16 November 1996 3980-A2-GZ40-20
Both groups have DTE Rate, Compression Ratio, and Line Efficiency measurements. Each measurement is an
average of the last ten seconds’ data, and is updated every second. See Table 6.
Table 6
SDC Performance Measurements
Group DTE Rate Compression Ratio Line Efficiency
TX The number of bits per second
received from the DTE. This
number takes into account only
the data between the opening and
closing flags of the HDLC frames.
Flags and DTE interframe time fill
are ignored.
The ratio between the number of
bits in the input and output of the
compression algorithm. The
calculation does not include the
overhead of the LAPM frame or
the DTE flags and DTE
interframe time fill.
The ratio between the number of
bits per second sent on the line,
and the line rate. The calculation
does not include the DCE
interframe time fill.
RX The number of bits per second
sent to the DTE. This number
takes into account only the data
between the opening and closing
flags of the HDLC frames. Flags
and DTE interframe time fill are
ignored.
The ratio between the number of
bits in the output and input of the
decompression algorithm. The
calculation does not include the
overhead of the LAPM frame or
the DTE flags and DTE
interframe time fill.
The ratio between the number of
bits per second received from
the line, and the line rate. The
calculation does not include the
DCE interframe time fill.
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.