Conference Voice Quality Issues with CP7936 Phones

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Jul 17th, 2006
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Hi Pros,


I have a setup :

CCM 4.1(3)with SR3B

Cisco 2811 Router with PVDM2-64 configured with DSP Resources

8 * CP7936 Phones

Codec: G.729


All phones are spread over the WAN network, link speeds range from 256K - 2 MB

Most links are 50% to 75% congested.


My problem is that when I speak individually to each locations, the voice is clear & with no breakage, but when I join them say 4-5 conferees, then the voice quality degrades & breakage is there.


Now I want to know what could be the reason for this degradation (I dont have QOS configured on my WAN at all)

I want to make sure I am not hitting on any bugs or any misconfiguration issues.

I have applied CCM SR3b because it addresses 2 sec jitter problem with 7936 phones.

Besides that any recommendations would be useful.


Second thing I wanted to know is that I make a single call to one of my locations which is connected with a 2MB Link. The out is 550K & in is 150K traffic. The problem is I am able to hear the other side properly (from where 150K traffic is coming) but other side cant hear me properly (where 550K traffic is going).

I know that 2MB link doesnt require LFI to be configured, but I wonder because there is no QOS configured, would serialization or queueing delay be causing a problem??


Hoping to hear some answers...


Pratik




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gpulos Tue, 07/18/2006 - 03:06
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with WAN links of 256k - 2MB, if any of these calls are traversing the WAN link it is seriously recommended that you implement some QoS.


at the very least LLQ should be implemented for all of the links under 2mb.


you MUST prioritize your voice traffic and even voice signaling to ensure your voice gets the required resources and queuing that provides for the quality users are expecting.


see this link for more info:

http://www.cisco.com/en/US/tech/tk543/tsd_technology_support_category_home.html

pratik.rb Tue, 07/18/2006 - 03:29
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Thanks for replying. But if the link of 2 MB is only 25% to 50% utilized, do you think that voice quality could still be affected with jitter ?


Also, is there any optimization or further config needing to be done for improving quality of the conference ?


gpulos Tue, 07/18/2006 - 03:52
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yes, the 25-50% utilization could and most likely will have an affect on voice quality if no QoS.


as far as the conference, there is more than just QoS in play here. chances are that you need to use transcoders between some if not all of your endpoints. also, the 2811 although capable of handling upto 36 ip phones, you've not specified how many phones you use or how many calls the 2811 must handle simultaneously.


see these specs for the 2811 for comparison:

http://www.cisco.com/en/US/products/ps5854/products_data_sheet0900aecd8016fa68.html


guarantee your voice packets their order on the network (qos) and you should definately see a change in your quality.


p.s. qos is not just for slow links, but also inside the network. assure that your switching environment is marking packets appropriately with CoS/DSCP where applicable.


(if you have a later version of IOS, try to use AutoQOS; this feature will handle most of the QoS shaping for you based on what the device(s) see as far as traffic) autoQOS is an excellent way to start.


autoQOS in switches: (minimum IOS/catOS versions)

12.1(12c) 2950/3550

12.1(19)E 4500

catOS 7.5.1 6500

IOS 12.2(15)T

pratik.rb Tue, 07/18/2006 - 04:18
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Thanks again for the reply,but I wanted to know anything in specific can cause distortion (one conferees voice overwriting others) or jitter with these CP 7936 IP Phones. I am using G.729 codec & PVD2-64 resources on 2811 router...

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