This can be a significant disadvantage because mobile stations often lose the signal for short periods as they are on the move. Another widely used system is the Continuous Tone-Coded Squelch System (CTCSS) also referred to as subaudible tones or PL tones (a Motorola trademark). This uses single audio tones in the range from 67 to 257 Hz to address each message to a specific radio. Each radio is assigned a different tone frequency. The code tone is transmitted continuously throughout the audio transmission along with the voice modulation. Since the tones are below the audio passband of the receiver, roughly 300–3,000 Hz, they are filtered out in the receiver's audio amplifier and therefore not heard. Only when the correct tone for the required station is transmitted will the squelch for that receiver be opened and the transmitted audio be heard. The advantage of this system is that the code tone is transmitted during the entire transmission, instead of just at the beginning as in the DTMF system above, so the system works in spite of noise or signal dropouts. Systems typically are able to provide up to 37 different tones between 67 Hz and 254.1 Hz, allowing up to 37 separately addressable radios or groups of radios. TETRA is a modern standard for digital private mobile radio (PMR) and public access mobile radio (PAMR). Work started on the development of the TETRA standards in 1990 and has relied on the support of the European Commission and the ETSI members. Experience gained in the development of the GSM cellular radio standard, as well as experience from the development and use of trunked radio systems has also been used to fashion the TETRA standard. In addition to this the process has gained from the co-operation of manufacturers, users, operators and industry experts. With this combined expertise the first standards were ready in 1995 to enable manufacturers to design their equipment to interoperate successfully. TETRA allocates the channels to users on demand in both voice and data modes. Additionally national and multi-national networks are available and national and international roaming can be supported. For civil systems in Europe the frequency bands 410–430 MHz, 870–876 MHz / 915–921 MHz, 450–470 MHz, 385–390 MHz / 395–399.9 MHz, have been allocated for TETRA. Then for the emergency services in Europe the frequency bands 380–383 MHz and 390–393 MHz have been allocated.