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=== Variations ===P25 Phase 1: The phase 1 Project 25 radios can operate in 12.5 kHz analogue, digital, or a mixed mode format. The Phase 1 radios use a form of modulation known as Continuous 4 level FM, C4FM.Phase 1 P25 compliant systems are backward compatible and interoperable with legacy FM systems. In addition to this, the P25 standards provide an open interface to the RF Sub-System to facilitate interlinking of different vendors' systems.P25 Phase 2 : Phase 2 P25 radio systems are based around a 6.25 kHz channel bandwidth> within this they achieve one voice channel or a minimum 4800 bps data channel. P25 Phase 2 uses FDMA - frequency division multiple access and the modulation format is CQPSK. An alternative 2 slot TDMA solution is currently under development. Phase 2 implementation achieves the goal of improved spectrum utilization. Also being stressed are such features as interoperability with legacy equipment, interfacing between repeaters and other sub-systems, roaming capacity and spectral efficiency/channel reuse.P25 Phase 3: With the growing need for high speed data in many areas, Phase 3 of the P25 radio standards will address this requirement. Additional features are aimed at providing functionality for a new aeronautical and terrestrial wireless digital wideband/broadband public safety radio standard that can be used to transmit and receive voice, video and high-speed data in wide-area, multiple-agency networks.=== C4FM ===Yaesu C4FM amateur radio is not compatible with APCO-25 Phase I C4FM. The Yaesu C4FM deviation levels are wider than that of Phase 1's levels (approximately 600 Hz of difference).