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Project 25

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Project 25
Also calledP25, APCO-25
Developed byAPCO, NASTD and US federal agencies, starting in 1989
StandardsTIA-102 series, published by the Telecommunications Industry Association (TIA)
Main usersPublic safety agencies, chiefly in North America
Technical
Phase 1FDMA, one voice path per 12.5 kHz channel; C4FM at 9,600 bit/s
Phase 2Two-slot TDMA in a 12.5 kHz channel, for trunked systems
VocodersIMBE or compatible full-rate AMBE+2 (Phase 1); half-rate AMBE+2 (Phase 2)
Access codeNetwork access code (NAC), 12 bits; default $293
Amateur use
BandsMostly 2 m and 70 cm
RadiosUsed and surplus commercial radios
NetworkingRepeaters and hotspots linked to P25 reflectors

Project 25, usually shortened to P25, is a family of digital two-way radio standards designed for police, fire, emergency medical and other public safety services. It is widely used by public safety agencies in North America and in other parts of the world.[1] Retired P25 handhelds and mobile radios turn up on the used market, and a community of radio amateurs puts them to work on repeaters and internet-linked networks.

History and standards

The project began in October 1989 as a joint effort of APCO (the Association of Public-Safety Communications Officials), NASTD (the National Association of State Technology Directors) and several US federal agencies.[2] The aim was a common digital standard, so that radios from different manufacturers and agencies could talk to each other. The standards are written by the TIA's TR-8 committee and published as the TIA-102 series.[1] The P25 Technology Interest Group (PTIG) promotes the standard.[1]

How it works

Phase 1 carries one conversation per 12.5 kHz channel, a method called frequency-division multiple access (FDMA).[2] The signal uses C4FM, continuous four-level FM, at 4,800 symbols per second, giving 9,600 bits per second.[3] Some public safety simulcast systems instead transmit a compatible variant called CQPSK (also known as LSM) from their towers; Phase 1 radios decode both, but handhelds and mobiles always transmit C4FM.[3] System Fusion uses the same type of modulation, but the two systems cannot decode each other. Speech was originally compressed with the IMBE (Improved Multi-Band Excitation) vocoder.[3]

Phase 2 fits two conversations into one 12.5 kHz channel by time-division multiple access (TDMA), much as DMR does. It was designed to give the spectrum efficiency of a 6.25 kHz channel, a goal set by US FCC narrowbanding policy.[2] It uses a half-rate version of the AMBE+2 vocoder.[4] Phase 2 is built for trunked systems.[3]

P25 radios can work in two ways. In conventional operation, a channel is a fixed frequency, used directly between radios or through a repeater. In trunked operation, a computer-controlled system assigns each call a free channel from a shared pool, and users select talkgroups rather than frequencies.[2]

Every P25 Phase 1 transmission carries a network access code (NAC), repeated throughout, a 12-bit number written as three hexadecimal digits, often with a $ sign. It does the job a CTCSS tone does on analog FM: a receiver or repeater ignores signals with the wrong NAC. The default is $293. A receiver set to $F7E accepts any NAC, and a repeater set to $F7F repeats any signal and passes its NAC through unchanged.[3] Most P25 radios sold to agencies also work on analog FM.

The standard supports encryption, including AES with 256-bit keys and the older DES. Some agencies encrypt some or all of their traffic.

P25 in amateur radio

A Motorola XTS 2500, a commercial handheld for P25 and analog FM, programmed for the amateur bands.

Hams mostly use secondhand commercial radios such as Motorola's XTS series. The radio must cover the band: commercial VHF models typically tune 136 to 174 MHz, which includes all of 2 meters,[5] while UHF radios come in several ranges, and only some include 70 centimeters. Radios are programmed with the manufacturer's software and a cable, which can be costly or hard to obtain.

P25 repeaters are found in many areas, and most publish their NAC as an FM repeater publishes its tone. Repeaters and hotspots running the open-source MMDVM software can link to the amateur P25 reflector network.[6][7] Each reflector carries a single talkgroup and is identified by number.[8][6] Amateur activity is almost entirely Phase 1, because MMDVM supports only Phase 1 and Phase 2 is used only on trunked systems.[7][3]

Several rules matter on the air:

  • No encryption. In the US, Part 97 §97.113(a)(4) forbids messages encoded to obscure their meaning, and the ITU Radio Regulations bar it between amateur stations of different countries.[9][10] Any encryption feature must stay off. Ordinary P25 digital voice is not encryption: its coding is not used to hide the meaning, and the air interface is a published standard that scanners and open-source software can decode.
  • Identify by voice. A P25 radio sends a numeric unit ID, not a call sign, so operators give their call signs by voice as station identification rules require.[11]
  • Commercial gear is allowed. In the US, most amateur equipment needs no FCC certification,[12] so a Part 90 commercial radio may be used on amateur bands if its signal meets Part 97's technical standards.

Rules in other countries follow similar principles but differ in detail.

Comparison with other digital modes

Two Motorola commercial handhelds programmed for ham use: an XTS 2500 for P25 (left) and a DMR model (right). Both also work on analog FM, but neither digital mode can talk to the other.
Mode Origin Channel and access Voice codec
P25 Phase 1 Public safety standard C4FM, FDMA, 12.5 kHz IMBE or AMBE+2
DMR ETSI commercial standard 4FSK, two-slot TDMA, 12.5 kHz AMBE+2
D-STAR Japan Amateur Radio League GMSK, about 6.25 kHz AMBE
System Fusion Yaesu C4FM, 12.5 kHz AMBE+2
NXDN Icom and Kenwood 4FSK, FDMA, 6.25 or 12.5 kHz AMBE+2
M17 Open amateur project 4FSK, about 9 kHz Codec 2

P25 offers hams rugged radios and common ground with public safety, which appeals to those active in emergency communications. Its drawbacks are cost, harder programming and fewer users than DMR or Fusion.

Listening with scanners

Digital scanners sold to the public decode P25 Phase 1 and, on many models, Phase 2. They cannot decode encrypted traffic.[13] Laws on owning or using scanners, especially in vehicles, vary by country and in some places by state or province, so listeners should check local rules.

Before keying up on a P25 repeater, check its NAC and set your radio to match. Hotspot software such as WPSD offers a parrot, reflector 10, which plays back your transmission so you can check your audio.

See also

References

  1. ↑ 1.0 1.1 1.2 What is P25 Technology?, P25 Technology Interest Group.
  2. ↑ 2.0 2.1 2.2 2.3 Benefits of Project 25, TIA.
  3. ↑ 3.0 3.1 3.2 3.3 3.4 3.5 P25, Signal Identification Wiki.
  4. ↑ Standards, Digital Voice Systems, Inc.
  5. ↑ XTS 2500 Digital Portable Radio, Motorola Solutions.
  6. ↑ 6.0 6.1 P25, WPSD Manual.
  7. ↑ 7.0 7.1 MMDVMHost README, G4KLX, GitHub.
  8. ↑ DVReflectors, GitHub.
  9. ↑ 47 CFR 97.113, eCFR.
  10. ↑ Amateur Radio and the ITU Radio Regulations, USTTI.
  11. ↑ 47 CFR 97.119, eCFR.
  12. ↑ FCC Warns that Use of Authorized Equipment Must Comply with All Laws and Rules, ARRL, June 2016.
  13. ↑ Digital Police Scanners & APCO P25 Scanner Radios, Scanner Master.
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