ABSTRACT
This chapter discusses the basic principles of modern relaying communications equipment. A frequency change is associated with "frequency-shift" channels. Some frequency shift systems operate at a lower power output level in the guard state, and shift to a higher power level when in the trip state. The shift of frequency initiated at the transmitter and detected by the remote receiver allows a logical decision to be made for the need of a relaying system to operate or to refrain from operating. An important variation of the frequency shift concept is one in which a continuous variation of frequency is accommodated. In quadrature phase shift keying, the shift in phase is implemented at specific angles, with the pertinent angle being the difference between the phase angles at the edge of the bit boundary compared with the phase shift at the edge of the previous bit boundary.
