Calculating Packets Per Second of Devices

So here's a topic which just keeps cropping up and I look it up every time, so I'm sticking it on the blog so that I've got a reference for the future and hopefully I'll remember it a bit better for next time!

You will often have to calculate the required packets per second of a device to make sure it is "powerful enough" to handle the job given to it. Alternatively you may be sizing a device for a link, for example a router for a WAN connection, and you want to make sure it can handle wire speed on the link. Here is my calculations for this.

I'll use the standard example of a 1 gigabit WAN connection 1Gbps, this is 1,000,000,000 bits:

In order to work out the required pps (Packets Per Second) of a device for the WAN link we need to consider the maximum and minimum packet sizes. Bear in mind the packet size in reality will vary so the actual number is anyone's guess but this gives you a great boundary.

The minimum packet size is 84 bytes (46 payload, 4 CRC, 2 MAC type, 6 MAC source address, 6 MAC destination address, 8 preamble, 12 inter frame gap).
The maximum packet size is 1538 bytes (same as above but with 1500 payload instead of 46)

The calculation is:
Convert bits into bytes -- 1,000,000,000 bits per second / 8 = 125,000,000 bytes per second
Convert bytes into packets -- 125,000,000 bytes per second / 84 = 1,488,096 packets per second

The above works out the minimum packets per second, changing 84 to 1538 works out the maximum packets per second:
Convert bytes into packets -- 125,000,000 bytes per second / 1538 = 81,274 packets per second

From this we know if all the packets were the minimum sized we'd need a more powerful device to handle wirespeed, but this is theoretical because you wont be able to guarantee the size of all packets, apart from in very special cases.

If you can work out the average packet size within the environment you will be able to workout more accurately the device requirements, but from here is it a bit of a guessing game do you plan for the lowest possible packet size to ensure the device can handle wirespeed or do you pick a more realistic but unknown value somewhere in the middle. That answer is up to you.