[asterisk-dev] Reducing RTP overhead

John Todd jtodd at loligo.com
Mon Feb 13 00:10:06 MST 2006

>Hello everyone,
>	A long, long time ago someone somewhere told me about a 
>feature with RTP to reduce overhead (and bandwidth).  By trunking I 
>mean trunking as in "IAX trunking" - stuffing multiple voice 
>channels into the same UDP packet on connections between the same 
>systems to reduce UDP/etc overhead (you know what I am talking 
>about). :)
>	I was told that such a feature exists for use with RTP.  The 
>closest thing that I have been able to find is RTP header 
>compression (RFC 2508).  With the calculations that I have seen, RTP 
>header compression can increase call capacity by over %50:
>	Are there any other ways to improve bandwidth usage with 
>SIP/RTP? Perhaps something more like IAX trunking?  RFC 2508 appears 
>to only apply to PtP serial links (it also compresses the IP header, 
>but that may be optional).  I'll continue to read the spec.
>	Lets just say that RFC 2508 (or something like it) is the 
>only way to reduce RTP bandwidth usage.  I have several questions:
>	If an ideal implementation for Asterisk was created, would it 
>stand a chance of being put in CVS?  What equipment/vendors also 
>support it? This is key.  If I'm using just Asterisk (I wish) I 
>would just use IAX2!
>Kristian Kielhofner

There is the protocol called RTPC (RFC2508) which is rumored to 
compress ~40 byte IP/UDP/RTP headers down to <5 bytes when used with 
RFC2509.  I haven't seen it in in action other than on T1 connections 
point-to-point, and I don't know what the requirements are for 
intermediate systems to understand and route the IP layer datagrams 
with the shortened packet data.  I have only been a witness to what I 
was told was a compressed link, and I've not actually set it up 
myself.  Cisco seems to be the only player in this field currently, 
though it is an open standard.

   Interleaving data into single, larger packets MUST be the best way 
to do it with VoIP, though I read that there are "drafts" 
(un-discoverable) about a better way called Enhanced CRPT (ECRPT) 
which describe a better way to do it with VoIP packets - links, 


There may be a typo on some Grandstream literature that was perhaps 
supposed to say "RTCP" instead of "RTPC", but there is an off-chance 
that the Budgetones support this protocol (though it is almost 
certainly "RTCP" which is a whole different bugtracker subject.)


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