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Modems Glossary



Modems - Networking; Glossary of Terms



Adaptive Speed Leveling (ASL)
A Courier( feature, first used by V.32 bis and V.32 terbo modems in November 1990, that detect interference on a line or improved line conditions and shift accordingly. This feature allows the send and receive channels of a modem connection to adapt independently to varying line conditions. Each channel shifts down in speed when interference is detected and back upward again when line conditions improve without affecting the performance of the other channel. The modems at both ends of the connection adapt independently, each detecting and adjusting to line conditions. ASL keeps the modems online, always operating at the highest possible speed, and constantly ensuring data integrity.

Adaptive Differential Pulse Coded Modulation. This is used in voice compression algorithms.

Acoustic Echo Cancellation. This is used in Speakerphone technology.

Automatic Gain Control. This is used in Speakerphone technology.

Analog Interface Circuit. Used on all modems.

One of many sound file formats for the Macintosh computer.

Anonymous FTP
A manner of logging into an FTP site as a public user. Many FTP sites will allow anonymous logins to access some files.

American National Standards Institute - Generic body for U.S. Standards.

Answer Mode
A state in which the modem transmits at the predefined high frequency of the communications channel and receives at the low frequency. The transmit/receive frequencies are the reverse of the calling modem which is in Originate mode.

Automatic Repeat Request. A general term for error control protocols which feature error detection and automatic retransmission of defective blocks of data. See HST(, MNP, and V.42.

American Standard Code for Information Interchange. The format used for standard unformatted text. A 7_bit binary code (0's, 1's) used to represent letters, numbers, and special characters such as $, !, and /. Supported by almost every computer and terminal manufacturer.

Applications Specific Integrated Circuit. Used on all U.S. Robotics modems.

Asymmetrical Modulation
A duplex transmission technique which splits the communications channel into one high speed channel and one slower channel. During a call under asymmetrical modulation, the modem with the greatest amount of data to transmit is allocated the high speed channel. The modem with less data is allocated the slow, or back channel (450 bits per second (bps)). The modems dynamically reverse the channels during a call if the volume of data transfer changes.

Asynchronous Transmission
Transmission in which the length of time between transmitted characters may vary. Because the time lapses between transmitted characters are not uniform, the receiving modem must be signaled as to when the data bits of a character begin and when they end. The addition of Start and Stop bits to each character serves this purpose. Compare to Synchronous Transmission.

Asynchronous Transfer Mode. A network technology based on transferring data in cells or packets of a fixed size. The cell used with ATM is relatively small compared to units used with older technologies. The small, constant cell size allows ATM equipment to transmit video, audio, and computer data over the same network, and assure that no single type of data hogs the lines. Current implementations of ATM support data transfer rates of from 25 to 622 Mbps (megabits per second).

A sound file format for UNIX systems.

Auto Answer
A feature in modems enabling them to answer incoming calls over the phone lines without the use of a telephone receiver.

Auto Dial
A feature in modems enabling them to dial phone numbers over the phone system without the use of a telephone transmitter.

Analog Wide Area Network 

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Baud Rate
The number of discrete signal events per second occurring on a communications channel. The communications carrier frequency. Due to advanced coding methods, each signal can carry more than one bit. At 300 baud, approximately 300 bits are transmitted per second. For speeds higher than 2400, baud rate and bit rate are no longer equivalent. Although not technically accurate, baud rate is commonly used to mean bit rate (bps). In communications software, baud rate refers to the computer’s serial port rate. See UART.

Electronic Bulletin Board System

Bell 103
A standard for modem data communication at 300 bps. See V.21.

Bell 212A
A standard for modem data communications at 1200 bps. See V.22.

Beta Test
Second stage testing performed by dependable customers who volunteer to use a product and report any problems, before the product is actually released.

Binary Digit
A 0 or 1, reflecting the use of a binary numbering system (only two digits). Used because the computer recognizes either of two states, OFF or ON. Shortened form of binary digit is bit.

Binary Synchronous Control. An earlier protocol developed by IBM for software applications and communicating devices operating in synchronous environments. The protocol defines operations at the link level of communications, for example, the format of data frames exchanged between modems over a phone line. See Protocol, HDLC, SDLC.

