Modem Glossary
Version 2.0 November, 1997
©Jeff Powell, Ascend Communications



A/D Converter Converts an analog signal to digital data.
ABS Analysis-By-Synthesis, a group of voice compression algorithms. See LPC.
Adaptive A technology that enables high-speed modems to operate on poor lines by compensating for the types of lines. It changes the amplitude and phase to remove the distortion introduced by the channel.
ADM Adaptive Differential Modulation, an example of waveform coding voice compression algorithm.
ADP Answer Detection Pattern, a bit sequence under V.42 that answers the originating modem's ODP bit sequence.
ADPCM Adaptive Differential Pulse Code Modulation, an example of waveform coding voice compression algorithm.
AGC Automatic Gain Control
ANSI American National Standards Institute, a private U.S. standards organization.
API Applications Programming Interface.
ARAM Audio-grade DRAM with "bad bits." An ARAM can be mapped to avoid using bad bits, and therefore deliver good performance.
ASL Adaptive Speed Leveling.
Asynchronous Random transfer of data in bytes, with "start" and "stop" identifying bits. See Synchronous.
Baud Unit of signal frequency in signals per second in other words, the number of signal events per second occurring on a communications channel. Not synonymous with bits per second since signals can represent more than one bit. Although not technically accurate, baud rate is commonly used to mean bit rate.
BBS Bulletin Board Service.
Bell 103 US & Canada, 0-300 bps, 2-wire, full-duplex modem standard, Bell, Dial up Modems. Uses FSK
Bell 212A US & Canada, 1200 bps, 2-wire, full-duplex modem standard, Bell, Dial up Modems, (Bell Pub #41214). Uses PSK
Bell Standard A voice-circuit standard for US & Canadian phone systems only
BER Bit Error Rate
Binary Characteristic of having only two states, such as current on and current off. The binary number system uses only ones and zeros.
Bit The smallest unit of data processing information. A bit (or binary digit) assumes the value of either 1 or 0. Baud equals bits per second only when the signal represents a single bit.
Bit Rate The number of binary digits or bits transmitted per second.
Blacklisting Call blocking capability for specific phone numbers.
BLER Block Error Rate.
BPS Bits per second, units of transmission speed.
BRI Basic Rate Interface. See ISDN
BTLZ British Telecom Lempel-Ziv, data compression
Byte A data unit of eight bits.
Call Progress Network signals that allow for monitoring the call
CCD Charge-Coupled Device.
CCITT International Telegraph and Telephone Consultative Committee . Older international standards body. Now called the ITU, it has been superseded by a new organization, the TSB, which will continue to recommend and publish worldwide communications standards. - See ITU-TSB
CD signal Carrier Detect signal.
CDPD Cellular Digital Packet Data, a technology for wireless mobile communications.
CELP Codebook Excited Linear Prediction, a popular form of an LPC voice compression algorithm.
CIS Contact-Image Sensor, a type of scanning device.
Codec Voice coder and decoder.
Command State When your modem will accept AT commands. Synonym: Escape State. See also Online State.
Computer Rate The rate at which a modem [DCE] and computer [DTE] communicate. Synonyms: Serial Port Rate, Modem-to-Computer Speed, DTE-DCE Rate, Communications Rate.
Connection Rate The rate at which modems communicate over a phone line. Synonyms: Transmission Speed, Line Speed, Carrier Rate, Modem-to-Modem Speed, DCE-DCE rate.
CRC Cyclic Redundancy Check.
DAA Data Access Arrangement.
Data Compression The process of reducing the size of data parcels to transmit more data in less time. In scanners, a bit encoding process that removes redundancy from the scanned page, resulting in fewer bits to transmit. See also Error Correction/Data Compression Standards.
DCE Data Communications Equipment. ex: a modem. See also DTE.
DFE Decision Feedback Equalization.
DIP Dual In-line Package.
