Tone and Status Monitoring - Shielded <DLE>Statuses
The modem can detect specific tones and other status information, and
report these to the DTE while in any of the three voice submodes. The modem
simultaneously looks for 1300 and 1100 Hz calling tones when answering,
and for ITU and Bell answer tones when originating. The modem can also detect
dial or busy tones in any of the three voice submodes.
When in Online Voice Command Mode or Voice Transmit Mode, the codes in the
following table are sent to the DTE immediately upon verification by the
modem of the associated tone, status, or cadence. In this mode, the 2-character
code is not buffered, nor does the DTE have the ability to stop the code
with flow control. If the DTE has started (but not completed) sending any
AT command, the Tone Monitoring function is disabled until the command has
been received and processed.
The modem can discriminate between single and multiple DTMF tones received.
If calling tone, dial tone, busy tone, or answer tone is detected, this
detection is reported repeatedly (at reasonable intervals) if the DTE takes
no action, and the tone continues to be detected.
Shielded Code Sent to DTE: <DLE>0, <DLE>9, <DLE>*,
<DLE>#, <DLE>A, <DLE>D Meaning: Digits 0 through
9, *, #, or A through D detected by the modem, i.e., user has pressed a
key on a local or remote telephone. The modem sends only one <DLE>code
per DTMF button pushed.
Shielded Code Sent to DTE: <DLE>a
Meaning: Answer Tone (ITU) - Send to the DTE when the V.25/T.30 2100
Hz Answer Tone (Data or Fax) is detected. If the DTE fails to react to the
code, and the modem continues to detect Answer tone, the code is repeated
as often as once every half second.
Shielded Code Sent to DTE: <DLE>b
Meaning:
Busy - Sent in Voice Receive Mode when the busy cadence is detected,
after any remaining data in the voice receive buffer. The modem sends the
busy <DLE> code every four seconds if busy continues to be detected
and the DTE does not react. This allows the DTE the flexibility of ignoring
what could be a false busy detection.
Shielded Code Sent to DTE: <DLE>d
Meaning: Dialtone - Sent in Voice Receive Mode when dial tone is detected
after any remaining data in the voice receive buffer. The modem sends the
<DLE> code every three seconds if dial tone continues to be detected
and the DTE does not react. This allows the a false dial tone detection.
Shielded Code Sent to DTE: <DLE>e
Meaning: European Data Modem Calling Tone - Sent when the V.25 1300
Hz Calling Tone (Data Modem) is detected in a voice submode. The modem assumes
that the calling tone is valid, and sends this code only after four seconds
of proper cadence has been detected. If the DTE does not react to the <DLE>e
and calling tone continues, the code is sent again as often as once every
four seconds.
Shielded Code Sent to DTE: <DLE>f
Meaning: Bell Answer Tone - Sent when Bell 2225 Hz Answer Tone (Data)
is detected. If the DTE fails to react to the code and the modem continues
to detect Answer tone, the code is repeated as often as every 1/2 second.
Shielded Code Sent to DTE: <DLE>h
Meaning: Hung Up Handset - Sent immediately when the modem detects that
the local handset has hung-up (i.e., has transitioned from off-hook to on-hook)
while in any of the three voice submodes.
Shielded Code Sent to DTE: <DLE>o
Meaning: Overrun - Sent in Voice Receive Mode if the voice receive data
buffer overflows, in which case, the latest data is lost in favor of retaining
the oldest data in the buffer. (The DTE may not have been reading data from
the modem lately.)
The modem inserts the <DLE>o as the end of the buffer, thus marking
the location where data was lost by the DTE. The modem does not append <E><D><DLE>o
codes to the buffer contiguously. If more than one data byte is lost before
the DTE begins reading again, there is only one <DLE>o code in the
data stream. However, if the DTE resumes reading, and then once again allows
the buffer to overflow, a new <DLE>o is appended to the data by the
modem. Thus, the <E><D><DLE>o does not indicate how many
bytes of contiguous data has been lost, but may report multiple gaps of
lost data in the data stream.
Shielded Code Sent to DTE: <DLE>q
Meaning: Quiet - Sent in Voice Receive Mode following any remaining
data in the receive voice buffer when the silence detection time (#VSP)
expires and there has been voice data passed to the DTE.
Note: With #VSS=0, the Quiet message never occurs. But with silence
deletion turned on, #VSS may not in itself be sufficient since "cracks
and pops" for a short duration directly after a connection should not
beconsidered voice.
In general, <DLE>q is sent if first there was sound, and there is
no sound, and means that somebody has stopped talking, and according to
the criteria selected in the #VSP and #VSS settings, is probably not going
to start again. If the DTE ignores the <DLE>q 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 silence
detection.
Shielded Code Sent to DTE: <DLE>s
Meaning: 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.
Shielded Code Sent to DTE: <DLE>t
Meaning: 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.
Shielded Code Sent to DTE: <DLE>u
Meaning: 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 on 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.
End of Stream - <DLE><ETX>is sent to denote the end of
a voice data <DLE><ETX>stream, similar to FAX Class 1 and 2.
This occurs in Voice Receive Mode only after the DTE issues the key abort.
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