Bit Rate
The number of binary digits, or bits, transmitted per second (bps). Communications channels using telephone channel modems are established at set bit rates, commonly 300, 1200, 2400, 4800, 9600, and 14400.

Bits per Second
The bits (binary digits) per second rate. A 14,400 Sportster(, for example, is capable of transmitting 14,400 bits per second to another modem capable of at least 14,400 bps.

Data communications software made by CRG, mainly for file transfer and the name of the file transfer protocol.

Bits Per Second.

Basic Rate Interface - 2B+D ISDN connection

A memory area used as temporary storage during input and output operations. An example is the modem's command buffer. Another is the Transmit Data flow control buffer used for flow control and to store copies of transmitted frames until they are positively acknowledged by the receiving modem.

A group of binary digits stored and operated upon as a unit. A byte may have a coded value equal to a character in the ASCII code (letters, numbers), or have some other value meaningful to the computer. In user documentation, the term usually refers to 8-bit units or characters. 1 kilobyte (K) is equal to 1,024 bytes or characters; 64K indicates 65,536 bytes or characters. 

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Call Link
Software based phone system

Formerly, an international organization that defined standards for telegraphic and telephone equipment. It has been incorporated into its parent organization, the International Telecommunication Union (ITU). Telecommunication standards are now covered under Telecommunications Standards Sector (TSS). ITU-T replaces CCITT. For example, the Bell 212A standard for 1200 bps communication in North America was referred to as CCITT V.22. It is now referred to as ITU-T V.22.

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

Characters Per Second
A data transfer rate generally estimated from the bit rate and the character length. For example, at 2400 bps, 8-bit characters with Start and 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 USR-HST and MNP, employ advanced techniques such as longer transmission frames and data compression to increase cps.

Class 1/EIA-578
An American standard used between facsimile application programs and facsimile modems for sending and receiving Class 1 faxes.

Class 2.0/EIA-592
An American standard used between facsimile application programs and facsimile modems for sending and receiving Class 2.0 faxes.

A computer that connects to a server which exchanges information.

Compressed Serial Line Internet Protocol- A SLIP protocol that compresses the amount of information in the packet header. Also see SLIP.

Cyclic Redundancy Checking (CRC)
An error-detection technique consisting of a cyclic algorithm performed on each block or frame of data by both sending and receiving modems. The sending modem inserts the results of its computation 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. In the ARQ protocol implemented in U.S. Robotics high speed modems, the receiving modem accepts no more data until a defective block is received correctly. 

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Data Communications
A type of communications in which computers and terminals are able to exchange data over an electronic medium.

Data Compression (Computer)
Storage of programs, files, and data on a hard drive or other storage device in compressed format so as to require less storage space. Can be done manually using a compression utility such as PKZIP, or can be done automatically using a utility such as Stacker or DoubleSpace.

Data Compression (Modem)
When the transmitting modem detects redundant units of data, it recodes them into shorter units of fewer bits. The receiving modem then decompresses the redundant data units before passing them to the receiving computer.

Data Communication (or Circuit-Terminating) Equipment. In this manual, the term applies to dial-up modems that establish and control the data link via the telephone network.

Dedicated Line
A user-installed telephone line used to connect a specified number of computers or terminals within a limited area, for example, one building. The line is a cable rather than a public-access telephone line. The communications channel may also be referred to as nonswitched because calls do not go through telephone company switching equipment.

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

A subsection of the internet of an IP address which is most commonly used to refer to a group of computers whose hostnames share a common suffix: the domain name. The primary domains are: .COM, .ORG, .NET, .MIL.

Domain name
The information located to the right of the @ in an address, this is also referred to as DNS. e.g. In this case is the domain

Domain Name Server- A system used to translate hostnames like USR.COM to a numeric Internet address. The DNS will also provide naming for clients as well as hosts.

Digital Signal Processor. Frequently called the “data pump.” A chip in the modem’s circuitry that handles all of the actual work of sending and receiving data.

