This is an old revision of the document!
SUMMARY:
When evaluating or measuring an Ethernet device's (switches, routers, firewalls) performance capabilities, the main indicator that most will consider is the raw bandwidth that the device backplane can provide.
However it is also important to make sure that the device has the capacity or the ability to switch/route as many packets as required to achieve wire rate performance. This metric is called the ‘Packets per Second’ or PPS for short.
This article details how to calculate how many packets per second processing capabilities is required from a port to achieve wire-rate performance.
SOLUTION:
Note: This article focuses on Ethernet; other mediums such as ATM will have other considerations for calculating PPS.
To calculate the amount of packets per second a port must be able to handle to achieve wire-rate performance we need to take into consideration the fact that the IP protocol allows for variable payload sizes which in turn plays a part in our PPS calculation.
The smaller the packet passing on the wire the more packets that need to be switched to achieve wire-rate performance; while on the other hand, larger packets will require less PPS throughput to achieve wire-rate. As such to calculate how many PPS needed to achieve wire-speed we need only be concerned with small packet sizes since they will be the most taxing for the switch and will yield the larger PPS number. Naturally we will assume that no collisions occur on the medium.
We need to see how much ‘space’ each packet will occupy so we will look at the frame size in which the smallest packet will be encapsulated, as well as the inter-frame gap, and the preamble since they occupy ‘space’ in between frames.