User Manual

Modems Modem and SCS Interaction
9-10
Before compression can be enabled, flow control must be enabled (see Flow Control on page 8-18). In
addition, the modem’s serial speed must be set higher than the line speed. This enables the SCS to keep the
modem’s internal data buffer filled with data to compress. As lower compression ratios decrease the
effective line speed, the modem will flow control the SCS more often. When compression ratios and the
effective line speed rise, the modem will flow control the SCS less often.
Note: On many modems, error correction must be enabled for data compression to
work properly. Error Correction is discussed on page 9-10.
To enable modem compression, use the following command:
Figure 9-9: Enabling Modem Compression
Note: For this command’s complete syntax, see Define Ports Modem Compression on
page 12-95.
When modem compression is enabled on a port, the SCS will send a string to the modem to instruct it to
enable modem compression. When compression should be disabled, a disable string may be sent. The
default enable and disable strings vary, depending upon the modem profile used. To display the default
strings for a particular modem profile, use the List Modem command.
To modify these strings, use the Define Ports Modem Compression command. The first string specified
is the disable string; the second is the enable string.
Figure 9-10: Changing the Disable and Enable Strings
The compression mode used varies from modem to modem, however, the most common mode is V.42bis.
This is the recommended method of data compression.
V.42bis encoding offers an automatic 20% savings on all data send, regardless of how compressible it is.
Some text files can be compressed down to 1/4 or less of their original size. In addition, V.42bis will enable
or disable compression according to whether or not it’s required.
Other compression modes, such as MNP, may not give the same results as V.42bis. To obtain the best
results, experiment with different modes of compression.
9.4.6 Error Correction
A modem profile’s Error Correction setting enables or disables the modem’s error correction mode. Error
correction modes enable modems to ensure data integrity in the presence of telephone line noise. These
modes work by checking the data for errors at the receiving modem. If an error is detected, the receiving
modem requests that the sending modem retransmit the data.
When errors are not detected, data flows through the modem at a normal rate. When an error occurs, the
sending modem must retransmit the data and not send any new data. The sending modem must be able to
flow control the SCS during the retransmission. Ensure that flow control is enabled on the SCS before
enabling error correction.
Note: To configure flow control, see Flow Control on page 8-18.
Local>> DEFINE PORT 2 MODEM COMPRESSION ENABLED
Local>> DEFINE PORT 2 MODEM COMPRESSION "s46=12b" "q5"