100Mbps Cable Speed

[hs08]

Hello,

My PC ethernet and my switch running already have Gbps speed, but actually when i plug my pc to the switch i only got 100Mbps.

Then i try to re-crimp my RJ45 and suddenly the speed change to 1Gbps. 

I just want to know why bad RJ45 crimp affected the speed? If there are one or more RJ45 cable from 4 paired is not connected to the pin inside the RJ45, it shouldn't be not connected rather than speed become lower?

[Leo Laohoo]

Pair D is for Gig.  

If this pair is not terminated properly, the port will negotiate down to 10/100 Mbps.

[hs08]

Pair D is 'white-brown and brown'?

[Gopinath_Pigili · Accepted solution]

Hello hs08,

As Leo Laohoo mentioned,  if it is (any pair) not terminated properly....the port will negotiate down to 10/100 Mbps.

If port want to operate 1000 mbps/1 gbps....all 4 pairs (8 wires) needed to  participate in communication. If any of the pair is not terminated( not pushed properly to the copper contacts inside RJ-45) then port speed down to 100 mbps

Please go the video below...it may help you to understand:

https://www.youtube.com/watch?v=sB7Vaed8NIs

Best regards
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[Joseph W. Doherty]

"If there are one or more RJ45 cable from 4 paired is not connected to the pin inside the RJ45, it shouldn't be not connected rather than speed become lower?"

No, as you've seen.

This because gigE needs all the wires but FE does not, so NICs run at 100 Mbps because they are multi-speed Ethernet NICs.  If NICs were only gigE, link would not connect.

[RAdamWilliams]

https://en.m.wikipedia.org/wiki/Ethernet_over_twisted_pair

This page provides solid details about the cabling and capabilities. Note in particular that speeds slower than 1Gig can't just use any four pairs.