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Chapter 4 - Command Mode
Bell/V.21 Tone B
The
B
command selects the frequency that the modem uses for its answer tone. (The answer tone is
the tone transmitted by a modem receiving a call to the modem that called it, which initiates the
handshaking between the two modems.) At higher speeds there is no conflict, because all use CCITT
frequencies. At lower speeds (0-1200 bps), in the U.S., some modems use the Bell frequency of
2225 Hz. However, the CCITT specification for V.22 has an answer tone frequency of 2100 Hz.
The
ATB0
command enables CCITT frequencies including V.21 (300 bps) and V.23 (1200/75 bps).
The
ATB1
command enables Bell frequencies including Bell 103 (300 bps). The modem default is
ATB0
.
Phone Line Monitoring Speaker M
The modem has an internal on-board, speaker which functions like the speaker in a telephone
handset to enable you to monitor phone line activity. The
M
command can be used to determine
when the speaker is operational.
Entering
ATM0
disables the speaker completely, while
ATM1
causes the speaker to be on only until
a carrier signal is detected.
ATM2
causes the speaker to remain on at all times.
ATM3
causes the
speaker to be on during dialing and off during handshake.
ATM1
is the default setting, and the one
that should be used in most applications.
Enable or Disable Recognition of Remote Digital Loop Signal &T
The modem has several self-test features (covered in Chapter 8). The tests are activated with
different
U
commands, such as
ATU1
, and so forth. The
&T
command is a phone line conditioning
command that enables or disables the modem’s ability to recognize the Remote Digital Loop (
RDL
)
test signal.
The
AT&T4
command lets the modem respond to a RDL signal, and places itself in digital loop. The
AT&T5
command causes the modem to ignore the RDL signal. The factory default is
&T5
.
Enable/Disable Trelis Coded Modulation #T
This command enables or disables the Trellis Coded Modulation of the modem. There is usually no
need to disable (turn off) Trellis codings except under an unusual line condition called impulse noise.
AT#T0 turns Trellis coding off and AT#T1 turns Trellis coding on (factory default).
Fallback Modes When On-Line #F
If line conditions deteriorate, the modem automatically drops its transmission speed (
“fallback”
). The
#F
command controls the different ways the modem falls back. During operation, if the error rate
becomes too great, the modem performs a retrain. If after the retrain, the error rate is still too high for
33600 bps operation, the modem initiates a retrain at 4800 bps. If after the first retrain the modem
returns on line at 33600 bps , the modem then starts a counter and a timer. If three retrains occur
within a two minute period, the modem falls back to 4800 bps.
Entering
AT#F0
(or
AT#F
) allows no fallback when on-line.
AT#F1
allows the modem to fallback (based on the error rate or if three retrains have occurred within
a two minute period) from:
33.6K to 31.2K to 28.8K to 26.4K to 24K to 21.6K to 19.2K to 16.8K to 14.4K to 12K
to 9.6K to 4.8K bps.
The
AT#F2
command enables incremental fall back, but also enables incremental fall forward if the
phone line improves.
AT#F2
is the factory default setting.