OT? cRTP header compression

Thu Apr 11 17:47:40 UTC 2002

At 08:55 PM 4/10/2002 -0700, Pena, Antonio wrote:
>I had some kind of experience doing cRTP over Cisco routers, we use Cisco
>7204 & 7206 Routers on the IP Gateways and Cisco's 3600 and 5300 as VoIP
>gateways, as well we had a small setup using a Cisco 2611 router on the
>termination router.

As well as increasing sample size (thereby increasing the payload:header 
ratio), the other thing to try is turning on voice activity detection (VAD, 
AKA silence suppression). For human conversation, this typically reduces 
packet rates by around 60%, enabling you to squeeze more conversations onto 
the link. It also has the side effect of reducing CPU utilization per call 
on your Cisco voice gateways. Note that turning on VAD does decrease the 
perceived voice quality a little, so whether it is worth it depends on 
where you want to make the trade-off between cost and voice quality.

Also, cRTP is not CEF switched on the 5300 in 12.2, AFAIK. It was on 
26xx/36xx, but 5300 architecture (and hence switching code) is different. 
That may have changed since I last looked 6 months ago -- best bet is to 
ask on the Cisco-NAS mailing list at

http://www.cisco.com/warp/public/471/cisco-nas.html

  Cheers,

Mathew

>The trick is change the VoIP payload size of each packet to reduce the
>packets per second in a half improving the  performance over the routers,
>also we are using as Cisco recommends TCP & RTP headers compression over the
>circuits using only MLPPP encapsulation.
>
>Also please note that using cRTP and Compression you have increased the
>switching delay over the circuit and for that reason you may need also to
>have more processing power of the router.
>
>Below you can see a Cisco site where you can check the recommendations for
>this setup and also based on that information I created a Bandwidth
>calculator on an excel sheet, if you want it, just drop me an email, I will
>send it you.
>
>http://www.cisco.com/warp/public/788/voice-qos/voip-mlppp.html
>
>http://www.cisco.com/warp/public/788/pkt-voice-general/bwidth_consume.html
>
>http://www.cisco.com/univercd/cc/td/doc/product/software/ios122/122cgcr/fqos
>_c/fqcprt6/qcfcrtp.htm
>
>http://www.cisco.com/warp/public/105/compression-qos.html
>
>
>
>Bye
>
>Antonio J. Pena
>Manager, Network Engineering
>
>(  /_ _ _  __/_ _
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>1901 Main street
>Santa Monica, CA, 90405
>Phone(310)382-3300
>Direct(310)382-3409
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>http://www.verestar.com
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>
>-----Original Message-----
>From: Thomas Kernen [mailto:tkernen at deckpoint.ch]
>Sent: Wednesday, April 10, 2002 1:11 PM
>To: nanog at merit.edu
>Subject: OT? cRTP header compression
>
>
>
>
>I'm looking for real world experience related to deploying cRTP header
>compression on Cisco routers related to VoIP flows. We are trying to
>evalute what type of hardware (ie: CPU power since cRTP is CEF switched
>since 12.2x IIRC) is required to handle 96/192/384 VoIP calls over a
>single circuit (HDLC/PPP/FR). This is related to specific overseas
>circuits where the cost of the circuit is still very expensive vs the
>cost for the extra hardware to handle the header compression. I'm
>disregarding all QoS info at this stage.
>
>Cheers
>Thomas