From: eicc tester (reto_ccie@yahoo.com)
Date: Thu Aug 30 2007 - 12:35:38 ART
Hi,
The issue with fragmentation is related with the TS , the time that you use for shaping, in order to keep the CIR rate in the TS. The fragmentation have to be small enough in order to fill completely between the TS time-space.
For example a 512K CIR using a TS of 10 ms, implied a BC of 5120 bits, which correspond with a maximum of 640 bytes fragment. You transmit rate are T1 , then you are able to transmit 5120 bits in less of 10 ms (aprox 4ms) the rest 6 ms are idle (no transmit) for that pvc.
The fragmentation to 640 is used to cut big data packet (1500 bytes) in fragments in order to match the shapping. voice packet with less of 640 will not be fragmented.
reto
Scott Smith <hioctane@gmail.com> wrote:
I'm hoping someone can shed some light on why my thinking is wrong on this.
Config example: 2 512k PVCs on a T1 port. Each PVC is shaped via FRTS
to 512k with no burst and the physical interface can run at T1 speed.
Cisco says "you do not need fragmentation because the port speed is
T1, irregardless of the shaping."
I know all the in and outs of fragmentation/interleaving,
serialization delay, etc and how this impacts voice traffic. Where
I've got a problem is the seeming disregard for the amount of traffic
that will actually pass through the interface. T1 port or not a single
PVC cannot send more than 512k since it is shaped to that rate.
Granted, once the traffic passes through shaper and its time to place
the bits on the wire it'll be serialized at T1 rate (512k of traffic
will be serialized at T1 rate) . Since a single PVC can only place
512k on the T1 port how is it that we completely ignore this fact when
deciding to frag or not frag? It seems to me, in this case, the port
speed is irrelevant because we can only send at 512k... OK, I could go
on and on but I'll spare you all :-)
So, obviously I'm wrong and or confused... lord knows it isn't the
first time and will not be the last but I'd really like to know why my
logic is flawed. Thus far no one who says I'm confused (including
Cisco) has been able to explain why.
TIA!
-- Scott CCIE #17040 (R&S)
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