Indicates a communications channel capable of carrying signals in both directions. See Half Duplex, Full Duplex. 

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(Electronically Eraseable Programmable Read Only Memory) replaced ROM in our new models of modems. This is permanent memory too. The only way that a user can write to this memory is through an executable program that erases the code written there and new code files contained in the program are written to the EEPROM. (Also known as EPROM, which is the way it is listed under ATI7) This makes flashing a modem for an upgrade or update possible.

EIA Standards Committee
Electronic Industries Association, which defines electronic standards in the U.S.

Error Control
Various techniques which check the reliability of characters (parity) or blocks of data. V.42, MNP and HST error control protocols use error detection (CRC) and retransmission of frames in error (ARQ). Error control is a required component of data compression.

Error Detection
The process of determining whether errors have been introduced to transmitted data. Although some people use the terms “error detection” and error correction” interchangeably, it is important to note that the modem does not perform error correction. If errors are detected, the receiving modem sends a request for the sending modem to start over again where the errors where first encountered. Formerly parity was used as a method of error detection, but as the speed increased and the amount of data increased, this was not effective. 

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A system set up to monitor and block certain types of traffic between a network or networks and the internet. This is mainly set up for security reasons, to prohibit unauthorized access of confidential information.

Flash ROM
Read Only Memory that can be erased and reprogrammed with software. Most U.S. Robotics Courier modems can be updated or upgraded using a program downloadable from the U.S. Robotics BBS when available.

Flow Control
A feature that compensates for differences in the flow of data input to and output from a modem or other device. Necessary to keep a modem from sending data faster than a computer can handle it, to prevent overflow and data loss.

File Transfer Protocol- A protocol used for sending files from one computer to another.

Full Duplex
Signal flow in both directions at the same time. In microcomputer communications, may refer to the suppression of the online Local Echo. Telephones operate in full duplex, as they are capable of sending and receiving signals at the same time. See also Simplex, Duplex, Half Duplex. 

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A system by which two incompatible networks or applications can communicate with each other.

Graphical User Interface- An interface that is accessed by icons and other graphical images. Windows and Macintosh systems use a GUI. 

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Half Duplex
Signal 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. Walkie talkies use Half Duplex transmission, because they cannot send and receive signals at the same time.

The part of a data packet which contains the source and destination address of that data packet. A header is like an envelope for a letter.

High Level Data Link Control. A standard protocol developed by the International Standards Organization for software applications and communicating devices operating in synchronous environments. The protocol defines operations at the link level of communications, for example, the format of data frames exchanged between modems over a phone line. See Bisync, Protocol, SDLC.

A unique name by which a computer is known on a network. The hostname is also used to identify it in electronic mail or other forms of electronic information interchange over a network or internet.

HST(High Speed Technology)
U.S. Robotics' proprietary signaling scheme, design and error control protocol for high-speed modems. HST incorporates trellis-coded modulation, for greater immunity from variable phone line conditions, and asymmetrical modulation for more efficient use of the phone channel at speeds of 4800 bps and above. HST also incorporates MNP-compatible error control procedures adapted to asymmetrical modulation.

Hyper Text Markup Language- The language or format in which Web pages are written.

Hyper Text Transport Protocol- The system used to request HTML documents from the World Wide Web.

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

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Intranet is a TCP/IP network inside a company that links that company to the Internet. Most Intranets are only accessible from within the location or company that runs that Intranet.

IP Address
All computers on the Internet use IP address for identification purposes. The format of this address is three dots and four groups of numbers. There is a network number, host number, and a subnet address. i.e. 1.22.333.44

Internetwork Packet eXchange- A protocol used to link Local Area Networks to each other with a router device.

Internet Relay Chat- The live chat area of the internet.

Integrated Switched Digital Network- A digital telephone system which provides noise free high speed communications. The service is a custom service that must be installed by the phone company. You will need a Terminal Adapter, a telephone provider which supports ISDN, and a service provider. Usually telephone exchange carriers use digital lines to communicate with one another, but it is possible to have one of these digital lines installed directly into your home or office. Results in higher speed, fewer errors, and allows for separate lines and communications channels. No analog signal is modulated (no modem), but usually a Terminal Adapter is required because digital signals differ.

