Adaptive vs Fixed Jitter Buffer - Round Trip Delay issue

[mdimarzo]

I've set up a test-lab configuration with an MGX8850 with VISM and a 3661 with NM-HDV. When I measure the round trip delay with different settings of jitter buffer the results are incomprehensible:

Fixed Mode (jitter buffer VISM) to 10ms => RTD = 100-103ms

Adaptive Mode with initial value of 10ms => RTD = 86-92ms

I can't understand theese results because Cisco told me that the real minimum value of Jitter buffer is 10ms, and so it's impossible that an adaptive buffer gives a higher RTD than fixed.

What's wrong? Is my line of thought correct?

mdm

[carenas123]

I am also puzzled with your observations. You could refer to this document detailing the fixed and adaptive mode operation. May be this gives you some explanation. Let us know what you find out.

http://www.cisco.com/en/US/products/sw/iosswrel/ps1834/products_feature_guide09186a008008033c.html

[mdimarzo]

Thank you for your suggestion, but I already knew that document and I think it does not help me. I'll try to specify better my problem.

Here are the exact settings of 3661 and MGX:

Cisco 3661:

IOS Rel. 12.2.(13)T3

Coder: Clear Channel G.711

Packetization: 5ms

De-Jitter buffer: Fixed 20ms (Unlike what is written in the document you refer, Cisco declares, at the moment, that fixed mode at 20ms is the only setting possible).

MGX 8850-VISM-PR:

Sw Rel. 3.1(1.0)

Coder: Clear Channel G.711

Packetization: 5ms

De-Jitter buffer: Fixed 10ms or Adaptive with initial value 10ms (Cisco declares that 10ms is the minimum value the buffer can set, in the Adaptive mode too.

Maintaining cisco 3661 configuration fixed, if I change only the settings on the VISM from Fixed to Adaptive mode, the RTD value change from ~100ms to ~90ms.

Moreover so higher values in the depicted configuration are very strange but, at this time, I don't wonder at anything.