US Robotics COURIER - Error Control and Throughput

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COURIER HIGH SPEED MODEMS
efficient line operation. If the impairments are severe, the
modems use normal retraining.
Dual Standard Handshaking
Dual Standard modems always connect with each other, either
in HST mode or in V.32 bis mode. However, we recommend
that Dual Standard modems be set to BO and to &NO, so that
they can connect with V.32 bis, HST and lower-speed modems,
in both Originate and Answer modes.
A
Dual Standard modem set to Bl (HST) does not send out any
V.32 bis or V.32 answer tones, and therefore cannot originate or
answer calls in those modes. A Dual Standard modem set to BO
will switch to HST mode for that call only if the other modem is
operating in HST mode.
ERROR CONTROL AND THROUGHPUT
Overview
Error control is available for calls at 1200 bps and above. It can
be disabled, although high speed calls (above 2400 bps) should
always be under error control. The operations defined in an error
control protocol include the following:
• Establishment of compatibility
• Data frame formatting
• Error detection
through Cyclic Redundancy Checking
(CRC)
• Retransmission of
corrupt data frames
The
Courier is set at the factory to &M4, causing it to try for an
error control connection and, if that isn't possible, to proceed
with the call in Normal mode. The modem first tries for a V.42
connection,
then an MNP connection. The following information
is
based on the Courier's setting of &M4.
Link Negotiation and Error Control A-5