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10–4
Bidirectional Compression
Processing data compression consumes a lot of the modem controller CPU's com-
putation 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 2864 series modems use a powerful 16-bit processor that does not slow
down during bidirectional file transfers.
Negotiation Fail Fall-Back
There are two options you can choose from if the error control negotiation fails at
the initial modem connection handshaking:
DTE to DCE Connection
No matter what control level is enabled, DTE speed can always be set separately
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 of the speed the modem is linked to. The DTE
speed options range from 300 bps to 460800 bps. What DTE speed is optimized
for your application depends on how fast your computer and communication soft-
ware can process the serial port data. 2864 series modems use certain controls and
commands to enable the modem to optimize its performance.
Averaging Throughput
At high DTE speeds, slow computers tend to lose characters. Even though most
transfer programs offer protocols with error correction capabilities, it will still take
time to recover from errors. ZyXEL modems average the throughput before sending
characters to the DTE. This will decrease the probability of the computer losing
characters due to "burst" high-speed character strings.
Since new computers are much faster and there is an increasing use of serial port
chips with internal buffers like the 16550A UART, the computer's speed has
become less of a problem. The default setting for throughput averaging is disabled.
This will reduce data delay and increase throughput in Unix applications. You can
enable throughput averaging by setting S42b1 to 1.
STAYS ON-LINE AT*E0 Maintains the non-error control connection, if
error control negotiation fails. (Default)
DISCONNECT AT*E1 Disconnects the line if not error control linked.