Can anyone explain why this cable into a router is split between the sockets?
The left gray socket isn't even an RJ45 socket - it looks like an RJ11 socket for a DSL connection.
The purpose of the setup is likely that the twisted-pair PSTN wall socket is located physically near one of the hosts on the network, while it was required to move the router to another location - probably due to WiFi coverage issues. There was likely already a single CAT5 cable running between the location of the PSTN wall socket and where the router was to be relocated to, and then running another line was considered problematic. So the installer likely thought it is a good idea to use one of the wire pairs on the CAT5 to run the DSL signal through the existing cable from the PSTN socket to the router, and you'd still be left with a "standard" 2-pair connection that is sufficient for a 100MBps Ethernet connection (and 1 pair is left unsused).
This tells you that:
- The installer doesn't believe in Gigabit Ethernet.
- They didn't have access to a wiring lead (electrician's fish tape) - otherwise installing an RJ11 cable would have been a snap - but have a cable crimp (I'd expect a wiring lead to be easier to come by).
Interestingly enough, with a standard DSL filter that you get from your provider in most VDSL2+ setup, running the DSL signal on an active CAT5 wire pair - that is also being used for full Gigabit Ethernet - would work just fine because the DSL signal is on a different frequency than standard Ethernet signals and while DSL is very sensitive to noise - the filter will take care of that and the Ethernet NICs are much more resilient to high frequency line noise. And doing that they wouldn't need to remove wire pairs from the RJ45 contact as was clearly done here.
The installer has just been lazy imo and used one single cable instead of two, it really doesn't take that much longer to just use another copper cable. Albeit some may the installer was just being creative, as well... It technically still works.
Cat5e (which is normally used as the bare minimum so using this as an example) Has 4 pairs (8 wires) , when using cat5e for 1gbps speeds you must use all four pairs.
The installer has used one pair for the xDSL connection and one pair for the network connectivity going to device X, which means you can only now use 100mbps on each of those pairs, which may not be an issue if you don't need more than 100mbps.
Using one cable for 2 services could mean if the cable is damaged it could take out two devices instead of one, but also if you did require 1gbps speeds you'd need to replace the cable anyways. Also because the wires are exposed it could technically add crosstalk / noise interference Vs if there was 2 cables and not exposed.
If the xDSL service is dropping / low speed, would suggest trying a new rj11 cable, would also find out what speed your expected to receive, how far your away from the local exchange (distance affects speed / quality) and where the socket in the wall goes to (it likely goes to a Comms room? Or building intake room) as you may want to see the state of that cabling to, just incase that's been "frakinsteind"
Also xDSL service can easily be interfeared by power sources emitting noise (REIN), which can often cause service to drop frequently, and cause poor speed, you can also spot problems by looking at the modems stats (errors and resyncs)
Hope that helps.