Cisco Support Community
cancel
Showing results for 
Search instead for 
Did you mean: 
Announcements

Welcome to Cisco Support Community. We would love to have your feedback.

For an introduction to the new site, click here. And see here for current known issues.

New Member

Compressed VoIP over Ethernet

Hello All

I am a bit confused about this and would be gratefu to have your thoughts on this.

I am managing a link where everything is Ethernet but it connects to a client router which has serial connection and a voice link. They are compressing the voice over the Serial link using Frame Relay Fragmentation before it hits my router. I just wanted to know since I cannot use Fragmentation over ethernet will the Voice traffic BW will get increased on my links or will it stay compressed.

Any thoughts / advice will be very appreciated.

Thanks a lot in advance.

2 ACCEPTED SOLUTIONS

Accepted Solutions
VIP Super Bronze

Compressed VoIP over Ethernet

cRTP is per-hop in nature; it must be uncompressed at either end of the serial link.

http://www.cisco.com/en/US/docs/ios-xml/ios/qos_hdrcomp/configuration/12-4t/config_rtp_hdr_comp.html

Also, keep in mind that G.729 adds 30ms of delay purely due to the nature of the codec. If this is a satellite link where there is high delay using G.729 will only exacerbate it.

Please remember to rate helpful responses and identify helpful or correct answers.

Please remember to rate helpful responses and identify helpful or
Hall of Fame Super Gold

Compressed VoIP over Ethernet

It is not a matter of IOS.

There is no standard or method for compressing IP header over Ethernet.

10 REPLIES
VIP Super Bronze

Compressed VoIP over Ethernet

You are talking about three unique points here:

  1. LFI which takes a larger packet and fragments it into smaller transmission units to avoid introducing unacceptable jitter/delay on low speed interfaces, typically below 768kbps.
  2. cRTP which only impacts the overhead (e.g. RTP,IP,MLPPP) not the actual audio payload. Also only recommended on links below 768kbps due to the CPU overhead.
  3. Codec selection (e.g. G.711 vs. G.729) which is the largest contributing factor to per-call bandwidth.

To recap: LFI doesn't impact bandwidth, only getting the packet on the wire in low-speed situations. cRTP can influance bandwidth, taking what is normally 40bytes of overhead down to as little as two. Codec choice also influances bandwidth.

Please remember to rate helpful responses and identify helpful or correct answers.

Please remember to rate helpful responses and identify helpful or
New Member

Compressed VoIP over Ethernet

Thanks Jonathan for your views -- actually the link i am dealing with is a 256Kbps WAN link so the client has setup the cRTP on their side and is using G.729 for around 13-15Kbps bandwidth for voice calls. I am not sure how to ensure that the voice calls stay at that bandwidth level when traversing through my WAN link (256Kbps) as it is Ethernet on my side (Satelllite Link so cannot increase the link). or will it stay at that bandwidth once it is compressed on the Serial interface -- if you could please share your thoughts on this.

VIP Super Bronze

Compressed VoIP over Ethernet

cRTP is per-hop in nature; it must be uncompressed at either end of the serial link.

http://www.cisco.com/en/US/docs/ios-xml/ios/qos_hdrcomp/configuration/12-4t/config_rtp_hdr_comp.html

Also, keep in mind that G.729 adds 30ms of delay purely due to the nature of the codec. If this is a satellite link where there is high delay using G.729 will only exacerbate it.

Please remember to rate helpful responses and identify helpful or correct answers.

Please remember to rate helpful responses and identify helpful or
New Member

Compressed VoIP over Ethernet

Thanks again: one thing i am confused about is whether Ethernet WAN link is able to keep the traffic (which has gone cRTP compression on a previous back to back serial hop) compressed?

VIP Super Bronze

Compressed VoIP over Ethernet

No. You must decompress the cRTP at the receiving interface because it's per-hop in nature. The Ethernet interface should support cRTP as well but the router would be performing a separate compression operation. Be careful to watch the CPU under load.

Please remember to rate helpful responses and identify helpful or correct answers.

Please remember to rate helpful responses and identify helpful or
New Member

Re: Compressed VoIP over Ethernet

Thanks

This document says cRTP is not available over Ethernet.

http://www.cisco.com/en/US/tech/tk652/tk698/technologies_tech_note09186a0080094ae2.shtml

I tried a router as well and it is not giving me the option:

Router(config-if)#ip rtp?

% Unrecognized command

Router(config-if)#ip rtp

the IOS version on it is flash:c2801-spservicesk9-mz.124-15.T7.bin

Is there a higher version of IOS that support this?

Hall of Fame Super Gold

Compressed VoIP over Ethernet

It is not a matter of IOS.

There is no standard or method for compressing IP header over Ethernet.

New Member

Compressed VoIP over Ethernet

Thanks Paolo

Hall of Fame Super Gold

Compressed VoIP over Ethernet

Thank you for the nice rating and good luck!

VIP Super Bronze

Compressed VoIP over Ethernet

Thanks for keeping me honest Paolo! That's what I get for skimming a document vs. reading.

Please remember to rate helpful responses and identify helpful or
221
Views
15
Helpful
10
Replies