Internet Service Provider- A company which provides access to the Internet whether it is a personal dial-up account or a cooperate networked connection. Most ISPs will provide extra services include help with design, creation and administration Web sites, and intranet support for companies.

International Telecommunication Union-Telecommunication sector. Formerly referred to as CCITT. 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. 

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Local Area Network- A computer network located in one or more buildings.

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

Leased Line
A phone line, leased from the phone company, set up specifically as a direct connection between two fixed locations. There is no dial tone on these lines. Only Courier modems support leased lines.

A term used to describe products that are

A reference point from one site or location which will point you to another location for that information.

Listserv an automatic email server. This server processes electronic mail requests for addition to or deletion from mailing lists. Some listservs provide facilities such as retrieving files from archives and a database search.

Local Echo
A modem feature that enables the modem to send copies of keyboard commands and transmitted data to the screen. When the modem is in Command mode (not online to another system) the local echo is invoked through the ATE1 command. The command causes the modem to display your typed commands. When the modem is online to another system, the local echo is invoked through the ATF0 command. This command causes the modem to display the data it transmits to the remote system. 

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Multipurpose Internet Mail Extension- A file format that will allow you to post non-text data in a mail message. Graphic files, video, audio, and executables file can be sent via email using this manner.

Microcom Networking Protocol, an asynchronous error control protocol developed by Microcom, Inc. and now in the public domain. The protocol ensures error-free transmission through error detection (CRC) and retransmission of errored frames. U.S. Robotics modems use MNP Levels 4 and Level 5 data compression. MNP Levels 4 have been incorporated into ITU-T Recommendation V.42. Compare HST.

The word modem is actually an acronym for modulate and demodulate. A modem is a serial device that modulates the computer’s digital data into analog signals compatible with the Public Switched Telephone Network (PSTN) to communicate over an analog telephone line and demodulates analog signals into digital data to communicate with the computer.

Multi-User Dungeon- This goes by many other names as well. A format in which games are played over the internet. This is usually a text based game, but it can be used for conferencing and educational purposes. They are accessed via telnet or other specialized client programs. 

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Nonvolatile Memory (NVRAM)
User-programmable random access memory whose data is retained when modem power is turned off. Used in Courier modems to store a user-defined default configuration loaded into random access memory (RAM) at power on.

A company that develops software to allow PC’s to link together using today’s networking hardware.

(Non-volatile Random Access Memory) is user-programmable random access memory whose data is retained when modem power is turned off. Used in modems to store a user-defined configuration which can be loaded into RAM at power on. 

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Original Equipment Manufacturer.

Online Fallback
A feature that allows high speed error-control modems to monitor line quality and fall back to the next lower speed if line quality degrades. The modems fall forward as line quality improves. 

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A unit of data which is sent through a network. A packet will consist of many layers of information. Each layer of information will address an aspect of that data packet, from the “where” and “who” information to the type of information that is being sent.

Parallel Transmission
The transfer of data characters using parallel electrical paths for each bit of the character, for example, 8 paths for 8-bit characters. Data is stored in computers in parallel form, but may be converted to serial form for certain operations. See Serial Transmission.

An error-detection method that checks the validity of a transmitted character. Character checking has been surpassed by more reliable and efficient forms of block-checking, including Xmodem-type protocols and the ARQ protocol implemented in Courier modems. The same type of parity must be used by two communicating computers, or both may omit parity. When parity is used, a parity bit is added to each transmitted character. The bit's value is 0 or 1, to make the total number of 1's in the character even or odd, depending on which type of parity is used.

Personal Computer. Generally refers to an IBM compatible computer (386, 486, Pentium, etc.).

Post Office Protocol- A protocol designed to read electronic mail from a server. There are three versions: POP, POP2, and POP3 which are commonly used on today’s computers.

Point to Point Protocol- An internet protocol used to send packets of internet information over serial points to point links.

Plug-ins are software programs that extend the capabilities of your Web browser. They allow you to play audio, video, and view 3D graphics through your browser.

