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.
|
