Thursday, 23 May 2013

The quickest way to verify SIP connectivity

As simple as this:

sipsak -s sip:

sipsak will send an OPTIONS request to , defaulting to UDP port 5060.

Tuesday, 21 May 2013

The sound of silence (encoded with G.711)

There are times where you check the RTP streams in your VoIP capture and everything looks sane, although one or more parties in the call reported no audio.

Those times, in particular when G.711 is in use, can be made a little less frustrating by looking at the RTP payload.

In fact, it's possible that one party is just sending silence. With G.711 silence has its own coding, and it's easy to spot:

a-law: silence is either a payload entirely populated with 0x55 or 0xD5 (depending on the sign applied).
u-law: silence is a payload entirely populated with 0xFF.

Monday, 13 May 2013

debian packages - what version is available and from which repo?

'apt-cache' madison is the answer.


giavac@myhost$ apt-cache madison vim
       vim | 2:7.3.547-7 | wheezy/main amd64 Packages

From apt-cache man page:

 madison pkg...
           apt-cache's madison command attempts to mimic the output format and a subset of the functionality of the Debian archive
           management tool, madison. It displays available versions of a package in a tabular format. Unlike the original madison, it can
           only display information for the architecture for which APT has retrieved package lists (APT::Architecture).

About ICE negotiation

Disclaimer: I wrote this article on March 2022 while working with Subspace, and the original link is here: