Why does just splitting an Ethernet cable not work?

Question

I thought the Ethernet is logically a one-line communication bus (for argument's sake, I am excluding hubs). All machines attached on the bus hears the same signals and the machines themselves try to avoid collisions by randomly backing off.

http://computer.howstuffworks.com/ethernet6.htm

If so, why would splitting one Ethernet line from my home router into two and connecting two computers not work? Why do I have to add a switch to it?

What the Internet said would not work.

                                                           +----------+
                                               +---------->|computer 1|
                                               |           +----------+
    +------------------+       +---------------+
    |4 port home router+------>|simple splitter|
    +------------------+       +---------------+
                                               |           +----------+
                                               +---------->|computer 2|
                                                           +----------+

What the Internet said I should do

                                                           +----------+
                                               +---------->|computer 1|
                                               |           +----------+
    +------------------+       +---------------+
    |4 port home router+------>|    switch     |
    +------------------+       +---------------+
                                               |           +----------+
                                               +---------->|computer 2|
                                                           +----------+

Is this because of the signal degradation (reduced electric current)?

---

Thank you for all the answers! The reason why I did not just use the two ports of my home router is...

The 4-port gigabit router is in my room, and I had put a computer in another room (also my room, though). Since a wired network is far more reliable and secure, I had bought a long Ethernet cable and and connected the computer to the router. Now I was thinking about adding another computer to that room. I could buy another long Ethernet cable, but then there will be two cables between the rooms. The one line already is a minor annoyance, so I thought if I could share the one line between the two computers in that room. A switch would work, but it requires power and is a little bit pricey. That is why I wondered why it would not work to simply split the physical Ethernet cable.

Apparently I do not completely understand how Ethernet and a switch work. I just have some bit of knowledge I heard in my college class.

Answer 1

In 10BASE-T and 100BASE-TX, one pair of wires is used for transmitting, and one for receiving. That is, one pair is the pair the Ethernet host transmits on, and the hub or switch receives on, and the other pair is the pair that the the hub/switch transmits on, and the Ethernet host receives on.

If you split the cable with a simple passive splitter, you're hooking up those two Ethernet hosts transmitter-to-transmitter and receiver-to-receiver. That's like holding the phone handset upside down and trying to speak into the speaker and listen to the microphone—it just doesn't work. So even if both were in half-duplex mode (like they were hooked to a hub, not a switch), neither of the Ethernet hosts would be able to sense when the other was transmitting, because neither one's receiver was hooked up to the other one's transmitter. So they would have undetectable collisions. Not to mention that they'd both be connected to the same port of the hub, probably confusing the hub's autonegotiation ability, because hubs don't expect to autonegotiate with two separate hosts on the same port.

In many ways, things are even worse in your case of hooking them both up to a switch, because they could both end up thinking they can do full-duplex, which means even more undetectable collisions, on what's supposed to be a collision-free link (properly-wired full duplex links can't possibly have collisions).

With 1000BASE-T (Gigabit Ethernet over Cat5 or better UTP copper cabling), the situation is even worse, because all 4 pairs of wires are used for both transmit and receive (simultaneous, full-duplex), and the transceivers are sophisticated enough to enable that. But if you suddenly have a third party on the line transmitting and receiving all at the same time, it completely blows away the way the simultaneous bidirectional signaling scheme works. With three devices all transmitting at the same time, even when you subtract out your own transmission, you can't differentiate the other two devices' transmissions in the signal you're receiving.

Some early flavors of Ethernet, such as 10BASE-2 a.k.a. "thinnet" a.k.a "cheapernet", featured a bus topology where all the hosts on the LAN literally shared the same wire (the same coaxial cable). Because the same wire was used for both Tx and Rx and there could be any number of hosts on the bus, it had to be half-duplex. But a 10BASE-2 transceiver was expecting it to be that way. And since all the transmitters and receivers were hooked up to the same wire, everyone could hear each other (unlike your split 10/100/1000BASE-T example).

Answer 2

The original Ethernet spec called for coaxial cables that were tapped (split) to each workstation (hence the "ether" in ethernet). But we're talking ancient history here. Technically it's still possible with RJ-45 cables since the ethernet protocol still supports the collision detection mechanisms, but why in God's name would you want to set it up that way? Especially since your router has 4 ports to work with in the first place.

Answer 3

I'm surprised I must disagree with Spiff -- in a sense it does work. We were hunting the cause of excessive packet errors in the factory. Among other things we found where some electrician had simply spliced a Y into a 100BASE-T network cable.

The two computers involved sometimes had network errors, but since this persisted for a long time while the users used a program that was on the network and all its data (except stuff written to the temp directory) was on the network I can conclusively say it's possible.

The switches are the traffic lights of the network -- without them packets run into each other badly. Normally the network protocol makes up for the lost packets, though.

Answer 4

If one were to split a cable such that two device's receive inputs got data from the third device's transmitter, and the first two devices' transmitters fed the third devices' receiver, then data transmitted by the third device might be received by the first two, and it's even possible that the third device might hear data transmitted by one of the first two, but reliability in either case would be poor.

Imagine a cable as a Slinky brand spring toy which is hanging vertically and floating at the bottom. If one briefly jostles the top of the spring, a wave will travel down the spring to the bottom, whereupon it will be reflected back up. Fastening the bottom end to the floor won't solve the problem. It will reverse the polarity of the reflected wave, but the reflection will still be there. The only way to avoid a reflection at the bottom of the spring would be to have enough give to prevent a like-phase reflection, but not so much give as to cause an anti-phase reflection.

