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extended AT-command . Extended commands were developed to
provide greater functionality and control over modem operations
than is available from the basic AT command set.
F
fax mode
. The modem is in fax mode when, through use of fax
communications software, it can send and receive faxes, print and
display fax files, convert files to fax-files, and set certain fax-related
features. Note: the modulation protocol used by the modem in fax
mode is also different from the usual data mode modulation.
flow control
. Compensates for the difference between the rate at
which data reaches a device and the rate at which the device
processes and transmits. This is controlled by the extended AT
command &K. The two common types of flow control are RTS/CTS
signaling (a hardware based method, employing an electrical signal)
and XON/XOFF (a software-based method using standard ASCII
control characters to pause or resume transmission).
full-duplex . Two-way simultaneous transmission between modems,
which may occur via a four-wire circuit on a leased line, or with a
two-wire connection when the frequency bandwidth is divided into
two distinct channels, or when echo cancellation is employed (e.g.,
Bell 103, 212, and V.22 use frequency division, while V.32 uses echo
cancellation).
G
guard tone
. Guard tones are used in the United Kingdom and other
countries. This requires that the modem transmit an 1800-Hz tone
after it sends an answer tone. The guard tone is controlled by the &G
command. Guard tones are not used in the U.S.A.
H
half-duplex . Signal flow in both directions, but only one way at a
time with each modem alternating between send and receive.
Hayes-compatible . Hayes Microcomputer Products, Inc. developed
the AT command set which has become a de facto industry standard.
Hayes commands are always initiated with an AT (attention code)
prefix.
hook flash . The dial modifier “!” causes the modem to go on-hook
(hang-up) for one-half second. Also controlled by the ATH command.
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