This section of IEC 835-2 deals with measurements pertaining to the service channels used in digital microwave radio-relay systems. In digital radio-relay systems, two basic methods are commonly used for the transmission of service channel signals (that is voice, supervisory and control signals) (see annex A, ref. [6]). The first transmission method uses analogue transmission techniques where the i.f. or r.f. carrier signal of the digital modulator is directly frequency-modulated by the analogue service channel signal, which is then recovered at the receive end by an f.m. demodulator (for example in the carrier recovery circuit of a coherent digital demodulator). In the second method of transmission, digital transmission is used and the service channel signal is transmitted in the form of separate bits inserted into the main bit stream by a multiplexer (possibly together with a bit stream carrying additional information). For digital transmission, the service channel signal may be either an analogue signal which is digitally encoded for bit insertion or a digital signal, in which case suitable code conversion is normally applied in order to convert the line code into a code suitable for the radio equipment. At the receive end, a demultiplexer is used to recover the service channel bits and thus supply, after suitable decoding or code conversion, the service channel output signal. Normally, more than one serv ice channel is transmitted. Sometimes an additional bit stream (for example alarm control signals for protection switching and supervisory equipment) may also be transmitted. Suitable multiplexing and demultiplexing equipment is then used, such as frequency division multiplexing for analogue transmission, and time division multiplexing for digital transmission. The use of time division multiplexing also allows the simultaneous digital transmission of analogue and digital service channel signals. Figure 1 shows the simultaneous application of all the above possible service channel transmission systems. However, in a given radio-relay link, only one or two of these systems is used at a time.