RTP Packet Loss in Transmission

Unanswered Question

Hi all,

I got this problem where I think is more on UDP protocol limitation when it come to losses.

2 voice media GW, the user calls is transports via IP RTP protocol. As we knew RTP is UDP packet, where no handshake and no retransmission. Well this advantage for voice to avoid jitter and delay.

Well I got his problem, the STM-1/OC3 link speed 155Mbps, and sometime I believed the transmission quality is degrades sometime.

When this happened, the RTP packet that lost in the transmission cloud I believed will not received by the other end of router of course voice gateway facing lost RTP packets ( call drops).

My question for voice expert here

1) What router can do or help if transmission drop the packet, can router recover it?

2) What tools can we test and monitor and measure that transmission is dropping packets, this is to prove to customer nothing wrong with router.

3) Field experience how to handle this problem if the transmission is the root caused.

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Joseph W. Doherty Sat, 10/31/2009 - 05:29
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#1 Not that I'm aware of.

#2 You could configure on Cisco devices SLA tests to emulate real-time voice traffic and record the results.

#3 Since you note the drops on the STM link, yet have QoS enabled, where are these drops happening? If the drops are at the ingress interface to the STM link, verify your QoS is correct to insure packets aren't being dropped (this also assumes you have sufficient bandwidth to support the voice bandwidth). If the drops are happening on the link, you have a line quality issue to take up with your Telco.

hakkyokujp Sun, 11/15/2009 - 22:11
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1) RTP is a UDP protocol. but RTP have a retransmission.

2) Use sniper or wireshark on SPAN port. SPAN does not support on Router, need switch and configuration.

3)If packet was droped, check firewall configuration or access-list.

paolo bevilacqua Mon, 11/16/2009 - 05:18
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1) RTP is a UDP protocol. but RTP have a retransmission.

Incorrect, RTP does not retransmits packets.


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