orbit
array: an antenna system composed of two or more simpler antenna elements
artificial satellite: a structure that orbits the earth and was built by humans (the moon is a natural satellite); usually referred to as just a satellite when the context is clear
asynchronous transfer mode (ATM): a versatile data-transmission system using 53-byte packets and designed to enable various kinds of data, including live audio and video, to be multiplexed
attenuation of free space: the reduction in signal strength due to spreading of the waves at a distance from the transmitter
audio frequency-shift keying (AFSK): data transmission by varying the frequency of an audio tone used to modulate a higher-frequency carrier See a Simulation
automatic request for retransmission (ARQ): an error-control system based on the repetition of data blocks that contain errors
B (bearer) channel: 64 kb/s ISDN channel used for voice or high-speed data
baseband: the band of frequencies occupied by an information signal before it modulates the carrier
baud rate: number of symbols transmitted per second in a data transmission scheme
beamwidth: the angle between the points on the major lobe of an antenna at which the radiated power density is one-half its maximum value
bent-pipe configuration: a satellite transponder design that receives signals and retransmits them at higher power and at a different frequency, without any other processing
bipolar code: a data code that uses both polarities of voltage or current
bit overhead: bits that do not carry the message, for example, those used for timing and error control
bit rate: number of bits transmitted per second
bit stuffing: addition of extra bits to a data block to avoid the accidental generation of a flag pattern
bit stuffing: synchronization of digital signals by adding extra bits to one signal
bridge: a network-to-network connection that passes only data addressed to a node in the other network
broadband ISDN (B-ISDN): variation of ISDN for use with higher data ratesbuncher: in a klystron, a cavity that velocity-modulates the electron beam
carrier: a signal that can be modulated by an information signal
carrier-to-noise ratio: the signal-to-noise ratio in a receiver at a point before the detector
catcher: in a klystron, a cavity that removes some of the energy from the electron beam and transfers it in the form of microwave energy to the output
cavity: a space in which microwaves can resonate by means of in-phase reflections from the walls
channel: a path for the transmission of signals
character code: a set of rules that translates alphanumeric characters into binary numbers
character set: see character code
circuit switching: a method of organizing a network in which a physical path is dedicated to communication between two nodes for the duration of the communication
circulator: a device with three or more ports that allows an input to one port to emerge only at the next port in order
codec: data coder and decoder combined into one integrated circuit
coding: conversion of an analog signal into logic ones and zeros for digital transmission
collector: in a klystron or traveling-wave tube, the element that receives the electron beam; in a conventional tube, this element is called the anode
combo chip: an integrated circuit that combines coding and decoding with data compression and expansion
companding: a combination of compression at the input to a system and expansion at its output
compression: amplification of a signal in such a way that there is less gain for higher-level input signals than for lower-level input signals
computer bulletin board: a computer set up for unattended operation, with one or more modems to enable remote users to read information and download and upload files
constellation diagram: a pattern showing the possible states of a modem on a plane where amplitude is represented by distance from the center of the pattern and phase angle by the angle made by an imaginary line from the center to a dot representing a particular state (with respect to a reference angle)
crossed-field tube: a microwave tube in which the electric and magnetic fields are at right angles
cyclic redundancy check (CRC): an error-detecting method in which the binary number corresponding to the group of bits to be checked is divided by a predetermined binary number, and the remainder is transmitted as a check
data communications equipment (DCE): a modem data link layer: the second layer of the OSI model, responsible for detecting and correcting errors within frames of data and providing the flags that indicate the beginning and end of frames
data terminal equipment (DTE): a terminal or computer that communicates via a modem
dBd: a measure of antenna gain: decibels with respect to a lossless half-way dipole
dBi: a measure of antenna gain: decibels with respect to an ideal isotropic radiator
DEC Network Architecture (DNA): a data-communications protocol created by Digital Equipment Company for its minicomputer products
decoding: conversion of a digital data stream to an analog stream
delta modulation: a digital pulse-modulation scheme that transmits changes in analog signal level, rather than the level itself
delta phase-shift keying (DPSK): a phase-shift keying system that uses the change in phase angle from one symbol to the next to avoid the need for a reference phase
dial-up line: a telephone connection via the public switched telephone network
dibit system: a data communication system that transmits two bits per symbol
dielectric strength: the magnitude of the electric field required to cause breakdown and arcing in a dielectric
diffraction: the deviation of a wave as it passes an obstacle or passes through a small aperture
digital signal processing (DSP): manipulation of analog signals by first digitizing them and then performing computations with the digits
dipole: any antenna consisting of a single conductor with zero current only at its two ends
directional coupler: a device that launches or receives a wave in a transmission line or waveguide, in one direction only
