This leaflet sets out general technical considerations applicable to the radio service links covered in UIC Leaflet 750 , with the exception of ground-to-train radio. Key points handled include modes of operation and choice of frequencies. The leaflet also contains technical requirements for the minimum reception voltage, transmitting power and rules for the aerials of fixed and mobile units. Different radio network structures are described: single frequency (with selective call function in some cases), multi-frequency, combined, fixed unit networks working in relay and monitored links. Finally, a technical solution is specified for closing off discrete networks working on the same radio frequency.