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While reading about the Ethernet protocol, I noticed that it has bus features like collision resolution. I've only ever seen Ethernet networks using a star topology (a bunch of PCs connected to a switch) or point to point (the switch connected to a router).

Can you connect office equipment with standard NICs into an Ethernet bus? And if so, how would it actually be wired up? Can you use standard Cat5e?

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Yep, believe it or not, the old-school ethernet "hubs" (note, not a switch) would actually just broadcast everything received on any RX pair out all the other TX pairs.

As such, you did indeed have issues with collisions.


From wikipedia:

An Ethernet hub, active hub, network hub, repeater hub, multiport repeater or hub is a device for connecting multiple Ethernet devices together and making them act as a single network segment. It has multiple input/output (I/O) ports, in which a signal introduced at the input of any port appears at the output of every port except the original incoming. A hub works at the physical layer (layer 1) of the OSI model. The device is a form of multiport repeater. Repeater hubs also participate in collision detection, forwarding a jam signal to all ports if it detects a collision.

Emphasis mine.


There is an article about building a passive hub (that only supports three devices) here.

enter image description here

There was a question about this particular passive-hub-topology on electonics.stackexcahnge here.

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Great answer, thanks. Out of interest, do you know if there any commercial passive hubs? I did a quick Google and I couldn't find any. –  C Nielsen Nov 17 '12 at 5:52
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It looks like you can't get more then 3 ports without active devices. The drawing above is managing to work due to some cleverness with the diode-drop voltage versus the signaling level of the Ethernet interface. I would imagine that it rather strongly violates the official Ethernet specifications. –  Connor Wolf Nov 17 '12 at 6:14
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Yes, Ethernet supports a bus, but you need coaxial cable. The variant is called 10base2 (thin coax) and 10base5 (thick coax). It runs at 10mbps.

The problem is that if the cable is broken somewhere, the entire network stops working (even the PCs that are on the same side of the break cannot talk to each other because there is no terminator at one end of the cable.

Also, if you make a star topology using a hub (you can find hubs that work on 10mbps or 100mbps) instead of a switch you can also have collisions. The reason is that a hub forwards the packet to all ports (except the one it came from), compared to a switch which forwards the packet only to the required port. Hubs were used because switches were expensive at the time, so you only used them if you really needed the performance (and you can make a really simple 10mbps hub with a few transistors that just repeat the electrical signals). Now switches are cheap and I don't think they make hubs anymore.

A network that uses hubs (or a bus) is quite limited in speed - if you have 100M Ethernet then that is the total speed of the network, if PC1 is transferring data to PC2 and PC3 is transferring data to PC4 both transfers will be at ~50mbps (on a switch both transfers would go at 100mbps). So, if you have 100 PCs and they all use the network at the same time there will be lots of collisions and the network will be really slow. Also, it is half duplex (a PC cannot send and receive data at the same time) and relatively insecure - all PCs can see all packets and it's up to the network cards to filter the unwanted packets out (and you can set most of them to promiscuous mode and capture all packets).

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