GLOSSARY

GLOSSARY

Some of the definitions in this glossary are from the American National Standard

Dictionary of Information Technology, ANSI Standard X3.172, 1995; these are

marked with an asterisk.

Abstract Syntax Notation One (ASN.1) A formal language used to define syntax.

In the case of SNMP, ASN.l notation is used to define the format of SNMP

protocol data units and of objects.

Aloha A medium access control technique for multiple-access transmission

media. A station transmits whenever it has data to send. Unacknowledged

transmissions are repeated.

Amplitude The size or magnitude of a voltage or current waveform.

Amplitude modulation* A form of modulation in which the amplitude of a carrier

wave is varied in accordance with some characteristic of the modulating signal.

Amplitude-shift keying Modulation in which the two binary values are represented

by two different amplitudes of the carrier frequency.

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.

Analog signal A continuously varying electromagnetic wave that may be propagated

over a variety of media.

Analog transmission The transmission of analog signals without regard to content.

The signal may be amplified, but there is no intermediate attempt to recover

the data from the signal.

Angle modulation* Modulation in which the angle of a sine wave carrier is varied.

Phase and frequency modulation are particular forms of angle modulation.

Application layer Layer 7 of the OSI model. This layer determines the interface

of the system with the user.

Asymmetric encryption A form of cryptosystem in which encryption and decryption

are performed using two different keys, one of which is referred to as the

public key and one of which is referred to as the private key. Also known as

public-key encryption.

Asynchronous transfer mode (ATM) A form of packet transmission using

fixed-size packets, called cells. ATM is the data transfer interface for B-ISDN.

Unlike X.25, ATM does not provide error control and flow control

mechanisms.

Asynchronous transmission Transmission in which each information character

is individually synchronized (usually by the use of start elements and stop

elements).

Attenuation A decrease in magnitude of current, voltage, or power of a signal in

transmission between points.

ATM adaptation layer (AAL) The layer that maps information transfer protocols

onto ATM.

Authentication';' A process used to verify the integrity of transmitted data, especially

a message.

Automatic repeat request A feature that automatically initiates a request for

retransmission when an error in transmission is detected.

Balanced transmission A transmission mode in which signals are transmitted as a

current that travels down one conductor and returns on the other. For digital

signals, this technique is known as differential signaling, with the binary value

depending on the voltage difference.

Bandlimited signal A signal, all of whose energy is contained within a finite frequency

range.

Bandwidth* The difference between the limiting frequencies of a continuous fi-equency

spectrum.

Baseband Transmission of signals without modulation. In a baseband local network,

digital signals (Is and 0s) are inserted directly onto the cable as voltage

pulses. The entire spectrum of the cable is consumed by the signal. This

scheme does not allow frequency-division multiplexing.

Bit stuffing The insertion of extra bits into a data stream to avoid the appearance

of unintended control sequences.

Bridge'" functional unit that interconnects two local area networks (LANs) that

use the same logical link control protocol but may use different medium

access control protocols.

Broadband The use of coaxial cable for providing data transfer by means of analog

(radio-frequency) signals. Digital signals are passed through a modem and

transmitted over one of the frequency bands of the cable.

Broadcast The simultaneous transmission of data to a number of stations.

Broadcast address An address that designates all entities within a domain (e.g.,

network, internet).

Broadcast communication network A communication network in which a transmission

from one station is broadcast to and received by all other stations.

Bus:': One or more conductors that serve as a common connection for a related

group of devices.

Carrier A continuous frequency capable of being modulated or impressed with a

second (information carrying) signal.

CATV Community Antenna Television. CATV cable is used for broadband local

networks, and broadcast TV distribution.

Cell relay The packet-switching mechanism used for the fixed-size packets called

cells. ATM is based on cell relay technology.

Checksum An error-detecting code based on a summation operation performed

on the bits to be checked.

Ciphertext The output of an encryption algorithm; the encrypted form of a message

or data.

Circuit switching A method of communicating in which a dedicated communications

path is established between two devices through one or more intermediate

switching nodes. Unlike packet switching, digital data are sent as a continuous

stream of bits. Bandwidth is guaranteed, and delay is essentially limited

to propagation time. The telephone system uses circuit switching.

Coaxial cable A cable consisting of one conductor, usually a small copper tube or

wire, within and insulated from another conductor of larger diameter, usually

copper tubing or copper braid.

Codec (Coder-decoder) Transforms analog data into a digital bit stream (coder),

and digital signals into analog data (decoder).

Collision A condition in which two packets are being transmitted over a medium

at the same time. Their interference makes both unintelligible.

Common carrier In the United States, companies that furnish communication services

to the public. The usual connotation is for long-distance telecommunications

services. Common carriers are subject to regulation by federal and

state regulatory commissions.

