How can different CAT5/6 cables affect sound.

@richardbrand ​Switched  Ethernet typically does not use CSMA/CD (Carrier Sense Multiple Access with Collision Detection) for its primary operation. While CSMA/CD is a fundamental part of Ethernet technology and was essential for early Ethernet networks using hubs, it is obsolete in modern switched Ethernet environments. 

TCP/IP guarantees bit perfect delivery. Packet sequencing, checksum and other header data answer the remaining questions about dropped, deformed or other anomalous packet behavior. When i was first introduced to TCP/iP in the late 1980s I was overwhelmed at everything the protocol did compared to competing non-routeable protocols like NetBEUI and IPX, but it became very clrar very shortly that TCP/IP was the future.

When Microsoft made its big push into networking, it threw tons of money into free training for Microsoft Certified Systems Engineers. And the class that separated those who would make it and those who couldn't? TCP/IP.  I took the week long class in LA - MS flew me from Seattle, housed, and fed me on their dime - with the provisions that if you failed, you wouldn't get reimbursed until you passed. Highly motivational. We had over 20 people in that class and about half failed the exam on their first try. It was a tough class, their toughest, and luckily I passed and became MCSE #410. There are now tens if not hundreds of thousand MCSEs.

A few years later Cisco Systems created the Cisco Certified Network Engineer, a curriculum so challenging that one CCNE i knew said "Next time I'll do something easy, like medical school." He wasn't kidding. So I went into Systems Architecture instead, and the CCNEs essentially worked for me. I knew the network architecture and dealt with big picture stuff , budgets, and management, shielding the CCNEs so the could concentrate on building and operating a 5-9s global infrastructure with multiple Enterprise-class data centers.

But boil it all down, and Switched/Routed Ethernet and TCP/IP is at the heart of networking as we know it today. I still like getting my hands dirty, I pull my own cable, terminate all my own connectors, and am even a certified fiber splicer. And I am very confident about what networking can and cannot add or subtract to sound quality.

So I'll say it again, whatever SQ differences people think they hear, it has nothing to do with anything happening at Layer 1 Physical, Layer 2 DataLink, or Layer 3 Network Layer 4 Transport, Layer 5 Session, Layer 6 Presentatio or Layer 7 Application. Or the condensed and simplified 4-Layer model. It is all happening above that. The entire DAC process rides above all the networking, and the analog output simply isn't even in the same domain.

@jea48 I depended on Fluke gear for years and even did some consulting on the original LanMeter. We even suggested "If it works, it's a Fluke" for an advertising slogan, but Marketing wasn't amused. I still think it was a lost opportunity. 

I agree with what @panzrwagn just wrote above as I was also in that field before, but with all that was said, how can one hear a change in the way music is presented?

As some of the posters have written, measurements vs subjective is a debatable topic. I for one is a measurement guy but after I have gotten to the level of my gear which can discern differences in musical tone/Prat, I for now is a believer of the subjective point of view. 

I believe more solidly of changes in cable on the analog side, I.E., speaker and interconnect cables, phono cable and even power cables. The cable itself presents a L/R/C network at given frequencies, so Yeah, it does. The fanboys in AudioScience don't believe it, but hey, that's their take. 

But in ethernet? Hmm, that's a take. However, when I change the Cat6e cable from my Cisco switch to the Eversolo, there was a change (i can't pinpoint, but there was) in how the overall performance of the music. Was it better or worse, I don't know. I just put back the old one as the evaluation cable was $$$.

So, in the end, is all about synergy, which one in the chain makes the most impact and does it steer you to your goal.