Exclusively owned, made, and sold by one holding a trademark or patent. HST is a proprietary protocol of U.S. Robotics.

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. 

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A waiting line. For example, in a print queue, if a document is sent by person A, then by person B, then by person C, the jobs to be printed would be stored in a queue, job A would be printed, then job B, then job C. 

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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 Access
A feature that allows a remotely-located user to view the Courier's configuration screens and change the Courier's configuration. Password protection is available.

Technical name for a standard phone cable used by telephones and modems. One RJ11 cable is supplied with each of our desktop modem products.

(Random Access Memory) is available for use when the modem is turned on, but clears of all information when the power is turned off. User may write to this memory. The modem’s RAM holds the current settings.

(Read Only Memory) is permanent memory and the user may not write to this memory. The modem’s factory settings are stored in ROM and can be read (loaded) into RAM as an operational configuration or current settings.

A device which passes information from one network to another. 

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Serial Port
A nine- or 25- pin male port found on the back of a computer, which, via an RS-232C cable, connects to the serial port on a modem. Most computers have two serial ports, COM1 and COM2. A computer’s serial ports are run by UART chips, which come in different speeds. See UART.

Serial Transmission
The transfer of data characters one bit at a time, sequentially, using a single electrical path. See Parallel Transmission.

Signal Ground

Software that is offered free to try. If the software is used beyond the trial period, it will need to be registered with the company or individual that produced it. There is usually a fee for the registration of the software. Most shareware applications are posted on the Internet, on-line services, and Bulletin Boards.

Simplex (Transmission)
Indicates a comunications system capable of transmitting a signal in only one direction, for example, an FM radio channel.

Subscriber Line Interface Circuit - A standard RJ11 telephone jack

Serial Line Internet Protocol- Is used to connect a remote computer to the Internet using a modem.

U.S. Robotics line of desktop modems.

Start/Stop Bits
The signaling bits attached to a character before the character is transmitted during Asynchronous Transmission.

Synchronous Data Link Control. A protocol developed by IBM for software applications and communicating devices operating in IBM's Systems Network Architecture (SNA). The protocol defines operations at the link level of communications, for example, the format of data frames exchanged between modems over a phone line. See Bisync, Protocol, HDLC.

Simple Mail Transfer Protocol- One of the protocols under the TCP/IP group of protocols used to transfer electronic mail between computers.

Synchronous Transmission
A form of transmission in which blocks of data are sent at strictly timed intervals. Because the timing is uniform, no Start or Stop bits are required. Compare Asynchronous Transmission. Some mainframes only support synchronous communications unless their owners have installed a synchronous adapter and appropriate software.

A SYStems OPerator. A person who runs a bulletin board (BBS). 

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Terminal Adapter - Used in ISDN connections

Transmission Control Protocol / Internet Protocol- A protocol that allows you to access services on the internet such as Telenet, FTP, email and other services.

A TCP/IP application that allows you to connect to other host systems on the internet. Some Bulletin boards can be access via Telnet.

A device whose keyboard and display are used for sending and receiving data over a communications link. Differs from a microcomputer in that it has no internal processing capabilities. Used to enter data into or retrieve processed data from a system or network.

Terminal Mode
An operational mode required for microcomputers to transmit data. In Terminal mode the computer acts as if it were a standard terminal such as a teletypewriter, rather than a data processor. Keyboard entries go directly to the modem, whether the entry is a modem command or data to be transmitted over the phone lines. Received data is output directly to the screen. The more popular communications software products control Terminal mode as well as enable more complex operations, including file transmission and saving received files.

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

Portable modem company which USR acquired in 1991, therefore giving U.S. Robotics its WorldPort product line.

Transmission Rate
Same as Bit Rate.

Transmit Data 

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Universal Asynchronous Receiver- Transmitter. The controlling component of a serial port. There are three common types of UARTs: 8250, 16450, and 16550, which are capable of safe transmission speeds of 19,200 bps, 38,400 bps, and 57,600 bps respectively, though higher speeds are sometimes possible. Our internal modems have 16550 UARTs built in. External modems do not have UARTs.

