Cat 8 is extended frequency range 7 and 7a. The SHIELDS are an issue for impedance stability if done poorly, and ISTP shields are the worst to do wrong as they are intimately in contact…or not, with the shielded wires. The proximity CHANGES the impedance with varying distance…not good. The GEOMETRY stbility needs to be VERY good with shield.
Here is why we use shields when we go WAY up in frequency. It isn’t external ANEXT noise, it is internal NEXT! ACR is the attenuation level relative to the NOISE, mostly internal cable crosstalk. A shield will get you about -100dB down noise between pairs. This saves our pretty small, even with larger 22 AWG wire, signal from the noise.
The second unimagined advantage at the time was indeed ANEXT, or cable to cable crosstalk, not inside the cable but coming in from the outside…INGRESS as it is called. 10G likes low ingress cross talk. An ISTP, individually shielded twisted pair, cable is great at ANEXT reduction so 7, 7a and 8 are 10G compliant by initial design. This is nice as all the installed cable can still be used after three decades! Remember CAT7 goes to 600 MHz and 10G needs 500 MHz.
This is all worst case, though. The signal goes up dramaticlly with shorter lengths and the NIC card only understands the ACR “RATIO”, not how it was derived. A short cable or shielded longer cable looks the same to a NIC card. The length based SKEW and DELAY are automatically adjusted for so those are immaterial.
You can use about any cable in a 1 meter or so interconnect, and it is the CONNECTOR quality that is 90% of your channel problem.
This is why I use CAT6 PS6+ level patch with pre terminated and tested connectors intalled. The impedance stability and RL response with good RJ connectors is better than CAT7, 7A or 8 as they are nor DESIGNED to use RJ, but are supplied that way. They use a TERA style connector and plug that ISOLATES each pair with METAL and that is different than RJ to better leverage the cable’s design attributes. These have far better RL geometry through the connector to not waste everything that was done to make the cable so much better with lower ANEXT and NEXT and lower attenuation (22 AWG wire).
We have PLENTY of signal in short reach audio with standard Ethernet 5e, and 6. We want to mitigate reflections as best we can in short “channels”. In some cases, adding length can improve things as it attenuates RL reflections bouncing around in the cable from crappy connectors and cable.
I recommend super good BONDED pair CAT6+ patch with preterminated and tested RJ’s. I use these 100% in my system as the impedance stability is really, really, good and that’s what we need in short lengths to remove RL reflections inside the cable as we technically have TOO MUCH signal!
Best,
Galen
