COURIER HIGH SPEED MODEMS
A-4 Link Negotiation and Error Control
Dual Standard Handshaking
We recommend that Dual Standard modems be set to retain the
default B0 and &N0 settings. This allows them to connect with
V. protocol modems and HST modems in both Originate and
Answer modes at a variety of speeds.
An originating Dual Standard modem set to B1 sends out a Bell
answer tone, which is the prevalent standard in the U.S. and
Canada for connections at 2400 bps and lower. At higher
speeds, the Dual Standard also recognizes the ITU answer tones
necessary for connecting with V. protocol modems, and adjusts
to the answering modem.
However, in Answer mode, a Dual Standard modem sending
out the Bell answer tone (B1) won't be recognized by V. protocol
modems. The calling V. protocol modem, instead, will wait
until it detects a tone it recognizes, the V.22 bis tone used at 2400
bps. If you wish to have your Dual Standard connect with V.
protocol modems at high speeds, make sure it is set to B0 for the
ITU answer tones. It will also connect with HST modems at
speeds up to 16.8K bps.
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.
