Can RTP header compression be enabled on a per sub-interface/ VCI basis?

Unanswered Question
Mar 13th, 2008

I and trying to enable RTP header compression on a 7206 ATM DS3 card. I working with 80 VCI's and need to be enable it one sub-interface/ VCC at a time. I can't find a configuration example. 12.3 IOS

I have this problem too.
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Paolo Bevilacqua Thu, 03/13/2008 - 14:56

HI, RTP header compression is not supported on ATM (aal5snap), only PPP or frame-relay.

sundar.palaniappan Thu, 03/13/2008 - 15:08

Actually, I just noticed you are trying to do this on an ATM interface.

Yes, you can only do it on PPP or frame-relay. However, I believe you do have a workaround, configure ppp over ATM (virtual-template) and configure RTP header compression there.



sundar.palaniappan Thu, 03/13/2008 - 15:38

This document does indicate PPP over ATM should give you the ability to do RTP header compression over ATM links.

""This feature does not support RTP or TCP header compression directly over ATM links, even when header compression is configured in a policy map. To support RTP or TCP header compression, PPP over ATM is required.""



GrayH Thu, 03/13/2008 - 16:07

I apoligize for not giving more details in my question. We are using the ATT ATM/FR interwork network. If we change from aal5snap encapsulation to PPP would the network also have to be reconfigured?

GrayH Thu, 03/13/2008 - 16:00

Thank you, Paulo,

I had just found the note in the configuration guide. But this may present a serious problem. My ATM DS3 main site circuit is oversubscribed with 50 T1 remote sites. We have an ATT ATM/FR interworking network. NEC 2400 and 2000 PBX's and Vanguard branch voice GW routers doing TCP encapsulation using G723 codecs. These Vanguard GW's sit behind our branch 2811 data routers. We are moving to Cisco analog E&M cards as an interum solution until we go to CM and Cisco phones. Our plan was to use G729A codecs with the new E&M cards. I though that this would have a negative impact on effective throughput and the existing applications unless we used RTP header compression. I have enclosed a diagram showing the inefficent use of the 53 byte ATM cells. Am I over thinking this? Or do I need to use G723 with the Cisco E&M cards. If so it might reduce the port density capabilities of the 4-T1 2851 GW/GK at the main site that connects to the NEC switch. The higher complexity codec would not allow as many voice channels. Thank you for any advice.

Paolo Bevilacqua Thu, 03/13/2008 - 17:22

Thanks for sharing the complete picture. It helps a lot.


If you think it's essential for you to switch away from G.723, there is an alternative to using E&M cards on PBXs and GWs (kind of a step back), that is called IP-to-IP GW with transcoding.

The 2811 in branch is good at that. Basically on the PBX you would configure, send all calls to the 2811 in g711u. The 2811 would reapply a codec of your choice and send across the wan.

Viceversa on he hub, where I would separate the wan router with the voice gw, thing that probably you've done already.

Doing so you would able to use the iLBC codec that is ages ahead of G.(anything), as it is robust to packet loss and jitter, and very low bandwidth. It was really designed for the Internet. See about transcoding:

Note Ip-to-IP GW requires a special images that ends with _isv in the filename.

Then, after a good codec optimization, you're left with the rtp overhead. There's a solution for that too as others mentioned, need to run PPP on the VC as the other fellows mentioned. Here's the application note:

As you can see it has been written with IWF in mind. There other relevant documents but I think you've enough food for thought now.

What you want to do is not easy, but with dedication and testing, can be done and you will end with a cutting edge voice and data design. Do not be afraid in asking help from your cisco account team.

Hope this helps, please rate post if it does!

GrayH Thu, 03/13/2008 - 19:52

Thank you Paulo,

You are an invaluable resource. I honestly know of no where else I can get such expert and timely advice. I will begin reseaching your recommendations immediately.


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