APCO 25: American standard for digital trunking.
Previous issues of the journal presented the TETRA and Tetrapol digital trunking radio communication standards, aimed at building mobile communication systems for law enforcement agencies and public safety services (see “Tetrapol – “an unnoticed standard for digital trunking radio communication for public safety services”, N3, 1999 and “TETRA standard, open to all”, N4, 1999).
These standards were developed in Europe: TETRA by the European Telecommunications Standards Institute (ETSI), Tetrapol by the French company Matra Communications.
However, the story of the standards fighting for the market of digital trunked radio systems for public safety services would be incomplete without introducing the third competitor — the popular APCO 25 standard.
History of creation
The APCO 25 standard was developed by the Association of Public Safety Communications Officials-international (APCO), which unites users of communication systems working in public safety services.
APCO is an international organization and unites representatives of law enforcement agencies from about 70 countries.
APCO's headquarters are located in South Daytona (Florida, USA), in addition, this organization has representative offices in Canada, the Caribbean region and Australia.
The first specifications of trunking radio communication standards, on the basis of which the EDACS (Ericsson), SmartNet (Motorola), LTR (E.F. Johnson) systems were created, were developed by APCO back in the late 70s.
However, these specifications did not ensure compatibility and interoperability of various systems, which was the reason for starting a new project to develop a digital trunking radio communication standard, called APCO 25.
Work on creating the standard began in late 1989, and the last documents establishing the standard were approved and signed in August 1995 at the APCO International Conference and Exhibition in Detroit.
Currently, the standard includes all the main documents defining the principles of constructing the radio interface and other system interfaces, encryption protocols, speech coding methods, etc.
In 1996, a decision was made to divide all standard specifications into two implementation phases, designated Phase I and Phase II. In mid-1998, functional and technical requirements for each phase of the standard were formulated, highlighting the new capabilities of Phase II and its differences from Phase I.
Basic principles of construction
The fundamental principles of development of the APCO 25 standard, formulated by its developers, were the requirements:
- to ensure a smooth transition to digital radio communications (i.e. the possibility of joint operation at the initial stage of base stations of the standard with subscriber analog radio stations currently in use);
- to create an open system architecture to stimulate competition among equipment manufacturers;
- to ensure the possibility of interaction between various departments of public safety services when conducting joint events.
While the TETRA and Tetrapol standards do not support the operation of analog stations in their systems, the APCO 25 standard is specifically designed for the joint operation of digital and analog radio equipment.
At the same time, the developers of the standard declare that systems based on APCO 25 allow, on the one hand, a strict delimitation of the frequency resources used (analog and digital), and on the other hand, they also allow flexible channel configuration, i.e. joint use of a common frequency resource by digital and analog subscriber stations.
If this capability is actually conveniently implemented in practice (and not only for systems manufactured by Motorola), it will provide a serious advantage for APCO 25 over other standards, since it will allow for a smooth transition to digital radio communications by gradually re-equipping public safety units with digital radio stations.
This is especially relevant for Russia, where law enforcement agencies and public safety services still use a fairly large fleet of outdated analog radio equipment.
The system architecture of the standard supports both trunking and conventional radio communication systems, in which subscribers interact with each other either in direct communication mode or through a repeater.
The main functional block of the APCO 25 standard system is the radio subsystem, defined as a communication network that is built on the basis of one or more base stations.
In this case, each base station must support the Common Radio Interface (CAI — Common Radio Interface) and other standardized interfaces (intersystem, with PSTN, with a data port, with a data network and network management).
A trunking system uses a dedicated control channel.
Radio interface
The strength of the APCO 25 standard is that it provides the ability to operate in any of the standard frequency ranges used by mobile radio systems: 138-174, 406-512 or 746-869 MHz.
The main method of access to communication channels is frequency (FDMA), however, at the request of Ericsson, the possibility of using APCO 25 time-division multiple access (TDMA) in systems of the standard is included in Phase II.
In Phase I, the standard frequency grid step is 12.5 kHz, for Phase II — 6.25 kHz.
In this case, with a bandwidth of 12.5 kHz, four-position frequency modulation is performed using the C4FM method at a rate of 4800 symbols per second, and with a bandwidth of 6.25 kHz, four-position phase modulation with phase smoothing using the CQPSK method.
The combination of the specified modulation methods allows the use of the same receivers in different Phases, supplemented by different power amplifiers (for Phase I — simple amplifiers with high efficiency, for Phase II — amplifiers with high linearity and a limited width of the emitted spectrum).
In this case, the demodulator can process signals using any of the methods.
For speech coding, the standard uses the IMBE (Improved MultiBand Excitation) codec, which is also used in the Inmarsat satellite communications system.
The coding rate is 4400 bits/s. After noise-resistant coding of speech information, the speed of the information flow increases to 7200 bits/s, and after forming speech frames by adding service information — to 9600 bits/s.
