Introduction

Gigabit Ethernet allows network transfers up to 1.000 Mbps using standard Cat 5 UTP (unshielded twisted pair) cabling. How can this be accomplished, since Cat 5 cables can run only up to 100 Mbps? We will explain this and also other very interesting issues regarding Gigabit Ethernet performance.

Ethernet Cat 5 cables have eight wires (four pairs), but under 10BaseT and 100BaseT standards (10 Mbps and 100 Mbps, respectively) only four (two pairs) of these wires are actually used. One pair is used for transmitting data and the other pair is used for receiving data.

Pin	Color	Function
1	White with Green	+TD
2	Green	-TD
3	White with Orange	+RD
4	Blue	Not Used
5	White with Blue	Not Used
6	Orange	-RD
7	White with Brown	Not Used
8	Brown	Not Used

Ethernet standard uses a technique against electromagnetic noise called cancellation. As electrical current is applied to a wire, it generates an electromagnetic field around the wire. If this field is strong enough, it can create electrical interference on the wires right next to it, corrupting the data that were being transmitted there. This problem is called crosstalk.

What cancellation does is to transmit the same signal twice, with the second signal “mirrored” (inverted polarity) compared to the first one, as you can see on Figure 1. So when receiving the two signals, the receiving device can compare the two signals, which must be equal but “mirrored”. The difference between the two signals is noise, making it very simple to the receiving device to know what is noise and to discard it. “+TD” wire standards for “Transmitting Data” and “+RD” wire standards for “Receiving Data”. “-TD” and “-RD” are the “mirrored” versions of the same signal being transmitted on “+TD” and “+RD”, respectively.

Figure 1: Cancellation technique.

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