In the pre-DSL days, you had two options to get a short-haul high-speed link (at least in Europe): take E1 (or fractional E1) from a telecom (which was more expensive than a highway robbery, as the cost was recurring) or use baseband modems with proprietary encoding techniques on physical copper wires (assuming you could get them). As it turned out, some of these encoding techniques were not as good as the others (but the equipment was relatively cheap, so the budget limits usually forced the decision). We had our own share of modem-related problems, but they were never as bad as what I've heard from one of my students: his modems would lose synchronization when transmitting a long string of zeroes over a regular synchronous serial interface; ping ip 188.8.131.52 size 1000 data 0000 would be enough to bring down the link.
And now two questions for you:
- What could you do on the router to fix this problem?
- Why was the synchronization retained when transmitting a long string of ones?