US Robotics COURIER - Page 105

COURIER HIGH SPEED MODEMS
highest possible speed, usually 14.4K bps. (If the calling
modem is a V.32 modem, its maximum speed is 9600 bps,
and the modems may connect at that rate. But see
paragraph 3.)
2. If the calling modem doesn't recognize the answer tone, the
answering modem then sends a 2250 Hz answer tone used
by 2400, 1200 or 300 bps modems. If the calling modem is
operating at one of those rates, the modems connect at the
calling modem's rate.
3. If the modems don't connect as in paragraph 1 or 2, the
answering V.32 bis modem then sends extra V.32 answer
tones. If the calling modem is a V.32 modem, the answering
V .32 bis modem shifts its speed down to 9600 bps and the
modems connect at that speed. This extended V.32
handshaking ensures connections with V.32 modems if they
did not connect as in paragraph 1.
It may even be necessary to extend the length of these tones
if the
modem is to answer older, "dumb" V.32 modems.
Register 528 is
used to modify the duration of these extra
V.32 tones. See 528
in Appendix B.
USRobotics V.32 bis to V.32 bis
1. The modems first negotiate a V .42 error control connection.
Speed negotiation is
part of the V.42 detection/LAPM
sequence. If they fail to make a V.42 connection, these
modems follow the procedures described under V.32 bis,
above.
2. The Receiver of each modem dictates its modem's highest
speed,
with negotiation beginning at 7200 bps and
proceeding upward to the 14.4K bps maximum. This means
that your modem's receiver may be operating at a different
speed from your modem's transmitter, which operates at the
same rate as the remote modem's receiver. In other words,
the rate at your modem's Originate frequency may be
different from the rate at your modem's Answer frequency.
Because of this asymmetrical design, one frequency may
suffer line hits or other impairments, and fall back to a
slower speed, while
the other does not. The result is more
A-4 Link Negotiation and Error Control
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