I live in an older house that has some in-wall cabling the previous residents installed. The plate here Closed Plate says ISDN, as you can see, but the ports are RJ-45 and ethernet "works" over it, albeit at 100 Mbps. I assumed it was CAT 5 and not CAT 5e, because of these speeds.

However, it recently stopped working. So I opened it up to see what I could see and saw this: Open Plate

I have some experience with ethernet and cabling but have never seen anything like this before. I have to assume it's something ISDN related, but don't know much about it. When I search for ISDN information, I mainly get results about ISP ISDN connections and nothing about this type of connector. I was able to find shops online selling this type of wall-plate and connector, but those offered no explanations as to what this is.

The cable behind it looks like a normal UTP CAT 5e cable, but I can't be completely sure myself, as I can't see enough of the cable to see any markings on it. The rest is deep in the wall. Cable

My main question is can I re-terminate this into a normal RJ-45 CAT 5e connector? And hopefully get gigabit speeds?

Secondary question is, what is this type of connector used for? Why in the world would you want to split one perfectly good UTP cable into two connectors and get reduced speeds?

  • Presumably you know where this cable goes and can access the other end ? If you pull one end does the other end move a little? Is one single gigabit cable useful here or do you need more runs ?
    – Criggie
    Commented Jan 25, 2023 at 22:40
  • 1
    @Criggie Yes, I can access the other end. It terminates to the exact same kind of connector/plate as in the photos. And unfortunately, when the previous residents installed this, they plastered the cables into the wall. So the only way to replace them is to do some semi-major wall renovation which I'm not interested in doing. The cable in question serves the upstairs WiFi access point, so just the one is sufficient.
    – Iceape
    Commented Jan 26, 2023 at 12:19
  • 1
    I'm amazed that you even managed to get 100Mbps over that cable with the green/blue pairs being split like that.
    – brhans
    Commented Jan 26, 2023 at 21:40
  • You say it's "not working" - I'd check that out before doing much else. Just checking continuity of each wire across the length would be a good start. It'd be a shame to reterminate just to find the cable has been chewed by a rodent or something! Commented Jan 27, 2023 at 9:28
  • 1
    @richardb each socket has one green wire & one blue wire. That's splitting the green & blue pairs between each socket. T568B (and every other related spec) requires that the paired wires be kept together. You'd expect to see the orange pair and both wires from the green pair on one socket, and the brown pair and both wires from the blue pair on the other socket.
    – brhans
    Commented Jan 27, 2023 at 16:31

4 Answers 4


Just off-hand, that looks like Cat 3 to me. That was likely an old ISDN BRI circuit (2 concurrent phone lines or one datastream at 128 Kb/s if you bound them).

That said, you could probably re-terminate the cable at both ends and use it for Gigabit Ethernet. The certifications of the various categories of telecom cables only mean that you can run the rated speeds in the harshest of RFI/EMI conditions at the longest supported length. At short distances in a home, you should be able to run Gig over that without issue.

As for your second question - those speeds didn't exist yet so nobody was the wiser. Ethernet only required 4 wires until Gigabit, ISDN was 4 wire, and analog telephone and DSL only required 2 wires. A single cable could support up to 4 jacks depending on the service.

  • 1
    That explains a lot. When I got started with home networking, CAT 5e was the most popular cable. Anything lower than that was not on my radar.
    – Iceape
    Commented Jan 25, 2023 at 15:31
  • 3
    Yeah, the color scheme is typical of CAT3 4-pair to my eyes. Most 5/5e I've worked with has whites with color, not colors with white; not that it's definitive. All you can do it try it, might get lucky.
    – Ecnerwal
    Commented Jan 25, 2023 at 17:25
  • 1
    @Ecnerwal Could be old Cat5 (before 5e). Some of them used solid/white stripe. Cat3 and Cat5e both use 24 AWG wires so there's not a lot of practical difference there
    – Machavity
    Commented Jan 26, 2023 at 19:44
  • 1
    The new connectors arrived today and I got them set up. I was not able to determine exactly which type of cable it is, but after connecting them to Cat 5e keystones, they are showing gigabit speeds through iperf (~949 Mbps)
    – Iceape
    Commented Feb 13, 2023 at 16:05

One reason that you were limited to 100Mbps is that each jack only has two pairs (4 wires) connected. Gigabit requires all four pairs.

