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New Member

PSTN calls using 6608 blade over WAN breaking up

I have remote users connected to a Centalzed CM cluster using 6608 blades in a Cat6500 for inbound and outbound PSTN calls. Recently these PSTN calls started to breakup. IP Phone to IP Phone between central and remote site do NOT have an issue but PSTN calls do. I have already configured QOS and we have two T1s in a multilink bundle between sites with utilization around 20%.

Any ideas?


Cisco Employee

Re: PSTN calls using 6608 blade over WAN breaking up

I have a couple of questions:

1. is your only problem choppy voice? do you hear static/echo/robotic etc?

2. Can you please check that there are no qos drops/ interface drops etc?

3. Make sure you have tx-ring-limit 2 (or 3 depending on the platform) on all ther wan interfaces.

4. Does telco have echo cancellers enabled on the circuit?

5. If you query the 6608 T1 (http://ipaddress), do you see any lost packets? or slips etc? If you see layer one issues such as CRC/ Slips etc, you need to fix that.

6. Make sure you have enough bandwidth (check locations, regions and wan bw allocation)

7. When the problem occurs, hit the i button on the phone twice in quick succession. Record the numbers you see. That will give a good hint.

8 Check if you are using only G711/g729 packetization period of 20 ms.

New Member

Re: PSTN calls using 6608 blade over WAN breaking up

To answer your questions:

1) It sees like the last syllable is left off on each word on PSTN calls only.

2) No interface drops. I have been working with TAC regarding QOS. They cannot find any trouble.

3) Not done at this point.

4) Not sure but this has been working for over a year. The problem just started a couple of weeks ago.

5) I see a few but these are the same Ts that we use in our HQ office and nobody has ever complained of this issue. The difference is that these PSTN calls stay as G711 versus the remote PSTN call which has to be converted to G729. I also had Telco check all the T1 lines and no trouble was found.

6) I have two T1 between these facilities and have set the priority queue to 1024K. We are not using locations. I am not sure how to check bandwidth regarding regions.

7) I have done this and avg jitter and max jitter vary greatly on PSTN calls.

8) This is where I think my problem is and maybe you can lend further assistance. I'll explain.

IP phone to IP phone between these two locations is always fine. Jitter values are low which tells me that my QOS settings across the WAN must be OK (also verified with TAC switching and routing group) since this traffic follows the same path as the problem PSTN Calls.

Now these IP Phone to IP Phone calls are G729. When a PSTN call occurs it has to go from G711 at the Hub site to G729 at the remote site over the WAN so I assume that some device has to be performing the transcoding. Not sure if the 6608 blade itself is doing this or not.

Any addition thoughts.

Thanks for your reply

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