Direct Connection Any standard modem connection, without error correction, data compression or flow control. Computer rate must equal connection rate.
DIS (1) Digital Identification Signal

(2) Dynamic Impedance Stabilization

Dithering Gray scale images through fax systems. Also known as half-toning.
DMA Channel Direct Memory Access Channel
DNMS Dial-Up Network Management System.
DPCM Differential Pulse Code Modulation.
DPSK Differential Phase Shift Keying, a modulation technique.
DQPSK Differential Quadrature Phase Shift Keying, a modulation technique.
DRAM Dynamic RAM.
DSP Digital Signal Processing.
DSR signal Data Set Ready signal.
DTE Data Terminal Equipment. ex: a computer serial port of a dumb terminal. See also DCE.
DTMF Dual Tone Multiple Frequency tone generation and detection.
DTR signal Data Terminal Ready signal.
Echo Cancellation A full-duplex transmission technique requiring the receiver to cancel out its own transmitted signal which occupies the same frequency band as the receiver.
ECM Error Correction Mode sends the fax bit stream in page sections or blocks, with an error checking code for each block.
EEPROM Electronically Erasable PROM.
EIA Electronic Industries Association, a private U. S. standards organization.
EIA/TIA-578 Asynchronous facsimile DCE control standard, EIA/TIA, Dial up Modems, Class 1.
EIA/TIA-592 Asynchronous facsimile DCE control standard, EIA/TIA, Dial up Modems, (Class 2) (Proposed)
EPROM Erasable PROM, which can be programmed, erased and reprogrammed by the user.
Error Correction The process of verifying the accuracy of transmitted data and retransmitting data that did not transfer successfully.
Error Correction/ Data Compression Standards Standards that increase the amount of data that can be transferred (data compression) and/or improve the accuracy of transmissions (error correction). ex: MNP 2-5, V.42, V.42bis.
Error Detection and Correction Errors sometimes occur on the switched telephone network and can show up as streaks on the fax output. In modem technology, the basic means of "correcting" errors is actually by retransmission. See ECM.
Fallback Ability of a modem that can operate at different speeds to fall back to its next lower speed if line conditions cannot support the higher speed. To work, both sending and receiving modems must shift to the lower rate.
FAPI Facsimile API.
Fax Standards Standards that let fax devices [faxmodems and fax machines] communicate at specific rates. some are ITC-TSS V.17, V.29 and V.27ter.
FAXENGINE A Rockwell proprietary two-device chipset that contains the fax modem and fax machine functions like machine control and monitoring. It can serve as the functional core of Group 3 office and personal facsimile machines. The only additional parts needed to implement a complete, full-featured system are external memory chips and a telephone line interface.
FDM Frequency Division Multiplexing, the most common and simplest technique supporting full-duplex transmission.
FEES FAXENGINE Evaluation System.
FIR Finite Impulse Response.
Flash memory Frequently ROM is provided as "flash ROM," which can be reprogrammed if the software needs updating.
Flow Control A system that regulates the flow data between a DCE and DTE devices. Can be done in software [XON-XOFF] or through hardware [CTR/DTR, etc.]
FSK Frequency Shift Keying modulation method.
Full Duplex Simultaneous transmission and reception of signals.
GPIO General Purpose Input/Output
Group 3 Refers to ITU digital standard that insures compatibility among fax machines.
Group 4 The ITU digital recommendation, first adopted in 1984, designed for transmission over ISDN. Group 4 fax machines transmit at 64Kbps, and are well suited to computer-controlled network communications.
GSTN General/Global switched telephone network.
Half Duplex Alternating transmission or reception of signals at one time.
Handshake The process of two modems exchanging information via predetermined signals on how they will send data to one another, such as transmission speed, size of data packets, whether the operation will be full- or half-duplex, etc.
HDLC High-level Data Link Control framing.
