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Slow / synthetic RTP stream playback

Unanswered Question
Aug 30th, 2006
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Hi all,


I'm using a packet capture tool to analyze a problem with recording voicemails over a customer's VoIP network. G.711 is the codec being used.


The RTP packets seem to get duplicated and the result is a slow / synthetic sounding voice during playback.


Could anyone confirm this ? I'm not sure what device is generated the duplicate packets. However, I do see them within the packet-capture output.


Thanks,


Mike

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Anonymous (not verified) Tue, 09/05/2006 - 11:52
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To resolve this issue, determine the cause of the delay and try to eliminate it.


The causes of drops or delays in a packet telephony network can be numerous and varied.


These are common examples:


Misconfigured Low Latency Queuing (LLQ)


Misconfigured fragmentation for low speed links


Misconfigured traffic shaping or Frame Relay Committed Information Rate (CIR) being exceeded


Links with over committed bandwidth in the path of the call (for example, Poor Connection Admission Control [CAC] for voice calls). A simple example would be a G.711 call with no Compressed Real-Time Protocol (CRTP) or Voice Activity Detection (VAD) across a 64-Kbps link.


Duplex mismatches in an Ethernet environment


CPU intensive operations on a router in the path of the call. For example, debugs to a console or saving the router configuration can cause high CPU utilization that delays packets traversing it.


For more information, refer to:

http://www.cisco.com/en/US/tech/tk652/tk698/technologies_tech_note09186a00800f6cf8.shtml

anup.anand Tue, 09/05/2006 - 15:06
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The packet duplication could be because of the way you have configured SPAN in the switch to capture the packets.


For example, if you have a phone connected to fastethernet 1/1 on the switch and the uplink port on the switch is gig 1/0, and you configure SPAN to capture packets on both fastethernet 1/1 and Gig 1/0 in both directions (TX and RX), this could lead to duplicate. The reason being that, the switch will forward to the SPAN port the packet coming in the RX direction in fast 1/1 and also the packet in the TX direction in Gig 1/0 .


To resolve this issue, configure SPAN to either do the capture on the phone ports or on the uplink ports. It is better to do so on the phone ports so that you can see the packets on calls between phones connected to the same switch.


Regards,


Anup

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