Voice quality degradation

Unanswered Question
Sep 17th, 2008

Hi all,

I have an issue with voice quality in calls. When yo call the call is established correctly, but after 2 or 3 seconds, the voice quality degrades until it is almost impossible to hear something. The call is never droped.

I'm new in VoIP troubleshooting, soif somebody can guide me trough this problema I have. Also, in the Data network I start to configure QoS. In switches I'm configuring mls qos trust dscp. But in routers this command is not available. Does the routers will forwards packet and keep the EF mark attached to it?

thanks a lot and regards....

I have this problem too.
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hypnotoad Wed, 09/17/2008 - 11:57

Are these on LAN calls, WAN calls, or PSTN calls?

Routers don't need the trust command like switches. You do however have to deploy a QoS model on all of your WAN egress ports to prioritize your voice traffic. It will be overrun by all the other larger TCP packets otherwise.

Here is an example from one of my routers:

class-map match-any QoS-VoIP-Control

match ip dscp cs3

match ip dscp af31

class-map match-any QoS-VoIP-RTP

match ip dscp ef

policy-map QoS-Policy

class QoS-VoIP-RTP

priority percent 15

class QoS-VoIP-Control

bandwidth percent 5

class class-default


interface Multilink1

ip address xxx.xxx.xxx.xxx

ip tcp header-compression iphc-format

ppp multilink

ppp multilink interleave

ppp multilink group 1

ppp multilink multiclass

service-policy output QoS-Policy

ip rtp header-compression iphc-format


hypnotoad Wed, 09/17/2008 - 12:09

One more thing. Don't forget to trust dscp on your switch trunk ports.

A protocol analysis tool is great for troubleshooting. I use Omnipeek Pro and like it very much. There are a number of free ones that will do as well.

ccnp_seeker Wed, 09/17/2008 - 14:49

Ok, thanks!!

I'll be cheking tomorrow on switches to see if packets are marked as I need.

These voice travels trough a DS-3 link.


hypnotoad Wed, 09/17/2008 - 15:11

Configure the switchport where the router connects for port mirring ('monitor session' command) and use see if voice packets going in and out of the router are properly marked.

If they are marked at that point, run this command on the router to see if queueing is working:

show policy-map interface


ccnp_seeker Thu, 09/18/2008 - 09:20


thanks for your help....

I put a packet sniffer and i see packets marked as EF in both ways.

If I asign a COS of 5 in a switch, the router will recognize this as a DSCP EF?


hypnotoad Thu, 09/18/2008 - 09:37

COS is a layer two designation and DSCP is layer three. You can use a COS to DSCP map in the config.

What do you mean by "assign a COS of 5 in the switch"? The phones already mark (or color) the packets. You just need to use QoS to prioritize the EF marked packets on a per hop basis through your network.

Start with the basics. Be sure you are trusting DSCP on all of the switch ports that carry voice traffic. Then get QoS configured on your routers. Just this much will do wonders for your voice quality.

BTW - What codecs are you using? I typically use G.711 on high speed LAN segments and G.729a on slower WAN links. Have you defined regions on your CallManager cluster?



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