Elmar
Posts: 1
Joined: Sun Sep 26, 2021 8:09 am

FYI: Raspberry Pi 4 / 400 network cable length (<2m)

Sun Sep 26, 2021 9:13 am

I sometimes had problems with the network speed of the Pi 4 / 400, it won’t connect full 1Gbit. I always had tried different cables and/or switches which had solved the problem.

But when I switched a cable from my laptop to my Pi 400 recently, I wondered why the Laptop has no trouble running full 1Gbit but the Pi insists on 100Mbit on the very same cable and switch.
I had looked online for known issues with the Pi 400 and found a solved topic where someone had replaced a CAT7 cable with a CAT5e one to solve the problem. Also there were topics where certain brands and models are declared as incompatible.

So I did some experiments and measured the signals. From what I can see I think that the Pi has serious trouble with the clock recovery if the reflections return very early while receiving 1Gbit signals. Also the SNR on the transmission side is a bit worse than most (more expensive) gear has but I don’t think that this would cause any problems except when there is a lot of cable length between the Pi and the switch.

The longer the cable, the more time the reflections need to bounce back and forth along the cable. When there is at least 2m of cable in between, it is much easier to recover the clock timing of the data signals.

So if your Pi 4 / 400 won’t connect with 1GBit, make sure you use a longer cable before trying to replace your switch. Don’t use cables which are shorter than 2 meters! Also you shouldn’t use cables <2m to connect any active components to each other but they are fine to connect to patch panels which add extra cable length anyway.

I did some more experiments with my Pi 400. All 0.1m patch cables I tried failed. Some 0.75m cables worked and more than half of my 1m cables had worked 1Gbit. When patching several cables that had failed together, they all worked fine as long as they add up to more than 2 meters. Even when using cheap ISDN F2F adapters (CAT 2 or 3 at best) to patch a lot of 0.1m and 0.75m cables together, the Pi 400 was happy.

So I guess that the guy who downgraded his cable from 7 to 5e used a <2m CAT7 and replaced it with a >2m 5e cable which had fixed the problem because it was longer, and not because it was rated lower.

In Germany, the routers which were sold by your provider often came with patch cables which only had 2 pairs of wire inside instead of 4 until the routers had 1GB capability. Most of them are easy to spot since they have yellow plugs. So I am not sure about other countries, but if you spot a patch cable with yellow plugs, be very suspicious and check it.

cleverca22
Posts: 4663
Joined: Sat Aug 18, 2012 2:33 pm

Re: FYI: Raspberry Pi 4 / 400 network cable length (<2m)

Sun Sep 26, 2021 2:02 pm

Elmar wrote:
Sun Sep 26, 2021 9:13 am
So I did some experiments and measured the signals. From what I can see I think that the Pi has serious trouble with the clock recovery if the reflections return very early while receiving 1Gbit signals. Also the SNR on the transmission side is a bit worse than most (more expensive) gear has but I don’t think that this would cause any problems except when there is a lot of cable length between the Pi and the switch.
of note, the bcm2711 soc, only spits out RGMII traffic, 125mhz 4bit DDR

there is a PHY on the board, to convert that digital traffic into analog, and deal with all of that clock recovery stuff
so changing out the PHY for a new one, is a relatively simple task, compared to if the problem was in the soc itself

the CM4 is also using a different PHY that has some phy-side timestamping features, so you may find different results on a CM4

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