User's Guide

The Operational View SB300 Series
Page 62 Proprietary and Confidential 2110059 Rev 1.1
Remember that the factory setting above sets the speed for the initial negotiation at 4800 bps but
the modem can train up to a maximum set in S37. The default value for S37 is to connect no
faster than the DTE connection speed (currently fixed at 19200 bps). This will happen only if the
line conditions allow it. Otherwise, the modem will connect at a lower speed.
If the modem is found to be training up and down too often due to a constantly changing line
quality, it may be better to reduce the upper limit to 7200 bps (s37=8) or 4800 bps (s37=7). This
will result in a more stable connection and eliminate excessive speed negotiation. This is feasible
if speed is not critical to the application or if small amounts of data are transferred at any one time.
Alternatively, if retraining is too frequent, the line quality threshold for retraining can be shifted
down using S108. This should only be tried under controlled trial conditions to determine the best
balance between signal threshold and maximum line speed. Transmitting a 100k test file gives a
reasonable measure of throughput efficiency.
Based on field experience using the &F5 settings, the following limits are recommended:
• V.32bis talking to cellularly aware landline modem – 9600 bps (S37=9)
• V.32bis talking to standard wireline modem – 4800 bps (S37=7)
• V.34 talking to cellularly aware modem – 12000 bps (S37=10)
• V.34 talking to standard wireline modem – 9600 bps. (S37=9)
If only a small amount of data is being transferred, it is recommended that a data rate of 4800 bps
(S37=7) is used. This will give optimal performance and coverage area. Error Correction should
always be enabled. Flow control should be used (hardware recommended).
For situations where even 4800 bps does not hold a connection, the last suggestion is to try a data
link at 1200 bps (s37=5).
6.5.2. Connections to ETC Enabled Modems
Enhanced Throughput Cellular (ETC) is a protocol enhancement designed to optimize cellular
data communications. It is described in Section 4.5.2.1.
This is the best case scenario and it should be possible to achieve speeds of 9600 – 19200 bps if
the host supports V.34 and ETC. The table below lists some of the steps for troubleshooting bad
connections at both the SB320 end and the Host modem end. As a rule, when starting the
troubleshooting procedure, both modems should be reset to their respective factory defaults
settings. If problems still persist, then a more thorough analysis of each modem’s settings needs
to be done as outlined below.
6.5.3. Connections to Non-ETC Compatible Modems
If the remote modem does not support LAPM and V.22bis then you will have to allow the SB320
to use alternative protocols. The &F0 command is used. This AT command configures the
modem in CSC data mode but without ETC enabled.
An additional series of AT commands and S-Register settings should also be set to optimize the
modem to operate in a cellular environment without the benefits of the ETC protocol. This non-
ETC configuration will then allow the modems to negotiate other protocols such as MNP, etc.
The following additional commands should be sent to configure the modem for CSC mode use
with non-ETC compatible modems:
\N3 Enables V.42 LAPM error control but also allows MNP and will even tolerate a
non-error controlled link. A setting of 2 is also possible if you want to
disconnect if neither LAPM nor MNP error control can be negotiated.
-K0 This is a companion to the above command indicating a preference for LAPM
when using V.42.