One of many computer operating systems which are used to control a computer. UNIX was the main operating system of the Internet at one time. There are other operating systems used on the Internet however UNIX is still popular with many Web developers. UNIX is offered in many different varieties some are OSF, Version 7, Linux, Xenix, and GNU.

Fixes to existing code of the modem and there is no charge associated with updates.

User’s Network also referred to as “Newsgroups.” A large collection of discussion groups on all topics. Messages are posted to these groups as mail messages.

Uniform Resource Locator - Used to point to a particular internet site

Universal Serial Bus - Specification for increasing serial bus throughput. 

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An ITU-T standard, included with V.34, that sends a signal to the receiving modem to make the process of connecting faster.

An ITU-T standard for facsimile operations that specifies modulation at 14.4K bps, with fallback to 12K bps.

V.21 190 Fax
An ITU-T standard for facsimile operations at 300 bps. U.S. Robotics or compatible fax devices then transmit or receive at higher speeds.

V.21 190 Modem
An ITU-T standard for modem communications at 300 bps. Modems made in the U.S. or Canada follow the Bell 103 standard. However, the modem can be set to answer V.21 calls from overseas.

A ITU-T standard for modem communications at 1200 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 2400 bps. The standard includes an automatic link negotiation fallback to 1200 bps and compatibility with Bell 212A/V.22 modems.

An ITU-T standard for modem communications at 1200 bps with a 75 bps back channel. Used in the U.K.

An ITU-T standard for modem communications. Among other things, V.25 specifies an answer tone different from the Bell answer tone. All U.S. Robotics modems can be set with the B0 command so that they use the V.25 2100 Hz tone when answering overseas calls.

V.25 bis
An ITU-T standard for synchronous communications between the mainframe or host and the modem using the HDLC or character-oriented protocol. Modulation depends on the serial port rate and setting of the transmitting clock source, &X.

V.27 terbo
An ITU-T standard for facsimile operations that specifies modulation at 4800 bps, with fallback to 2400 bps.

An ITU-T standard for facsimile operations that specifies modulation at 9600 bps, with fallback to 7200 bps.

An ITU-T standard for modem communications at 9600 bps and 4800 bps. V.32 modems fall back to 4800 bps when line quality is impaired, and fall forward again to 9600 bps when line quality improves.

V.32 bis
An ITU-T standard that extends the V.32 connection range: 4800, 7200, 9600, 12K and 14.4K bps. V.32 bis modems fall back to the next lower speed when line quality is impaired, and fall back further as necessary. They fall forward to the next higher speed when line quality improves.

V.32 terbo
Modulation scheme that extends the V.32 connection range: 4800, 7200, 9600, 12K, 14.4K, 16.8K, 19.2K, and 21.6K bps. V.32 terbo modems fall back to the next lower speed when line quality is impaired, and fall back further as necessary. They fall forward to the next higher speed when line quality improves.

An ITU-T standard that allows data rates as high as 28.8K bps. See V.8.

An ITU-T standard for modem communications that defines a two-stage process of detection and negotiation for LAPM error control. V.42 also supports the MNP error control protocol, levels 4.

V.42 bis
An extension of ITU-T V.42 that defines a specific data compression scheme for use with V.42 and MNP error control.

V.Fast Class (V.FC)
Proprietary modulation scheme developed by Rockwell International for data communication speeds up to 28.8K bps.

Voltage, Alternating Current 

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Wide Area Network- A network that is not limited to a specific location. A WAN may link up several LANs together as one network.

Word Length
The number of bits in a data character without parity, start or stop bits.

Portable fax and data modems that are designed for business people who travel. 

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The first of a family of error control software protocols used to transfer files between modems. These protocols are in the public domain and are available from many bulletin board services. Other related protocols are Ymodem and Zmodem.

Standard ASCII control characters used to tell an intelligent device to stop/resume transmitting data. In most systems typing <Ctrl>-S sends the XOFF character. Some devices, including the Courier, understand <Ctrl>-Q as XON; others interpret the pressing of any key after <Ctrl>-S as XON. 

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X.75 defines the connection between public networks such as larger computers and mainframes.

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

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