Looping RJ45 Network Sockets

[Winston Burns]

Hi all.
We have an internal network on our site, nothing special and it all works great. We recently put a presentation room in and it needed a network socket in each end of the room but only one socket would be used at any one time. As we are getting short on free ports on the router I decided to run one cable to the first socket and then parallel connect the second socket onto the first. I have tested all the wires and everything is connected correctly but only the second socket works with my laptop. If I disconnect the second socket from the first socket my laptop then works in the first socket but as soon as I re-connect the second socket and without anything plugged into it the first socket stops working. Has anyone any ideas why and what I could do to make them both work.
thanks,
winston

 

[Nigel Goodwin]

Sorry, but you can't parallel connect Ethernet sockets - if you need more ports then add an extra router, or replace the existing one with a larger one.

I've got two 16 way ones and one 24 way one in use here at work.

 

[Winston Burns]

I suspected that might be the answer. It's not a great problem. I will just have run a second cable and as you say when we run out of ports we will have to buy a bigger router. I just thought that as there would only ever be one computer plugged into one of the sockets at any one time it would have worked. Do you know why it doesn't work, is there small resistors inside the sockets that make the line the wrong impedance or something, just interested really.

 

[Nigel Goodwin]

I suspected that might be the answer. It's not a great problem. I will just have run a second cable and as you say when we run out of ports we will have to buy a bigger router. I just thought that as there would only ever be one computer plugged into one of the sockets at any one time it would have worked. Do you know why it doesn't work, is there small resistors inside the sockets that make the line the wrong impedance or something, just interested really.

I would have thought it 'might' be reflections, back on coaxial networks you had to terminate the end of the line with a dummy load to prevent that.

 

[KeepItSimpleStupid]

Ethernet cables and terminations are known as a "transmission line". If they aren't terminated in the characteristic impedance of the cables, a portion of the signals reflect back to the input. So, one device is better able to handle these reflections from the hanging cable.

Any switch and especially ones with an auto-MDIX port make it easy to expand the capability of a jack. Some early switches had UPLINK ports which reversed the transmit and receive pairs. auto-MDIX is required for Gigabit Ethernet.

Your best bet is to wire a patch panel for the premise wiring and to upgrade to a switch like Nigel suggested. Allocate internal (LAN) stuff to the switch and the high bandwidth WAN stuff to the router ports.

If you want cheap, then a 5 port switch which gives you 4 extra ports for every one. You don't buy a router with more ports. The earliest router I had had one LAN and one WAN port, because that;'s the function of a router and that;s what it needs. They are now combo-boxes and contain firewalls, switches, routers etc,

 

[Winston Burns]

I remember doing some fourier analysis on the reflection speeds and heights of signals on coaxial many many years ago at University. Never used it in practice and couldn't remember one single bit of it now. Thanks for the reply's.

 

[heydonms]

There is another reason not to do this quite aside from whether or not it works. In a few years time, when you have left or the company has moved to new offices and someone else has moved in to your current one, it will cause someone major headaches. If they are lucky they will just have to deal with extremely unusual behaviour when they plug in two ethernet devices, if they are unlucky they might plug in one PoE ethernet device (which would negotiate for power from the switch) and one digital phone or other non-ethernet device which isn't expecting to have 48V across one of the pairs.

As someone who has had to deal with other people's "creative" cabling, just do it properly the first time and save everyone a whole heap of suffering in the long run.

 

[picbits]

As above, bang a 5 or 8 port switch on the end of your existing single socket then you have an extra 4 or 7 free ports to distribute around the presentation room as needed. You could put a CAT5e socket next to the switch, run the cable to the other end and another CAT5e socket then patch the first to the switch with a short patch cable.