directivity: the ratio of the maximum to the average radiation intensity for an antenna
dispersion: variation of velocity as a function of frequency in a waveguide or medium
distortion: any undesirable change in an information signal
diversity: use of more than one frequency or transmission path, to improve system reliability in the presence of fading
domain: in a semiconductor, a concentration of charge
Doppler effect: the change in frequency that occurs when a wave reflects from a moving object
downlink: transmission of signals from a satellite to an earth station
ducting: a means of propagation in which the waves are confined within a refractive region of the troposphere or between such a region and the ground
dumb terminal: a terminal for a mainframe or mini-computer system that has a monitor, a keyboard, and the means to communicate keystrokes and sometimes cursor movements to the host computer while displaying data from the host; it does no computing itself
effective area: for a receiving antenna, the ratio of the available output power to the power density of the received wave
effective area: the area from which a receiving antenna can be considered to extract all the energy in an electromagnetic wave
effective isotropic radiated power (EIRP): the product of the power supplied to a transmitting antenna and the gain of the antenna with respect to an isotropic radiator
effective radiated power (ERP): the product of the power supplied to a transmitting antenna and the gain of the antenna with respect to a lossless half-wave dipole
electric field intensity (or strength): the ratio of the electric force on a charge to the charge, at a given point (units are volts per meter)
electronic mail (e-mail): a method whereby messages can be left for individual network users; messages are generally stored on the service in such a way that only the designated recipient can access them
element: in an antenna array, an individual conductor or group of conductors
energy per bit: energy received in the time taken to transmit one bit
expansion: amplification of a signal in such a way that there is more gain for higher-level input signals than for lower-level input signals
fading: variation in received field strength over time due to changes in propagation conditions
fallback: a transmission speed that is less than the maximum of which a modem is capable and that the modem resorts to when line conditions do not permit transmission at its maximum speed
Faraday rotation: the change indirection of polarization of signals passing through the ionosphere
far-field region: a distance far enough from an antenna that local inductive and capacitive effects are insignificant
fax modem: a device for the transmission and reception of facsimile documents; it may be used with a micro-computer or built into a stand-alone fax machine
fiber distributed data interface (FDDI): a 100 Mb/s LAN signaling standard intended for use with fiber optics but also used with coaxial cable
fiber in the loop (FITL): use of fiber-optic cable for telephone subscriber connections
fiber to the curb (FTTC): use of fiber for all of a telephone system except for the subscriber loop
figure of merit: for a microwave receiver, the ratio, expressed in decibels, of gain to noise temperature
file transfer protocol: a system to allow the transmission of binary data files between computers, incorporating some form of flow control and error control
file-transfer protocol (FTP): a program in the TCP/IP system that allows for the transfer of both binary and text files between computers with otherwise incompatible operating systems
flag: a bit or sequence of bits that indicates the beginning or end of a packet
flat-topped sampling: a sampling scheme that maintains the same amplitude for the duration of a given sample
flow control: a means of ensuring that a transmitter sends data only when the associated receiver is ready to receive it
foldover distortion: see aliasingforward error correction (FEC): an error-correcting system in which errors are corrected at the receiver using redundant transmitted data without using retransmission requests
Fourier series: a way of representing periodic functions as a series of sinusoids
frame relay: a packet-transport protocol, similar to X.25 but with less error correction, making it faster over low-error-rate channels
frame: a group of bits sent between framing signals in a bit-oriented synchronous communication system
framing bits: extra bits used in a digital signal to ensure synchronization of transmitter and receiver
free space: a vacuum that allows radio waves to propagate without any obstruction
freenet: a computer system, normally run by a non-profit organization, that provides modem access to the public at no charge (though donations are encouraged); a typical freenet has bulletin-board areas for local messages, and many have Internet access
frequency domain: a representation of a signal's power or amplitude as a function of frequency
frequency-shift keying (FSK): data transmission by shifting the transmitter frequency; two frequencies are generally used, one for mark (binary 1) and on for space (binary zero)
Fresnel lens: a lens that is stepped in order to reduce its size
Fresnel zone: a region near an object in which diffraction effects are significant
front-to-back ratio: the ratio between the radiation intensity in an antenna's direction of maximum radiation and the intensity at an angel of 150 to this direction
gateway: a device used to connect computers or computer networks running incompatible operating systems
geostationary orbit: an orbit in which a satellite appears to remain in one position as seen from the earth; a geostationary orbit is also geosynchronous
geosynchronous orbit: an orbit in which a satellite remains above the equator and has a period equal to one day. If the satellite moves in the same direction as the earth's rotation, the orbit is also geostationary . Since in practice all geosynchronous satellites are geostationary, the two terms are often used as synonyms.
