What happens if you 'T' off a cat5e cable used for internet?

[HappyHippyDad]

I don't tend to get involved in anything out of the ordinary with data cables as I don't fully understand how it all works. However, a builder friend has asked if I can connect up some ethernet sockets which is fine, but, he asked if he could have just one cable leaving his router which plugs into a 1g ethernet socket. Then... 2 cat5e cables inside the socket which go off to separate ethernet sockets.

What happens if you do this? Will only one socket work at any one time? Will both sockets work but have 1/2 speed? etc etc

Cheers

 

[pc1966]

No, it wont work properly.

The old "yellow spaghetti" style of coax networking was designed that way - just a long cable with network card's T-eed off at the points you wanted and the far ends both terminated with 50 ohm loads. But they operated expecting clashes of data so were designed to back off and retry on that:

Carrier-sense multiple access with collision detection - Wikipedia

en.wikipedia.org

That is not very efficient at moving data around if many machines want to talk to many others, also a single fault would take down the whole segment of cable. So from 90s onward almost everyone moved to using switches that would route the data from port to port as necessary, and the only traffic on a given cable was actually intended to go between its end points. That results in a lot of cable (think single socket radials per computer) but due to the separation of traffic it was much faster and more reliable.

Initially the switches were expensive to the migration quite costly, but now you get switches built in to routers or you can buy small ones for a tenner or so.

 

[UNG]

A cat5 has 4 pairs if you don't need POE then you can get splitters that allow all 4 pairs to be used for data

If you were talking a number of cat5's then I could understand your problem but in the grand scheme I don't see why seeing 2 cables leaving the router would be an issue

 

[loz2754]

It could be the case that the router has only one spare port. In which case a separate network switch will be needed, as pc1966 says.

 

[plugsandsparks]

mmm, sounds like the builder should have run 2 cables and is now trying to find some magic solution to make it look like there are really two outlets when any solution will be a compromise, in either speed or POe capability. What you would normally do is simply fit a very small switch at the socket, they are so small they easily blend into the rest of the IT stuff. Suggest ask what actual problem is trying to be solved (apart from the magic bit), lol

 

[Avo Mk8]

Provide the multiple feeds from one of these.
A single cable from his router into a socket on this, and then he plugs whatever into the remaining sockets. Does need mains power - if that's a problem then you could use a POE alternative.

https://www.amazon.co.uk/NETGEAR-Ethernet-Unmanaged-Internet-Splitter/dp/B07PYSNSDD

If it's all about fixed cabling that is already installed, but with inadequate numbers of cables, you have the option already suggested of using different twisted pairs for twin feeds down one cable, but you would have to use two ports on the router, not one (or put the switch with the router)!

 

[Lucien Nunes]

As above it's not clear what problem the bodge is supposed to overcome; we could advise better if we knew. But the idea of a true Y-connection core-to-core in twisted-pair data wiring is so crazy it's quite interesting and tempting to try. Of course it won't work properly both at an electrical level and a datacomms level, but would it work at all?

Layer 1 breaks because there's an impedance mismatch at the Y-joint, leading to reflections, crosstalk and attenuated, corrupt data. Layer 2 breaks because of contention between multiple devices talking at once. Whenever intelligble data does get through, layers 3, 4, and 5 are all subject to ambiguity.

It's like sneakily joining three very busy, independent railway lines together, with dozens of driverless trains passing through every minute, but without any control over the junction and none of the three separate systems even knowing that the junction exists. Imagine being at the next station, announcing in advance what train is coming. Will it be one from line A, line B, bits of both smooshed together, half a broken train pushing half another broken train in front of it? Or are they all completely derailed 100 yards downstream of the junction so that nothing arrives at all?

I feel an experiment with Wireshark is needed.