I put a connector on an unterminated RJ-45 cable to connect a new system to a LAN. I used the T568B standard (see Twisted Pair Connectors for an explanation of the differences between T568A and T568B) for the order of the pins in the connector. I use a ByteBrothers TVR10/100 for cable testing. I plugged the end of the cable onto which I had just placed a connector into the remote unit and plugged the main unit into the patch panel at the other end of the able using one of the cables that came with the TVR10/100 test devices. The remote unit showed all 4 pairs were ok, but at the remote unit, as the LEDs cycled green, I saw that the 4,5 pair was skipped. I disconnected the remote unit and found that the 4,5 LED was still lit on the main unit, which was odd.
I double-checked the connector I had put on the cable; it looked fine. I punched down the end at the patch panel again without pulling the cable out of the punchdown block, but the problem remained. I then wondered whether I really needed pins 4 and 5 working for a 10 Mbs or 100 Mbs Ethernet connection. Turns out I didn't. I ran a patch cable from the port on the patch panel to the network switch and plugged the other end of the cable into my laptop; the network connection worked.
The manual for the TVR10/100 LAN Tester provides the following information on the cable pairs required for 10 Mbs and 100 Mbs Ethernet connections.
If a cable problem disables data communications at 100 MB/s.
The problem could be caused by not enough connected pairs: 10Base-T data communications only requires two pair cables. There are two 100Base-T standards, one requires two pair cables and the other requires four wire pair cables. If a two pair cable is used, when four pair cables is required, a slow 10 MB/s connection will be permanently established. The cable problem could be caused by inverted pairs. A pair exists, but the pins are inverted (e.g. 1,2 is 2,1). Or the problem could be the cabling is not rated for 100 MB/s speeds ("category 5" cable).
LAN Type Cable Pairs Required 10Base-T 1,2 3,6 100Base-T (Type 1 or TX) 1,2 3,6 100Base-T (Type 2 or T4) 1,2 3,6 4,5 7,8
As shown in the above table, 10Base-T or 100Base-T (Type 1 or TX) LAN ports use two pair cables. 100Base-T (Type 2 or T4) LANs require all four pairs. It is best to use and install Category 5 cables with all four pairs to ensure compatibility with all three types of Base-T LANs.
If there is a short or open on pairs 1, 2 and 3,6 all communications will be prevented. If there is a short or open on pairs 4,5 or 7,8 the data rate may drop to 10 MB/s.
A faulty cable with missing or faulty pairs 4,5 or 7,8 may cause the data rate on that cable to drop to 10 MB/s If this faulty cable is between a PC and hub, all data going to and from that single PC will be at a slow rate. If the faulty cable is between two hubs then communications will some times be quick and other times it will be slow. Communications between PC connected to the same hub will be quick. Communications betwen a PC on one hub across a faulty cable to a PC on another hub will be slow. This type of problem can be very difficult to find without a TVR10/100.
So, I should probably fix the problem when I have time, even though the cable provides network connectivity at the moment.
There is a clear explanation of how to build an RJ-45 Ethernet cable at Building a RJ-45 Ethernet cable of a specific length (light version) . A source explaining the difference between 568A and 568B standards is Twisted Pair Connectors. How to wire Ethernet Cables is another good reference for Ethernet cables.