General Computing
networking home-networking cable network-discovery
Updated Mon, 01 Aug 2022 02:00:20 GMT

Cat7 cable doesn't work on only one computer


I'm running into a strange issue with a Cat7 cable seemingly not working for just one computer.

I picked up a a new computer yesterday and connected the Cat7 cable from my current desktop to the new one. Windows 10 showed the network as unidentified. The first thing I did was test the cable with my old desktop and my laptop again, and both instantly connected with a full gigabit connection, no issue. Next I tried all of the usual steps (release DHCP lease, reinstall drivers, disable wireless NIC, etc) and finally got to where I had manually assigned an IP address to this computer in my router. This made it work for about an hour, then it kicked back to unidentified again. It keeps bouncing between unidentified, connected, and network cable unplugged.

Finally, I grabbed an old Cat5e cable I had in a drawer next to me and hooked it up. It recognized the network instantly and hasn't disconnected once. I tried reconnecting the Cat7 cable and it went to unidentified again. I've had the Cat7 cable plugged into my old desktop since last night and it hasn't had a single connectivity issue.

While it's working fine now with the 5e cable, I'm completely baffled as to why this would make such a difference? Do I just have a weird faulty cable, or is something else going on?




Solution

I had a feeling it would be a Realtek RTL8168 based NIC.
I have had my share of issues with them too.
It seems that this particular chipset is often paired with a transceiver (that is the part that converts the line-signal in the cable to/from something the chip itself can handle) that is quite picky about the signal quality on CAT6 and CAT7 cables.

The stiff solid core cables usually work fine, but the thin supple ones (these operate often quite close to the edge of the electrical specifications) can be problematic.
It seems these transceivers are not always sensitive enough to accept the full range of signal quality that is valid for CAT6/CAT7.
Cat6a or Cat5e is usually no problem. Sometimes it also depends on the switch on the other end. Same cable works with one switch and not with another. Probably has to do with signal quality over the entire transmission path.

In most cases the NIC will detect an active link, but there is no actual traffic (or unreliable traffic) across the cable.

There is really nothing you can do about it, except experiment with different cables to get the best result.





Comments (3)

  • +0 – That's really unfortunate about this NIC, but this makes it sound like the problem also is partly with the cable? So based on what you're saying and my doing a bit more reading on Cat7 and Cat6a, it seems that I really should just be using 6a and forget about 7 completely? If I'm understanding correctly, the small potential performance benefit of Cat7 will make no difference in a home network, and the Cat7 "standard" isn't actually a standard. Whereas Cat6a is an official standard, meaning (hopefully) more consistency between cable manufacturers. Does that sound about right? — Nov 30, 2016 at 18:11  
  • +1 – @PseudoPsyche You've got the right of it. CAT7 is a proper standard, but only for fixed wiring in building installations. Not for patch-cables. And for a home-network there is really nothing to be gained. Cat6a will happily do 10Gb/s over 100 meter and even 40Gb/s over short (<=10 meters) runs. — Nov 30, 2016 at 22:59  
  • +0 – Alright, thanks. I'll pick up some 6a replacements and should hopefully be good to go! — Nov 30, 2016 at 23:01