A/D Converter
Analog-to-digital Converter. Used to convert an analog signal into digital  format.

Amplitude Modulation (AM)
A form of transmission in which the level of constant-frequency carrier is changed
to determine the encoded information.

Analog
Any continuous and variable quantity of information.

Analog Transmission
A transmission method by which the signal varies continuously in sympathy with
the information. Most commonly used to transmit  electrical voltages representing
variations in sound levels.

Answer Modem
The modem that has been designated to answer a call initiated by an originate
modem.

Asynchronous (Start/Stop Transmission)
A transmission method by which transmitting and receiving equipment are not
permanently synchronised and variable idle time exists between succeeding
characters. The beginning  and end of each character are indicated by start and
stop bits.

Attenuation Distortion
A parameter used by the PSTN to specify the limits of amplitude variations in the
passband of the telephone lines. Ideally, a flat frequency response is desired.
 
 

   B

Bandwidth
The data-carrying capacity of a medium expressed as the difference between the
highest and the lowest frequencies which may be transmitted.

Baseband
The frequency band occupied by the aggregate of the transmitted signals when first
used to modulate the carrier. Also used to describe transmissions which are not
modulated on a carrier.

Broadband
A network which can support a wide range of frequencies, typically  employing
frequency division multiplexing on a coaxial cable.

Broadcast
A transmission path with several stations directly attached, each receiving the same data at the same time (apart from propagation delay).
 
 

  C

Carrier
A continuous frequency which is modulated to transmit information.

Cell
A geographical area that services cellular mobile telephones.

Cell Site
A base station used in cellular telephony that is equipped to transmit and receive
calls from any mobile unit within the cell.

Cellular Radio
A low-power radio transmission system with a cellular network of base stations
which may be used by stationary or mobile users for voice or data communication
linking into the public switched telephone network.

Central Office
The local telephone switching exchange.

Channel
A path for the transmission of information which may be physical or logical.

Circuit
A transmission medium linking two or more electronic devices.

Circuit Switching
A technique which makes a temporary connection between two or more stations
or telephone users on demand giving the callers exclusive use of the circuit until the
connection is released.

Cluster Controller
A device which provides a remote communications capability to several terminals
located in a cluster.

Coaxial Cable
A transmission medium consisting of a central conducting wire surrounded by an
insulator and encased in a conductive tubular sheath.

Concentrator
A communications device which shares a number of transmission channels
between a large number of data sources.

Crossbar Switch
An electromechanical switch used in older central office switching systems. The
crossbar switch consists of a lattice of horizontal and vertical crossbars with contact
points that make and break contact for telephone calls.

Crosstalk
Unwanted coupling of electrical signals from one transmission medium to another
adjacent medium.
 
 

  D

Data
A digital repersentation of information.

Demodulation
The extraction of information from a modulated carrier signal.

Dial Tone
An audible tone acknowledging connection to the central office when a caller goes
off-hook.

Dial-up
A circuit switching technique used to establish a temporary connection through a
public switched telephone network.

Digital
A communications procedure in which information is encoded as eithera discrete
binary 1 or a binary 0, as opposed to a continuous analog representation.

Distrotion
Corruption of a signal by modifying the waveform.

DTMF
Dual Tone Multi-Frequency. The touch-tone signalling method employed in
telephony throughout the USA. Each key depression generates two audible tones
which are transmitted to the exchange.
 
 

  E

Echo
A reflected signal. In long-distance transmission over the PSTN, impedance
mismatches can cause a signal to be reflected back to the caller. This echo is
especially annoying when the round-trip delay time exceeds about 50 ms. This
occurs at distances beyond 1500 miles.

Equalizer
A circuit used by the PSTN to equalize any variations in gain or phase across the
passband of the telephone lines.

Equalisation
A function performed by high-speed modems which compensates for the
differences in attenuation at different frequencies in a telephone line.
 
 

F

Fibre Optics
The transmission method which uses modulated light transmitted down glass or
plastic optical fibres.

Filter
An electronic or optical device which removes energy of unwanted frequencies
from a transmission system.

FM
Frequency Modulation. A method for encoding a carrier by changing the frequency
according to the data being transmitted.

Frequency
The number of repetitions per unit time of a periodic waveform. The number of
cycles per second for an electromagnetic waveform is expressed in Hertz (Hz,
kHz, MHz or GHz).

Full-Duplex
A mode of operation on a data link in which simultaneous transmission is possible
in both directions between devices at both ends of the link.
 
 

  G

Gateway
A communication link between two heterogeneous computers or networks with the
ability to convert from one protocol set to another.
 
 

  H

Half-Duplex
A mode of operation on a data link in which transmission can take place in both
directions but not simultaneously. Transmission direction is alternately switched to
permit two-way operation.

Handoff
When a mobile unit employing an active cellular telephone approaches the
perimeter of the cell it is travelling in, its signal strength diminishes. As it enters a
new cell, the mobile telephone switching office (MTSO) automatically assigns it a
new channel within the new cell. This is called a handoff. Handoffs are transparent
to the user.
 
