DIGITAL TRUNK SYSTEM is a digital PMR/PAMR land radio system. The control channel data, user speech and data traffic in the system is in digital format. The roaming, call set-up, mobile location and call maintenance and call release functions are fully automatic.
DIGITAL TRUNK SYSTEM as GSM uses trunking technology, i.e. the computer based switching system allocates free radio channels from a common pool to users when they are needed, and makes necessary switching functions.
In the DIGITAL TRUNK SYSTEM trunking system, which uses TDMA, the carrier contains several traffic channels, thus making the use of frequencies even more efficient.
DIGITAL TRUNK SYSTEM uses an ACELP- codec (Algebraic-Code-Excited Linear-Prediction) for speech compression. The speech compression is needed to lower the required data bit rate i.e. to reduce the needed transmission bandwidth especially in the air interface. The coding is very efficient thus allowing one 25 kHz carrier to contain four speech channels.
In DIGITAL TRUNK SYSTEM four timeslots are multiplexed into one TDMA frame. The slot can carry control signalling, speech or data calls.
DIGITAL TRUNK SYSTEM uses /4 DQPSK (/4 Differential Quaternary Phase Shift Keying) modulation. The base station works with a pre-set power level (downlink). The power level of the mobile station (uplink) is adjusted by the mobile station itself or alternatively by the base station, if the system supports such a function.
The physical characteristics of the DIGITAL TRUNK SYSTEM carriers are as follows:
• /4 DQPSK (Differential Quaternary Phase Shift Keying) modulation
• 25 kHz carrier spacing
• 36 kbit/s gross transmission rate
• 10 MHz (45 MHz on 900 MHz band) duplex spacing
The DIGITAL TRUNK SYSTEM spectrum efficiency for speech channel usage is better than in any popular land mobile radio system or cellular system. In DIGITAL TRUNK SYSTEM the carrier gross bit rate compared to the used bandwidth is also very efficient (36 kbit/s / 25 kHz = 1.44 bit/s / Hz).
1.1.1 DIGITAL TRUNK SYSTEM Network Elements
The DIGITAL TRUNK SYSTEM system consists of digital exchanges, base stations, dispatching equipment and mobile as well as hand-held mobile stations. The exchanges and base stations are scaleable in size and capacity to produce cost-efficient networks.
• Digital Exchange for DIGITAL TRUNK SYSTEM - DXT
• DIGITAL TRUNK SYSTEM Base Station - TBS
• Dispatcher System for DIGITAL TRUNK SYSTEM
• Network Management Centre – NMS/400 and/or NMWS
Interfaces to external communications and management systems:
• public and private fixed telecommunications networks
• command and control systems - CCS
• public and private data communications networks
• conventional PMR radio systems
• charging and customer care systems
1.1.2 Services of DIGITAL TRUNK SYSTEM
The DIGITAL TRUNK SYSTEM standard defines the following services for voice and data:
• Bearer Services
• Teleservices
• Supplementary Services
The bearer service provides communication capability between network terminal interfaces, excluding terminal functions. It is characterised by some lower layer attributes (i.e. OSI layers 1...3).
These services are divided into:
• Circuit mode data service
• Packet data service
• Short data service
• Status messages
Because of the success of the Internet packet data will be implemented by using the Internet Protocol over the Air Interface. The gateways will be used to connect DIGITAL TRUNK SYSTEM network to the external systems.
The teleservice provides the complete capability, including terminal functions, for communication between end-users. Thus in addition to the lower layer attributes it also includes higher layer attributes (i.e. also OSI layers 4...7). The teleservice is a circuit mode service. The following services are provided for voice communication:
• Individual call (point to point)
• Group call (point to multipoint)
• Direct mode operation
• Broadcast call (point to multipoint one way)
The supplementary service modifies or supplements a bearer- or teleservice. The supplementary services are applicable to most of bearer- and teleservices and they cannot be used alone.