Common channel signaling Technique in which network control signals (e.g., call

request) are separated from the associated voice or data path by placing the

signaling from a group of voice or data paths on a separate channel dedicated

to signaling only.

Communications architecture The hardware and software structure that implements

the communications function.

Conlmunication network A collection of interconnected functional units that provides

a data communications service among stations attached to the network.

Connectionless data transfer A protocol for exchanging data in an unplanned

fashion and without prior coordination (e.g., datagram).

Connection-oriented data transfer A protocol for exchanging data in which a logical

connection is established between the endpoints (e.g., virtual circuit).

Contention The condition when two or more stations attempt to use the same

channel at the same time.

Conventional encryption Symmetric encryption.

Crosstalk'!' The phenomenon in which a signal transmitted on one circuit or channel

of a transmission system creates an undesired effect in another circuit or

channel.

CSMA (Carrier Sense Multiple Access) A medium access control technique for

multiple-access transmission media. A station wishing to transmit first senses

the medium and transmits only if the medium is idle.

CSMNCD (Carrier Sense ultiple Access with collision detection) A refinement

of CSMA in which a station ceases transmission if it detects a collision.

Current-mode transmission A transmission mode in which the transmitter alternately

applies current to each of two conductors in a twisted pair to represent

logic 1 or 0. The total current is constant and always in the same direction.

Cyclic redundancy check An error detecting code in which the code is the remainder

resulting from dividing the bits to be checked by a predetermined binary

number.

Data circuit-terminating equipment (DCE) In a data station, the equipment that

provides the signal conversion and coding between the data terminal equipment

(DTE) and the line. The DCE may be separate equipment, or an integral

part of the DTE, or of intermediate equipment. The DCE may perform

other functions that are normally performed at the network end of the line.

Datagram* In packet switching, a packet, independent of other packets, that carries

information sufficient for routing from the originating data terminal

equipment (DTE) to the destination DTE without the necessity of establishing

a connection between the DTEs and the network.

Data link layer'!' In OSI, the layer that provides service to transfer data between

network layer entities, usually in adjacent nodes. The data link layer detects

and possibly corrects errors that may occur in the physical layer.

Data terminal equipment (DTE)'x Equipment consisting of digital end instruments

that convert the user information into data signals for transmission, or

reconvert the received data signals into user information.

Decibel A measure of the relative strength of two signals. The number of decibels

is 10 times the log of the ratio of the power of two signals, or 20 times the log

of the ratio of the voltage of two signals.

Decryption The translation of encrypted text or data (called ciphertext) into original

text or data (called plaintext). Also called deciphering.

Delay distortion Distortion of a signal occurring when the propagation delay for

the transmission medium is not constant over the frequency range of the

signal.

Demand-assignment multiple access A technique for allocating satellite capacity,

based on either FDM or TDM, in which capacity is granted on demand.

Differential encoding A means of encoding digital data on a digital signal such

that the binary value is determined by a signal change rather than a signal

level.

Digital data Data consisting of a sequence of discrete elements.

Digital signal A discrete or discontinuous signal, such as voltage pulses.

Digital signature An authentication mechanism that enables the creator of a message

to attach a code that acts as a signature. The signature guarantees the

source and integrity of the message.

Digital switch A star-topology local network. Usually refers to a system that handles

only data, but not voice.

Digital transmission The transmission of digital data, using either an analog or

digital signal, in which the digital data are recovered and repeated at intermediate

points to reduce the effects of noise.

Digitizesk To convert an analog signal to a digital signal.

Encapsulation The addition of control information by a protocol entity to data

obtained from a protocol user.

Encrypt'To convert plain text or data into unintelligible form by the use of a

code in such a manner that reconversion to the original form is possible.

Error detecting code* A code in which each expression conforms to specific rules

of construction, so that if certain errors occur in an expression, the resulting

expression will not conform to the rules of construction, and thus the presence

of the errors is detected.

Error rate* The ratio of the number of data units in error to the total number of

data units.

Flow control The function performed by a receiving entity to limit the amount or

rate of data that is sent by a transmitting entity.

Frame A group of bits that includes data plus one or more addresses, and other

protocol control information. Generally refers to a link layer (OSI layer 2)

protocol data unit.

Frame check sequence An error-detecting code inserted as a field in a block of

data to be transmitted. The code serves to check for errors upon reception of

the data.

Frame relay A form of packet switching based on the use of variable-length, linklayer

frames. There is no network layer, and many of the basic functions have

been streamlined or eliminated to provide for greater throughput.

Frequency Rate of signal oscillation in hertz.

Frequency-division multiplexing The division of a transmission facility into two or

more channels by splitting the frequency band transmitted by the facility into

narrower bands, each of which is used to constitute a distinct channel.

Frequency modulation Modulation in which the frequency of an alternating current

is the characteristic varied.