HDSL High-bit-rate Digital Subscriber Line, an enhanced form of the T1/E1 transmission system technology that allows telephone companies to use existing copper-cable plants while meeting the demands for updated services, including faster modem transmissions.
ICM Incoming Message.
IFC Integrated Facsimile Controller.
Image Processing Dither coding is a method of improving the output quality of transmitted gray scale images through fax systems.
ISDN Integrated Services Digital Network.
ITU or ITU-TSB International Telecommunications Union, a special agency of the United Nations based in Geneva. Sets international standards for telecommunications. Standards concerning modems, fax machines and similar devices begin with V. [for voice]. Was CCITT
JBIG Joint Bi-level Imaging Group of the TSB, working on image compression enhancements for black and white, as well as for gray scale and color.
Jitter A type of analog communications line distortion due to signal variation, and causing data transmission errors.
Kbps Kilobits per second.
LAP-M or LAPM Link Access Procedure for Modems, an error detection and correction method. See V.42
LCD Liquid Crystal Device.
LCS Loop Current Sense (signals).
LCU Line Connection Unit.
LED Light-Emitting Diode.
Line Probing V.34 (28.8Kbps) handshaking signal that allows modem to determine the response of the PSTN channel.
Local modem Your modem
LPC Linear Predictive Coding, a technique for compressing voice and an example of an ABS algorithm.
Mbps Megabits per second.
MCI Media Control Interface.
MCU Micro Controller Unit.
MDP Modem Data Pump.
MH Modified Huffman data compression coding, a CCITT Recommendation T.4 for Group 3 fax machines.
MLSE Maximum Likelihood Sequence Estimation.
MMR Modified Modified Read data compression coding is about 50 percent faster than MR. MMR is also a two-dimensional coding scheme but of higher efficiency than MR.
MNP Microcom Networking Protocol. A widely used proprietary standard providing error correction [MNP 2,3, 4 and 10] and 2:1 data compression [MNP 5 & 7], and enhanced capability (MNP 10 for adverse channel reliability / MNP10EC) commonly used by cellular modems.
MNP 2,3,4 & 5 Error correction/data compression standards from Microcom for dial up modems & DCEs using the public switched network.
MNP 4 Microcom Networking Protocol - Class 4: Configures the modem to use error correction by resending blocks of data that are corrupted during transmission. Uses hardware flow control.
MNP 5 Configures the modem to use an advanced data compression protocol that increases throughput by up to 200% (a 2-to-1 compression ratio). Once enabled, stays on for duration of connection. Includes error correction (MNP Classes 2-4) and hardware flow control.
MNP 10 MNP Class 10 is an extended service to MNP4 error correction. MNP10's purpose is to provide adverse channel enhancements to optimize performance of modems in harsh environments such as cellular links and rural telephone service because it can handle short interruptions [dropped carrier] which occur when switching cells. Part of the error correction/data compression suite from Microcom. Used in dial up modems and other DCEs using the cellular telephone network
MNP 10 EC Incorporates existing MNP 10 protocol combined with Rockwell's new Cellular Optimized Processor (COP) data pumps.
Modem A device that transmits and receives computer data through a communications channel such as radio, telephone lines, or cellular phones. Modem comes from MOdulate/DEModulate
MONOFAX A Rockwell proprietary single chip fax modem.
MR Modified Read data compression.
MTBF Mean Time Between Failure, the average time a device runs until it fails.
MTF Modulation Transfer Function.
MTTF Mean Time To Failure (same as MTBF).
Negative Compression Compression resulting in more pixels [in fax] or data than originally present.
Negotiation The process where your modem and the remote modem determine what type of connection to make. Synonyms: Detection, Detection phase.
NVRAM Non-Volatile RAM.
ODP Originator Detection Pattern, a bit sequence under V.42 transmitted by the originating modem to the receiving modem, and answered by an ADP.
OGM Outgoing Message.
Online State When you are communicating with the remote host. See Command State
PAM Pulse Amplitude Modulation.