ground plane: an artificial ground consisting of a conduction surface or an equivalent (such as wire mesh or a group of wires) at the base of a vertical antenna
ground wave: a vertically polarized electromagnetic wave that propagates along the surface of the earth
group velocity: the speed of transmission of a signal along a waveguide
Gunn device: a slab of N-type gallium arsenide that can operate as an oscillator or amplifier by means of domain information
header: a sequence of bits at the beginning of a packet, containing information about the type of packet and/or routing
helix: a spiral
hop: a single transmission path from transmitter to receiver
Huffman coding: a data-compression scheme that uses fewer bits to represent more frequently occurring characters or bit patterns and more bits to represent those that occur less frequently
hybrid tee: a combination of E-plane and H-plane tees
Hypertext Transport Protocol (HTTP): a data-transfer method that allows quick transitions by simply clicking on a highlighted word or picture with a mouse or moving a cursor to the highlighted word and pressing one key on the keyboard; the protocol determines the type of file and the way to display it, depending on the user's computer software
IMPATT diode: a junction device that can operate as an oscillator or amplifier, by means of avalanche breakdown
index of refraction: the ratio of the phase velocity of a wave in free space to that in the medium under consideration
initialization string: a series of commands transmitted to the modem whenever a communications software program is loaded
integrated services digital network (ISDN): a completely digital telephone system that can provide both voice and data by both high speed and low speed channels simultaneously
Internet: a worldwide public network of networks that connects a very wide variety of computers, applications, and users
ionosphere: the ionized region of the earth's atmosphere
isolator: a waveguide device that has low loss in one direction and high loss in the other
isotropic radiator: a hypothetical antenna having zero physical size and no loss and radiating equally in all directions
isotropic radiator: a hypothetical antenna that would radiate all the energy supplied to it, with equal intensity in all directions
magnetic field intensity (or strength): magnitude of the magnetic field vector (units are amperes per meter)
magnetron: a crossed-field microwave-tube oscillator in which electrons circle around the cathode under the influence of a magnetic field
main lobe: the lobe in the direction of maximum radiation
mark: a line condition corresponding to a binary one
maximum usable frequency (MUF): the highest frequency that will be returned by the ionosphere at a given point
metropolitan-area network (MAN): a network that encompasses a city and its environs
microstrip: a microwave transmission line constructed on a printed-circuit board, consisting of a single conductor on one side of the board and a ground plane on the other side
microwave: conventionally, electromagnetic radiation in the range above approximately 1 GHz
minor lobe: a lobe with less intensity than the main lobe
mobility: the speed of electron drift in a conductor or semiconductor
mode: in a waveguide, a specific configuration of electric and magnetic fields that allows a wave to propagate
modem: acronym for modulator-demodulator, a device used to allow digital data to be transmitted over an analog channel See a Simulation
modulation: the process by which some characteristic of a carrier is varied by an information signal
monopole: an antenna with a current null at one end and a maximum at the other, with no other nulls in between
multipath reception: a situation in which a signal arrives at a receiving antenna via two or more paths (usually one of these paths is direct from the transmitting antenna and the other(s) involves reflections)
multiplexing: the transmission of more than one information signal over a single channel
natural sampling: a sampling scheme in which the amplitude of a sample varies during the sample time to follow the amplitude of the sampled signal
near-field region: the region close to an antenna, where local inductive and capacitive effects predominate
negative logic: a logic system in which a low level represents logic one and a high level represents logic zero
network layer: the third layer of the OSI protocol model; the network layer sets up the path to transmit data between terminals and arranges data into packets
newsgroups: electronic bulletin boards devoted to a wide variety of subjects, accessible via the Internet
noise figure: ratio of the input and output signal-to-noise ratios for a device
noise temperature: equivalent temperature of a passive system having the same noise-power output as a given system
noise: any undesired disturbance that is superimposed on a signal and obscures its information content
normal: a line drawn perpendicular to the interface between two media
NRZ (non-return-to-zero) code: a data line code in which the voltage or current does not necessarily return to zero between bits
