[c-nsp] GSR12008|GRP-B|4OC12/ATM-MM-SC|3GE-GBIC-SC throughput?

[Ibrahim Abo Zaid]

Dear Jason


I think ATM cell tax will be about 13% on average based on the following

ATM cell tax is composed of 2 parts

1-  ATM over-header (5 bytes for each 53 byte cell and that is a fixed
percnt )
2- cell padding which depends packet distribution

so ATM overhead will be 5/53 = ~ 4%
and cell padding can be calculated as
IP Packet size is 690 bytes will be padded with 30 bytes and transported as
720 bytes (15 cell x 48 payload size) so padding percentage will be ~ 9% (30
/ 720 )

so overall ATM cell tax will be 13% based on the given packet size and for
sure it will vary for other packet size values

best regards
--Ibrahim

On Thu, Apr 16, 2009 at 12:35 AM, Jason Lixfeld <jason at lixfeld.ca> wrote:

>
> On 15-Apr-09, at 2:58 PM, Lamar Owen wrote:
>
>  Incidentally, the 'show fabric' undocumented command shows internal
>> latencies
>> across the fabric.
>>
>
> Highest latency on the fabric is 84ms, over two months ago.
>
>  The ATM SAR tax may be hitting you, too.
>>
>
> Not being an ATM guru, I hope someone will clue-bat me if I get too far
> gone with my calculations below...
>
> An ATM cell payload is 48 bytes long.  On top of each cell, there's a 5
> byte ATM header.
>
> If my average packet size is 690 bytes, one packet would be stuffed into 15
> cells.  Each of those 15 cells would have an additional 5 bytes of overhead
> for the header.
>
> So, 5 bytes header for 15 cells = 75 bytes per 1 690 byte packet = 765
> bytes/6120 bits.
>
> At the time the sample was taken, I was pulling in 27131pps over my two GSR
> ATM interfaces and pushing 32478pps over the same two interfaces.  If my
> cell tax calculations are right, that would equal 166041720bps in and
> 198765360bps out across the GSR, but equally importantly, that would equal
> the same amount being put over the OC12 to Toronto.
>
> Correct me if I'm wrong, but based on this estimation, ATM cell tax
> wouldn't be an issue, would it?
>
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