I have been using a Netsys NV-600L/NV600R pair to get my backup ISP feed from an outbuilding to the server room for several years. I’ve got a 25-pair Category 3 (ordinary telephone unshielded twisted pair) cable between the two locations, which are about 100 feet apart. The boxes have Ethernet RJ-45 connectors and RJ-11 telephone connectors, and act as modems to extend Ethernets at speeds up to 100 Mb/s over Cat 3 cable connected between the RJ-11 ports. It’s been working fine. Because in the presence of large distances or noisy lines Netsys offers a higher data rate from the Local (L) unit to the Remote (R) unit than the other way around, I put the L unit at the ISP modem, and the R unit in the server room. Seems backwards, but that’s the way Netsys set it up.
A few days ago, I installed another NV-600L/NV600R pair to feed a webcam signal between the same two locations. I couldn’t use the same one, because the ISP feed is on a WAN port on the firewall, and the camera needs to be on my local LAN. I hooked up the new units, with the R unit in the server room and the L unit in the outbuilding.
It didn’t work. Not only could the new modems not establish a communications link, just plugging them in killed the old link. Either pair of modems could work over either pair of wires.
I email Netsys support, thinking that maybe the modems were not designed to share Cat 3 bundles. The answer came quickly. It should work just fine. If it didn’t work, I could mess with the “band plans”, but I shouldn’t have to. I should check to make sure that the wiring between the modems was indeed over a twisted pair, not two wires from different pairs.
I spent several hours checking and rechecking, and emailed SNR plots to Netsys. No luck.
I went to work out. On the stationary bicycle, it occurred to me that both the Netsys tech and I had been making the implicit assumption that the problem was far-end crosstalk (FEXT). The tech never asked, and I never thought to volunteer, that I was using one R and one L in each location. A lighbulb went off in my head. The problem had to be near-end crosstalk (NEXT)!
Here’s how it had to be working. The Netsys boxes use frequency division multiplexing with multiple carriers – I intuited that from the SNR plots. It appeared from those plots that the uplink frequencies and the downlink frequencies were disjoint, although there was a provision for some sharing. So, if you had a Local unit transmitting one one set of frequencies, the Remote unit at the far end would be listening on those frequencies, but the crosstalk from the Local unit right next to that remote unit could swamp out the signal from the other end, since it was only a few feet of cable away.
I swapped one pair of modems end for end, so that I had two Local units in one location and two Remotes in the other.
Problem solved.
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