Cat5 Cables - straight-through vs crossover

The Sweeper

Hey folks,

Can anybody give me a straightforward reason why you'd use a crossover cable instead of a straight-through? Have just had a problem in work where a contractor has cabled six ports on a client's site using crossover cables, which promptly failed to work with the cisco router. When I sent him back to the site today he argued every step of the way that he'd done what was required of him properly and that there was nothing wrong with him assuming they should be crossover cables... I thought that the straight-through were default and the natural choice for wiring a LAN??

Is the chap daft, or is it me?

Snowbat

It's not you.

The only situations I could think of where you need crosswired cables are:

• You need to connect two PCs directly without a hub/switch
• You need to connect two hubs/switches together but there are no uplink ports available

These are exceptions - normal by far is straight-through.

irlirishkev

Isn't it generally like-to-like requires a crossover?

For example, a network card on a pc to a network card on a pc = crossover.
Network card on a pc, to a hub = straight through?

Praetorian

Straight through is definitely the standard. I had 6 offices wired with cat5 a couple of years ago and I didn't have to specify the way they were wired; the company installing them knew what they were doing.

seamus

The way I understand Cat 5 cables:

Each cable has a number of individual wires in it. 2 (I think) of these wires are used for input signals and two are used for output signals. As such, when you connect it to the PC, the input connectors in the LAN card connect to the input wires and the
output connectors connect to the output wires.

If you use a straight-through cable, and connect two PCs together, you are essentially connecting the input channels on each machine to eachother and the output channels on each machine to eachother.
So when one machine attempts to send some data using the output channel, it's not received, because the other machine is not receiving any data on it's input channel.

Crossover cables switch the input and output on one end of the cable, allowing you to connect each input to each output.

Hubs work using straight-through cables. The inputs/outputs on a hub are essentially reversed. So the hub listens for input on each output line connected to it, and then outputs that data to all inputs lines connected to.

Hubs will not work with crossover cables unless you're connecting two hubs together.

</end_convoluted_explanation>

The Sweeper

Cool - thanks guys. I order a lot of LAN cabling for a particular client through this supplier, and I've never had this problem before, so I have a feeling it's just this one van-man. I mean, I would have though that, seeing a router connected to an RJ45 socket and with four ports awaiting cables, and four PCs around the building that need to connect to the router, it'd be fairly obvious, even WITHOUT my order paperwork, to see what was required.

Your responses just reinforce that. Anyway - when I phoned him after he left site yesterday he first response was "oh, er, um, well I didn't have a Cat5 cable tester with me..."

Random

I wouldn't invite the guy back. Count your losses and hire a professional, get the job done right.

Capt'n Midnight

Snowbat wrote:

It's not you.

The only situations I could think of where you need crosswired cables are:

• You need to connect two PCs directly without a hub/switch
• You need to connect two hubs/switches together but there are no uplink ports available

Nowadays in many cases you don't even need a crossover for those two cases.
It's rare to find a rack mount switch without an uplink ports or a little button to change the last port. A few 100Mb switches and AFAIK ALL Gigabit switches and network cards don't need crossover as they can autodetect the connection at the far end and match it.

More to the point I can't think of how a single site on a WAN could need six crossovers.

lomb

cant u just swap the output cables from the socket to the pc with crossovers insted of the normal straight thrus, this will then cross over the cross over and make it a straight thru.

Capt'n Midnight

seamus wrote:

The way I understand Cat 5 cables:
</end_convoluted_explanation>

10/100 uses 4 wires, the Green White and Orange coloured ones, now if only there was some aide memorie for that

IIRC on CAT5 Gigabit uses all 8 wires to spread the data as a way of getting more thoughput.