engineering it is usually called eye closure (a reference to the clutter in the center of a type of oscilloscope trace called an eye diagram). When the bit period is shorter than the flight time, elimination of reflections using classic microwave techniques like matching the electrical impedance of the transmitter to the interconnect, the sections of interconnect to each other, and the interconnect to the receiver, is crucial. Termination with a source or load is a synonym for matching at the two ends. The interconnect impedance that can be selected is constrained by the impedance of free space (), a geometric form factor and by the square root of the relative dielectric constant of the stripline filler (typically FR-4, with a relative dielectric constant of ~4). Together, these properties determine the trace's characteristic impedance. is a convenient choice for single-end lines, and for differential. As a consequence of the low impedance required by matching, PCB signal traces carry much more current than their on-chip counterparts. This larger current induces crosstalk primarily in a magnetic or inductive mode as opposed to a capacitive mode. To combat this crosstalk, digital PCB designers must remain acutely aware of not only the intended signal path for every signal, but also the path of returning signal current for every signal. The signal itself and its returning signal current path are equally capable of generating inductive crosstalk. Differential trace pairs help to reduce these effects. A third difference between on-chip and chip-to-chip connection involves the cross-sectional size of the signal conductor, namely that PCB conductors are much larger (typically or more in width). Thus, PCB traces have a small series resistance (typically 0.1 Ω/cm) at DC. The high frequency component of the pulse is however attenuated by additional resistance due to the skin effect and dielectric loss tangent associated with the PCB material. The main challenge often depends on whether the project is a cost-driven consumer application or a performance-driven infrastructure application. They tend to require extensive post-layout verification (using an EM simulator) and pre-layout design optimization (using SPICE and a channel simulator), respectively. Routing topology The noise levels on a trace/network is highly dependent on the routing topology selected. In a point-to-point topology, the signal is routed from the transmitter directly to the receiver (this is applied in PCIe, RapidIO, Gigabit Ethernet, DDR2/DDR3/DDR4 DQ/DQS etc.). A point-to-point topology has the least SI-problems since there is