APCO P25

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APCO P25 (short for Project 25 from APCO International ) is a transmission standard that was developed on behalf of the government for the security authorities in North America and meets similar requirements as ETSI - TETRA in Europe.

history

Due to the limitations of analog radio, there was a trend towards the introduction of digital radio in North America. The Association of Public Safety Communications Officials "APCO" (Association of Liaison Officers for Safety Tasks) created "Project 16" for this purpose, which has accompanied the introduction of digital radio with standardization proposals since the late 1970s.

With the increasing use of digital radio in the BOS services, considerable restrictions on interoperability became apparent. During catastrophes in the following years, it showed numerous problems, so that in 1988 the US Congress started a public inquiry for further development. "Project 25" was then initiated in 1989 on the basis of the results.

In addition to APCO, the development was now supported by other partners: the National Association of State Telecommunications Directors (NASTD), the National Telecommunications and Information Administration (NTIA), the National Communications System (NCS), the National Security Agency (NSA) and the United States Department of Defense (DoD). In the steering committee, the development was accompanied by FPIC, DHS Coast Guard and the Department of Commerce's the National Institute of Standards and Technology (NIST), Office of Law Enforcement Standards .

The introduction of APCO P25 is slow in the US due to the cost, but after the establishment of the Department of Homeland Security , a swift change is pushed. The introduction of APCO P25 is mandatory for new infrastructure projects in public authority radio. In addition to North America, P25 is also used in Australia, India, Russia and Singapore. Some regional organizations in South and Central America also use P25, a total of P25 is used in 54 countries in 660 service networks (as of mid-2004).

At the same time in 2005, however, ETSI TETRA was already in use in 60 countries, and outside of North America P25 is hardly to be found. The main reason is that the P25 terminals are many times more expensive than TETRA terminals (6,000 US dollars compared to 900 US dollars) and have therefore hardly been used outside of the BOS services in corporate radio. However, P25 is preferred in North America because it has the same range and frequency bandwidth as the existing analog systems. Since it is also backwards compatible with analog radio, individual BOS channels can be gradually converted from analog to digital operation. In addition, P25 can cover a larger area with one station, which is advantageous in sparsely populated regions. It is therefore more related to the francophone Tetrapol, which takes a similar technical approach, than to Tetra.

APCO P25 in amateur radio

APCO P25 is also used in amateur radio . Standard APCO-P25 devices are modified and reprogrammed by radio amateurs. In Germany there is an APCO-P25 network consisting of nine repeaters in the 70 cm band between Lower Saxony , North Rhine-Westphalia and Hesse , which is implemented via monitor links, i. H. Via directional radio links on the output and input frequencies of the repeaters.

Radio procedures

The P25 intercoms can be used in a wide variety of radio channels, including conventional analog radio. Digital voice transmission allows a range of voice encodings (Vocoder = voice encoder / decoder ) and data encodings (for example DES, AES, RC4). Due to the variability, interoperability is not mandatory - it is achieved by comparing the acquisition criteria of the individual BOS services.

In the P25, the various regional services are addressed by a digital network code NAC ( Network Access Code ) (no longer by selecting the radio channel). The 12-bit long key allows up to 4096 different services to be set up; the NACs are usually written as three hexadecimal digits. The standard value is NAC 293 for receiving devices, NAC F7F is used by devices in open repeater mode.

Phase 1

In the current "Phase 1" in the USA, P25 is used with 12.5 kHz wide channels. Phase 1 digital devices use Continuous 4 level FM ( C4FM ) with a symbol rate of 4800  baud and 2 bits per symbol, and thus 9600 gross bits. The receivers are also compatible with CQPSK, which only requires 6.25 kHz bandwidth.

Phase 2

A TDMA process with two slots is currently being tested for "Phase 2" . Advanced Multi-Band Excitation (AMBE), which manages with 4800 baud, is to be used as a vocoder . In phase 2, the repeater stations are also improved, which can then be reconfigured from the control center according to the application scenario.

Parts of standards

  • Common Air Interface (CAI) describes the radio interface for end devices.
  • Subscriber Data Peripheral Interface for connecting laptops and passing through to data networks.
  • Fixed station interface describes the base station in the P25 network
  • Console Subsystem Interface describes the control protocols in the P25.
  • Network management describes the station control in the P25
  • Data Network Interface describes the connection of data services.
  • Telephone Interconnect Interface describes the connection in analog and ISDN networks.
  • Inter RF Subsystem Interface describes the connection of public data networks.

See also

literature

  • Steve Ford: VHF digital handbook. ARRL, Newington CT 2008.

Web links

Individual evidence

  1. ↑ a b Is this finally P25's year? Interview with Don Pfohl of the Project 25 Steering Committee and Bill Belt of the Telecommunications Industry Association's wireless division, May 1, 2005
  2. ↑ DF0MOT APCO P25 repeater and network. Taunus Relais Group, accessed on August 21, 2014 .
  3. ↑ DF0MOT 70 cm APCO25 digital relay - Großer Feldberg. Taunus Relais Group, accessed on August 21, 2014 .