[nsp] High delay of modem connection over VoIP

[Bender, Andrew]

Luis-

You are introducting delay through the packetization time (sampling interval) of the 5850, and the playout buffer at the receive side of the packet network in the 2600. 

In default configurations, this is likely to be 20ms in + 40ms (or more) out... if you are measuring RTT with ping, I'd expect to see at least 120ms added. 

The bad news is that you will never be able to eliminate the additional latency that is introduced. 

The good news is that this scales with the sample size, so you'll want to reduce the packet size on that link, with something like:

codec g711alaw bytes 80

If 5850 / 2600 would do 5ms packets, that would be ideal, but I don't think you have that option. This will at least get you to 10ms... and *should* cut the latency in half. Be aware that it will increase the required bandwidth for these calls accordingly also.

Also in contemprary IOS you can now (finally!) adjust the jitter buffer to minimize its size, with something like:

playout-delay nominal 20
playout-delay minimal low

Be more careful with this however, because it may affect the quality of voice calls, depending on the quality of transit network you are using. As always, RTFM is your firend...

Regards,
Andrew Bender
taqua.com 


-----Original Message-----
From: Luis E [mailto:lucho1111 at hotmail.com]
Sent: Wednesday, July 09, 2003 2:13 PM
To: cisco-nsp at puck.nether.net
Subject: [nsp] High delay of modem connection over VoIP


Sirs:

We have a CISCO VoIP Network to provide IP telephony to our customers.   The 
CISCO VoIP Network is interconnected to PSTN over E1 lines using 5850 
gateways.
We want to provide dial-up Internet access to our customers. Most customer 
are on the PSTN side with standard phone lines and they dial using a modem 
to a Access Server (TNT MAX box) which is located at other side the VoIP 
network. We deliver the link to TNT box over an E1 R2 using a 2600.
Basically the scenario is as follow:

Remote PC--phone_line----PSTN---CISCO5850---VoIP Network----2600----E1 
line---TNT Access Server

We have made several test calls and the modem are interconnecting well but 
we noted a higher delay when doing a ping from the Remote PC to TNT box.   
Typically delays over a dialup connection is 110 ms over PSTN lines but in 
our scenario VoIP the average delay starts at 280 ms and sometimes the 
average goes up to 480 ms. The modems remain connected but this delay 
affects the browsing.

Why is a higher delay over a modem connection through a VoIP network?
We need to decrease this delay to acceptable values similar as a modem 
connection over PSTN Network.

Regards,
Luis

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