CONTROL LEVEL
ERROR CONTROL
DATA COMPRESSION
None
AT&K0 Disabled
Disabled
MNP4(MNP3)
AT&K1 MNP4 Disabled
MNP4+MNP5
AT&K2 MNP4 MNP5
V.42(MNP4)
AT&K3 V.42 or MNP4 Disabled.
V.42(b)+MNP4(5)
AT&K4 V.42 or MNP4 V.42bis or MNP5
&4K is the default. For some applications, like BBSs, where transmission of already compressed
files is usual, the MNP5 data compression process actually produces more redundant data and
slows down transmission. Setting S38 bit 5 will disable MNP5 protocol negotiation regardless of
error control setting.
Bidirectional compression
Processing data compression consumes a lot of the modem controller CPU's computation power.
Though data compression protocols of V.42bis and MNP5 are bidirectional, most modems slow
down when doing bidirectional file transfers because of limited processing power.
ZyXEL U-1496 series modems use the powerful 68000 processor that do not slow down during
bidirectional file transfers.
NEGOTIATION FAIL FALL-BACK
There are two options you can choose if the error control negotiation fails at the initial modem
connection handshaking:
STAYS ON-LINE
AT*E0 If error control negotiation fails, keep the no error control
connection (Default).
DISCONNECT AT*E1 If not error control linked, disconnect the line.
No matter what control level is enabled, DTE speed can always be set different from the
modem's speed, except in the case where you set 'FOLLOW LINK RATE'. For maximum
throughput, it is better to set AT&B1 (default) `FIXED AT DTE RATE'. The DTE speed will be
independent from the speed the modem is linked to. The DTE speed options range from 300 bps
to 76800 bps. What DTE speed is optimized for your application depends on how fast your
computer and communication software can process the serial port data. U-1496 series modems
use some controls and commands to make the modem optimize its performance.
ADAPTIVE BLOCK SIZE
Depending on the line condition, ZyXEL U-1496 series modems will automatically adjust the
block size to minimize the error rate and re-transmission time.
AVERAGING THROUGHPUT
At high DTE speeds, slow computers tend to lose characters. Even though most software
programs offer protocols with error correction capability, it will still take time to recover the
errors. The U-1496 averages the throughput before sending characters to DTE. This will
decrease the probability of the computer to lose characters due to 'burst' high-speed character
