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can refer to frequency, bit rate, or velocity of propagation. High speed is ambiguous; high speed to a PC designer may be 400 MHz but to a telecom designer it may be 2.5 GHz. As velocity of propagation, it refers to the speed at which the signal travels from one point to another along a conductor. The materials surrounding the conductor affect this speed. For example, in air, the velocity of propagation is 300 million meters per second and in FR-4 printed circuit boards it is about half that speed. See dielectric constant.

Speedsignal = Co /(√K), where Co = the speed of light and K = dielectric constant

Speedsignal » 1 foot/nsec in air and 6 inches/nsec in FR-4

Therefore, a 1 nsec risetime is 6 inches long in a PCB and 12 inches long in air