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GLS.TXT
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595 lines
COURIER 1200e
GLOSSARY
Cross-references in the following definitions are printed in
boldface.
Analog Loopback
A modem self-test in which data from the keyboard is sent to the
modem's transmitter, modulated into analog form, looped back to
the receiver, demodulated into digital form, and returned to the
screen for verification. Tests either the modem's originate or
answer frequency.
Analog signals
Continuous, varying waveforms such as the voice tones carried
over phone lines. Contrast with digital signals.
Answer Mode
A state in which the modem transmits at the pre-defined 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.
ARQ
Automatic Repeat Request. A general term for error control
protocols which feature error-detection and automatic
retransmission of defective blocks of data. See HST and MNP.
ASCII
American Standard Code for Information Interchange. A 7-bit
binary code (0s, 1s) used to represent letters, numbers, and
special characters such as $, !, and /. Supported by almost
every computer and terminal manufacturer. See Appendix B-6 for
decimal and hexadecimal code equivalents.
Asynchronous Transmission
Data 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 Synchronous Transmission.
Auto Answer
A feature in modems enabling them to answer incoming calls over
the phone lines without the use of a telephone receiver.
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Auto Dial
A feature in modems enabling them to dial phone numbers over the
phone system without the use of a telephone transmitter.
Baud Rate
The number of discrete signal events per second occurring on a
communications channel. Although not technically accurate, baud
rate is commonly used to mean bit rate.
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."
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 110, 300, 1200, 2400,
4800, 9600, and 144000.
BPS
The bits (binary digits) per second rate.
Buffer
A memory area used as temporary storage during input/output
operations. Two examples in the Courier HST are the command
buffer (last issued command) and the flow control buffer (for
data flow control and storing copies of transmitted frames until
they are positively acknowledged by the receiving modem).
Byte
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), for example, or have some other value
meaningful to the computer. In user documentation, the term
usually refers to 8-bit units or characters. 1 kilobyte = 1,024
bytes or characters; 64k = 65,536 bytes or characters.
Carrier
A continuous frequency capable of being either modulated or
impressed with another information-carrying signal. Carriers are
generated and maintained by modems via the transmission lines of
the telephone companies.
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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 CCITT V.22. For 2400 bps communication, most
U.S. manufacturers observe V.22bis. The initials CCITT represent
the French name; in English it's known as the International
Telegraph and Telephone Consultative Committee.
Character
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-Service
Class 3, employ advanced techniques to increase cps. When the
Courier HST is set for maximum throughput, as described in
Chapter 3, 140 characters in addition to the standard 960 can be
transmitted for a total of 1100 cps.
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 the Courier HST, the receiving
modem accepts no more data until a defective block is received
correctly.
Data Communications
A type of communications in which computers and terminals are
able to exchange data over an electronic medium.
Data Compression Table
A table of values assigned for each character during a call under
data compression. Default values in the table are continually
altered and built during each call: the longer the table, the
more efficient throughput gained.
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If a destructive Break is sent during a call (see the &Y
command), causing the modems to reset the compression tables, you
can expect diminished throughput.
DCE
Data Communication (or Circuit-Terminating) Equipment. In this
manual, the 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.
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Default
Any setting assumed, at startup or reset, by the computer's
software and attached devices, and operational until changed by
the user.
Digital Signals
Discrete, uniform signals. In this manual, the term refers to
the binary digits 0 and 1.
DTE
Data Terminal (or Terminating) Equipment. The device, such as a
personal computer, terminal, or mainframe, that generates or is
the final destination of data.
Duplex
Indicates a communications channel capable of carrying signals in
both directions. See Half Duplex, Full Duplex.
Equalization
A compensation circuit designed into modems to counteract certain
distortions introduced by the telephone channel. Two types are
used: fixed (compromise) equalizers and those that adapt to
channel conditions.
Error Control
Various techniques which check the reliability of characters
(Parity) or blocks of data. The Courier's error control protocol
provides error control through error detection (CRC) and
retransmission of errored frames (ARQ).
Flow Control
A mechanism that compensates for differences in the flow of data
input to and output from a modem or other device.
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 data.
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Full Duplex
Signal flow in both directions at the same time. In microcomputer
communications, may refer to the suppression of the online Local
Echo. The receiving computer may provide a Remote Echo.
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.
HST
High Speed Technology, part of the trademark for USRobotics' USR-
HST, a proprietary signaling scheme, design and protocol for 9600
bps modems. USR-HST incorporates trellis-coded modulation, for
greater immunity from variable phone line conditions, and asym-
metrical modulation for more efficient use of the phone channel.
HST also represents the enhanced error control protocol at 9600
bps that is similar to and compatible with MNP error control at
2400/1200 bps.
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.
MNP
Microcom Networking Protocol. An error control protocol
developed by Microcom, Inc. and now in the public domain. The
protocol ensures error-free transmission at 1200 and 2400 bps
through error detection (CRC) and retransmission of errored
frames.
Modem
A device that transmits/receives computer data through a
communications channel such as radio or telephone lines. The
Courier is a telephone channel modem that modulates, or
transforms, digital signals from a computer into a form that can
be carried successfully on a phone line. It also demodulates
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signals received from the phone line back to digital signals
before passing them to the receiving computer.
Intelligent modems with built-in software, such as the USRobotics
modem described in this manual, simplify using phone networks for
data communications.
Nonvolatile Memory (NRAM)
User-programmable random access memory whose data is retained
when modem power is turned off. Used in the Courier HST to store
a user-defined default configuration loaded into random access
memory (RAM) at power on.
OFF/ON Hook
Modem operations which are the equivalent of manually lifting a
phone receiver (taking it off hook) and replacing it (going on
hook).
Originate Mode
A state in which the modem transmits at the pre-defined low
frequency of the communications channel and receives at the high
frequency. The transmit/receive frequencies are the reverse of
the called modem which is in Answer Mode.
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.
Parity
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, a public domain protocol used in some telecommunications
software, and the ARQ protocol implemented in the Courier HST.
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 1s in the character
even or odd, depending on which type of parity is used.
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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 handling are only a few of the operations
that may be defined in a protocol.
RAM
Random Access Memory. Memory that is available for use when the
modem is turned on and clears of all information when the power
is turned off. The Courier HST's RAM holds the current
operational settings, a flow control buffer, and a command
buffer.
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. Permanent memory, not user-programmable. The
Courier HST's factory settings are stored in ROM and can be read
(loaded) into RAM as an operational configuration.
Serial Transmission
The transfer of data characters one bit at a time, sequentially,
using a single electrical path. See Parallel Transmission.
Start/Stop Bits
The signaling bits attached to a character before the character
is transmitted during Asynchronous Transmission.
Synchronous Transmission
A form of transmission in which groups of data bits are sent at
regular intervals. Because the timing is uniform, no Start or
Stop bits are required. See Asynchronous Transmission.
Terminal
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.
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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.
Throughput
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.
Transmission Rate
Same as Bit Rate.
V.22bis
The international CCITT standard for modem communications at 2400
bps. The standard includes an automatic data rate fallback to
1200 bps and compatibility with Bell 212A/V.22 modems.
Word Length
The number of bits in a character without parity, start or stop
bits.
XON/XOFF
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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