Internet cables operate much the same way--a device sends out pulses and expects that the other device will have just enough "give" to absorb them cleanly. Anyplace the characteristics of a cable change will cause reflections and other such unwanted effects unless proper measures are taken to prevent them. If packets are sufficiently short, and code waits long enough before sending a packet than any reflections which were propagating through the cable have died down enough, it may be possible for some data to be sent through the cable. Since Ethernet communications generally don't include such delays, however, communications are apt to be unreliable. It's possible that a device might transmit e.g. the first ten packets of data it wants to send, resulting in the first two being received and the rest being garbled by the first; the receiver might, upon receiving the second packet, hold off on acknowledging it until it decides no more data is coming immediately (conveniently not making that determination until after the noise has died down). Upon getting the acknowledgment from the second packet, the transmitter would send the third through twelfth packets (again, with only two of the ten making it), the receiver would acknowledge the fourth, etc. Data might make it through, but slowly at best.

Answer 5

If your network is 100BASE-TX with a Cat 5 cable you can split the cable, but you do so by using an adapter to use all four pairs in your existing long Cat 5 cable (usually only two of the four pairs are used).

So you'd need two very short patch cables at the router's end, two RJ45/Cat 5 LAN splitters, and some longer patch cables at the computers' end. In the diagram below, 'Y' signifies a splitter and an arrow signifies one Cat 5 cable.

.----------.                                         __  _ 
| Router   |                            .---------> [__]|=|
|          |       .----.               |           /::/|_|
|          |------>| Y  |             .----.
|          |       |    |------------>| Y  |
|          |------>|    |             '----'
|          |       '----'               |
'----------'                            |           __  _ 
                                        |          [__]|=|
                                        '--------->/::/|_|

The above solution won't require an extra power source, but it will require use of two ports on your router, but it would at least be tidier than two separate cables running in parallel.

See also the discussion on 100BASE-TX and its wiring in Fast Ethernet, subsection Copper.

Answer 6

Splitting an Ethernet cable to connect two clients is not as impossible as some answers here may suggest. Nor it is unheard of.

1 Issues

The main issues you will introduce by wiring your network like this is:

• the clients can't make use of the CSMA/CD collision detection and thus making them
• the clients can't talk to each other

1.1 Clients can't talk to each other issue

Both clients can talk to the router just fine (when we ignore the collisions) but they can't directly talk to each other because one client's transmitter wire is connected to the other client's transmitter pin. The correct wiring would be transmitter-to-receiver and vice versa.

I didn't dive into this particular problem too deep, but it seems possible that the router would traverse the communication between the two clients, so you really just need to solve the collision problem.

On the other hand the router may discard traffic addressed for endpoint that is physically on the same router port as origin. The router may rightfully expect hub downstream.

1.2 Collision issue

This is really just consequence of previous problem. The clients and the router listen on their own receiver pin before and during a transmission. If they detect somebody's else transmission they postpone or interrupt own transmission. This and how to resolve collision describes CSMA/CD.

So router can use the collision detection, but the clients can't. This will mumble some or all clients traffic.

There is one important point to mention - the CSMA/CD listens on receiver pin and it does not and cannot listen on transmission pin. One could argue that both clients could prevent collisions by detecting traffic on the other client's transmission wire which seems sensible as that is how we split the cable. This would be holy grail for us as it would let us split the cable as we like, with no collisions. However that is not the case because:

• your own transmission would trigger false positive collision detection
• you need to detect collisions even when you currently transmit

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2 Solutions

Let's discuss some options.

2.1 No-effort solution

Just splitting the wire and see what happens. It could work as long as there are no or little collisions maybe because:

• the collisions are not that bad (i.e. yes the network is flaky but it works)
• the clients are not that chatty (causing little collisions)
• or they receive only (e.g. Wireshark wire tap)

Collision is dirty word, but actually I have no idea how bad they are on real world network.

2.2 Solving collisions by other means

• the two clients are never powered at the same time
• (few more ideas came across my mind, but nothing practical or interesting)

If the collisions are too bad and you can't utilize the built in CSMA/CD you are pretty much screwed.

2.3 Using Ethernet splitter MYWA-04, MYWA-08

This is not real solution, rather workaround. Those splitters sacrifice 1 Gbps speeds in favor of two 100 Mbps independent ethernet channels in one wire. It comes with some issues discussed elsewhere, but I list it as option.

2.4 Solving collisions by on-wire hub

Hub is the solution to your problem. Its main function is to resend incoming traffic to all other ports except the traffic's origin (which would trigger false positive collision detection). That's it and it also summarize the problem we dealing with.

You could use couple of diodes to clone one clients transmission to the other client's receiver and vice versa. That would create simple unpowered passive hub.

It would be cool to modify MYWA-07 for this:

2.5 Three-port unpowered passive hub

This is similar to previous idea, but for 3 clients. Kudos to Miroslav Adzic.

The internals are explained in Building a passive ethernet hub with anti-parallel diodes

2.6 PoE hub or switch

One can also consider using PoE hub or switch if for some reason you are constrained on the other side of wall (so to speak).

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3 Notes

• this all assumes half duplex mode, which really is synonym to communication over shared media (e.g. walkie talkie)
  • full duplex (e.g. phone call) connection prevents and assumes no collisions, so the CSMA/CD algorithm used on shared half-duplex Ethernet channels is not used on a link operating in full-duplex mode at all

Answer 7

Keep in mind that an Ethernet network deals with the transmission of signals, which is a matter several orders of magnitude more finicky than the matter of transmission of electrical power.

The Wikipedia article 10BASE2 outlines the pros and cons of the 'old' Ethernet systems. While additional hosts could, in fact, be added without a hub, it was never quite as easy as 'splicing' another segment in.