p-25

Project 25 (P25 or APCO-25) is a suite of standards for interoperable digital two-way radio products. P25 was developed by public safety professionals in North America and has gained acceptance for public safety, security, public service, and commercial applications worldwide. P25 radios are a direct replacement for analog UHF (typically FM) radios, adding the ability to transfer data as well as voice for more natural implementations of encryption and text messaging. P25 radios are commonly implemented by dispatch organizations, such as police, fire, ambulance and emergency rescue service, using vehicle-mounted radios combined with repeaters and handheld walkie-talkie use.
Starting around 2012, products became available with the newer phase 2 modulation protocol, the older protocol known as P25 became P25 phase 1. P25 phase 2 products use the more advanced AMBE2+ vocoder, which allows audio to pass through a more compressed bitstream and provides two TDMA voice channels in the same RF bandwidth (12.5 kHz), while phase 1 can provide only one voice channel. The two protocols are not compatible. However, P25 Phase 2 infrastructure can provide a "dynamic transcoder" feature that translates between Phase 1 and Phase 2 as needed. In addition to this, phase 2 radios are backwards compatible with phase 1 modulation and analog FM modulation, per the standard. The European Union has created the Terrestrial Trunked Radio (TETRA) and Digital mobile radio (DMR) protocol standards, which fill a similar role to Project 25.

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