[Kurt] likes to understand what’s going on with his network. He already utilizes bandwidth inspecting software application on his DD-WRT capable router, however he desired a second opinion. So he developed his own network monitor. [Kurt] started by building a passive Ethernet tap. He then needed a network interface chip that would serve his purposes. The typical Wiznet chips utilized with Arduinos didn’t enable sufficient manipulation of raw packet data, so he changed to a Microchip ENC624J600 (PDF). The Microchip controller enabled him to count the bytes in the raw Ethernet packets.
With the Ethernet interface complete, [Kurt] turned his interest to a microcontroller to run the show. He started with an Arduino, however the lack of debugging quickly sent him to an Atmega128 in Atmel Studio. After getting the fundamental circuit working, [Kurt] changed over to a PIC24F chip. With data lastly coming out of the circuit, he was able to tell that his original back-of-the-napkin calculations for bandwidth were wrong. [Kurt] produced a PCB to hold the microcontroller, then composed a Python program to plot the data output from his circuit. The bandwidth plot matched up well with the plot from DD-WRT. now he just needs a giant LED matrix to show off his present network stats!