asynchronous transmission
Data transmission in which the length of time between transmitted
characters may vary. Since the time lapses between transmitted characters
are not uniform, the receiving modem must be signalled as to when
the data bits of a character begin and then they end. The addition
of start/stop bits to each character serves this purpose.
|
auto answer
In this setting the modem can pick up the phone line when it detects
a certain number of rings. See S-register S0 in the "Technical Reference"
section. |
autodial
A process where your modem dials a call for you. The dialling process
is initiated by sending an ATDT (dial tone) or ATDP (dial pulse) command
followed by the telephone number to dial. Autodial is used to dial
voice numbers. See command Dn. |
baud rate
A term used to measure the speed of an analogue 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, which reflects the use of the binary numbering system.
It is used because the computer recognises either of two states, OFF
or ON. The shortened form of binary digit is bit. |
bit rate
This refers to the number of binary digits, or bits, transmitted
per second (bps). It is also referred to as transmission rate. Communications
channels using telephone channel modems are established at set bit
rates, commonly 2400, 4800, 9600, 14,400, 28,800 and higher. |
bits per second (bps)
This is the bits (binary digits) per second rate. Thousands of
bits per second are expressed as kilobits per second or kbps. |
buffer
A memory area set aside to be used as temporary storage during
input and output operations. An example is the modem's command buffer. |
byte
A group of binary digits stored and operated upon as a unit. In
user documentation, the term usually refers to 8-bit units or characters.
One kilobyte (KB) is equal to 1,024 bytes or characters; 640 KB indicates
655,360 bytes or characters. |
carrier
A tone signifying a connection the modem can alter to communicate
data across telephone lines. |
character
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 between 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 code and responds with either a positive or negative
acknowledgement. |
data communications
A type of communications in which computers are able to exchange
data over 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
The mode in which the faxmodem is capable of sending and receiving
data files. A standard modem without fax capabilities is always in
data mode. |
DCE
Data Communications Equipment (or Circuit-Terminating Equipment)
is equipment such as dial-up modems that establish and control the
data link via the telephone network. |
default
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. |
detect phase
In the ITU-T V.42 error-control protocol, the first stage in establishing
if both modems attempting to connect have V.42 capability. |
dictionary
The term used for compression codes built by the V.42 bis data
compression algorithm. |
digital loopback
A test that checks the modem's RS-232 interface and the cable
that connects the terminal (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
Signals that are discrete and uniform. In this manual, the term
refers to the binary digits 0 and 1. These signals are in contrast
with analogue signals. |
DTE
Data Terminal (or Terminating) Equipment is a computer that generates
or is the final destination of data. |
duplex
Duplex indicates a communications channel capable of carrying
signals in both directions. See half duplex, full duplex. |
Electronic Industries Association (EIA)
This association is a group which defines electronic standards
in the U.S. |
error control
A variety of 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). |
facsimile
A method for transmitting the image on a page from one point to
another. This is commonly referred to as fax. |
fax mode
The mode in which the faxmodem is capable of sending and receiving
files in a 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 commands &Hn, &In,
&Rn. |
frame
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. |
full duplex
These signals will flow in both directions at the same time over
one line. In microcomputer communications, may refer to the suppression
of the online local echo. |
half duplex
These signals will 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. |
Hz
Hertz is a frequency measurement unit used internationally to
indicate cycles per second. |
ITU-T
An international organisation 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. |
LAPM
Link Access Procedure for Modems is 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 command En. |
MNP
Microcom Networking Protocol is 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 erred
frames. |
modem
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)
A user-programmable random access memory which retains data when
power is turned off. On some modems, 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 a high-speed, error-control modem 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 modem
switches 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. |
parity
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. |
protocol
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. |
RAM
Random Access Memory is 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. |
ROM
Read Only Memory is permanent memory, which is not user-programmable. |
serial transmission
The consecutive flow of data in a single channel. Compare it to
parallel transmissions where data flows simultaneously in multiple
channels. |
start/stop bits
These signalling bits are attached to a character before and after
the character is transmitted during asynchronous transmission.
|
terminal
A device whose keyboard and display are used for sending and receiving
data over a communications link. This device 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.
This mode is also known as command mode. |
throughput
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 it with characters per second. |
V.8
The ITU-T standard specification that covers the initial handshaking
process. |
V.17
An ITU-T standard for making facsimile connections at 14,400 bps,
12,000 bps, 9600 bps, and 7200 bps. |
V.21
An ITU-T standard for modems operating in asynchronous mode at
speeds up to 300 bps, full-duplex, on public-switched telephone networks. |
V.22
An 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. |
V.23
An ITU-T standard for modem communication at 1200 bps with a 75
bps back channel. |
V.27ter
An ITU-T standard for facsimile operations that specifies modulation
at 4800 bps, with fallback to 2400 bps. |
V.29
An ITU-T standard for facsimile operations that specifies modulation
at 9600 bps, with fallback to 7200 bps. |
V.32
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. |
V.32 bis
An ITU-T standard that extends the V.32 connection range: 4800,
7200, 9600, 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 back up) when line conditions
improve.
See online fall back/fall forward. |
V.34
An ITU-T standard that currently allows data rates as high as
28,800 bps and 33,600bps. |
V.42
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. |
V.90/V.92
The ITU-T standard for 56 Kbps modem communications. |
Xmodem
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. |
Xon/Xoff
Standard ASCII control characters used to tell an intelligent
device to stop/resume transmitting data. |
Ymodem
An error-checking protocol that can send several files of data
at a time in 1024-byte (1K) blocks. This protocol can use either checksums
or CRC for error checking. |
Ymodem G
This is similar to the Ymodem, except it relies on the modem for
error checking, which makes it faster. |
Zmodem
This is similar to Xmodem and Ymodem, except it includes batch
transfer, the ability to recover from a partially complete transfer,
an autostart feature, and improved efficiency. |