Way back, in the dim and distant past, a wan link would be implemented between proprietary equipment at each end supplied by us over a bit of wire supplied by the telco.
When a failure occurred an argument would ensue between the telco and us.
We knew our equipment and ran diagnostics over the link to show fault with the telco.
No-one else knew our equipment, we didn't tell anyone else how it worked, it was confidential.
The telco could probably run a protocol analyser on the link.
We were running video conferencing over multiple links but the sharing was accomplished by a proprietary switch at each end. The switch had some custom ICs which performed all the required magic.
Most equipment I worked on were just boring processors with boring peripherals implementing boring protocols, ho hum. Some equipment had some custom trickery which made life a bit more interesting. I worked with a few guys in networking who were absolute wizards. I was lucky to learn a bit from some really clever people.
My knowledge will pass away when I die, I hope you are passing on your knowledge.