User's Guide

User’s Guide CSC Operation
2110059 Rev 1.1 Proprietary and Confidential Page 63
*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 should be set higher than the default 50 seconds to allow for
the added time of cellular (and possibly modem pool) connections. This sets the
wait to 120 seconds.
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. Higher values are possible if needed.
S106=32 The frame size for error correction protocols 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.
The data rate discussion in Section 6.5.1.1 also applies to this type of connection.
6.5.4. Connections to Modem Pools
This scenario is also a best case scenario and it should again be possible to achieve speeds of
19200 – 9600 bps in most circumstances. Modem pools are discussed in Section 6.1.3 above.
The SB320 connection is a cellular one and therefore its pool counterpart will be an ETC-capable
modem. This means that the SB320 should be configured in its factory default ETC mode using
AT&F5 as described earlier.
The destination modem’s connection to the pool is a standard landline connection and the remote
modem must be configured as such. In this case the destination modem should be configured to
support the full array of protocols to give it the best possible options for connecting to the cellular
carrier’s modem pool on the landline side.
If any configuration adjustments are to be made, understand that you are either optimizing the
connection from the SB320 to the pool modem or from the destination modem to the pool and not
directly between the SB320 and the destination.