User's Guide

User’s Guide CSC Operation

2110059 Rev 1.0 Proprietary and Confidential Page 57

*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

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.