I tried to wire a ISDN/DSL and a Ethernet back over one Cat cable. (everything connects over RJ45 plugs) The reason is that there is only one Ethernet wire from where the router and phone are to the closet with all the ethernet connections are and the DSL comes in.
I used an adapter like you can see in the image below.
Now the ISDN connection (using pin 4,5) is working fine, but the ethernet doesn't work. As far as I know ethernet should work in 100 mbit/s mode when it only has the 4 pins (1,2,3,6), but it does not.
Any ideas how I could get this to work?
1 Answer
Some Gigabit Ethernet NICs fall back to 100 Mbit/s when there are only two pairs connected (mostly Broadcom-based). Many don't.
Primarily, Auto Negotiation is used to select speed and duplex. When both sides support Gigabit, they'll try to link with it. With only two pairs in a cable, that is bound to fail. If none of the link partners falls back to 100M ("Ethernet at wire speed"), Auto Negotiating and failing to link repeat indefinitely.
If one of the sides supports "Auto 100" it stops advertising Gigabit and the link should succeed with 100M.
Probably all NICs support manual speed and duplex, disabling Auto Negotiation (AN) entirely. Without AN, duplex negotiation fails and falls back to half duplex. So, you need to configure manual speed and duplex on both sides identically to avoid a duplex mismatch. Failing that, you could use manual 100M/half duplex on one side and the other will follow up.
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