Frequency-shift keying Modulation in which the two binary values are represented

by two different frequencies near the carrier frequency.

Full-duplex transmission Data transmission in both directions at the same time.

Half-duplex transmission Data transmission in either direction, one direction at a

time.

Hash function A function that maps a variable-length data block or message into

a fixed-length value called a hash code. The function is designed in such a way

that, when protected, it provides an authenticator to the data or message. Also

referred to as a message digest.

HDLC (high-level data link control) A very common, bit-oriented data link prot

o c o l - ( ~la~ye~r 2) issued by ISO. similar protocols are LAPB, L A P D , ~ ~

LLC.

Header System-defined control information that precedes user data.

lnlpulse noise A high-amplitude, short-duration noise pulse.

Integrated services digital network A planned worldwide telecommunication service

that will use digital transmission and switching technology to support

voice and digital data communication.

lntermediate system (IS) A device attached to two or more subnetworks in an

internet and that performs routing and relaying of data between end systems.

Examples of intermediate systems are bridges and routers.

Intermodulation noise Noise due to the nonlinear combination of signals of different

frequencies.

Internetwork A collection of packet-switching and broadcast networks that are

connected together via routers.

lnternet protocol An internetworking protocol that provides connectionless service

across multiple packet-switching networks.

Internetworking Communication among devices across multiple networks.

Layer'" A group of services, functions, and protocols that is complete from a conceptual

point of view, that is one out of a set of hierarchically arranged groups,

and that extends across all systems that conform to the network architecture.

Local area network A communication network that provides interconnection of a

variety of data communicating devices within a small area.

Local loop Transmission path, generally twisted pair, between the individual subscriber

and the nearest switching center of the public telecommunications network.

Longitudinal redundancy check The use of a set of parity bits for a block of characters,

such that there is a parity bit for each bit position in the characters.

Manchester encoding A digital signaling technique in which there is a transition in

the middle of each bit time. A 1 is encoded with a high level during the first

half of the bit time; a 0 is encoded with a low level during the first half of the

bit time.

Medium access control (MAC) For broadcast networks, the method of determining

which device has access to the transmission medium at any time.

CSMAICD and token are common access methods.

Microwave Electromagnetic waves in the frequency range of about 2 to 40 GHz.

Modem (Modulator/Demodulator) Transforms a digital bit stream into an analog

signal (modulator), and vice versa (demodulator).

Modulation* The process, or result of the process, of varying certain characteristics

of a signal, called a carrier, in accordance with a message signal.

Multicast address An address that designates a group of entities within a domain

(e.g., network, internet).

Multiplexing In data transmission, a function that permits two or more data

sources to share a common transmission medium such that each data source

has its own channel.

Multipoint A configuration in which more than two stations share a transmission

path.

Network layer Layer 3 of the OSI model. Responsible for routing data through a

communication network.

Network terminating equipment Grouping of ISDN functions at the boundary

between the ISDN and the subscriber.

Noise Unwanted signals that combine with and, hence, distort the signal intended

for transmission and reception.

Nonreturn to zero A digital signaling technique in which the signal is at a constant

level for the duration of a bit time.

Octet A group of eight bits, usually operated upon as an entity.

Open systems interconnection (OSI) reference model A model of communications

between cooperating devices. It defines a seven-layer architecture of

communication functions.

Optical fiber A thin filament of glass or other transparent material, through which

a signal-encoded light beam may be transmitted by means of total internal

reflection.

Packet A group of bits that include data plus control information. Generally

refers to a network layer (OSI layer 3) protocol data unit.

Packet switching A method of transmitting messages through a communication

network, in which long messages are subdivided into short packets. The packets

are then transmitted as in message switching.

Parity bit4: A check bit appended to an array of binary digits to make the sum of

all the binary digits, including the check bit, always odd or always even.

PBX Private branch exchange. A telephone exchange on the user's premises. Provides

a switching facility for telephones on extension lines within the building

as well as access to the public telephone network.

Phase The relative position in time within a single period of a signal.

Phase n~odulation Modulation in which the phase angle of a carrier is the characteristic

varied.

Phase-shift keying Modulation in which the phase of the carrier signal is shifted

to represent digital data.

Physical layer Layer 1 of the OSI model. Concerned with the electrical, mechanical,

and timing aspects of signal transmission over a medium.

Piggybacking The inclusion of an acknowledgment to a previously received

packet in an outgoing data packet.

Plaintext The input to an encryption function or the output of a decryption

function.

Point-to-point A configuration in which two stations share a transmission path.

Poll and select The process by which a primary station invites secondary stations,

one at a time, to transmit (poll), and by which a primary station requests that

a secondary receive data (select).

Presentation layer:% Layer 6 of the OSI model. Provides for the selection of a common

syntax for representing data and for transformation of application data

into and from the common syntax.

