Editing Project 25 (section)

====Phase 1====
Phase 1 radio systems operate in 12.5&nbsp;kHz digital mode using a single user per channel access method. Phase 1 radios use Continuous 4 level [[FM broadcasting|FM]] (C4FM) modulation—a special type of 4[[Frequency-shift keying|FSK]] modulation<ref>{{Cite web |url=http://www.aeroflex.com/ats/products/prodfiles/appnotes/Understanding%20P25%20Modulation%20Fidelityiss1.pdf |title=Aeroflex: Application Note - Understanding P25 Modulation Fidelity |access-date=2012-03-26 |archive-url=https://web.archive.org/web/20120320083432/http://www.aeroflex.com/ats/products/prodfiles/appnotes/Understanding%20P25%20Modulation%20Fidelityiss1.pdf |archive-date=2012-03-20 |url-status=dead }}</ref>—for digital transmissions at 4,800 [[baud]] and 2 [[bit]]s per symbol, yielding 9,600 bits per second total channel throughput. Of this 9,600, 4,400 is voice data generated by the [[Improved multi-band excitation|IMBE]] codec, 2,800 is forward error correction, and 2,400 is signalling and other control functions. Receivers designed for the C4FM standard can also demodulate the "Compatible quadrature [[phase shift keying]]" (CQPSK) standard, as the parameters of the CQPSK signal were chosen to yield the same signal [[Absolute deviation|deviation]] at symbol time as C4FM. Phase 1 uses the [[Improved multi-band excitation|IMBE]] voice codec.

These systems involve standardized service and facility specifications, ensuring that any manufacturers' compliant subscriber radio has access to the services described in such specifications. Abilities include [[backward compatibility]] and interoperability with other systems, across system boundaries, and regardless of system infrastructure. In addition, the P25 suite of standards provides an open interface to the radio frequency (RF) subsystem to facilitate interlinking of different vendors' systems.