Glossary CS601

Glossary CS601

LAN (Local Area Network) : The data communications facilities used to provide communications within a limited geographical area, normally up to 6 miles, using medium to high data rates between 9600 bps to 100 mbps. LAN 's may have bridges or gateways to other networks but are usually confined to a building or cluster or buildings, frequently referred to as a "campus".

Latency : The waiting time, or delay between a stations request to a transmission channel, and completion.

Layer : A term used to define one level of a hierarchy of functions, as specified by the OSI reference model. Divisions of other protocols, such as IBM 's SNA, sometimes will correspond to one or more OSI layers.

Line : A multipoint or point-to-point communications medium .

MAC (Media Access Control) : An access control protocol defined under IEEE 802 which includes variations for the token ring, token bus and CSMA/CD.

Mainframe : A large-scale computer, capable of processing large amounts of data with very fast processing, including control units and peripheral devices supplied by one vendor, examples of which would include IBM, Unisys, Control Data, and others. Often, mainframe systems will have a closed architecture.

Manchester Code, Manchester Encoding : A binary signaling mechanism in which each bit period is divided into two complementary halves, combining data and clock pulses.

Mbps : Millions of bits per second ( bps).

Medium : The material used to record or transmit data.

Message : Two definitions exist in normal usage: 1) A communication containing some information. 2) Several packets that comprise a complete transmission

Message Routing : Selecting a path or channel for message transmission.

MHz (MegaHertz) : A unit equal to one million cycles per second (Hz).

Microsecond : One millionth of a second

Microwave, Microwave Transmission : Transmitting voice or data via any electromagnetic wave in the radio-frequency spectrum above 890 mHz, in a line-of-sight fashion, with repeaters every 20-30 miles.

Millisecond : One thousandth of one second.

Modem : A device named from the contraction of the words "modulator - demodulator" and used to convert serial digital data from a transmitting terminal to a signal suitable for transmission over a telephone line. The term may be used when the modulator and demodulator are physically in the same signal-conversion equipment, since the device must also reconvert the signal to serial digital data for the receiving terminal.

Modulator, Modulation : The function or process by which a carrier is varied to represent an information-carrying signal. This is the technique used in modems to allow computer signals compatibility with communications facilities.

Multiplexing, Multiplexer(or) (Mux) : To divide or have a device that divides a composite signal between several channels. Devices such as FDM's, TDM's, concentrators, etc. are different types of multiplexors.

Nanosecond : One billionth of a second.

Network : Generally, a network can be said to be any inter-connection of computer systems facilities (including control units, modems, terminals, etc.) although three network categories exist in common usage: 1) Switched networks, in which the telephone network is the telephone lines normally used for dialed telephone calls for voice or data. 2) Any series of points connected by communications channels. 3) Dedicated, Leased or Private networks, reserved for the use of one user or customer.

Network Layer : The third entity in the OSI model that is responsible for addressing and routing between sub networks and servicing the transport layer.

Network Topology : Outlining all network nodes and their physical/logical relationship, such as ring, bus, star, etc.

Network Virtual Terminal : The usage of numerous data terminals having different protocols, formats, data rates and codes on the same network.

Node : An inter-connection point to a data communications network, however, examining further, the following definitions also apply: 1) In a packet-switched environment, one of the switches that forms the networks backbone. 2) Any unit that is polled on a multipoint network. 3) A LAN station or any unit on a ring topology

Noise : The random electrical signals, a communications line impairment which can either be inherent in the line design or induced by natural disturbances and therefore corrupting transmitted data.

NRZ (Non-Return to Zero) : A transmission encoding scheme where the "zeros" and "ones" are represented by alternating and opposite high and low voltages. Two basic forms of NRZ coding exist: 1) In a Unipolar NRZ code the voltages would vary between 0v and +5V. This code works well for the shielded and short travel paths within a machine, but is not suited for long distances due to the residual dc shifts of the "zero" level. 2) In a Polar NRZ code, less power would theoretically be required to transmit the signal (one half) since polar shifts the signal reference level to the midpoint of the signal amplitude, say for example, +2.5V and -2.5V. The disadvantage is that most of the energy in the signal is concentrated around zero frequency.

NRZI (Non-Return to Zero Inverted) : Also known as "invert-on-zero" coding, NRZI is an SDLC encoding technique where a change in state represents a binary 0 and no change in state represents a binary 1.

