isdn_audio - audio extension of ttyI ISDN character devices
DESCRIPTION
This document describes the audio related extensions of
the modem emulation, provided by /dev/ttyIx.
The ISDN tty devices are equipped with a modem emulation
implementing a set of traditional and some special AT com
mands. This provides easy use together with almost all
communication software that uses modem commands: mini
com(1), seyon(1), XCept(1), uucico(8), mgetty(8), dip(8),
pppd(8) and more. When enabled during kernel configura
tion, the emulator is capable of a reduced set of commands
to support audio. To use audio features, an ISDN card with
a audio-capable low-level driver is needed. Currently the
only audio-capable drivers are the teles and HiSax driver.
The port speed of the ISDN tty devices is always 64000
bps.
AUDIO RELATED AT COMMAND SET
This description does not include the standard AT command
set. For a description of the standard AT command set,
provided by the Linux ISDN subsystem, see ttyI(4). All
commands starting with AT+ except AT+FCLASS are enabled
only, if audio mode is active. The following audio
related AT commands are supported by ISDN tty devices:
AT+FCLASS=0
Disable audio mode (default). This command sets
register 18 to 4.
AT+FCLASS=8
Enable audio mode (default). This command sets reg
ister 18 to 5.
AT+FCLASS=?
Show possible modes.
AT+FCLASS?
Show current mode.
AT+VNH=x
No function, provided for compatibility only.
AT+VNH=?
Always showing "1".
AT+VNH?
Always showing "1".
AT+VLS=x
Select line x, where x is one of the following:
0 = No device.
2 = Phone line.
AT+VLS=?
Always showing "0,2".
AT+VLS?
Show current line.
AT+VRX Start audio recording. See below for more informa
tion.
AT+VSD=x,y
No function, provided for compatibility only. Pos
sible parameters:
x = 0 ... 31
y = 0 ... 255
AT+VSD=?
Show possible parameters.
AT+VSD?
Show current parameters.
AT+VSM=x
Select audio data format to x, where x is:
2 = ADPCM-2
3 = ADPCM-3
4 = ADPCM-4
5 = aLAW
6 = uLAW
AT+VSM=?
Show possible audio data formats.
AT+VSM?
Show current audio data format.
AT+VTX Start audio playback. See below for more informa
tion.
GENERAL BEHAVIOUR
On incoming calls, if the application responds to a RING
by sending ATA, depending on the calling service, the emu
lator responds with either CONNECT (data call) or VCON
(voice call).
When starting audio playback or recording, the emulator
responds with CONNECT. After that, the data transfer
in a single command. In this case, no CONNECT message is
sent to the application and the protocol described below
is started immediately.
When stopping audio playback or recording, the message
returned by the emulator depends on how the data transfer
is stopped. If the data transfer is stopped by a remote
hangup, NO CARRIER is sent to the application, while stop
ping data transfer with one of the DLE sequences, VCON is
sent. In any case, the message is preceeded by the appro
priate DLE sequence for end of audio data and/or abort
respectively.
On outgoing voice calls, the emulator responds with VCON
upon connection setup.
AUDIO DATA FORMAT
When recording or playing back audio data, a kind of modi
fied bisync protocol is used. When recording audio data,
the modem emulator is considered to be the sender, while
the application is the receiver. When playing back, this
relationship is reversed: The modem emulator acts as
receiver, while the application is the sender. With the
bisync protocol the character DLE (0x10) has a special
meaning and is used like an escape character. It is
always followed by a second character which has the fol
lowing meanings defined:
ETX (0x03)
End of audio data. This code has to be sent by the
sender, indicating end of audio data.
DC4 (0x14)
Abort sending. This code has to be sent by the
receiver, when it wishes to abort receiving data.
The sender has to reply by stop sending data and
finally indicating end of audio data.
DLE (0x10)
a single byte with value 0x10 in the data stream.
0 Touch-tone "0" received. Valid only in direction
from emulator to application.
1 Touch-tone "1" received. Valid only in direction
from emulator to application.
2 Touch-tone "2" received. Valid only in direction
from emulator to application.
3 Touch-tone "3" received. Valid only in direction
from emulator to application.
from emulator to application.
5 Touch-tone "5" received. Valid only in direction
from emulator to application.
6 Touch-tone "6" received. Valid only in direction
from emulator to application.
7 Touch-tone "7" received. Valid only in direction
from emulator to application.
8 Touch-tone "8" received. Valid only in direction
from emulator to application.
9 Touch-tone "9" received. Valid only in direction
from emulator to application.
# Touch-tone "#" received. Valid only in direction
from emulator to application.
* Touch-tone "*" received. Valid only in direction
from emulator to application.
A Touch-tone "A" received. Valid only in direction
from emulator to application.
B Touch-tone "B" received. Valid only in direction
from emulator to application.
C Touch-tone "C" received. Valid only in direction
from emulator to application.
D Touch-tone "D" received. Valid only in direction
from emulator to application.
AUTHOR
Fritz Elfert <fritz@wuemaus.franken.de>
SEE ALSO
ttyI(4).
