AT Command Reference Manual
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Table 7-2. Shielded Codes Sent to the DTE (Cont’d)
Code Sent to DTE
Meaning
<DLE>s
Silence. Sent in Voice Receive Mode after the silence detection timer (#VSP) expires and
if valid
voice has not been detected (#VSS). In general, <DLE>s is sent if no sound was detected at all. This
differs slightly from Quiet (<DLE>q) in that it can be used by the DTE in its discrimination algorithms
as a means of determining if anything at all is coming from the remote station. If the DTE ignores the
<DLE>s code, the modem automatically resets its timer and sends the code again if the #VSP time
period again expires. This allows the DTE time to react to what may be a false detection of silence.
<DLE>t
Handset Off-Hook. Sent one time when the local handset transition from on-hook to off-hook is
detected in any of the three voice submodes.
<DLE>u
Underrun. Sent in Voice Transmit Mode when the voice transmit buffer becomes empty without
receiving a <DLE><ETX> or <DLE><CAN> command first. The modem remains in transmit mode,
but issues the <DLE>u to indicate that silence is being generated. When the DTE resumes sending
bytes, the modem allows a small number of bytes to build up in the buffer before re-enabling the
transmitter to avoid repeated underruns. However, with the underrun condition, the modem does not
wait for the XOFF threshold (or timer) to be met before resuming, as it does on an initial transmit or
resume operation because the remote listener may be unaware of the underrun, and keeping the
silence short may serve to mask the underrun altogether.
<DLE>T
Timing Mark. Sent in Voice Receive Mode, and placed in the data stream at 1 second intervals,
when enabled by #VTM = 10.
<DLE><ETX>
End of Stream. <DLE><ETX> is sent to denote the end of a voice data stream, similar to fax Class 1
and 2. This occurs in Voice Receive Mode only after the DTE issues the key abort.
7.2.5 Shielded <DLE> Commands from the DTE
Most commands issued for voice are implemented as extensions to the AT command set, and are accepted only if the
modem is in a valid command mode, such as Online Voice Command Mode. However, there are some operations which are
available to the DTE without necessitating a switch to Online Voice Command Mode. The most obvious of these is the
<DLE><ETX> which the DTE issues during Voice Transmit Mode to terminate an output message. Table 7-3 lists shielded
<DLE> commands recognized by the modem.
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