When HN users recommended it
1 HN user recommended this in 2022, in one comment.
20072026
Each tall tick is one HN user's first recommendation. A short tick is the same user again. Click a tick to jump to the comment.
1 HN user recommended this in 2022, in one comment.
Each tall tick is one HN user's first recommendation. A short tick is the same user again. Click a tick to jump to the comment.
I know a little bit about Ethernet, having used it since the early 10Base-2 days as well as having done a bit of "hacking" in an effort to decode 1000Base-T.
This article is focused on the QSGMII PHY chip, without which it would be practically impossible to use an oscilloscope for any meaningful troubleshooting.
1000Base-T is the most commonly used Ethernet variant in use today. It uses four balanced differential pairs for signalling. Each pair transmits and receives simultaneously. This works because 1000Base-T is a point-to-point connection and the PHY contains a hybrid that subtracts the voltage being transmitted from what is received. This makes the problem of "tapping" the interface harder because the third party observer sees a jumble of data and cannot easily determine what data is coming from either of the two connected devices. Each pair can send/receive five possible symbols, represented by voltages of +3.5, +2, 0, -2, -3.5 volts. In addition, the PHY "scrambles" the data being sent with a LFSR (https://en.wikipedia.org/wiki/Linear-feedback_shift_register) seeded with either a "master" or "slave" value. (The master/slave identity is negotiated between the two peers when they detect each other.) It gets even more complex with hamming codes and an abstraction layer to separate the control/data planes. There's also a complicated DSP for line equalization.
BTW, I do own one of these: https://www.amazon.com/midBit-Technologies-LLC-SharkTapUSB-1... which can make the job very easy, but it was not an available option for my "hacking" application.