From: Peter (g4bbu@whittle-systems.demon.co.uk)
Date: Wed Feb 05 2003 - 17:28:41 GMT-3
>lillemor
Take a look at the headers.
G.729 codec produces an 8k payload.
To this you have to add RTP, UDP, IP, & L2 headers.
Typically this amounts to 40 plus bytes.
So depending upon how many 10 mS segments you pack into each RTP packet
(10 bytes a time) you can quite easily have 3 or 4 times the payload in
overheads!
Potential solutions:
1) use VoHDLC, or VoFR => lower overheads.
2) use cRTP header compression over point to point WAN links. The 40
bytes of RTP, UDP & IP mainly get compressed into a 2 byte identifier or
4 bytes if you have UDP checksums enabled. You can only do this across a
point to point link, when it travels over ethernet the overheads will go
back up again. But then who cares with 10 or 100 Mbps to play with.
3) increase the number of 10 mS segments you pack into each RTP packet.
Change the payload size.
(Look at the 'bytes' option on the end of the codec statement)
Hope this helps
Some interesting documents
Watch the wordwrap
1)
http://www.cisco.com/en/US/tech/tk652/tk701/technologies_tech_note09186a
00800b6710.shtml
2)
http://www.cisco.com/en/US/tech/tk652/tk701/technologies_tech_note09186a
00801149b3.shtml
3)VoIP per call bandwidth consumption
http://www.cisco.com/en/US/tech/tk652/tk698/technologies_tech_note09186a
0080094ae2.shtml
Peter
In message <03d301c2cc9b$b483d110$0a6a320a@cnf.com>, Tony Schaffran
<tschaffran@cconlinelabs.com> writes
>Are you sure you are correctly configured for g729? It should only be
>approx. 8k.
>
>
>
>Tony Schaffran
>Network Analyst
>CCNP, CCNA, CCDA,
>NNCSS, NNCDS, CNE, MCSE
>
>www.cconlinelabs.com
>"Your #1 choice for Cisco rack rentals."
>
>
>----- Original Message -----
>From: "Lillemor Hamnqvist" <lillemor@hamnqvist.org>
>To: <ccielab@groupstudy.com>
>Sent: Tuesday, February 04, 2003 12:04 PM
>Subject: OT: Voice compression
>
>
>> Hi,
>>
>> I have a dedicated VoIP circuit, HDLC. What can I do to get the better
>> compression? I'm using g729, but it looks like each call takes about
>30kb.(I
>> look at the load in the serial line). I would expect it to be lower. Is
>the
>> load number in 'show interface serial' accurate ? What is your experience
>?
>> Thank you,
>> /lillemor
>> .
>.
-- Peter .
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