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Contents:
Answer Mode
You (or your software) puts the modem in Answer Mode when expecting a call from another computer. The modem uses "originate mode" when calling another computer. Although your modem can operate in either mode, some modems operate only in originate mode. To communicate with these modems, your modem can switch to answer mode after placing (originating) a call.
Asynchronous Communication
Communication between computers at irregular intervals. To handle this kind of communication, the modem adds start and stop bits to each character it sends. The start bit lets the receiving computer know when a new character has started. The stop bit indicates the end of the character.
ASCII
ASCII stands for "American Standard Code for Information Interchange." This is a standard method of encoding characters that enables text generated by one program or computer to be understood by another program or computer.
AT Command Set
The set of industry-standard commands used to control the modem. Each command line must start with the two-character attention code AT (or at).
Attention Code
The two-character sequence AT or at. Signals the modem that one or more modem commands are to follow.
Automatic Answering (Or Auto Answer)
The modem automatically answers the phone after a certain number of rings. This is in contrast to manual answering, where a person sends an AT command to the modem to cause the modem to answer a ringing telephone.
Automatic Dialing
The modem automatically dials the telephone. This is in contrast to manual dialing, where a person dials the number.
Auto-Reliable Mode
The modem automatically negotiates with the remote modem for a connection, determines whether or not it can use error control and data compression during a transmission, and determines which error control and data compression protocol it can use.
Baud Rate
The number of symbols transmitted per second. This is not always the same as the bps rate (see "Bits per second"), because a given symbol, or baud, may have more than one bit.
Bits Per Second (BPS)
The speed at which a modem sends or receives information. For example, a modem that operates at 2400 bits per second can transfer 2400 binary digits each second. A character normally consists of seven or eight of these binary digits, plus the start and stop bits that separate the character from other transmitted characters.
Buffer
A storage location in its internal RAM where the modem can temporarily store data it receives until it can process the data.
Carrier
The base signal used to transmit data across a telephone line. The modem modulates this signal (alters its frequency or phase) to encode the data to be transmitted.
Clear To Send (CTS)
An RS-232C signal that tells the computer it can start sending information. See also Request to send (RTS).
Command Mode
One of the two operating modes of the modem, sometimes called local mode or terminal mode. (The other is on-line mode.) In command mode, the modem interprets any information it receives from the local computer (or terminal) as modem commands. It tries to perform the commands sent to it, and it returns result codes indicating the results of the commands. The modem enters command mode when you turn on or reboot your computer (or terminal), or when you type the escape sequence (normally +++) while the modem is in on-line mode.
Communication (COM) Ports
The connectors and accompanying circuitry on your computer that let you connect serial devices (usually serial printers, modems, or mice) to your computer and communicate with these devices. Communication ports are also called serial ports. To keep track of the devices, DOS assigns names that begin with the letters COM to communication ports (such as COM1 and COM2).
Communications Software
A program that sets up your modem and gives you a human interface to the various functions of your modem.
Configuration Profile
The current operating characteristics of your modem, stored in the modem's S-registers. Whenever you issue a modem command to change one of the operational characteristics (such as setting the volume control or turning the speaker on or off), the modem changes the values in the S-registers to reflect your changes.
Data Carrier Detect (DCD)
An RS-232C signal that indicates the modem is receiving a carrier signal from a remote modem.
Data Compression
The encoding of text or data so that it takes up less space (fewer bits). CCITT V.42bis and MNP class 5 are two different data compression protocols. Data compression allows your modem to transmit more information in a shorter period of time and thus increases your modem's data throughput.
Data Set Ready (DSR)
An RS-232C signal that tells the computer (or terminal) the modem is connected to the telephone line.
Data Terminal ReadY (DTR)
An RS-232C signal that tells the modem the local computer (or terminal) is ready for data transmission.
Dial Modifiers
AT commands that you enter along with the AT command D (dial) to further instruct the modem when dialing the telephone. Dial modifiers specify things like whether to use pulse or tone dialing, when and how long to pause between numbers, and whether to dial a stored number.
Duplex
The way modems exchange data: half duplex or full duplex. With half duplex transmissions, only one modem can send data at a time. Full duplex transmissions allow both modems to send data simultaneously.
Error Control
The encoding of text or data so that a receiving modem can detect and sometimes correct errors in data transmissions. LAPM and MNP classes 1 through 4 are two different error control protocols.
Escape Sequence
A sequence of three characters (normally +++) that switches the modem from the on-line mode to the command mode without breaking the telephone connection.
File Transfer Protocol
An error-checking protocol for file transfers (such as KERMIT, XMODEM, or YMODEM). It monitors information sent with each block of data. If the received data doesn't match the information used to check the quality of data, the system notifies the sender that an error has occurred and asks for a retransmission.
Flow Control
The mechanism that regulates the flow of data between two devices. Modems typically have two methods of flow control software flow control (XON/XOFF) and hardware flow control (CTS/RTS).
Guard Time
A period of time during which the modem must not receive characters. The escape sequence (normally +++) has a guard time to ensure that data sent from a remote modem isn't interpreted as an escape sequence. The factory setting for the guard time is one second. For the modem to interpret an escape code as valid, there must be one second before and one second after the escape sequence in which the modem doesn't receive any characters.
