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13-4 C
HAPTER
13: H
ANDSHAKING
,
ERROR CONTROL
,
DATA COMPRESSION
,
AND THROUGHPUT
If the remote device has V.FC capability, the Courier V.Everything mode
uses a line probing technique to determine the highest speed possible
under current line conditions, then completes the connection. If the
remote analogue device does not have V.FC capability, the Courier
V.Everything modem listens to the device's answer tones to identify what
standard rate the remote analogue device is operating at, and adjusts t
that rate.
When a Courier V.Everything modem answers a call, it sends out a series
of answer tone signals until both devices negotiate the best connection
rate.
HST
We recommend that Courier V.Everything modems retain the default B
and &N0 settings. This allows them to make analogue connections wit
"V." protocol and HST modems in both Originate and Answer modes at a
variety of speeds.
When originating an analogue call, the Courier V.Everything 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 Courier V.Everything modem also recognises the ITU answer
tones necessary for connecting with V. protocol modems, and adjusts to
the answering device.
However, when answering a call, a Courier sending out the Bell answer
tone (B1) won't be recognised by V. protocol modems. The calling
modem, instead, will wait until it detects a tone it recognises (the V.22
bis
tone used at 2400 bps).
If you want to have your Courier connect with V. protocol modems at
high speeds, make sure it is set to B0 for the ITU answer tones. It will als
connect with HST modems at speeds up to 16.8 Kbps.
USR V.32
terbo
to USR
V.32 terbo
On these analogue connections, Courier V.Everything modems have two
features that result in outstanding performance: Quick Connect an
Adaptive Speed Leveling (ASL).
■
Quick Connect allows two Courier V.Everything modems to connect in
approximately 7 seconds, a far shorter time than with most devices.
■
ASL (described below in Other V.Protocol Operations) is used by
Courier V.Everything modems operating in V.32
terbo and V.32 bis
modes.