Speech information in the radio channel is transmitted in 180 ms frames, which are called logical data units (LDU — Logical Data Unit).
A group of 2 frames forms a superframe with a duration of 360 ms.
Any transmission of speech information is preceded by a preamble with a duration of 82.5 ms and ends with an end-of-message marker (clearance signal). The structure of a speech message is shown in Fig. 1.
Fig. 1. Structure of a speech message in the APCO 25 standard
The speech preamble is intended for the initial synchronization of the transmitting and receiving radio stations, initialization of all encryption functions and transmission of address information.
The basis of the preamble is the header code word, which includes:
- message indicator (MI – Message Indicator), which characterizes the initial conditions for the encryption algorithm (72 bits);
- manufacturer identifier (8 bits);
- identifier of the type of encryption algorithm used (8 bits);
- identifier of the encryption key (16 bits);
- identifier of the talk group (16 bits).
The 120-bit code word is subjected to noise-resistant coding using Reed-Solomon and Golay codes, as a result of which its size increases to 648 bits.
After this, the following is placed at the beginning of the preamble:
- initial sync packet (FS – Frame Synchronization) of 48 bits;
- Network Identifier (NID), transmitted to prevent conflicts between radio stations of different networks operating on the same frequency (64 bits), and 10 zero bits at the end.
The final formation of the preamble structure is performed by inserting 2 bits of status information after every 70 bits of the preamble data packet (770 bits), obtained after adding the sync packet, network identifier and zero bits (a total of 22 status bits are added).
The final length of the preamble is 792 bits, so that at a channel information rate of 9600 bps, the preamble is transmitted over 82.5 ms.
Each logical data block consists of 9 speech frames of 144 bits, formed by 88 information bits obtained by converting a 20 ms speech signal segment using the IMBE codec, and 56 bits of the parity control correction code.
In addition, the LDU includes additional service messages.
The first logical block of the superframe transmits link control information (LC – Link Control), consisting of 72 bits of information and 168 bits of correction code) and low-speed signaling channel information (LSD – Low Speed Data), including 16 bits of data and 16 bits of correction code.
The second logical block of the superframe also contains low-speed signaling channel information LSD, and in addition, encryption algorithm information (ES Encryption Sync), including 96 information bits and 144 bits of correction code.
The structure of the speech superframe is shown in Fig. 2.
Fig. 2. The structure of the speech superframe in the APCO 25 standard
The channel control information includes a message indicator, a manufacturer identifier, an emergency call flag, a backup field, identifiers of the talk group (for an individual call of the called subscriber) and the transmitting subscriber.
The frames of the channel control embedded in the general information flow allow for increased reliability of the communication due to the possibility of restoring the connection after a short-term failure of the communication channel.
The encryption algorithm information contains a message indicator, an identifier of the type of encryption algorithm used, and an identifier of the encryption key.
The low-speed signaling channel can be used for various applications, in particular for transmitting signals about the location of moving objects.
The subscriber identification system incorporated into the APCO 25 standard allows addressing at least 2 million radio stations and up to 65 thousand groups in one network.
At the same time, the delay in establishing a communication channel in the subsystem, in accordance with the functional and technical requirements for the APCO 25 standard, should not exceed 500 ms (in direct communication mode — 250 ms, when communicating through a repeater 0-350 ms).
Data transmission
In APCO 25 standard systems, there are 2 options for data transmission: with confirmation of receipt and without confirmation.
When transmitting data, redundant trellis coding and interblock interleaving are used to correct errors.
The original data arrays are divided into fragments no longer than 512 bytes.
When transmitting with confirmation of receipt, fragments are divided into blocks of 16 bytes, and each block has its own number for the possibility of repetition.
When transmitting without confirmation of receipt, the blocks into which the fragments of the data arrays are divided contain 12 bytes.
The transmission of each data packet begins with a preamble containing a sync packet, a fragment number, the number of blocks in the packet, as well as network identifiers, manufacturer, access point, and logical connection identifier.
Data is transmitted over the same channels as voice messages and at the same speed of 9600 bit/s. Radio systems of the APCO 25 standard provide communication with fixed-line networks with the X.25, SNA, TCP/IP protocols.
It should be noted that the IP protocol is supported as a special IP service, which, with the help of a special network gateway, provides the ability to communicate between mobile terminals and wired infrastructure with applications using IP.
Communication security
When considering the model of a hypothetical adversary, the developers of the standard identified the following threats to communications security: interception of messages, repetition of messages with delay and distortion of information, creation of deliberate interference, analysis of subscriber traffic, creation of duplicate subscribers, introduction of an adversary as a legitimate user of the system.
Counteraction to most of these threats in the APCO 25 standard is ensured by three main mechanisms:
- confidentiality of communications, i.e. protection of information from any type of unauthorized access;
- authentication of subscribers and messages;
- key information management systems.
