Besides the pairs employed, data directions and data rates, another key dimension that differentiates these Ethernet schemes is the complexity of the signal present on each of the Ethernet pairs. As can be seen from this chart, 10Base-T, which operates from a 10MHz clock, employs a “Manchester” coding scheme. The three pictures across the slide are specialized oscillographs known as “eye patterns”. The data will be received correctly as long as the red traces do not touch the black diamond in the middle. All Ethernet signals are roughly 2V from minimum to maximum. With a period of 100 nanoseconds and a full 2V window to operate in, there are only two voltage levels to choose from making the 10Base-T signal very straightforward to decode. The 100Base-TX eye diagram is more complex. The receiver needs to decide which of three levels the signal is at. Furthermore, the levels are separated by only one volt and the signal is transitioning every eight nanoseconds. One aspect of this “MLT-3” coding scheme that does make the receiver’s job a bit easier is that the signal never transitions from the maximum to the minimum but only moves, at most, a single level in any clock cycle. The 1000Base-T coding scheme shows five possible levels which are only a half a volt apart.  This PAM-5 coding scheme is more challenging to the receiver because the signal can transition to any of the five levels during a single clock cycle. Notice how much smaller the “eyes” are in the 1000Base-T scheme that in the 10Base-T scheme. The only protection technologies that can be effectively used at these low voltages are the semiconductor devices. Semiconductor devices typically have a capacitance value that changes with the applied voltage. That means each of these levels will see a different capacitance from the protection. If those capacitance differences are large enough, they can cause data errors that the system cannot recover. As may be expected, this is more critical on the more complex coding schemes. Customers have noted that this effect is even more critical in systems operating at elevated temperatures where the “noise budgets” are considerably smaller than at room temperature.