User's Guide for ZyXEL Omni 56K USB Modem
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Software XON/XOFF Flow Control
This is a bi-directional flow control. XON and XOFF (flow control protocols for asynchronous
communication) character defaults are decimals 17 and 19.These can be changed by modifying
the S-Registers S31 and S32.Both the modem and the DTE treats XOFF as a signal to stop
transmitting data and treats XON as a signal to restart sending data. Modems does not send
these characters received from the local DTE to the remote modem.
Error Control
Error control keeps the modem data link error-free by detecting and re-transmitting erroneous
data. Omni 56K USB modem supports both MNP and V.42 error control protocols. The MNP
protocol was an industry standard developed and licensed by Microcom, Inc. Omni 56K USB
modem supports level 4 and 3 error control protocols, commonly denoted as MNP4 and
MNP3.
V.42 is a developed standard by CCITT (Consultative Committee for International Telephony
and Telegraphy). V.42 supports both LAPM (Link Access Procedure for Modems) and MNP4.
A V.42 handshaking tries an LAPM connection first, and if not successful, it tries MNP4.
Data Compression
In the modem, the data compression is activated in an attempt to reduce the number of bits
actually sent. The receiving modem applies these techniques in reverse to recover the actual
data from the compressed data stream.
Omni 56K USB modem supports both V.42bis and MNP5 data compression protocols. Data
compression needs an error-free data link to work correctly, otherwise the corrupted
compressed data stream ruins the decompression process. MNP5 is used with MNP4 error
control and V.42bis is used with V.42 error control. The compression efficiency of V.42bis is
generally higher than that of MNP5. In some cases, V.42bis can be 50% to 100% higher and in
other cases it is just slightly higher. In general, it is about 50% more efficient.
Repeat Dial
The modem dials the default number stored in non-volatile RAM (Random Access Memory),
EEPROM (electrically erasable programmable read-only memory) repeatedly if not
connected. (s38.0=1,*Dn)