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56K Faxmodem User's Guide and Reference


analog loopback

A modem self-test in which data from the keyboard or an internal test pattern is sent to the modem's transmitter, turned into analog form, looped back to the receiver, and converted back into digital form.

analog signals

A variety of signals and wavelengths that can be transmitted over communications lines such as the sound of a voice over the phone line.

answer mode

The mode used by your modem when answering an incoming call from an originating modem. The transmit/receive frequencies are the reverse of the originating modem, which is in originate mode.


A computer program designed to perform a specific task or set of tasks. Examples include word processing and spreadsheet applications.


Automatic Repeat reQuest. A function that allows your modem to detect flawed data and request that it be retransmitted. See MNP and V.42.


American Standard Code for Information Interchange. A code used to represent letters, numbers, and special characters such as $, !, and /.

asynchronous transmission

Data transmission in which the length of time between transmitted characters may vary. Because characters may not be transmitted at set intervals, start/stop bits are used to mark the beginning and end of each character.

Auto Answer

Sets the modem to pick up the phone line when it detects a certain number of rings. See S-register S0 in the "Technical Reference" chapter of this manual.


A process where your modem dials a call for you. The dialing process is initiated by sending an ATDT (dial tone) or ATDP (dial pulse) command followed by the telephone number. Auto-dial is used to dial voice numbers. See basic data command Dn in the "Technical Reference" chapter of this manual.

baud rate

A term used to measure the speed of an analog transmission from one point to another. Although not technically accurate, baud rate is commonly used to mean bit rate.

binary digit

A 0 or 1, reflecting the use of the binary numbering system. Used because the computer recognizes either of two states, OFF or ON. Shortened form of binary digit is bit.

bit rate

Also referred to as transmission rate. The number of binary digits, or bits, transmitted per second (bps). Communications channels using analog modems are established at set bit rates, commonly 2400, 4800, 9600, 14,400, 28,800, 33,600, and higher.

bits per second (bps)

The bits (binary digits) per second rate. Thousands of bits per second are expressed as kilobits per second (Kbps).


A temporary memory area used as storage during input and output operations. An example is the modem's command buffer.


A group of binary digits stored and operated upon as a unit. Most often the term refers to 8-bit units or characters. One kilobyte (KB) is equal to 1,024 bytes or characters; 640 KB is equal to 655,360 bytes or characters.


The basic signal altered or modulated by the modem in order to carry information.


A representation, coded in binary digits, of a letter, number, or other symbol.

characters per second (cps)

A data transfer rate generally estimated from the bit rate and the character length. For example, at 2400 bps, 8-bit characters with start/stop bits (for a total of ten bits per character) will be transmitted at a rate of approximately 240 characters per second (cps). Some protocols, such as error-control protocols, employ advanced techniques such as longer transmission frames and data compression to increase cps.

class 1 and 2.0

International standards used by fax application programs and faxmodems for sending and receiving faxes.

cyclic redundancy checking (CRC)

An error-detection technique consisting of a test performed on each block or frame of data by both sending and receiving modems. The sending modem inserts the results of its tests in each data block in the form of a CRC code. The receiving modem compares its results with the received CRC code and responds with either a positive or negative acknowledgment.

data communications

The transmission or sharing of data between computers via an electronic medium.

data compression table

A table containing values assigned for each character during a call under MNP5 data compression. Default values in the table are continually altered and built during each call: The longer the table, the more efficient throughput gained.

data mode

Mode used by a modem when sending and receiving data files.


Data Communications (or Circuit-Terminating) Equipment, such as dial-up modems that establish and control the data link via the telephone network.


Any setting assumed, at startup or reset, by the computer's software and attached devices. The computer or software will use these settings until changed by the user or other software.

digital loopback

A test that checks the modem's RS-232 interface and the cable that connects the terminal or computer and the modem. The modem receives data (in the form of digital signals) from the computer or terminal and immediately returns the data to the screen for verification.

digital signals

Discrete, uniform signals. In this manual, the term refers to the binary digits 0 and 1.


Data Terminal (or Terminating) Equipment. A computer that generates or is the final destination of data.


Indicates a communications channel capable of carrying signals in both directions. See half-duplex, full-duplex.

Electronic Industries Association (EIA)

Group which defines electronic standards in the U.S.

error control

Various techniques that check the reliability of characters (parity) or blocks of data. V.42 and MNP error-control protocols use error detection (CRC) and retransmission of flawed frames (ARQ).


A method for transmitting the image on a page from one point to another. Commonly referred to as fax.

fax mode

The mode used by a modem to send and receive data in facsimile format. See definitions for V.17, V.27ter, V.29.

flow control

A mechanism that compensates for differences in the flow of data into and out of a modem or other device. See extended data commands &Hn, &In, &Rn in the "Technical Reference" chapter of this manual.


A data communications term for a block of data with header and trailer information attached. The added information usually includes a frame number, block size data, error-check codes, and Start/End indicators.


Signals can flow in both directions at the same time over one line. In microcomputer communications, this may refer to the suppression of the online local echo.


Signals can flow in both directions, but only one way at a time. In microcomputer communications, may refer to activation of the online local echo, which causes the modem to send a copy of the transmitted data to the screen of the sending computer.


Hertz, a frequency measurement unit used internationally to indicate cycles per second.


An electronic communications network that connects computer networks and organizational computer facilities around the world.

Internet Service Provider (ISP)

A company that provides dial-up (modem) access to the Internet for a fee.


An international organization that defines standards for telegraphic and telephone equipment. For example, the Bell 212A standard for 1200-bps communication in North America is observed internationally as ITU-T V.22. For 2400-bps communication, most U.S. manufacturers observe V.22 bis.


Link Access Procedure for Modems. An error-control protocol defined in ITU-T recommendation V.42. Like the MNP protocols, LAPM uses cyclic redundancy checking (CRC) and retransmission of corrupted data (ARQ) to ensure data reliability.

local echo

A modem feature that enables the modem to display keyboard commands and transmitted data on the screen. See basic data command En in the "Technical Reference" chapter of this manual.


Microcom Networking Protocol, an error-control protocol developed by Microcom, Inc., and now in the public domain. There are several different MNP protocols, but the most commonly used one ensures error-free transmission through error detection (CRC) and retransmission of flawed frames.


A device that transmits/receives computer data through a communications channel such as radio or telephone lines. It also changes signals received from the phone line back to digital signals before passing them to the receiving computer.

nonvolatile memory (NVRAM)

User-programmable random access memory whose data is retained when power is turned off. On the USRobotics modem, it includes four stored phone numbers and the modem settings.

off/on hook

Modem operations that are the equivalent of manually lifting a phone receiver (taking it off-hook) and replacing it (going on-hook).

online fall back/fall forward

A feature that allows high-speed, error-control modems to monitor line quality and fall back to the next lower speed in a defined range if line quality diminishes. As line conditions improve, the modems switch up to the next higher speed.

originate mode

The mode used by your modem when initiating an outgoing call to a destination modem. The transmit/receive frequencies are the reverse of the called modem, which is in answer mode.


A simple error-detection method that checks the validity of a transmitted character. Character checking has been surpassed by more reliable and efficient forms of error checking, including V.42 and MNP 2-4 protocols. Either the same type of parity must be used by two communicating computers, or both may omit parity.


A system of rules and procedures governing communications between two or more devices. Protocols vary, but communicating devices must follow the same protocol in order to exchange data. The format of the data, readiness to receive or send, error detection and error correction are some of the operations that may be defined in protocols.


Random Access Memory. Memory that is available for use when the modem is turned on, but that clears of all information when the power is turned off. The modem's RAM holds the current operational settings, a flow control buffer, and a command buffer.

remote digital loopback

A test that checks the phone link and a remote modem's transmitter and receiver.

remote echo

A copy of the data received by the remote system, returned to the sending system, and displayed on the screen. Remote echoing is a function of the remote system.


