The document discusses the implementation of silence suppression in the Asterisk 1.8 telephony platform, aimed at saving bandwidth by detecting silence in conversations and sending comfort noise packets instead of continuous RTP traffic. Funded by Bressner Technology, the project includes detailed instructions for configuration and testing, with future plans for integration into later versions of Asterisk pending positive feedback. Key technical details are provided, including codec configurations and the necessity of testing for broader applicability in RTP channels.

1. Silence suppression in Asterisk 1.8
- Codename Roibos
oej@edvina.net * Twitter @oej
Copyright 2014, Edvina AB, Sollentuna Sweden.All rights reserved.
Want to save at least
40% of the bandwidth used for
media in your Asterisk server?

2. This work is funded by Bressner Technology GMBH
(bressner.de) that makes a series of Lync integration
products.
They also distribute Industrial PCs that are excellent as
Asterisk platforms.
Thank you, Bressner, for contributing to Asterisk.
Thank you
If you use Asterisk,
make sure you contribute back
whenever possible.
Thank you Josh and Matt
for the support!

3. An RTP session
Alice Bob
Typical RTP streams consists of UDP/RTP packets sent
every 20 millisecond. With Asterisk today, we need a constant
stream of packets.
But… In a normal
conversation one person
listens while the other one
speaks.

4. With silence suppression
Alice Bob
CN
CN
When the sender detects silence, it sends a CN - Comfort
Noise - request frame. After that no RTP traffic will be seen until
the audio comes back.
This saves a lot of bandwidth in a
normal conversation. Up to 40% bandwidth
savings in a normal call.

5. When we get no RTP from the other side,
Asterisk needs to produce some noise.
CN is agreed on in SDP Offfer/answer and
is only used if both parties support it.
After a few frames of silence, alerts RTP to send
a CN frame and starts dropping frames.
Listens to outbound audio stream and
detects silence. An Asterisk audiohook and DSP.
The various parts
Silence
detection
Silence
suppression
Comfort Noise
support in SDP and RTP
Noise generator
silencedetection.c
silencedetection.c
chan_sip.c, res_rtp_asterisk.c
channel.c

6. Configuration
Add the ”cn” codec to allowed codecs in sip.conf
Add ”silencesuppression=yes”
in the [general] section or device section of sip.conf
Note: If you enable the CN codec but not silence suppression,
we will accept CN but never send a CN frame.
This saves some CPU for silence detection,
but still uses CPU for noice generation.
1.
2.

7. Download roibos
svn checkout http://svn.digium.com/svn/asterisk/team/oej/roibos-cng-support-1.8"
cd roibos-cng-support-1.8
more README.roibos"
./configure"
make"
make install"
!
Test it and report bugs to the
bug tracker, as indicated in the
README file.
Issue: ASTERISK-23624

8. What next?
?
If this code get good feedback,
it’s going to get ported to 12 and trunk,
to be integrated into the standard
version of Asterisk."
!
We do need your help testing it to get there.
Due to release policies it can
not be part of Asterisk
1.8 or 11.

9. F.A.Q.
Why 1.8?
Simply because that’s what we use in the project
this was developed for. Funding for porting it to
other releases is welcome.
Why
chan_sip?
chan_pjsip doesn’t exist in 1.8 (see above). Most of the
work is in the core of Asterisk and will be updated to
other releases after testing and when funding exists.
Why not
IAX2? chan_iax2 does not use RTP media framing.

10. Other RTP-channels
Most of this work is generic in
Asterisk. Other channels may benefit too,
as long as they are using RTP.
Like chan_pjsip in 12 and the
XMPP Jingle drivers.
Time to start coding?

11. Do you need a new
Asterisk feature?
If you need a new Asterisk feature or want to
get general Asterisk or Kamailio
support or training from Edvina,
please contact us - info@edvina.net
Roibus will soon be part of our
version of Asterisk 1.8 - EDVx -
with improved RTCP support, Lync integration
PRACK support, Improved SRTP negotiation,
RTP packet loss concealment and much more.
http://edvina.net * Twitter @edvina