Specifications

AN93
20 Rev. 0.8
Error Correction
The Si2493/57/34/15/04 ISOmodem can employ error
correction (reliable) protocols to ensure error-free
delivery of data sent between two modems. The error
control methods are based on grouping data into frames
with checksums determined by the contents of each
frame. The receiving modem checks the frames and
sends acknowledgments to the transmitting modem.
When it detects a faulty frame, the receiving modem
requests a re-transmission. Frame length varies
according to the amount of data transmitted or the
number of re-transmissions requested from the
opposite end.
The Si2493/57/34/15/04 supports V.42 and MNP2–4
error correction protocols. V.42 (LAPM) is most
commonly used and is enabled in \N3 and \N4 modes.
In the default mode (\N3), the Si2493/57/34/15/04
attempts to connect with V.42 error correction and
V.42bis data compression (Si2457/34/15) and falls back
to either V.42 only, MNP 2–5, or no error correction
(wire mode) if necessary. In \N4 mode, the Si2493/57/
34/15/04 hangs up if a V.42 connection cannot be
established. If the ISOmodem hangs up in V.42 mode
after all data is successfully sent, the result code is
“OK”. If the modem hangs up before all data is
successfully sent, the result code is “No Carrier”. If the
modem connects without a protocol, “No Carrier” is
always sent.
The V.42 specification allows an alternate error
correction protocol, MNP2-4. MNP2-4 is enabled in \N2
mode. In \N2 mode, the Si2493/57/34/15/04 hangs up if
an MNP2, 3, or 4 connection cannot be established.
Wire Mode
Wire mode (\N0) is used to communicate with standard,
non-error-correcting modems. When optioned with \N3,
the Si2493/57/34/15/04 falls back to Wire mode if it fails
in an attempt to negotiate a V.42 or MNP2-4 link with the
remote modem. Error correction and data compression
are not active in Wire mode.
Fast Connect
The Si2493/57/34/15/04 supports several fast connect
modes of operation to reduce the time of a connect
sequence in originate mode.
V.29 Fast Connect
In addition to the low modulation speed fast connect
modes, the modem (only Si2493/57/34/15) also
supports a fast connect mode based on the 9600 bps
V.29 fax modulation standard. In order to provide a time-
critical interface from the host to the modem (only
Si2493/57/34/15), the ISOmodem uses an interface
derived from the fax class 1 AT command set. The
example below shows how the class 1 AT commands
for V.29 would commonly be used in a client-side
terminal (originating modem).
Calling Modem Example:
 AT+FCLASS=1
Set the modem in fax mode so that it can be
switched back and forth between Data and
Command mode after executing AT FAX commands.
 AT:UAA,2
Disable normal fax tone during handshaking
indicating to answer modem that V.29 fast connect
will be requested. Cause the execution of
AT+FTM = 2 at the beginning of connection.
 ATDT1234567
Dial the number and wait for <CONNECT> and
<OK> to establish connection.
 AT+FTM=2 (For reference only; there is no need to
send this command to the modem)
Transmit V.21(980 Hz) tone until Answer Tone(2100/
2225 Hz) is received for 100 ms, followed by <OK>.
 AT:UAA,0
Restore UAA to default value.
 AT+FRM=96
Put modem in V.29 receiving mode. Wait for
<CONNECT> and then receive data. Wait for <NO
CARRIER>.
 AT+FTM=96
Set modem to V.29 transmit mode and wait for
<CONNECT>. Send Data from DTE to DCE and wait
for <OK> after sending <DLE><ETX> characters
where DLE is a 0x10 character and ETX is a 0x03
character.
 AT+FRM=95
Set modem to V.29 short synchronous receiving
mode. Wait for <CONNECT> and then receive data.
Wait for <NO CARRIER> indicating transmission
has ended.
 AT+FTM=95
Send out short synchronous signal and wait for
<CONNECT>. Send Data from DTE to DCE and wait
for <OK> after sending <DLE><ETX> characters.
 ATH
Hang up the modem.
Legacy Synchronous DCE Mode/V.80
Synchronous Access Mode
The Si2493/57/34/15/04 supports two different DTE
interfaces to implement an Asynchronous DTE to
Synchronous DCE conversion.
Table 10 provides high-level options to choose between
the Legacy Synchronous DCE Mode and the newer
V.80 synchronous access mode.