Tuning Sdc Efficiency; Dte Configuration; Modem Configuration - Paradyne COMSPHERE 3920Plus Technical Reference Manual

COMSPHERE 392xPlus Modems

Tuning SDC Efficiency

SDC efficiency is dependent on the configuration parameters of the DTE and the modem.

DTE Configuration

The way DTE parameters are changed, and whether they can be changed, depends on your DTE.
See the documentation that came with your DTE.
DTE rate configuration parameter. The nominal DTE rate (the Tx/Rx clock's frequency at the
DTE/DCE interface) can be a bottleneck. Increase the nominal DTE rate if the:
• DTE rate measurement reported by your modem is close to the nominal DTE rate.
• Line Efficiency measurement reported by your modem is substantially less than 100 percent.
• Ratio between the nominal DTE rate and the line rate is less than the Compression Ratio.
DTE acknowledgment window size configuration parameter. This is the number of information
frames the DTE will send without awaiting positive acknowledgment. Increase the DTE
acknowledgment window size if your modem reports a low DTE rate and a comparatively high
Compression Ratio (and therefore a Line Efficiency measurement substantially less than
100 percent). Set it to the maximum possible value.
DTE frame size configuration parameter. If SDC modems are used between synchronous DTEs,
then the DCE frame size (and not the DTE frame size) must take into account the line error rate.
Set the DTE frame size to as large a value as your DTE and your DTE application can handle. This
reduces the significance of DTE frame overhead, and reduces the delay (expressed in number of
DTE frames) for positive acknowledgment.

Modem Configuration

There are also configuration options in the V42/MNP/Buffer branch of the 392xPlus modem
which affect SDC efficiency.
(LAPM) Max Frame Size. Except in environments where line impairments are known to be
unusually high, set this to the maximum (256).
Sync DTE CRC. If the CRC type of the DTE frame is the 16-bit CRC for HDLC frames as
defined by ITU-T, set this to CRC16.
SDC Delay Min. If clamping of the receive clock (circuit 115/Pin 17) is accepted by your DTE,
then set this to Rx_Clock.
Sync Flow Cntrl. The Tx_Clock selection is suitable for most synchronous DTEs. If your DTE
interprets transmit clock clamping or a drop in CTS as a problem (resulting in an aborted frame),
set Sync Flow Cntl to None. Selecting None should not be a problem if the entire acknowledgment
window of DTE frames does not exceed 5 kilobytes, so that it can be buffered by the modem.
H-6 October 1998 3920-A2-GH30-40