Network Cable Max Length

[Martin109]

What's the max length for a casually/occasionally used network cable, and can they be connected together to make a longer cable, and if so, how?

[Greenbank]

What's the max length for a casually/occasionally used network cable, and can they be connected together to make a longer cable, and if so, how?

100m usually (for copper), any longer and you'll get lots of wierdness. You can chain cables together using something clever like a hub (I think) or a switch but you can't just use a longer cable or several cables connected using dumb passive connectors.

The reason is to do with frame lengths and prorogation of electrical signals in copper which travel at about 2/3 of the speed of light (so about 200,000,000metres/sec).

So one bit (on a 100Mbit ethernet network) would take 2m of cable. If you factor in the smallest packet size, and slot sizes, it works out that any more than 100m and you can get to the point where data collisions (where the network devices spot that two people are trying to use the cable to transmit at the same time) become possibly undetectable.

With Gigabit Ethernet 100m of cable is almost enough to get a whole 64 byte (512bit) packet on the wire at one time. Kind of messes with your head that.

[Martin109]

Thanks Greenbank.

So if I go to somewhere like Maplins, or PCWorld, can I get, say, a 5m or 10m cable with plugs fitted to both ends?

[Greenbank]

Thanks Greenbank.

So if I go to somewhere like Maplins, or PCWorld, can I get, say, a 5m or 10m cable with plugs fitted to both ends?

Should be no problem, just make sure you get the right grade of cable.

Cat 3 is fine for 10Mbit or slower (so not 100Mbit or Gigabit)
Cat 5(e) is fine for 100Mbit or slower but not for Gigabit.
Cat 6 is fine for Gigabit and anything slower.

[iakobski]

What Greenbank said...


The official length is 90 m but you will get away with 100, or a bit more if you don't care about performance.

You can buy repeaters that allow you to extend up to 5 lengths in total, but because they are not exactly commodity they are a lot more expensive than using an ethernet switch (or hub if you have one).

After 460 m you have to think of something else.

[Zipperhead]

The reason is to do with frame lengths and prorogation of electrical signals in copper which travel at about 2/3 of the speed of light (so about 200,000,000metres/sec).

So one bit (on a 100Mbit ethernet network) would take 2m of cable. If you factor in the smallest packet size, and slot sizes, it works out that any more than 100m and you can get to the point where data collisions (where the network devices spot that two people are trying to use the cable to transmit at the same time) become possibly undetectable.

Are collisions an issue with anything except 10base2 & 10base5? Doesn't everything nowadays use full duplex, with only two devices (one at each end)?

Confused of wherever I am.

[Martin109]

Thanks Greenbank.

So if I go to somewhere like Maplins, or PCWorld, can I get, say, a 5m or 10m cable with plugs fitted to both ends?

Should be no problem, just make sure you get the right grade of cable.

Cat 3 is fine for 10Mbit or slower (so not 100Mbit or Gigabit)
Cat 5(e) is fine for 100Mbit or slower but not for Gigabit.
Cat 6 is fine for Gigabit and anything slower.

Ha.. every answer throws up more questions!  That's what's called a learning curve.

I've just got an old laptop running Vector Linux 5.8, connected to our Netgear router via LAN card, adapter and cable.  The internet seems to be running faster than the Windoze laptop connected wirelessly to the same router!  Does that tell you what grade of cable I should go for?

[Greenbank]

For home stuff almost certainly Cat 5 would be fine.

Anything marked "network cable" and is the right length would do, just make sure it's not a "crossover" cable.

[Martin109]

Many thanks GB!

[Martin109]

I'm coming across references to 'patch cable'.  Is this what I want?  ie does a patch cable have a plug at either end, and is it different from a 'crossover cable' for computer-to-computer.  I don't understand the meaning of 'patch', and Wikipedia hasn't come up with anything I can find to explain it.

[MSeries]

Patch cable connects a computer to a hub/router, RJ45 plug at both ends.  Crossover is for computer to computer, they look the same but are wired differently.

The LAN card that I supplied is 10BaseT (not fastest but faster than broadband), CAT5 cable is just fine.

NB I believe there is a minimum length for CAT5 of 0.5m

[pcolbeck]

Basically it depends what you are patching. A standard CAT5 or CAT6 cable is wired straight through i.e pin 1 at one end to pin 1 at the other and so on for all the strands in the cable. The thing is that in comms you need to connect transmit on one device  to receive on the thing it is connected to. One of the strands is transmit and the other is receive - this is known as a crossover.
Most connections in an Ethernet network are host (which can be a PC, a router, a firewall, a Unix box etc) to switch so switches were designed to implement the cross inside their Ethernet ports so that nice and simple straight through cables could be used for most connections.
Of course when you connect a switch to a switch or a host to a host you don't have any cross at all (there are none in host to host and the two inside the ports in switch to switch cancel each other out) so you have to have a crossover in the patch cable.
Actually these days quite a lot of switches are smart and can detect whether they need to use a cross inside the port or not and apply it if needed (auto MDX) but still it's best to use the correct cables as it stops confusion.
There is a nice Wikipedia page here with diagrams.

[Martin109]

Why the word 'patch' is what I haven't got my head round.

[delthebike]

Why the word 'patch' is what I haven't got my head round.

Because these cables used to be plugged into a patch panel. Basically a distribution board for networks.

[Martin109]

Ahh!  An old term that's stuck, like 'Hoover'.  I get it.

[Lonewolff]

Cat 5(e) is fine for 100Mbit or slower but not for Gigabit.

Cat5e (the e is for enhanced) will do 10/100/1000mbit without issue.

and as others have said maximum cable run is 100m

[JohnP]

Ahh!  An old term that's stuck, like 'Hoover'.  I get it.

and, of course, wiki does know that patch cables are used at patch panels

http://en.wikipedia.org/wiki/Patch_panel

I used to have a fantastic panel for joining WANG OIS and/or VS computers to WANG Work Stations.  They used dual cables, we had 4x OIS and around 75 w/stations - all joined together at a large panel.

[Martin109]

and, of course, wiki does know that patch cables are used at patch panels

http://en.wikipedia.org/wiki/Patch_panel

More fool me for doubting wiki!