Signalling on low-voltage electrical installations in the frequency range 3 kHz to 148,5 kHz - Part 1: General requirements, frequency bands and electromagnetic disturbances (english version)
33.040.30 Switching. Including telecommunication call charging and billing aspects / Системы коммутации и сигнализации. Включая аспекты начисления платы и выписки счета за вызов по телекоммуникациям
