An example of a decision level for PECL is shown in Figure 2a . Note that the decision voltage is 3.7V in an ideal academic system, where greater voltages are true and less voltages are false. Further corruption of this decision voltage occurs, particularly in single-ended logic, when adjacent circuitry moves current around on resistive power buses, resulting in local drops in supply voltage to which a switching threshold is referenced. As shown in Figure 2b , the real device threshold becomes fuzzy, which then translates to the time axis as uncertainty, or jitter. These considerations are applicable to both intradevice as well as interdevice characteristics.
Minimum jitter means greater eye opening, which means further distance in a given medium.
There were early times when devices existed in pairs, one as transmit and one as receive, and the conjugation of these devices was the forbidden fruit of all PHY knowledge. As frequencies of communication systems increase, the less capable technologies will fall back to that lower integration partitioning. With the Internet panic, this partitioning's rationale appears to have been forgotten at 622 Mbps, and partitioning has given way to board space considerations and CEO aspirations. A room temperature measurement of a transmitter circuit, performed with the receiver off, is shown in Figure 3 . This does not appear to have been an intentionally misleading gesture, rather the jitter measurements were made on very specific test equipment optimized for measuring transmit jitter or receiver performance only.