null modem: a cable used to make a direct connection between two devices using their serial ports; it is wired to simulate the presence of a link via modem between the devices
online service: a large computer that connects to many users simultaneously via modem and typically offers such services as electronic mail, databases, interest groups, and Internet access
Open Systems Interconnection (OSI) model: a system for organizing data-transmission protocols developed by the International Standards Organization (ISO) See a Simulation
orbital satellite: any artificial satellite that is not geostationary
packet switching: a way of organizing a network so that small blocks of data are routed individually from source to destination
parallel transmission: simultaneous transmission of multiple data bits using several channels
path loss: the ratio between the signal appearing at the transmitting antenna terminals and that at the receiving antenna terminals
perigee: the point closest to earth in a satellite orbit
phase velocity: the apparent speed of propagation along a waveguide based on the distance between wavefronts along the walls of the guide
phase-shift keying (PSK): a means of transmitting data by shifting the phase angle of the transmitted signal
phi: in an antenna pattern, the Greek letter phi denotes the angle in the horizontal plane, from the x axis toward the y axis
photon: a quantum of electromagnetic radiation
physical layer: the lowest level of the OSI protocol model, dealing with matters such as voltage and current levels
PIN diode: a three-layer diode (P-intrinsic-N) that can be used as a switch and an attenuator at microwave frequencies
polarization: the direction of the electric field vector of an electromagnetic wave
positive logic: a logic system in which a high level represents logic one and a low level represents logic zero
power density: the power flowing through a unit cross-sectional area normal to the direction of travel of an electromagnetic wave
precession: in a ferrite, rotation of the axis of rotation of the electrons
private branch exchange (PBX): a small telephone switch operated independently of the public network, to which it appears as one or a number of subscriber lines
propagation time: the time taken for a signal to travel through space from transmitter to receiver
propagation: the process by which waves travel through a medium
protocol: a formal set of conventions governing the format and timing of messages
pulse-amplitude modulation (PAM): an analog pulse-modulation scheme in which one pulse is transmitted per sample of the original signal. The amplitude of each transmitted pulse is proportional to the amplitude of the original signal at the corresponding sampling time
pulse-code modulation (PCM): a digital pulse-modulation scheme in which sample amplitudes are expressed as binary numbers, which are transmitted as a series of pulses
pulse-duration modulation (PDM): an analog pulse-modulation scheme in which one pulse is transmitted per sample of the original signal; the duration of each transmitted pulse is proportional to the amplitude of the original signal at the corresponding sampling time
pulse-position modulation (PPM): an analog pulse-modulation scheme in which one pulse is transmitted per sample of the original signal; the time at which each pulse is transmitted is a function of the amplitude of the original signal at the corresponding sampling time
pulse-width modulation (PWM): see pulse-duration modulation
pump laser: a laser used as an energy source to excite electrons into a higher energy state
quadrature amplitude modulation (QAM): a means of transmitting data by shifting both the amplitude and the phase of the transmitted signal See a Simulation
quadrature phase-shift keying (QPSK): phase-shift keying that employs four different phases and allows two bits of information to be transmitted simultaneously
quantizing: the division of a continuous range of values (for example, the possible voltages of an analog signal) into a number of discrete values
radar cross section: the equivalent size of a radar target, in terms of a perfectly conducting flat plate oriented toward the receiver
radial: in a monopole antenna, a wire extending along the surface of the ground or just below it, away from the antenna (a set of radials is used to improve the effective conductivity of the ground)
radiation resistance: equivalent resistance at the feed-point corresponding to the radiation of energy by an antenna
receiver: device to extract the information signal from the signal propagating along a channel
regenerative repeater: a device that decodes and recodes a digital signal as well as amplifying it
repeater: a device used to extend a network or other digital communication system by regenerating bits and restoring voltage levels and timings to their original values
repeater: a receiver-transmitter combination that amplifies and retransmits a signal
rise time: the time required for the voltage level at the beginning of a pulse to increase from 10% to 90% of its maximum value
router: a device used to interconnect networks; routers operate at level three, the network layer, of the OSI protocol and can change packets from one protocol to another
