US Robotics COURIER - Page 194

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COURIER HIGH SPEED MODEMS
Online Fallback
A feature that allows high speed error-control modems to mon-
itor line quality and fall back to the next lower speed if line
quality degrades. The modems fall forward as line quality
improves.
Originate Mode
A state in which the modem transmits at the predefined low fre-
quency 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 con-
verted to serial form for certain operations. See Serial
Transmission.
Parity
An error-detection method that checks the validity of a trans-
mitted 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 'sin the character even or
odd, depending on which type of parity is used.
Protocol
RAM
A system of rules and procedures governing communications
between two or more devices. Protocols vary, but communi-
cating 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 opera-
tions that may be defined in protocols.
Random Access Memory. Memory that is available for use
when the modem is turned on, but that clears of all information
Glossary G-9