running: Add note about DTMF support

osmo-sip-connector does not yet support full DTMF support. The current
implementation only supports DTMF tones to be send from MNCC to SIP,
but not in the opposite direction.

Change-Id: I578e50b0a42d88b05cf6da80443b71494b5eb26f
Related: OS#2777
This commit is contained in:
Philipp Maier 2018-10-01 12:56:23 +02:00 committed by Neels Hofmeyr
parent a001c591b3
commit 3130f0776a
1 changed files with 26 additions and 0 deletions

View File

@ -56,3 +56,29 @@ mncc
socket-path /tmp/msc2_mncc
=== DTMF signaling
In VoIP based telephony networks DTMF (Dual-tone multi-frequency signaling) can
be signaled through multiple methods. Common methods are in-band, RFC2833 and
sip-info messages.
Osmo-sip-connector is using sip-info messages to signal DTMF tones. When a DTMF
tone is signaled at the MNCC socket interface, osmo-sip-connector will generate
a matching sip-info message to forward the DTMF signal to the PBX. Depending on
the PBX software reconfiguring the DTMF signaling method to sip-info may be
While sending DTMF tones through the MNCC interface to a SIP leg, the current
implementation of osmo-sip-connector does not support sending DTMF tones in the
opposite direction. Any attempts to send DTMF tones to an MNCC leg will be
confirmed with a status 405 "Method not allowed".
The reason for this limitation is that in mobile networks, depending on the
signaling direction, the signaling of DTMF tones is implemented differently.
A mobile originated DTMF tone is signaled through out of band messages,
which arrive at osmo-sip-connector on as MNCC DTMF START/STOP messages. Those
messages can be directly translated to sip-info messages. However, in the
other direction (mobile terminated), an in-band signaling method is used. This
means that osmo-sip-connector would have to translate an incoming DTMF sip-info
message into an audio sample that then would have to be injected into the
voice stream. Currently this scheme is not implemented in osmo-sip-connector.