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In my line of work I recently encountered someone who had a DHCP problem, but only with certain hardware and only via certain cables. The cables that we found out caused the problems were in the walls and attached to regular RJ45 wall sockets. We tried several different nodes and several different cables from node to wall, so the problem is definitely in the wiring in the wall.

Long story short: problems were on the upper floor, bottom floor worked fine. On the upper floor, laptops got IP via all wall sockets, but STB boxes for IPTV consistently reported link but no DHCP traffic. The bottom floor worked for everything (both STB boxes and PC:s got IP and access just fine).

I'd love som input here from someone who knows more about cables, standards and installation than I do. What kind of cabling could cause this problem? It seems strange at first that the computers could get an IP but the IPTV boxes only got link, while everything worked great on the bottom floor.

Could it have something to do with old cable standard? Something about straight or cross-over cables?

Aside from this question, If anyone knows any good resources for reading specifically about what to expect in permanent installations in the way of cable turn diameter, shielding and cat4/5/6 standards and what problems might arise when you disregard these standards would be greatly appreciated :)

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Can you open the wall socket and look at the cable? Do they read CAT3 and do they work fine at 10mbit? (You can test the latter even without opening the wall sockets). – Hennes Aug 24 '12 at 12:27
I know for a fact they worked at 10 Mbit, but not much more than that. Sadly there's no possibility to go back and check :( – pzkpfw Aug 24 '12 at 12:28

There is a lot of information available on the Internet about network cabling. Depending when the original installation was, the cables may have been up to standard when installed but no longer meets today's demands. Or maybe improper cable was used.

Here is a source for you to start your research; Cabling Standards De-Mystified

This discusses TIA, ISO/IEC and IEEE standards.

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I believe the standards also specify a maximum length. The further you are from your switch the weaker the signal will get. It will be even worse with cables that don't meet the standards for the speeds you are trying to achieve. Some managed switches have utilities/commands that will do some tests on attached cables. The error counters and other diagnostic messages may also give you a better idea of what is going on. – chuck Aug 24 '12 at 13:00

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