As for if it will support Gigabit, unless you can find markings on the jacket (I don’t see anything in your photo), your guess is as good as mine. You can try removing the jacks on both ends and re-terminate with an RJ45 keystone on each end. If you have extra cable, you can strip off an extra inch of jacket to see if the pairs are twisted inside. When terminating in the new RJ45, keep the twists as close to the jack as possible.

  • 6
    Note that CAT5e and CAT6 are certified for gigabit speeds, but you can (unreliably) get faster than 100MB speeds on CAT5. There's no guarantee, of course, but you may be pleasantly surprised. OTOH, you may be bitterly disappointed...
    – FreeMan
    Commented Jan 25, 2023 at 15:05
  • 10
    I have personally run 100Mbit over flat 'satin cord' and gigabit over Cat3. It's unreliable, not impossible.
    – KMJ
    Commented Jan 25, 2023 at 17:22
  • 3
    No need to strip, even CAT3 is twisted. Want to remake the twist on the end as much as possible, but if you don't go crazy, enough untwisting to make the connections sanely is "the problem" very rarely in practice. Lots of folks get too excited about trying to minimize it beyond the point where it matters.
    – Ecnerwal
    Commented Jan 25, 2023 at 18:45
  • Isn't there markings visible in the second photo? That black line? Commented Jan 26, 2023 at 6:00
  • 1
    @DaniëlvandenBerg that's actually a different cable that's just running behind this assembly. I'm not sure what it is or where it goes.
    – Iceape
    Commented Jan 26, 2023 at 11:30

The ordering of wires follows the T568B standard, so you might be able to get two breakout boards connected to jacks and simply replace the termination of the wires using the order that they are connected to the two existing jacks.

Once you do that, you should absolutely get an ethernet tester -- generally when cables stop working it is because something has shifted, because there was a loose connection, or because a rodent ate your cable.

You may want to do this test first, as there's no point in re-terminating the cable if it is bad.

  • I'm not familiar with breakout boards. Is something like this what you are referring to? amazon.de/-/en/euroharry-Terminal-Adapter-Ethernet-Converter/dp/…
    – Iceape
    Commented Jan 26, 2023 at 12:22
  • Just stick them into a 5e or better (6, 6A etc are all fine) punchdown jack and see what you get - or do tests with a multimeter if you have one as detailed here. diy.stackexchange.com/a/265346/18078 No real need for an "ethernet tester" nor "breakout boards" If you have two functional pairs (color does not matter) you can get back to 100M, anyway.
    – Ecnerwal
    Commented Jan 26, 2023 at 14:31
  • @Ecnerwal was trying to conserve length of the wire. Punchdown tools are kind of special purpose and you can make mistakes with them (e.g. have the tool backwards and cut the wire on the inside)
    – gbronner
    Commented Jan 26, 2023 at 17:12

Secondary question is, what is this type of connector used for? Why in the world would you want to split one perfectly good UTP cable into two connectors and get reduced speeds?

In ISDN there were typically two different interfaces. The "U" interface ran over a single pair from the telephone exchange to a "Network terminator". In some countries the "Network terminator" was provided by the telco, while in others it was provided by the customer.

The "S" interface connected the network terminator to the individual devices. The S bus used two pairs for signals with optionally a third pair for power, but was nevertheless usually wired with 8P8C connectors.

Just as you could connect multiple analogue phones in parallel to an analogue phone line you could connect multiple items of ISDN equipment in parallel to the "S bus" of a single ISDN line. I suspect the socket you have is just a parallel-wired double socket. Sensible for ISDN, not sensible for Ethernet.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.