Hayes Compatibility
Software (and people) communicate with modems using the AT command set originally developed by Hayes Corporation. Modems that use the AT command set are said to be Hayes-compatible.
Hook Flash
The act of hanging up the telephone for a short period of time (one-half second). This operation is useful in PBX systems that interpret a hook flash as a signal to perform other services, such as transferring a call.
LAPM (Link Access Procedure For Modems)
LAPM is an error control protocol specified by CCITT V.42. LAPM provides error control when your modem is communicating with another modem that supports LAPM.
Line Noise
Random signal disturbances that sometimes occur over telephone lines. Noise can disrupt communications and corrupt the transmitted data.
Loopback Test
Diagnostic test where characters that are sent to the modem are immediately sent back from the modem so the computer can compare the characters sent with the characters received.
Manual Dialing
Dialing a remote modem from a telephone connected to the modem. This is in contrast to automatic dialing, where the modem dials the number.
Modem (Derived From Modulator/Demodulator)
A modem transforms (modulates) digital information into an analog signal that can be sent across a telephone line. It also demodulates an analog signal it receives from the telephone line, converting the information contained in the signal back into digital information.
Nonvolatile Memory
Memory that retains the information stored in it even if you turn off the computer. There are two kinds: ROM and NVRAM. ROM is programmed with information at the factory, and you can't change it. NVRAM is used to store configuration information, and you can changed it with modem commands.
NVRAM (Nonvolatile RAM)
See Non-volatile memory.
Off-Hook
The condition of a telephone line that corresponds to picking up the telephone receiver. A modem creates an off-hook condition when it tries to communicate on a telephone line.
On-Hook
The condition of a telephone line that corresponds to hanging up the telephone receiver. A modem creates an on-hook condition to break its connection to a telephone line.
On-Line Mode
One of the two operating modes of the modem, also called data mode. (The other is command mode.) In on-line mode, the modem interprets all information sent to it as data, not commands. The only exception is the escape sequence (normally +++), which returns the modem to command mode without breaking the connection. The modem is placed in on-line mode when it makes a connection with a remote modem, or when you enter the O command to return to a previously established connection.
Originate Mode
The mode used by the modem when calling another computer. See also "Answer mode."
Parity
A data encoding scheme that computers (and terminals) use to check the validity of transmitted characters. This scheme adds an extra bit to each character, which the transmitting computer sets or clears based on the type of parity the computers agree to use (odd or even). For example, if the computers use even parity, the transmitting computer sets or clears the parity bit so that there are an even number of bits set in each character it transmits. The receiving computer checks each character and flags a parity error if any character has an odd number of bits set.
Protocol
A set of rules that governs how data is transmitted. To communicate successfully, two computers must use the same protocol.
Pulse Dialing
A method of dialing the telephone where the modem sends pulses (which you hear through the handset as clicks) to represent the telephone numbers (one pulse for a one, two pulses for a two, etc.). Pulse dialing is normally associated with rotary-dial phones. See also "tone dialing."
Reliable Connection
A connection between two modems where they communicate using an error control protocol (such as LAPM or MNP).
Request To Send (RTS)
An RS-232C signal that requests the modem to send data. It initiates any data transmission between the computer (or terminal) and the modem. It is answered by a Clear to send (CTS) signal. See also "Clear to send."
Result Codes
When you enter most modem commands, the modem displays a message on your screen, called a result code, that tells you the results of the command. In the factory setting, result codes are returned as English words that describe the conditions. You can also set the modem to display corresponding numbers instead of words.
RS-232C
A standardized connection system for connecting a device to the serial port of a computer or terminal. This is the recommended standard of the Electronic Industries Association (EIA) for exchanging information between Data Terminal Equipment (such as computers) and Data Communications Equipment (such as modems).
S-Registers
RAM in your modem that is used to store the current configuration profile (operating characteristics) of your modem.
Serial Port
The circuits and connector that let your computer communicate with serial devices such as printers, modems, plotters, mice, and custom laboratory equipment. (Also called a COM or communications port.)
Software Flow Control
See XON/XOFF.
Start/Stop Bits
The bits at the beginning and end of a data block when using asynchronous data transmission. See also "Asynchronous Data Transmission."
Throughput
The total useful information processed or communicated in a specified amount of time. Data compression increases the throughput of your modem by allowing you to send more information in the same number of bits.
Tone Dialing
One of two methods of dialing the telephone. (The other is pulse dialing.) With tone dialing, the modem sends tones of different frequencies to represent the telephone numbers. Tone dialing is normally associated with push-button (touch-tone) phones and is also called Dual Tone Multi-Frequency (DTMF) dialing.
V.21, V.22, V.22BIS, V.32, V.32BIS, V.42, V.42BIS, V.54
CCITT specifications for modems.
XON/XOFF
A way of controlling the flow of data between a modem and its host computer and between two modems, also called software flow control. XON stands for "Transmitter On," and XOFF stands for "Transmitter Off." If the modem receiving data needs time to process the data or do some other task, it sends an XOFF signal to the host computer (or sending modem). The host computer (or sending modem) then waits until it receives an XON signal before sending more data.
This applies to:
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