run-length encoding: a data-compression scheme that replaces repeated characters or bit patterns with a code indicating the character or pattern and the number of repetitions
RZ (return-to-zero) code: a line code in which the voltage or current returns to zero at the end of each bit period
sample-and-hold circuit: a device that detects the amplitude of an input signal at a particular time called the sampling time and maintains its output at or near that amplitude until the next sampling time
serial transmission: data transmission using only one channel
sidelobe: a minor lobe at an angle of approximately 90 to the main lobe
signal-to-noise ratio (S/N): ratio of signal to noise power at a given point in a system
Simple Mail Transfer Protocol (SMTP): the part of the TCP/IP suite that allows for e-mail over the Internet and similar networks by specifying the control messages used in mail transfer
sky wave: an electromagnetic wave that is returned to earth by the ionosphere
slope overload: in delta modulation, the inability of the output to keep up with a rapidly changing input signal
slow-wave structure: in a microwave tube, any device that causes a wave to propagate at less than the speed of light, so that the electron beam and the wave move at approximately the same speed
solitons: single pulses that can travel through a medium with no dispersion
space switching: a method of telephone switching that provides a separate physical path for each cell
space wave: an electromagnetic wave that propagates directly from the transmitting to the receiving antenna
space: a line condition corresponding to a binary zero
spectrum analyzer: device for displaying signals in the frequency domain
start bit: in asynchronous communication, this bit alerts the receiver to the beginning of a transmitted character by changing the line from the mark to the space condition
station-keeping: the process of adjusting the orbit of a geostationary satellite so that it appears to remain stationary above a point on earth
stop bit: in asynchronous communication, this bit marks the end of a transmitted character and returns the line to the mark condition
store-and-forward network: a network in which nodes receive a packet of data from the source or a node closer to the source and then transmit it to the destination or a node closer to the destination
stripline: a microwave transmission line that consists of a conductor inside a circuit board, working against two ground planes, one on the top and one on the bottom of the board
symbol: a transmitted signal that can have two or more possible states
synchronous digital hierarchy (SDH): the European standard for synchronous transmission over a fiber-optic network
Systems Network Architecture (SNA): an IBM system for transferring data between IBM mainframes and between IBM mainframes and other computers
target: in radar, the object whose range, direction, and/or velocity is to be measured
Telenet: a system to allow users to log on to a distant host by emulating a dumb terminal; part of the TCP/IP suite
terminal adaptor (TA): a device that allows terminal equipment type 2 to be used with the ISDN system
terminal equipment type 1 (TE1): digital telephones and data terminals that are expressly designed for use with the ISDN system
terminal equipment type 2 (TE2): analog telephones and data modems that are designed to be used with a conventional analog telephone system
terrestrial propagation: propagation along or near the surface of the earth
theta: in an antenna pattern, the Greek letter theta refers to the angle from the horizontal (x-y) plane toward the zenith, represented by the z axis
time domain: representation of a signal's amplitude as a function of time
time switching: a method of telephone switching in which a call is moved from one time slot to another on a single time-division-multiplexed physical channel
topographic map: a map showing surface features, including the elevation of the terrain
track: to keep an antenna pointed at a satellite as it moves with respect to the antenna
training sequence: a series of tones transmitted by a modem to allow the automatic adjustment of line equalization
unipolar code: a line code in which the polarity of the voltage or the direction of the current remains the same at all times
Universal Asynchronous Receiver-Transmitter (UART): a device, generally an integrated circuit, that converts from parallel to serial format when transmitting and from serial to parallel format when receiving; it also adds start, stop, and parity bits and checks for errors
uplink: transmission signals from an earth station to a satellite
virtual circuit: a link between computers in which each recognizes a software connection to the other; the physical connection is not continuous but consists of packets routed as transmitted
Any opinions, findings and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect those of the National Science Foundation.
This material is based on work
supported by the
National Science Foundation
under Grant Number DUE 0302548.