Private key One of the two keys used in an asymmetric encryption system. For

secure communication, the private key should only be known to its creator.

Propagation delay The delay between the time a signal enters a channel and the

time it is received.

Protocol A set of rules that govern the operation of functional units to achieve

communication.

Protocol control inforn~ation:~I nformation exchanged between entities of a given

layer, via the service provided by the next lower layer, to coordinate their joint

operation.

Protocol data unit (PDU)" A set of data specified in a protocol of a given layer

and consisting of protocol control information of that layer, and possibly user

data of that layer.

Public data network A government-controlled or national-monopoly packetswitched

network. This service is publicly available to data processing users.

Public key One of the two keys used in an asymmetric encryption system. The

public key is made public, to be used in conjunction with a corresponding private

key.

Public-key encryption Asymmetric encryption.

Pulse code modulation A process in which a signal is sampled, and the magnitude

of each sample with respect to a fixed reference is quantized and converted by

coding to a digital signal.

Residual error rate The error rate remaining after attempts at correction are

made.

Ring A local-network topology in which stations are attached to repeaters connected

in a closed loop. Data are transmitted in one direction around the ring,

and can be read by all attached stations.

Router An internetworking device that connects two computer networks. It

makes use of an internet protocol and assumes that all of the attached devices

on the networks use the same communications architecture and protocols. A

router operates at OSI layer 3.

Routing The determination of a path that a data unit (frame, packet, message)

will traverse from source to destination.

Service access point A means of identifying a user of the services of a protocol

entity. A protocol entity provides one or more SAPS for use by higher-level

entities.

Session layer Layer 5 of the OSI model. Manages a logical connection (session)

between two communicating processes or applications.

Simplex transmission Data transmission in one preassigned direction only.

Sliding-window technique A method of flow control in which a transmitting station

may send numbered packets within a window of numbers. The window

changes dynamically to allow additional packets to be sent.

Space-division switching A circuit-switching technique in which each connection

through the switch takes a physically separate and dedicated path.

Spectrum Refers to an absolute range of frequencies. For example, the spectrum

of CATV cable is now about 5 to 400 MHz.

Star A topology in which all stations are connected to a central switch. Two stations

communicate via circuit switching.

Statistical time-division multiplexing A method of TDM in which time slots on a

shared transmission line are allocated to 110 channels on demand.

Stop-and-wait A flow control protocol in which the sender transmits a block of

data and then awaits an acknowledgment before transmitting the next block.

Subnetwork Refers to a constituent network of an internet; this avoids ambiguity

because the entire internet, from a user's point of view, is a single network.

Switched communication network A communication network consisting of a network

of nodes connected by point-to-point links. Data are transmitted from

source to destination through intermediate nodes.

Symmetric encryption A form of cryptosystem in which encryption and decryption

are performed using the same key. Also known as conventional encryption.

Synchronous time-division multiplexing A method of TDM in which time slots on

a shared transmission line are assigned to I10 channels on a fixed, predetermined

basis.

Synchronous transmission Data transmission in which the time of occurrence of

each signal representing a bit is related to a fixed time frame.

TeBematics User-oriented information transmission services. Includes Teletex,

Videotex, and facsimile.

Thermal noise Statistically uniform noise due to the temperature of the transmission

medium.

Time-division multiplexing The division of a transmission facility into two or

more channels by allotting the facility to several different information channels,

one at a time.

Time-division switching A circuit-switching technique in which time slots in a

time-multiplexed stream of data are manipulated to pass data from an input

to an output.

Token bus A medium access control technique for busltree. Stations form a logical

ring, around which a token is passed. A station receiving the token may

transmit data and then must pass the token on to the next station in the ring.

Token ring A medium access control technique for rings. A token circulates

around the ring. A station may transmit by seizing the token, inserting a

packet onto the ring, and then retransmitting the token.

Topology The structure, consisting of paths and switches, that provides the communications

interconnection among nodes of a network.

Transmission medium The physical path between transmitters and receivers in a

communications system.

Transport layer Layer 4 of the OSI model. Provides reliable, transparent transfer

of data between endpoints.

Tree A local network topology in which stations are attached to a shared transmission

medium. The transmission medium is a branching cable emanating

from a headend, with no closed circuits. Transmissions propagate throughout

all branches of the tree, and are received by all stations.

Twisted pair A transmission medium consisting of two insulated wires arranged in

a regular spiral pattern.

Unbalanced transniission A transmission mode in which signals are transmitted

on a single conductor. Transmitter and receiver share a common ground.

Value-added network A privately-owned, packet-switching network whose services

are sold to the public.

Virtual circuit A packet-switching service in which a connection (virtual circuit) is

established between two stations at the start of transmission. All packets follow

the same route; they need not carry a complete address, and they arrive

in sequence.

ACRONYMS