09-17-2008 09:48 AM - edited 03-15-2019 01:21 PM
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....
09-17-2008 11:57 AM
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
fair-queue
interface Multilink1
ip address xxx.xxx.xxx.xxx 255.255.255.252
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
--Patrick
09-17-2008 12:09 PM
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.
09-17-2008 02:49 PM
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.
regards...
09-17-2008 03:11 PM
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
--Patrick
09-18-2008 09:20 AM
Hi,
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?
regards...
09-18-2008 09:37 AM
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?
--Patrick
09-18-2008 11:48 AM
I'm using G.729a...
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