All of the above mechanisms for ensuring communication security are based on cryptographic encryption of information.
APCO 25 systems, in accordance with functional and technical requirements, must be designed to provide at least two of the four levels of cryptographic protection, depending on the type of communication system:
Type 1 – communications with guaranteed information classification at the national government level;
Type 2 – unclassified communications at the national level requiring protection of communications;
Type 3 – unclassified government communications requiring access restrictions;
Type 4 – for commercial and other applications (including exported modifications of systems).
The general model of cryptographic transformation (encryption/decryption) of information in a communications system is presented in Fig. 3.
Fig. 3. Model of cryptographic information transformation in the APCO 25 standard
On the transmitting side, the plaintext of the message is sent to the encoder, where, based on the key and a specific cryptographic algorithm, it is transformed into encrypted text of the same length, after which it is transmitted over the radio channel.
Along with the encrypted text, the message indicator MI is transmitted, which is intended to synchronize the operation of the encoder and decoder.
On the receiving side, after the synchronization procedure is completed, the encrypted text is converted into open text using a similar cryptographic algorithm and cryptographic key.
Various modifications of the general model of information cryptographic protection are used in different modes, shown in Fig. 4:
a) ECB – Electronic Code Book;
b) OFB – Output Feed Back;
c) CFB – Cipher Feed Back.
Fig. 4. Types of cryptographic transformations of information
Confidentiality of communication is achieved by encryption of speech and data traffic, which is carried out using the OFB method (Fig. 4b).
Authentication, designed to verify the authenticity of messages and subscribers, as well as to ensure subscriber privacy (i.e., to protect information about who the message is addressed to and from whom it originates), is carried out by transmitting message numbers that are encrypted using the OFB option (Fig. 4b) and adding a special authentication code (MAC – Message Authentication Code) to the message, which is generated using the CFB option (Fig. 4c).
The message number code and its identification code are temporary and change from message to message.
The key information management system is designed to generate, store, input, distribute, archive and delete cryptographic keys.
Keys are input into subscriber equipment using special KMF (Key Management Facility) equipment.
In addition, a special mode of key distribution via radio channel OTAR (Over-the-air-rekeying) is standardized in APCO 25 standard systems.
Information about keys sent via radio channel is protected using the ECB option (Fig. 4a).
Communication system equipment
Although APCO is an international organization with offices in a number of regions, the primary role in promoting this standard is played by American firms supported by the US government.
The Association's public sector members include the FBI, the US Department of Defense, the Federal Communications Commission, the police forces of a number of US states, the Secret Service, and many other government organizations.
Leading companies such as Motorola (the main developer of the standard), E.F. Johnson, Transcrypt, Stanlite Electronics and others have already announced themselves as manufacturers of APCO 25 standard equipment.
Motorola has already introduced its first system based on the APCO 25 standard, called ASTRO.
ASTRO radio networks can use a wide range of subscriber equipment that meets the needs of various users.
The first full-featured digital portable radio stations released by Motorola were called ASTRO Saber.
They can operate in both digital and analog modes in conventional and trunked radio networks in any of the frequency ranges allocated for land mobile communication systems (138-174, 406-512, 746-869 MHz).
The frequency grid step can have values of 12.5; 25 and 30 kHz.
Various modifications of the stations are produced, differing from each other in the number of operating channels, options for implementing controls and indicators, as well as some functional capabilities.
The latest development from Motorola is the XTS 3000 family of portable radio stations.
These radio stations support the same set of functions as the ASTRO Saber, but are made in a different design and have smaller dimensions and weight.
Due to the unification of design solutions with the MTS 2000 series of stations operating in the SmartNet system, the XTS 3000 stations can use the same accessories: headphones, a concealed headset, an external microphone and speaker, a charger, antennas.
ASTRO Spectra mobile radio stations are available in two versions: for cars and motorcycles. Motorola offers five versions of the stations, differing in some functional capabilities, the number of operating channels and the size of the indicator board.
The ASTRO CONSOLETTE station is used as a stationary radio station in the system.
In addition to supporting the APCO 25 standard protocol, the stationary radio station can work with the MDC-1200 and PL/PDL analog signaling systems.
The ASTRO CONSOLETTE station has two versions: for local and remote control via a telephone line.
For organizing data transmission, Motorola produces the FORTE wireless portable data terminal.
It is a pocket personal computer with a touch screen and a stylus, equipped with means for maintaining radio communication.
So far, APCO 25 standard systems have not been deployed in Russia, but specialists are showing great interest in this standard, the appeal of which lies in its continuity with existing analog radio communication systems, a large number of equipment manufacturers and the ability to build communication networks in all standard frequency ranges.
Representative offices of Motorola and ADI Limited are actively promoting systems of this standard in Russia. (Australia).
Ovchinnikov Andrey Mikhailovich