QoS for VoIP

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May 5th, 2010
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All,


I'm about to run our first site using voip through our ISP's mpls network. I was going to mark the traffic outbound, but I want to guarantee that the phones get a minimum amount of bandwidth when needed. I'm not sure if priority is a good choice since it polices the traffic also, so is using the priority command better than the bandwidth command under a  policy map for voip?


Also, should I mark the traffic outbound on the serial interface with the EF tag, or should I mark it inbound on the lan side of the router?


Thanks!

John

Correct Answer by spremkumar about 6 years 10 months ago

Hi John


You need to make sure that your ISP runs the QoS policies throughout their whole network till the egress PE after which the packets will be handed over to your remote end (CE) location. (thts the default case with most of the ISPs)


You need to apply the service policy command outbound on the serial interface.


Make sure you have similar configs at the remote end router as well so that you have policies applied on both the sides.


regds

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spremkumar Wed, 05/05/2010 - 07:01
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Hi John


You should apply priority to the voice class instead of bandwidth, thats the best practice being used and followed.

And of course if you dont have voice traffic then the bandwidth will be available for other traffic class.


regds

John Blakley Wed, 05/05/2010 - 07:10
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Is there a calculation that I can use to figure out what a good minimum would be? I'm not sure how many phones I'm going to end up with....


Thanks,

John

spremkumar Wed, 05/05/2010 - 07:27
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Hi


You need to arrive at a no of approx simultaneous calls which needs the WAN bandwidth, based on the same you can allocate the priority for the voice traffic.



Coding algorithm

BandwidthSampleIP bandwidth
G.711PCM64kbps0.125ms80kbps
G.723.1ACELP5.6kbps30ms16.27kbps
MP-MLQ6.4kbps17.07kbps
G.726ADPCM32kbps0.125ms48kbps
G.728LD-CELP16kbps0.625ms32kbps
G.729(A)CS-ACELP8kbps10ms24kbps


Above table for your reference, you can consider using G.729(A) codec and use it along with no of calls to arrive at the approx bandwidth required for voice traffic.


http://www.erlang.com/bandwidth.html


regds

Jon Marshall Wed, 05/05/2010 - 08:10
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John


Just to expand on Edwin's answer . You said -


I'm not sure if priority is a good choice since it polices the traffic also, so is using the priority command better than the bandwidth command under a  policy map for voip?


That's actually what you want ie. you want the priority traffic to be policed otherwise priority traffic could end up starving the other queues. And if you have not setup your trust boundaries correctly someone could in theory mark all their traffic as priority in which case the policer will save you from that user having all bandwidth, although it could degrade the VOIP calls themselves.


So the trick is to estimate how many calls together with the codec to work out how much bandwidth you will need in the prioriy queue.


Also bear in mind if this is across an MPLS network then unless the provider is also prioritising this traffic it will be treated as best effort across the MPLS cloud.


Jon

John Blakley Wed, 05/05/2010 - 09:38
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Yeah, we've contacted the ISP to put qos on the link. So, should I apply the policy inbound on the inside interface or outbound on the serial side, or both inbound and outbound on the serial side?


John

Jon Marshall Wed, 05/05/2010 - 09:52
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j.blakley wrote:


Yeah, we've contacted the ISP to put qos on the link. So, should I apply the policy inbound on the inside interface or outbound on the serial side, or both inbound and outbound on the serial side?


John


John


Presumably the traffic will already be marked by the time it reaches your MPLS router ? If so then apply your queueing policy outbound on the serial interface.


Jon

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spremkumar Wed, 05/05/2010 - 09:53
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Hi John


You need to make sure that your ISP runs the QoS policies throughout their whole network till the egress PE after which the packets will be handed over to your remote end (CE) location. (thts the default case with most of the ISPs)


You need to apply the service policy command outbound on the serial interface.


Make sure you have similar configs at the remote end router as well so that you have policies applied on both the sides.


regds

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