Helping people with computers... one answer at a time.

When it comes to network segments, there's really one pipe that carries all of the information from all of the computers. And the information is colliding all the time.

This question is a little bit in broken English, but I'm going to read to you verbatim:

"How signals travel by cable? One by one or side by side? From LAN, many computers send and receive packets on a single telephone line. Whether they travel one after another or in lanes?"

In this excerpt from Answercast #57, I look at the way that data travels along network wires.

How does data travel?

So, first of all - it's not a telephone line. It's an Ethernet connection.

When computers are on a LAN, the plugs may look kind of like a phone plug, but they're actually a little bit bigger and it's a completely different wiring scheme. A telephone works on exactly two wires, whereas Ethernet actually requires all eight connectors in that wire to be operational.

Now, with that little trivia out of the way, it's interesting to note that the answer is surprisingly "one by one." And in fact, there are no lanes.

Well... there's two lanes, I should say. On an Ethernet connection, there are in fact two lanes: a lane going out (in other words, a connection for transmitted data) and then one coming back (a connection for received data.)

Data collides

When you get those all connected together (in say, a hub or a switch or a router), everybody's sending and receiving data at the same time, and in fact, they collide.

It is very common. A lot of the Ethernet communications protocol is all about detecting and handling collisions. The reason you and I don't notice this is that it happens so gosh darn quickly.

Packets themselves (once they get started, once you start transmitting a packet) are a very short burst of information. The system finds out if there's a collision.

In other words... Two people try to talk at the same time (or to use your terminology, two computers tried to occupy the same lane at the same time) and unlike an automobile collision, the computers can simply try again. And in fact, they will even try again at different times. They'll delay.

If you think about it: if you had two identical computers running identical software following identical rules, if they collided, they would follow identical paths to try again, which would likely cause them to collide again. So, built into this protocol are things like a certain amount of randomization that says, "OK, I'm going to wait this much and that guy's going to wait that much." And as a result, the packets will then get sent at different times and not collide.

There's one pipe

But ultimately, the answer to your question is there's really one pipe. And that one pipe carries all of the information from all of the computers on a network segment.

Network segments can be isolated from one another. In other words if you cross a router, one side of a router may have a completely different set of traffic than the other. But when you take a look at a simple network connected by a hub (or several computers that are connected somehow directly together), it really does boil down to a single lane of traffic and everybody just sort of collides like crazy, recovers, and tries again.

Article C5869 - October 1, 2012 « »

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Leo Leo A. Notenboom has been playing with computers since he was required to take a programming class in 1976. An 18 year career as a programmer at Microsoft soon followed. After "retiring" in 2001, Leo started Ask Leo! in 2003 as a place for answers to common computer and technical questions. More about Leo.

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3 Comments
Alex Dow
October 2, 2012 11:58 AM

To amplify on Leo's answer, Ethernet is a CSMA system, ie Collision-Sensing, Multiple Access - and note the sequence of that title.

That is, the CS - Collision Sensing, is the more important aspect regarding getting access.

In general, it resembles accessing a Motorway (UK), Autoroute (France) or Inter-State Highway (USA).

Each driver looks out for and if necessary avoids other vehicles.

The other main system is Token Ring, which resembles railway/chemin de fer/railroad practice.

The driver/ingeneur/engineer is given a unique token or staff, then being the only one authorised to access the signal block or designated length of track.

There can be slight variations of this. The driver is shown the token at the entry signal box/tower, thus knows that the track section should be clear of opposing traffic.

This can be repeated for a number of trains, until the last driver is given the token, to carry through to the far end of section, where it is handed over to the exit signaller, who can then allow a train or trains to use that same section in the opposite direction etc.

The token itself is very clearly marked identifying it with the track section or block involved.

There are modernised version of this, involving radio tokens etc; but the principle remains the same.

Steven
October 2, 2012 2:04 PM

When DSL has been explained to the layman it has been related to a 'sideband'. You have to filter it to keep it out of your voice line. When you use a modem it takes the entire voice line.

While you do use the extra wires they still have to be converted to a hybrid form to travel on that voice network, once again to be separated for the ISP.

At least, that's how I understand it and my gateway, and I've been using computers since 1979 and online for more than 15 years. When the gateway receives the line signal it splits it off for a clean voice line but the extensions must be filtered.

Robin Clay
October 4, 2012 2:02 PM

O.K., a slightly different topic...

My old printer has a Centronics connection - a Parallel Port. My old computer also had a serial port. Both have now been replaced by USB - Universal SERIAL Bus.

*I* thought parallel connections were faster, with 8 wires, each carrying one byte, i.e. all 8 carried one byte, whereas with a serial connection, the signal went down one wire a bit at a time, so took eight times as long.

Now I'm lost.....

As my grandmother used to say, they don't make ink as black as they used to - and the bulbs aren't as bright, either...

Think of that old parallel interface as an 8 lane highway with a speed limit of 5 miles per hour. You're WAY better off travelling the one lane road on which you can do 180. Smile (Other than hardware costs I'm not sure why parallel interfaces like that aren't more common, and faster.)
Leo
05-Oct-2012

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