User's Guide

User’s Guide CSC Operation

2110059 Rev 1.1 Proprietary and Confidential Page 61

6.5.1. Factory Default for ETC

The &F5 command will configure the SB320 for optimal performance on most cellular systems.

This AT command is designed to make it easy for the user or application to set up the modem in

ETC mode. Sending just this one command will cause the modem to reconfigure all its registers,

including those that are not associated with ETC, to factory default and optimized for ETC

operation.

It is equivalent to sending AT&F0 followed by a series of additional AT commands and S-

command. A brief description and commentary of each of these additional settings is given

below.

\N5 Enables V.42 LAPM error control only. This is a requirement of ETC. The

modem will disconnect if the remote end does not support LAPM.

-K0 This is a companion to the above command indicating a preference for LAPM

when using V.42.

*H2 This sets the Link Negotiation Speed for the initial connection to 4800 bps. The

policy of “start slow and train up” guarantees that the initial negotiation phase

has more chance of success. Start-up speeds of 4800 bps and 9600 bps (*H3)

may be used. In addition if a connection has not been established after 60

seconds, the modems will fall back to 1200 bps.

S7=120 The Wait for Carrier is set to 120 seconds. The handshaking process with ETC

enabled and cellular generally can take longer than the default 50 seconds. This

is especially true of modem pools are being used.

S10=100 Lost Carrier Hang-up Delay is set to 10 seconds. The default is 0.6 seconds but

cellular links can be interrupted during cell changes and due to other “noise” so

the modem will wait longer to determine if the connection is truly lost.

S51=15 This sets the V.34 symbol rate to 2800 symbols per second and the carrier

frequency control to automatic for transmissions. The default would allow 3429

symbols per second, which is likely to lead to more failures and retransmissions.

S52=15 This does the same as S51 but for the V.34 receive side.

S55=37 V.34 options are set to enable asymmetric bit rates, and non-linear encoding. It

disables asymmetric rates, constellation expansion and receiver pre-encoding.

S70=20 This extends the maximum number of retransmissions from the default 12 up to

20.

S96=31 ETC Options are set to enable all features for ETC handshaking. This includes

enabling a fallback to 1200 bps if higher rates cannot be supported. It also

enables /ETC to be appended to the CONNECT message.

S106=32 The frame size for LAPM is set to the minimum 32. The default is the maximum

of 128. Use of a small frame size does reduce direct throughput, but gives a

more robust connection. This leads to fewer retransmissions and can improve

overall throughput in noisy connections that require frequent retransmissions.

S108=3 Signal Quality is selected to high, meaning that automatic re-training is activated

at the most sensitive setting. This means that the modem will re-train on slight

changes in line quality. This is intended to keep the rate as high as practical.

6.5.1.1. Data Rate

Even with optimal configuration, modems will have a tendency to attempt to communicate at a

data rate that is faster than optimal for overall throughput. For this reason it is recommended that