a repetition storage. The common send section circuit includes a code translator that changes the five bit code to the seven bit code. Next the seven bit character goes to an output shift register, and then to a polarity inverter. For a four channel system the bits would then be interleaved between AB and CD channels. The final send step is a tone modulator and output circuit. The tone is modulated using frequency-shift keying. The 0 (or space) signal picks out a lower frequency, and the 1 ( or mark signal) picks out a higher frequency. This signal to frequency conversion may be inverted, and this is common below 10 MHz. On the receive section the signal from the radio receiver comes to a tone demodulator, then a polarity inverter, and then to an error checking circuit, and at the same time to an input shift register that converts from serial to parallel. Next it goes to a code translator to convert from seven bits to five bits. From here the five bit code goes to a parallel to serial converter, controlled by a repetition cycle timer on the receive side. Interoperation Because the telex network does not give feedback to a sender about when a message is delivered, and a shortwave ARQ link may or may not be available, systems with ARQ links set them up to be store and forward half duplex systems. This means that a telex sender transmits their telegram in full to the message centre that then stores and forwards the message over the shortwave link when it is available. Call charges were calculated for the time when the link was running without errors, either at six or ten second increments. CCITT recommended not charging if reliability was below 80%, and to give a busy signal if reliability was not satisfactory. Signalling is needed to determine the start and end of a call, and the destination of the connection. Two systems have been used, signalling system B CCITT U1 and CCITT U20. An international telex exchange would have signalling conversion facilities. Monitoring For radio spectrum monitors, ARQ-M is classified as a digital mode, and radio stations that use it will be termed utility stations. Two stations communicating with ARQ-M will use two different frequencies as data is continuously transmitted in each direction. Channels are often running in idle mode and have a rhythmic sound, as