From: Church, Chuck (cchurch@wamnetgov.com)
Date: Sun Jul 18 2004 - 03:23:39 GMT-3
There's a good white paper on CCO (at least there used to be) on this.
The secret to clean VoIP is making sure a large frame being dumped onto
the wire won't make a VoIP frame behind it wait to long. The time
difference between a frame's first bit being sent and that same frame's
last bit being received is called 'serialization delay'. Cisco
recommends keeping this around 10ms. I found even 20ms keeps VoIP
running fine with the right RTP queuing. So, to keep serialization
delay to 10ms, take your bandwidth in bits/sec, and divide it by 100
(since there's a hundred 10ms slices in a second. Divide again by 8
(bits to bytes conversion), and that's your recommended fragment size.
So a 256kb PVC should have a fragment size of 320 bytes to keep the
delay to 10ms. HTH.
Chuck Church
Wam!Net Government Services - D&I Team
Lead Design Engineer
CCIE #8776, MCNE, MCSE
1210 N. Parker Rd.
Greenville, SC 29609
Office: 864-335-9473
Cell: 703-819-3495
cchurch@wamnetgov.com
PGP key:
http://pgp.mit.edu:11371/pks/lookup?op=index&search=cchurch%40wamnetgov.
com
-----Original Message-----
From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of
Scott Savage
Sent: Saturday, July 17, 2004 5:18 PM
To: ccielab@groupstudy.com
Subject: FRF.12
I have searched and looked through a number of documents covering
frame-relay fragmentation and have not been able to find a good answer
to this question.
What is the correct way of calculating the # of bytes for fragmentation?
In all the examples I have seen, it appears that the value is set to Bc
when it is small and relates to voice and the value tends to be a fairly
small number like less than half of the interface MTU or else 1/8th of
Bc if Bc is larger than the interface MTU. I have found no rhyme or
reason to this, though. Is it user preference as long as the number
isn't smaller than the framesize of the "realtime" traffic you're trying
to reduce latency for or larger than Bc?
I am assuming in the real world you are really only going to use
fragmentation when you are running something like voice/video across a
link smaller than 768K. But if a question comes along dealing with
fragmentation and the Bc value is greater than 1500, then what is the
right way to determine the fragmentation byte count or what are the
right details/requirements to pay attention to that would help you
arrive at an acceptable number?
=====
-- Scott Savage
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