[cisco-voip] Jitter Question

[J. Oquendo]

Patrick Diener wrote:
> bottom line:  jitter level should not be greater than 30 - 35ms...
> unfortunately on a quick search on cisco.com I could not find any docu
> to back that value...
>   
I don't know where you got the 30-35ms range from so I quote again from 
the DQoS book:

As per:

Cisco DQOS Exam Certification Guide
Wendell Odom and Michael J. Cavanaugh
Copyright © 2004 Cisco Systems, Inc.


ONE WAY DELAY BUDGET GUIDELINES
*1-Way Delay (in ms) Description*
0–150 ITU G.114’s recommended acceptable range
0–200 Cisco’s recommended acceptable range
150–400 ITU G.114’s recommended range for degraded service
400+ ITU G.114’s range of unacceptable delay in all cases

Some flows tolerate loss better than others do. For instance, the human 
ear can detect loss of
only 10 ms of voice, but the listener can generally understand speech 
with such small loss. Cisco
digital signal processors (DSPs) can predict the contents of lost voice 
packets, up to 30 ms when
using the G.729 codec.

Lost voice packets result in the receiver having a period of silence 
corresponding the
length of voice payload inside the lost packet(s). With two consecutive 
G.729 packets
lost, 40 ms of voice is lost; the G.729 codec cannot predict and 
generate replacement
signals when more than 30 ms of consecutive voice is lost. A single lost 
G.729 packet
would only cause a 20-ms break in the voice, which could be regenerated. 
So, a single
lost packet is not perceived as loss in a G.729 call.

No luck searching through the book on 35 ms or 35ms so I tried 40 ms & 40ms:

Page 862
Instantaneous buffer overrun occurs when a switch port TX queue fills 
for an instant
causing packet loss, which can adversely affect real-time applications. 
In a VoIP
conversation, for example, 40 ms of congestion causes an audible clip in the
conversation.

Page 82
In Figure 1-23, the playout begins at the statically set playout delay 
interval—40 ms in this
case—regardless of the arrival time of other packets. A 40-ms de-jitter 
playout delay allows
jitter to occur—because we all know that jitter happens—so that the 
played-out voice can
continue at a constant rate.


-- 
====================================================
J. Oquendo
http://pgp.mit.edu:11371/pks/lookup?op=get&search=0x1383A743
sil . infiltrated @ net http://www.infiltrated.net 

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John Adams

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