Giuseppe Larosa Sat, 04/25/2009 - 01:50
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Hello Shaymaa,


you can use the following formula


L2 frame size [B] = IPsize + 14 (header) + 4 (FCS)


you need to take in account the interframe GAP that is a silence interval between two frames that is equivalent to 20 -20,1 bytes on the line.


so to find the pps for a given rate on the wire

R bps

pps = R /[(ipsize+18+20)*8]


where 8 is the conversion between B bytes and b bits.


you can use the formula in an spreadsheet to build your own tool.


Hope to help

Giuseppe


Joseph W. Doherty Sat, 04/25/2009 - 03:55
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I haven't seen a tool, but if you're referencing Ethenet, for 64 bytes, the conversion is 1.488095 mpps for 1 gbps to support "wire speed". Don't forget the need to double for duplex, if necessary, and you can scale for 10/100 Ethernet.


PS:

As an example, to support "wire speed" on 48 gig ports for 64 bytes packets: 1.488095 Mpps * 48 ports = 71.42856 Mpps (NB: the Cisco 4948 switch is spec'ed at 72 Mpps, while 3560G-48 is spec'ed at 38.7 Mpps).


BTW, "wire speed" pps requirements drop, much, as packet size increases. For instance, gig with 1518 frames only requires about 81 Kpps.


Many switch vendors often used to document pps for a larger frame size, to make it appear it could support "wire speed". Which it could for the larger frames, although not for smaller. Although misleading, normally switches don't need to contend with every port running "wire speed" with minimum size packets.


PPS:

With switches, don't forget to also check what the fabric bandwidth is. "Wire speed" backplane bandwidth is often spec'ed at 2x the ports' bandwidth. E.g. 48 gig ports would need 96 Gbps (NB: the Cisco 4948 switch is spec'ed at 96 Gbps, while 3560G-48 is spec'ed at 32 Gbps).

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