Question Computer-modem connection had to be initiated with a crossover cable

Pimpom

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The short version first:
  • A new Windows 10 desktop failed to connect to a fiber modem with a straight-through LAN cable.
  • It connected immediately with a crossover cable.
  • After that, it also worked with the straight-through cable.
Long version:
My daughter works in a small newly established cell under a Central (federal) government program. She's provided with a new desktop computer and fiber internet supplied by local companies. These were installed in her absence and, while she could access the internet via Wi-Fi from her phone and laptop, the desktop couldn't connect to the modem with the supplied UTP patch cable.

I gave her a Wi-Fi dongle to try with the desktop and it worked but still refused to connect via LAN cable. When she called the ISP, they told her that her computer needed a "network driver" and that they couldn't install it because her computer lacked an optical drive. I checked the computer with TeamViewer and all drivers, including that for the network adapter, are installed.

She brought the computer home and it had no trouble connecting to our home network and internet. All network settings are at default.

The supplied patch cable was a straight-through one. So I gave her a crossover cable which worked. Then she tried the straight-through cable and it also works now.

All of this happened over a period of several days, during which my daughter unplugged and re-plugged the cable, restarted the modem and the computer several times. What could be the cause of the initial problem?
 
Last edited:

kanewolf

Titan
Moderator
The short version first:
  • A new Windows 10 desktop failed to connect to a fiber modem with a straight-through LAN cable.
  • It connected immediately with a crossover cable.
  • After that, it also worked with the straight-through cable.
Long version:
My daughter works in a small newly established cell under a Central (federal) government program. She's provided with a new desktop computer and fiber internet supplied by local companies. These were installed in her absence and, while she could access the internet via Wi-Fi from her phone and laptop, the desktop couldn't connect to the modem with the supplied UTP patch cable.

I gave her a Wi-Fi dongle to try with the desktop and it worked but still refused to connect via LAN cable. When she called the ISP, they told her that her computer needed a "network driver" and that they couldn't install it because her computer lacked an optical drive. I checked the computer with TeamViewer and all drivers, including that for the network adapter, are installed.

She brought the computer home and it had no trouble connecting to our home network and internet. All network settings are at default.

The supplied patch cable was a straight-through one. So I gave her a crossover cable which worked. Then she tried the straight-through cable and it also works now.

All of this happened over a period of several days, during which my daughter unplugged and re-plugged the cable, restarted the modem and the computer several times. What could be the cause of the initial problem?
Most likely bad straight thru cable. Just moving it or coiling it could make or break any of the 8 wires in the cable.
 

Pimpom

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Thanks for the reply but I still doubt that it's as straightforward as that. My daughter and I used the same straight-through cable throughout - the one that came with the modem. I have quite a bit of experience in electronics and I tested the cable thoroughly at home. I made it go through several plug-unplug cycles as well as a bit of pulling, twisting and bending. Connection was rock-steady at home and now also at my daughter's workplace.

I rather thought it might be something to do with the initial negotiation between modem and computer. I am not an expert in networking and was looking for enlightenment.
 

ex_bubblehead

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Thanks for the reply but I still doubt that it's as straightforward as that. My daughter and I used the same straight-through cable throughout - the one that came with the modem. I have quite a bit of experience in electronics and I tested the cable thoroughly at home. I made it go through several plug-unplug cycles as well as a bit of pulling, twisting and bending. Connection was rock-steady at home and now also at my daughter's workplace.

I rather thought it might be something to do with the initial negotiation between modem and computer. I am not an expert in networking and was looking for enlightenment.
Seeing as all Gigabit ports are autosensing and will work with either straight through or crossover equally well it's extremely likely a cable issue. Just because a cable works with one port does not automatically mean that it will work in another. This happens all the time. Replace the suspect cable and be done with it.
 

Pimpom

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OK, so the consensus seems to be that the cable is the culprit and there's no possibility of it's being anything else. Fine, then. A glitch in internet access is not mission critical at her workplace, so she'll keep using the same cable for now. She can always fall back on the WiFi dongle if needed. I'd rather see if the problem arises again before blindly replacing the cable.
 

Pimpom

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"My daughter works in a small newly established cell under a Central (federal) government program. "

Their IT department should be fixing any problems with it, if you mess around with it you may endanger her employment.
One advantage of living in a not-fully-developed country is that no one gets fired for something like that. You can have an idea about how things are from what the ISP guy said about needing to install a "network driver" from a CD - in a Win10 computer with all drivers installed. That doesn't mean that it's OK to be reckless, just that one doesn't have to tiptoe while trying to solve a problem.

There's no IT wing in the whole department, let alone the tiny cell where my daughter works. There's only her (she's a psychologist), a junior doctor and a lab technician.
 

gggplaya

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while she could access the internet via Wi-Fi from her phone and laptop, the desktop couldn't connect to the modem with the supplied UTP patch cable.
Is it a modem/router with a 4 port switch, or just a single ethernet port?

If it's just a modem that connects to a separate router, then you can't just unplug it from the router and plug it into a computer. You have to powercycle the modem anytime you do that.
 

Pimpom

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It's a modem with one output port, not a modem-router combination, and it goes straight to the desktop's LAN port without a separate router in between. Following my instructions, my daughter did plenty of power cycling. At least I know that much.

I didn't mean to sound unappreciative of what others said, But in several decades of experience with advanced technology, I've seen plenty of cases that seem to contradict logical deductions. If a = b = c, a doesn't always equal c. Or so it seems.
 

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