Multiple virtual UTP cables over one physical?

Question

Say there is a physical UTP cable running from point A to point B.

At A there are two physically isolated devices, a1 and a2, each running their own network and unaware of each other's existence.

So there are two devices at B: b1 and b2.

Device a1 is supposed to be on the same network as b1. Device a2 is supposed to be on the same network as b2.

What is the simplest way to connect a1↔b1, a2↔b2 so that they share that one physical UTP cable but essentially run two separate "logical" (or "virtual") cables over it and don't conflict? (Assuming that reasonable speed trade-off is acceptable?)

Is there some sort of smart splitter that does that?

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The only way that I currently know of is complicated: put devices a0 and b0 at each end, each having 3 network interfaces (0, 1, and 2) and connect them with the physical cable attached to interface 0 at each end. Configure two separate VPN networks inside a0 and b0, exposing them to interfaces 1 and 2 respectively. Plug:

• a1 into a0-1
• a2 into a0-2
• b1 into b0-1
• b2 into b0-2

Answer 1

Hi and welcome to Network Engineering

Is there some sort of smart splitter that does that?

While ressource recommendations are actually off-topic...

There used to be (and still are) splitters that split the 8wire/4pairs of a stretch of T-568A/B cable into two outlets of 2 pairs each (wires 1&2, 3&6). Even entire cable plants were done this way ("AMP ACO" might be a search term for your favourite search engine, in this context).

There are also products that do the same with a "Y" shaped cable adapter to plug into a 8P8C ("RJ 45") outlet.

The main downside for such an idea: If we're talking about Ethernet, you'll have a hard time running anything faster than 100Mbit/s Fast Ethernet on just two pairs - certainly not Gigabit.

In short: technically possible, but strongly discouraged. The split cable will come to haunt you over and over again. For an example, see Confused about the results from the cable tester

Totally different approach: VLANs

(actually this pretty much implements your idea with devices a0 and b0)

• install a VLAN capable switch at each end of the single cable you have
• define the link across that cable to be a 802.1q trunk
• on both switches, define (at least) two VLANs: e.g. VLAN 10 (for a1 <-> b1) and VLAN 20 (for a2 <-> b2)
• on each site, define an "access port" into VLAN 10 and connect hosts a1 resp b1
• on each site, define an "access port" into VLAN 20 and connect hosts a2 resp b2

The hosts in VLAN 10 cannot reach the hosts in VLAN 20 - this is what VLAN separation is all about.