Nyquist Theory : A communications theory recommending a two-sample per cycle process to characterize an analog signal limited by bandwidth. The rate of sampling must be twice the highest frequency component of the signal, so for example, a 4000 Hz analog signal would be sampled 8000 times.

Open-Air Transmission : A data communications technique relying on radio frequency (RF) signaling, including infrared, microwave and FM radio.

Optical Fiber : The thin filaments of glass, glass strands or glass-like material, each of which is an independent circuit for transmission of very wide frequency ranges. Optical fiber is contained in a shielded fiber optic cable for communications use.

OSI Model (Open Systems Interconnection Model) : The 7-layer reference model recommended by the ISO to provide a logical structure for network operations protocol.

Packet : A group of information and overhead bits sometimes referred to as a message, that is transmitted as a package on a packet-switched network, and is usually smaller than a transmission block.

Packet Switching, Packet-Switched Network : The sending of addressed packets containing data over a data communications network via a channel occupied for the duration of the packet transmission. Packets from different sources would be interleaved over channels (called virtual circuits).

Packet Terminal : Any DTE device able to transmit and receive packets.

Parallel Interface, Parallel Transmission : The interface or process designed to send each bit simultaneously over a separate line or wire, and usually used to send data one byte at a time to a high-speed printer or local peripheral. Contrast with serial interface, serial transmission.

TCP/IP (Transmission Control Protocol/Internet Pro : A communications protocol, functioning within the third and fourth OSI layers, used in a LAN environment for internetwork routing and reliable message delivery. TCP/IP is found in ARPANET and is endorsed by the DOD.

Telecommunications : The transmission (and reception) of signals producing sounds, images or information using a variety of media such as fiber optic, copper wire, infrared or radio frequency.

Amplitude : A term used to describe the relative height of a sine wave .

Amplitude Modulation : One of three methods to add information to a sine wave carrier signal. By varying the voltage level, the magnitude is modified in accordance with the information to be transmit ted. Contrast with phase modulation and frequency modulation.

Amplifier : A unidirectional, electronic device used to boost (amplify) signals, with the resulting performance measured in decibels.

Analog, Analog data - : Any data in the form of continuously variable physical qualities, which are "analogous" to the data source. Continuously variable as opposed to discretely variable. Contrast with digital data.

ANSI (American National Standards Institute) : A voluntary organization that represents the USA in the ISO, and is responsible for defining ASCII. Members include manufacturers, common carriers, and other standards organizations such as the IEEE.

Application Layer : In referring to the OSI model, this is the "top" or seventh layer containing all user or application programs. See OSI layers diagram in Section B.

ASCII : Usually pronounced "ask'-ee", this seven-bit-plus parity code set was established by ANSI to provide compatibility between data systems and services.

Asynchronous, Asynchronous Transmission : Also known a "start/stop", asynchronous refers to having a variable or random time interval between successive characters, operations, or events. Each character, word, or small block is individually synchronized by the use of start and stop bits. Contrast this concept with synchronous.

Attenuate, Attenuation - : A decrease in power over distance of a signal. Typically, attenuation increases (and the signal level decreases) with both frequency and cable length. Attenuation is measured in terms of levels called decibels. Contrast this concept with gain.

Automatic Baud Rate Detection (ABR) : A process that enables a receiving device to accept data from a variety of transmit ting devices, operating at different speeds, stop bits and code levels. ABR usually relies on a sign-on character to provide the information needed by the receiving device to make the necessary transition.

Bandwidth : The information-carrying capability of a communications channel or line, expressed in cycles per second ( Hz) between the highest and lowest frequencies of a band.

Baud : A unit of signaling speed or rate, taken from the name of French telegrapher, Emile Baudot. Baud is usually defined as the number of signal level changes per second, regardless of information content of those signals. If each signal event represents only one bit condition, baud is the same as bits per second when each signal event represents other than one bit

Bit Oriented : A communications protocol (such as IBM 's SDLC) where control information is encoded in one or more bits. Contrast with byte or character oriented.

Bit Rate : The speed at which binary digits (bits) would be transmit ted over a communications path and usually expressed in "bits per second" ( bps). Bit rate should not be confused with Baud which defines the rate of signal state changes.

Bit Stream : The continuous series of transmit ted bits through a transmission link

Block - : A group or quantity of information composed of any quantity of contiguous bits and/or bytes.