PCM Pulse Code Modulation, a technique for converting voice to digital form.
PCMCIA Personal Computer Memory Card International Association. Standards group for PCMCIA [PC Card] devices.
PCS Personal Communications Services.
PDA Personal Digital Assistant.
PDU Protocol Data Unit, a basic MNP message element.
PICA (1) PCMCIA Interface Control Adapter for PC card modems.

(2) A unit of measurement for type

Pixel Picture Element.
PLCC Plastic Chip Carrier Component, a type of surface-mount packaging.
PQFP Plastic Quad Flat Package, a type of surface-mount packaging.
PROM Programmable Read-Only Memory, which can be programmed once by the user and then becomes permanent storage.
Protocols A set of communications procedures.
PSK Phase Shift Keying modulation method, a more bandwidth-efficient method than FSK.
PSTN Public Switched Telephone Network.
QAM Quadrature Amplitude Modulation - Shift Keying modulation method, which, of practical modulation techniques, has the best bandwidth efficiency per information symbol (baud). It is used in most modems.
QFP Quad Flat Package, a type of surface-mount packaging.
QPSK Quadrature Phase Shift Keying
RAM Random-Access Memory, read-write volatile memory that is lost when power is discontinued; temporary storage.
RAPI Rockwell API.
RBT Remote Batch Terminal, used to communicate with mainframe computers.
Reliable Connection A connection that uses one of the error correction or data compression methods. Requires flow control.
Remote Modem The modem you connect to via your communication channel.
RHSI Rockwell High Speed Interface.
ROM Read-Only Memory, nonvolatile memory that retains its information even when power is removed; permanent storage. See PROM, EPROM.
RPACS Rockwell Proprietary Algorithm for Compression of Speech.
RPI Rockwell Protocol Interface. A low-end modem chipset.
RPICS Rockwell's Proprietary Image Corrections System.
RS232C The standard interface between modems and PCs for serial port communications. Its international equivalent is V.24.
RSSE Reduced-State Sequence Estimation.
RTC Real Time Clock.
SDC Synchronous Data Compression, a technique that preserves HDLC/SDLC frame integrity for transmission across an analog link without any jitter or delay.
SDI Silence Deletion/Insertion.
Shading Compensation Shading compensation corrects for nonuniformity in the scanner optical system, and includes distortions due both to light sources and to nonuniformity in the scanner sensor element.
SIO Serial Input/Output.
SMT Surface-Mount Technology.
SNR Signal-to-Noise Ratio.
SRAM Static RAM.
Synchronous Transfer of data or files in batches timed by a clock. See Asynchronous.
T.30 Facsimile protocol standard, CCITT, Facsimile
T.4 Compression/decompression standard for CCITT, facsimile applications
TAM Telephone Answering Machine.
TCM Trellis-Coded Modulation.
Throughput Effective overall transfer rate. Generally speaking, on a 14.4k connection, data will transfer at a maximum of 14,400 bps, so maximum throughput is 14,499 bps. But data compression changes throughput. On a 14.4k V.42bis connection, where V.42bis compresses the data [that's not already compressed] to one-fourth its size, throughput can be up to four times that of a non-V.42bis connection. But although the throughput is 57,600 bps, the transfer may not actually take one-fourth of the time because of the time it takes to compress and decompress the data.
TPH Thermal Print Head.
TQFP Thin Quad Flat Package, a type of surface-mount packaging.
Trellis Encoding An encoding technique for the bit stream sequence in the transmission signal.
TSB Telecommunications Standardization Bureau
UART Universal Asynchronous Receiver/Transmitter.
V.17 14,400 bps fax transmission (two wire, half-duplex, general switched telephone) CCITT/ITU standard.
V.21 0 to 300 bps data transmission using FSK. CCITT/ITU standard.
V.22 A&B 1200/600 bps, 2-wire, full-duplex modem standard, Dial up Modems. Uses DPSK. CCITT/ITU standard
V.22bis 2400/1200 bps, 2-wire, full-duplex Dial up Modems using QPSK/QAM. CCITT/ITU standard.
V.23 2-wire, asymmetric full-duplex modem standard, CCITT, Videotext utilizing a 1200 bps or 600 bps forward channel and 75 bps backward channel
V.25 Automatic answering equipment and/or parallel, CCITT, Dial and Leased automatic calling equipment on the PSTN, Line Modems including procedures for disabling of echo control devices for both manually and automatically established calls
V.25bis Command set interface between data terminal, CCITT, Dial and Leased, equipment (DTE) and data circuit terminating, Line Modems, equipment DCE)
V.26 2400 bps , 4-wire, full-duplex modem standard, CCITT, Leased Line, Modems
V.26bis 2400/1200 bps, 2-wire, half-duplex modem standard, CCITT, Banking for use on the PSTN Terminals
V.26ter 2400/1200 bps, 2-wire, half-duplex modem standard, CCITT, Leased Line for use on leased lines, Modems
V.27 4800 bps, 4-wire, full-duplex modem standard, CCITT, Leased Line, Modems
V.27 2400/4800 bps fax transmission (full duplex/four wire, half duplex/two wire)
V.27bis 4800/2400 bps, 4-wire, full-duplex fax & modem standard for leased lines. Uses adaptive equalization. CCITT/ITU standard.
V.27ter 4800/2400 bps, 2-wire, half-duplex fax & modem standard for GSTN. CCITT/ITU standard
V.29 4800/9600 bps, 4-wire, full-duplex modem standard, four wire leased line, ( a 2-wire half duplex implementation of the standard is used for Group 3 compatible facsimile machines & modems). CCITT/ITU standard
V.32 4800 / 9600 bps, 2-wire, full-duplex GSTN dial up modem standard, using QAM/QPSK. CCITT/ITU standard
V.32bis 4800 /7200 /9600 /12000 /14,400 bps, 2-wire, full-duplex dial-up modem standard. CCITT/ITU standard
V.32ter or V.32terbo Not actually an ITU standard - up to 19.2K data. Very poor connectivity with V.34 modems because the V.32ter signal is often mistaken for a fax tone.
V.33 14,400 bps, four-wire, full-duplex modem standard, CCITT, Leased Line, Modems
V.34 2400 /4800 /7200 /9600 /12000 /14400 /16800 /19200 /21600 /24000 /26400 /28,800 bps, 2-wire, full-duplex dial-up modems. CCITT/ITU standard
V.42 International standard for data error correction. Uses LAP-M (Link Access Procedure for Modems) as the primary error-control protocol and MNP Classes 1-4 as backup protocols. Uses hardware flow control.
V.42bis International standard for data compression and decompression. Increases throughput by up to 400% (a 4-to-1 compression ratio). Always uses V.42 error correction and hardware flow control. Also known as BTLZ
V.42bis+ features of V.42bis with addition of MNP 5
VFC or V.Fast Class (V.FC) Rockwell's pre-version of the modem protocol (V.34). Speeds vary from 19.2 Kbps to the theoretical maximum of 28.8 Kbps for analog dial-up lines. Non Standard. Works at the following rates: 14400 /16800 /19200 /21600 /24000 /26400 /28800. Will drop connection if rate falls below 14.4k baud
Video Processing Processing of the scanner data done on the analog scanner signal, the digital data, or both. It accommodates for the shading, distortions and other aspects of the original image so that reproduction can be as accurate as possible at the receiving end.
VSELP Vector-Sum Excited Linear Prediction, a patented voice compression algorithm. See CELP.
VSOP Very Small Outline Package, a type of surface-mount packaging.
Waveform Coding A type of voice compression that attempts to preserve the input waveform in the recovered (decompressed) speech. See DPCM, ADM and ADPCM.
WCEM World-Class Evaluation Module

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