 

  I

ISDN
Integrated Services Digital Network. A network which incorporates a number of
digital transmission services including speech, data and facsimile. CCITT standards
have been recommended for interfaces and operating procedures.
 
 

  J
 
 

K
 
 
 

  L

LAN
Local Area Network. A short-distance data communications network using
dedicated cables serving users within a building or group of buildings up to about
5 km in diameter.

Laser
Light amplification by stimulated emission of radiation. A device which produces
coherent single-wavelength light; use as a light source in high-performance fibre
optic systems.

LED
Light-emiting diode. A component which converts electrical energy into light.
Commonly employed as the light source in multimode fibre optic systems.

Line of sight
An open-air transmission system in which the path between the transmitter and
receiver must be unobstructed. Includes microwave, millimetre wavelength, optical
and infra-red transmission systems.
 
 

  M

Modem
Modulator/demodulator. A device which converts (by modulation of a carrier)
digital signals into analog form for transmission over analog transmission facilities.
At the destination the carrier is demodulated to convert the analog signal into digital
form.

Modulation
The systematic alteration of amplitude, frequency or phase of an analog carrier to
encode information for transmission.

MTSO
Mobile Telephone Switching Office. In cellular telephony, the MTSO coordinates
all mobile calls between cell sites and the central office.

Multidrop
A network configuration in which a master station communicates successively with
multiple end points by polling (inviting to transmit) each end point in turn.

Multiplexer (MUX)
A device which combines several signals to share a common physical transmission
medium. Multiplexed signals at the other end of the link. Common forms of
multiplexing are time division multiplexing (TDM), frequency division multiplexing
(FDM), wavelength division multiplexing (WDM), and code division multiplexing
(CDM).

Multipoint
A network configuration in which one communications channel is connected to
more than two stations. Multidrop operation is used to ensure that only one station
transmits at a time.
 
 

  N

Network
An interconnected group of points, nodes or stations linked by communications
channels or the assembly of equipment through which connections are made
between data stations.

NRZ
Non-Return to Zero. A binary encoding techniquein which binary 1 and 0 symbols
are represented by high and low voltages with no return to the 0 reference voltage
between bits.
 
 

  O
 
 

  P

PAM
Pulse Amplitude Modulation. A modulaiton technique that produces a series of
pulses whose amplitudes are in proportion to the modulating voltage at the time of
the sample.

Ping-Pong
A pseudo full-duplex mode of transmission. Full-duplex operation is simulated
between two modems by buffering each modem's data and rapidly turning their
modulated carriers on and off in a successive fashion using flow control
procedures.

Point-to-Point
A circuit which connects only two nodes directly, without intermediate nodes.

PSTN
Public Switched Telephone Network. Otherwise known as the dial-up telephone
network.
 
 

  Q

Quatization
The division of a signal's dynamic range into discrete numerically encoded binary
values.
 
 

  R

Real-time
A form of processing which is concurrent with a physical process and in which the results are used to influence the process while it is occurring.

Repeater
Equipment which receives a signal and amplifies, reconstructs or retimes it before retransmitting it to permit transmission over greater distances.

Rotary
An option offered by telephone companies which allows a block of dial-up telephone lines to be accessed by dialling a single number.

Routing
The selection of a suitable path through a network.
 
 

  S

Sidetone
The small amount of signal fed back from a telephone set's transmitter to its
receiver that allows a person speaking into the handset to hear himself or herself.

Signal-to-Noise Ratio (SNR)
The ratio between the magnitude of the transmitted signal and the noise on a
communications channel. Commonly measured in dB.

Simplex
Transmission in one direction only.

Single-Sideband Transmission (SSB)
A transmission in which one modulation sideband of the carrier is transmitted while the carrier frequency and the other sideband are suppressed to minimise the bandwidth required.
 
 

  T

Telegraphy
An early data transmission technique which used teleprinters to send data at speeds up to 50 bit/s using bipolar DC current signalling.

Telecommunications
Long-distance communications via a conglomeration of information-sharing networks tied together.

Telephony
Voice communication.

Teletex
A worlwide switched message exchange service operating at a higher speed than Telex. Intended to replace Telex.

Transceiver
A device capable of transmission and reception.

Trunk
A circuit interconnecting two telephone switching centers.
 
 

  U

Uplink
The data link conveying information froms the earth station to a geosynchronous communications satellite. The reverse direction link is called the downlink.
 
 

  V

Voice Frequency (VF)
Analog signals within the frequency range commonly employed to transmit speech (typically 300 Hz to 3.4 kHz).
 
 

  W

Wavelength
The distance between two successive peaks (or troughs) of a sinusoidal waveform.
 
 

  X
 
 
 

  Y
 
 

  Z
 
 
 
 
 
 
 
 
 
 
 

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