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Re: [Question #219552]: rohc 1.5.1 does not compress VoIP


Question #219552 on rohc changed:

    Status: Open => Answered

Didier Barvaux proposed the following answer:
Hello Florent,

> I am using rohc 1.5.1 on 2 servers Centos 4.x and 5.x.
> I installed from sources following your wiki and tunnel is up
> with data transfer OK.
> It seems to be working fine but I need rohc to compress
> RTP flow, so I set up a tunnel on 2 asterisks and created a
> sip trunk in this rohc tunnel.

Fine. Please note that the tunnel application is for testing purposes
only. Do not use it on production.

> However, I need to verify rohc's compression rate.
> Thus, I used 2 different tools to monitore bandwidth: iftop and
> a router calculating BP stats.
> My problem is that a call going through rohc takes as much BP
> as a non compressed one (about 30kb/s).
> I tried up to 3 calls and no difference were noticed.
> Compression logs during call shows that it is working:
> 206 O-mode SO 60 28 36 4 0

The logs show that ROHC compression is effective. On packet #206,
you saved 24 bytes for example. It could probably be better if you
configure the ROHC library to detect your RTP streams among all UDP

How do you compute the bandwidth rate with iftop and your router?
My guess is that you take into account the IP/UDP headers of the
tunnel in your computation.

As said before, the tunnel application is not for production use, but
only for testing. The overall gain (ROHC compression then IP/UDP
encapsulation) is not that great!


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