Read Only Memory. Permanent memory, not user-programmable.

serial transmission

The consecutive flow of data in a single channel. Compare to parallel transmissions where data flows simultaneously in multiple channels.

start/stop bits

The signaling bits attached to a character before and after the character is transmitted during asynchronous transmission.


A device whose keyboard and display are used for sending and receiving data over a communications link. Differs from a microcomputer or a mainframe in that it has little or no internal processing capabilities.

terminal mode

Software mode that allows direct communication with the modem. Also known as command mode.


The amount of actual user data transmitted per second without the overhead of protocol information such as start/stop bits or frame headers and trailers. Compare with characters per second.


Universal Serial Bus.


The ITU-T standard specification that covers the initial handshaking process.

V.17 fax

An ITU-T standard for making facsimile connections at 14,400 bps, 12,000 bps, 9,600 bps, and 7,200 bps.


An ITU-T standard for modems operating in asynchronous mode at speeds up to 300 bps, full-duplex, on public switched telephone networks.


An ITU-T standard for modem communications at 1,200 bps, compatible with the Bell 212A standard observed in the U.S. and Canada.

V.22 bis

An ITU-T standard for modem communications at 2,400 bps. The standard includes an automatic link negotiation fallback to 1,200 bps and compatibility with Bell 212A/V.22 modems.

V.27 ter

An ITU-T standard for facsimile operations that specifies modulation at 4,800 bps, with fallback to 2,400 bps.


An ITU-T standard for facsimile operations that specifies modulation at 9,600 bps, with fallback to 7,200 bps.


An ITU-T standard for modem communications at 9,600 bps and 4,800 bps. V.32 modems fall back to 4,800 bps when line quality is impaired.

V.32 bis

An ITU-T standard that extends the V.32 connection range: 4,800, 7,200, 9,600, 12,000, and 14,400 bps. V.32 bis modems fall back to the next lower speed when line quality is impaired, fall back further as necessary, and also fall forward (switch backup) when line conditions improve (see online fall back/fall forward).


An ITU-T standard that currently allows data rates as high as 28,800 bps.


An enhancement to V.34 that enables data transfer rates as high as 33,600 bps.


An ITU-T standard for modem communications that defines a two-stage process of detection and negotiation for LAPM error control.

V.42 bis

An extension of ITU-T V.42 that defines a specific data compression scheme for use during V.42 connections.


An ITU-T standard for modem data compression. It provides for a 6:1 compression ratio.


The ITU-T standard for 56 Kbps modem communications. This technology uses the digital telephone network to increase the bit rate of the receive channel by eliminating the analog to digital conversion commonly found in modem connections. V.90 connections require a modem with V.90 or x2 technology calling a digitally connected Internet Service Provider or corporate host site compatible with V.90 or x2 technology.


The ITU-T standard for advanced 56 Kbps modem communications. This technology offers three new features to enhance the V.90 standard. The first feature is V.PCM-Upstream, which allows a modem's upstream communication to reach speeds of 48,000 bps. The second feature provides quicker connection times by allowing the modem to remember the line conditions of a V.92 supported service provider. The third feature is the Modem on Hold technology, which allows your Internet connection to be suspended when there is an inbound telephone call, then return to the connection when the call is completed without losing the connection. The V.92 technology can only be utilized if a V.92 modem is dialing into an Internet Service Provider that supports and provides a digital V.92 signal.

World Wide Web (WWW)

A part of the Internet designed to allow easier navigation of the network through the use of graphical user interfaces and hypertext links between different addresses.

x2TM Technology

USRobotics's trademark for its proprietary technology that uses the digital telephone network to increase the bit rate of the receive channel by eliminating the analog-to-digital conversion commonly found in modem connections. x2 connections require a modem with x2 technology calling a digitally connected Internet Service Provider or corporate host site compatible with x2 technology.


Standard ASCII control characters used to tell an intelligent device to stop/resume transmitting data.