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I'm Trying to understand how the current flows and how network cards receive the electrons (packets) in a Ring network (all the servers are connected to the same cable).

My question is in the case of a broadcast one server will produce current and put it on the wire and then the electrons will flow and get captured by the first server down the line, but how do the electrons reach the second server ?

Can some provide some references to electrons flow in cables (some basic stuff) ?

Thanks

Eggplants
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3 Answers3

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In a ring, all communication is really one-directional and point to point. The ring topology only happens because each node has two ports, one for receiving from upstream and one for sending downstream. When a node does not consume a packet (and sometimes even when it does), it re-transmits the bits it receives from the upstream port to the downstream port. Electrically each link is point to point, making termination easier.

Even modern ethernet is all point to point. The illusion of a shared network segment is created by hubs and switches. Hubs merely pass on what they hear on one port to the other ports, and are not really used anymore today. Switches look at the destination addresses of packet and pass them on to specific ports as appropriate. Everything is point to point again, but this time bi-directional at the cable level at least. Inside each cable are two twisted pair, one for carrying the bits in each direction. That actually makes each pair point to point and uni-directional.

The original ethernet and then ThinLan were true busses, but you'll only find those in museums today. True busses are still in use, with CAN being the top example. However, it runs much slower (up to 1 Mbit/s) and is for a different purpose than general networking.

Olin Lathrop
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  • One more question please : So the network card receives voltage variations that it can interpret as bits, but how is this voltage used and where it goes ? (is it dissipated as heat ? ) – Eggplants Jul 04 '12 at 13:13
  • @user: In the case of point to point ethernet, the voltage and current on the cable drives a transformer. The secondary of that makes a voltage that special circuits for that purpose eventually interpret into a string of 1s and 0s. What exactly the voltages do depends on the logic technology. With CMOS, they change the electric field in FET channels so that they conduct or not. In the basic case, the signal on the cable is only meant to carry information, so any power is eventually dissipated in heat at the receiver and on the cable. Power over ethernet (POE) adds deliberate power. – Olin Lathrop Jul 06 '12 at 12:39
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When you transmit a message you don't send a current through the cable, but instead put a voltage on it. Each node will load that voltage with its impedance, causing a current to it. The transmitter will supply that current, but keep its voltage constant. The network hardware is specified in such a way that the transmitter can supply enough current for all nodes with at most a very minor voltage decrease.

edit
That's for a bus topology. Like Olin says in a ring topology each node has only two neighbors it can communicate with, which are on separate ports. So each port sees only one node.

enter image description here

stevenvh
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I think the simplest answer to your question:

[...]one server will produce current and put it on the wire and then the electrons will flow and get captured by the first server down the line, but how do the electrons reach the second server ?

is:

The second server will retransmit the voltage at its own. That way it can change it if it needs to. It acts exactly the same as the first server.

The receiving server listens at its input, buffers the digital data contained in the signal on its network interface card memory and then retransmits this data at its output.

Olin is right in that basically a ring topology is built from several identical point to point connections. The real trick eg. in Token Ring, FDDI or CDDI is in the digital data contained in the electric signal. The servers are passing a 'token' around, but this is at a much higher abstraction level than your initial question.

I can't really think of any ring topology networks for computers/servers that is current. The ones I know have all been replaced with Ethernet over the past decade or so.

jippie
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