Specifications

AN93
Rev. 0.8 117
Modem On Hold
The Si2493 supports modem-on-hold as defined by the
ITU-T V.92 specification. This feature allows a
connected Si2493 to place a server modem on-hold
while a second call, typically a voice call, uses the
phone line. The maximum time the modems will
remain on-hold is controlled by the modem receiving the
modem-on-hold request. Once the second call has
completed, the Si2493 will re-initiate the data
connection if the time elapsed has not exceed the time
negotiated by the two modems. The Si2493 can also be
placed on hold itself by a remote modem allowing a far-
end user to make or receive a voice call. Modem on
hold is only supported on the Si2493 for V.34 (14400–
33600 bps) and higher speed modulations. The
AT+PMH command is used to enable (+PMH = 0) or
disable (+PMH = 1) modem-on-hold.
Initiating Modem On Hold
Modem on hold is typically initiated when a connected
client modem receives a subscriber alert signal (SAS)
tone as described in "Type II Caller ID/SAS Detection"
on page 106. However, it may be initiated any time the
modem is on-line in command mode. The command
AT+PMHR is used to initiate a modem on hold request.
After this command is issued, the modem will place a
modem on hold request to the server and the command
response +PMHR: will indicate the server’s response to
the request. The possible responses may be seen in
Table 83.
If the server refuses to grant a modem on hold request,
the modem will use the +PMHT setting to determine
what to do. If +PMHT = 0 then the modem will remain
connected to the server. If +PMHT is set to a non-zero
value, the modems will disconnect. The Si2493 will
indicate these conditions with the result code “MHnack;
Disconnecting…” or “MHnack; Reconnecting…”.
Once modem on hold has been initiated, it may be
necessary for the Si2493 to perform a hook-flash to
indicate to the central office the incoming call may be
accepted. This is initiated with the AT+PMHF
command. The Si2493 will go on-hook for the time in
user register U4F and remain off-hook while on-hold.
Usually a second hook-flash is necessary to reestablish
a data connection with the remote modem
The Si2493 will attempt to reestablish a data connection
with the remote modem upon receipt of the ATO
command and will indicate the connection has been
reestablished with the CONNECT message. If the
modems fail to renegotiate the connection, the Si2493
will send the NO CARRIER message.
Table 83. Possible Responses to PMHR Command from Remote Modem
<value> Description
0 V.92 Modem on Hold Request Denied or not available. The modem may ini-
tiate another Modem-on-hold request later.
1 MOH with 10 second timeout Granted
2 MOH with 20 second timeout Granted
3 MOH with 30 second timeout Granted
4 MOH with 40 second timeout Granted
5 MOH with 1 minute timeout Granted
6 MOH with 2 minute timeout Granted
7 MOH with 3 minute timeout Granted
8 MOH with 4 minute timeout Granted
9 MOH with 6 minute timeout Granted
10 MOH with 8 minute timeout Granted
11 MOH with 12 minute timeout Granted
12 MOH with 16 minute timeout Granted
13 MOH with indefinite timeout Granted
14 MOH Request denied. Future requests will also be denied during this session.