JPL's Wireless Communication Reference Website Chapter: Network Concepts and Standards

Trunking Networks With contributions by Bernhard H.Walke For a long time, mobile networks for closed user groups all used different radio channels. This appeared to result in a massive waste of bandwidth, and more efficient use of the spectrum can be made by sharing radio channels. In trunked radio systems, the channel selection is performed for each call, considering the actual traffic intensity. Thus, cellular voice networks are not limited to public telephone services. Closed user-group 'trunking' networks are installed in a number of countries. The advantages to be gained from trunking mobile voice communication for closed user groups onto a set of common frequency channels are evident from the large quantity of traffic still handled on separate frequencies. Combining this traffic ('trunking') is seen as a new development in mobile radio, although the concept is a simple extension of Erlang's classical results for wired telephone service to the management of radio channel usage.

Closed user groups, such as railway companies, used to operate their own radio infrastructure, and each organization used a separate set of radio channels. Trunking systems allows different organizations to share radio spectrum and infrastructure resources.

Private trunked mobile radio systems have been operated many years by offices and organisations related to security tasks, airports, the major industry, energy-suppliers, public personal transport operators and taxi-offices. In a local range defined by the base station, voice and data radio is offered on half-duplex channels between one or many mobile users and a central dispatcher. Local systems with one base station only and (cellular) systems covering larger areas are in use. Base stations often are equipped also to connect to the fixed network.

In the Federal Republic of Germany of 1989, when trunking was not widely used, more than 100,000 mobile radio networks for closed user groups supported traffic from more than 700,000 voice terminals in the business sector, plus 200,000 terminals in the public-services sector. Nine frequency bands, with a total of 550 channels were allocated. Companies in a city like Munich operated a total of 40,000 terminals in 4500 nets. This corresponds to an average of 100 to 150 users, usually belonging to four or five companies, per frequency.

Since 1990 a public trunked mobile radio service (Chekker) is operated in 18 German economic centres ("A-Regions") by the Deutsche Bundespost TELEKOM. In addition, private operators have been licensed to operate competing networks there. These networks are based on analog signal transmission and operate at 410 MHz with a channel-spacing of 12.5 or 25 kHz, cf. Table 1. The user data rate is 2,4 kbit/s. The network follows a non-state-of-the-art standard.

Licenses for B-Regions (with expected lower traffic demand) will be granted soon. These radio networks cover locally restricted areas, typically, conurbational city-areas with a maximum diameter of 50 km and a few cells each of 10-25 km diameter). Through suitable address assignment closed user-groups, virtually private radio networks dedicated e.g. to a company, are possible. All mobile stations of a user-group can be called and connected directly within a region. Chekker permits a maximum call-length of 60 sec. Multicast, identification of caller, re-direction of calls and short data transmission up to 23 characters are offered optionally. Trunked radio is typically used point-to-multipoint and thereby differs from cellular or cordless, where users communicate point-to-point. Connection establishment is by push-button (push-to-talk).

European Standards

The European Telecommunication Standards Institute (ETSI) proposes two Trans European Trunked Radio (TETRA) standards, one for pure packet data services to single or multiple destinations, and another also supporting additional circuit-switched data and speech channels (similarly to the initial Mobitex systems in Scandinavia). Hybrid (data and voice) systems may be required for certain applications, e.g. in the transport sector. The European frequency allocations for TETRA will be in the UHF band (parts of 380-400 MHz, 410-430 MHz, 450-470 MHz and/or 870-890/915-933 MHz), and may adopt 25 kHz channels for co-existence with existing mobile services. TETRA is thus a typical narrowband data standard. European harmonization is deemed essential to achieve cross-border operation with this second-generation standard, in the interest of international courier services, railroads, and road and river transport companies.

Trunked European Telecommunications Radio Air interface, TETRA

Since 1988 ETSI RES 06 is working for the TETRA standard to be finished in 1994 [18]. Aims are the European harmonization of frequency-allocation and standardization of

- the radio interface (air-interface),
- the basic elements of a man-machine-dialogue,
- some network-internal interfaces,
- interfaces in mobile stations.

Two standards are prepared in parallel: Voice and data (V+D) and packet radio (data optimized, DO). The TETRA V+D standard aims at a succession of existing trunked mobile radio systems, while the DO-standard defines an optimized packet radio system. Both TETRA-systems will use the same bit transmission rate. It is not clear yet, if the same transmit/receive-equipment will be used. Table 3 shows the technical features. Table 4 shows the carrier- and tele-services available.

TABLE 3: Characteristics of TETRA

frequencies: 2*5 MHz, later 410-430 & 450-470

TABLE 4: Carrier and Teleservices with TETRA

• A)V+D:
  
  • Bearer-services:
    • a) 7.2 kbit/s TDM unprotected voice
    • b) 19.2 kbit/s TDM data
    • c) 28.2 kbit/s TDM unprotected data
    • d) conn.-oriented packet-transmission
    • e) connection-less packet.

  • Tele-services
    • f) 4.8 kbit/s voice
    g) coded voice
• B) DO: Only the packet-services
  d)-e) do exist

JPL's Wireless Communication Reference Website © Jean-Paul M.G. Linnartz, 1993, 1995.