COMSPHERE 3800Plus Modems
I-4 October 1998 3980-A2-GB30-40
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.
•If the Line Efficiency measurement reported by
your modem is substantially less than 100 percent.
•If the 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 3800Plus 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) 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.