Understanding the network.pdf - Back to Home

Typically, when evaluating switch and router performance,
manufacturers use the smallest legal frame size (Layer 2) to measure
the frame per second (FPS) throughput rating to delineate equipment
performance. The FPS rating is usually specified as an
interface-to-interface rate, or as the total throughput capacity for the
router or switch. It is measured by streaming frames through an
interface pair and counting how many frames were forwarded through
in one second. Based on this value, the device's throughput is then
calculated.
In many instances, the term "packet" is used interchangeably with the
term "frame," in which case, this measurement is expressed as
packets per second (PPS). However, the two terms technically refer to
different data transport structures. The term "frame" is commonly
used to describe a structured collection of ordered bits used by a
Layer 2 data transmission protocol to carry data. A frame has a base
minimum size that includes header, error checking, and delineation
information, with a variable data portion. The term "packet" refers to
the link control information, Layer 3/Layer 4 header information, and
the actual data. Layer 3 packets typically range from 4K to 64K in size.
These packets are transported in Layer 2 frames. You should recall
that when a packet exceeds the payload of a frame, the packet is
fragmented into sections and transported in multiple frames to the
packets destination. After all the frames have been delivered, the
packet is reassembled and handed to the ULP.
When you are looking at PPS or FPS measurements provided in
technical specifications disclosed in equipment documentation or
product literature, be sure you know what packet/frame is being used
to calculate the measurement. It is common today for vendors to use
PPS to describe FPS (Layer 2) data transmission rates.
When calculating the FPS measurement, the entire length of the
data-link transport frame must be used to get an accurate
measurement. It is important to keep this in mind when measuring
Ethernet performance because the frame preamble (the first 8 bytes)
and the Inter-Frame Gap (IFG), which adds an additional 12 bytes to
the frame, are not included as part of the minimum and maximum
frame size. When calculating the PPS measurement, only the frame's
data/information field is used. Needless to say, these two values will
be different from one another.
PPS measurements describe the transmission and volume of the
actual data being carried. FPS measurements describe the Layer 2
segment's transmission, throughput and, by extension, the segment's