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56K Faxmodem User's Guide and Reference
Glossary
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.
