Silicon Laboratories SI2493/57/34/15/04 - Manual (Page 164)

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6.7. Modem-On-Hold
The Si2494/93 supports modem-on-hold as defined by the ITU-T V.92 specification. This feature allows a
connected Si2494/93 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 ended, the Si2494/93 will reinitiate the data connection if the time elapsed has
not exceeded the time negotiated by the two modems. The Si2494/93 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 Si2494/93
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.
6.7.1. 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 "6.5. Type II Caller ID/SAS Detection" on page 151. However, it may be initiated any time the
modem is on-line in command mode. The AT+PMHR command 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 +PMHR: command
response will indicate the server’s response to the request. The possible responses are shown in Table 103.
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, the modem will remain connected to the server. If +PMHT is set to a non-zero value, the
modems will disconnect. The Si2494/93 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 Si2494/93 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 Si2494/
93 will go on-hook for the time set 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 Si2494/93 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 Si2494/93 will send the NO CARRIER message.
Table 103. Possible Responses to PMHR Command from Remote Modem
<Value> Description
0
Modem-on-hold request denied or not available.
The modem may initiate another modem-on-hold request at a later time.
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 in the same session will also be denied.