QoS for VoIP

[John Blakley]

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

[spremkumar]

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]

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]

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		Bandwidth	Sample	IP bandwidth
G.711	PCM	64kbps	0.125ms	80kbps
G.723.1	ACELP	5.6kbps	30ms	16.27kbps
	MP-MLQ	6.4kbps		17.07kbps
G.726	ADPCM	32kbps	0.125ms	48kbps
G.728	LD-CELP	16kbps	0.625ms	32kbps
G.729(A)	CS-ACELP	8kbps	10ms	24kbps

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]

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]

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]

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

[spremkumar · Accepted solution]

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