OBJETIVOS ESPECIFICOS

The Dimetra IP system is a digital radio communications system that provides radio users with voice and data services over a very wide geographical area. Users at any location within the coverage area can press the Push-To-Talk (PTT) button on their radios to make calls to any valid group or individual located anywhere in the coverage area (which can cover thousands of square miles).

The system requires a complex network of workstations, high-speed Local Area Network (LAN) and Wide Area Network (WAN) facilities, sophisticated databases, and management software.

The Dimetra IP system allows communication across multizones and allows users from different zones to be combined into talkgroups. This means that users can communicate across a wide geographic area and use a wide range of communication capabilities, provided that the user configuration is well-planned and systematically implemented.

2.1 Terrestrial Trunked Radio (TETRA)

The TETRA standard was developed by the European Telecommunications Standards Institute (ETSI) for private mobile radio. TETRA provides standardization for radio system services, system interfaces, and methods for registration and call processing. ETSI has defined TETRA as open standard to provide standardization and interoperability of telecommunication systems and their application.

Some of the TETRA standards include: • 25 kHz carrier spacing

• Pi/4-DQPSK modulation • ACELP voice compression • 36 kbps carrier data rate • 28.8 kbps user data rate

• TDMA with 4 timeslots per carrier

The TETRA standard defines the air interface between radios and the infrastructure. This enables radios from different suppliers to be used for basic TETRA operations regardless of the supplier of the infrastructure – see the figure below.

Figure 2-1 TETRA defined Air Interface

The most typical users of a TETRA system include public safety, transportation, utilities, industrial organizations, government agencies, and private telecommunication agencies.

2.1.1 Spectrum Management

Because frequency spectrum is a finite resource, the use of channels is authorized and licensed by government agencies in most countries. International regulations fall under the jurisdiction of the International

Telecommunications Union (ITU). The European Telecommunications Standards Institute (ETSI) produces standards and regulations for telecommunications in the European region.

How the spectrum is distributed among agencies is country specific.

In all cases, a license to operate radio equipment is required and must be applied for with the appropriate governing body. The license is granted to operate on a particular frequency, or set of frequencies, with specific eligibility rules that must be met.

2.1.2 Pi/4-DQPSK Modulation

A radio transmitter sends signaling across at a particular carrier frequency. To deliver intelligible traffic across this carrier, the radio transmitter modulates outbound traffic with the carrier signal. The receiver then demodulates the traffic from the carrier signal. The popular forms of modulation are amplitude modulation, frequency modulation, and phase modulation.

The Dimetra IP system uses pi/4-Differential Quadrature Phase Shift Keying (pi/4-DQPSK) modulation. This modulation method uses phase shifts in the transmit frequency to reflect different digital values.

As shown in the figure below, pi/4-DQPSK modulation uses eight separate carrier phases. Up to four possible phase shifts can be made from any phase point. Each possible shift in phase is assigned a two-digit binary value (00, 01, 10, 11). This is an improvement over many other phase shift keying methods which only provide a single-digit binary value for each phase shift.

The phase shift example shows a transmission shift from phase 0 (in phase) to 3pi/4. The receiver would recognize this shift and declare the received value as 01. The next phase shift shown is from 3pi/4 to pi/2. The receiver would recognize this shift as a binary value of 10. Each point offers four possible phase shifts for the next transmission.

2.1.3 ACELP Voice Compression

Figure 2-2 Pi/4-DQPSK Modulation

2.1.3 ACELP Voice Compression

Voice signaling must be compressed to maintain the high rates of traffic flow. The Dimetra IP system and its radios use the Algebraic Code Excited Linear Prediction (ACELP) compression method, according to the TETRA standard. Most voice compression methods are similar. The analog audio is first converted into some form of electronic or digital signaling, such as pulse code modulation (PCM). Any silence or redundant data is then noted and eliminated. Any remaining data is compared to a codebook. Each chunk of the data is replaced by an index number from the codebook. The receiver then receives the data, looks up the index numbers from its codebook, and reconstructs the data.

The ACELP compression method uses voice prediction algorithms and filters, along with its own particular codebook, which allows quality audio to be synthesized in as little as 8 kbps. As shown in figure below, the radio processes 30 msec blocks of speech. The final compressed signal is placed in a TDMA timeslot for transmission.

Figure 2-3 ACELP Voice Compression

2.1.4 Time Division Multiple Access

The Dimetra base stations and radios use Time Division Multiple Access (TDMA). Each carrier signal is divided into four timeslots, creating four channels for each 25kHz carrier.

2.1.4.1 TDMA for Dimetra IP Base Stations

Each base station can send a continuous stream of traffic for all four time slots. Since each base station is full-duplex, it is able to send and receive traffic simultaneously on its transmit and receive channels. As shown in the figure below, the first channel on the first base station is typically configured as the control channel. All other channels at the site (base stations 2-8) are configured as traffic channels or packet data channels as required. The first channel on base stations 2-4, however, are also configured with control channel capability, allowing any one of them to operate as a control channel if the primary control channel fails. The figure shows the typical channel configurations made for a site with up to eight base radios.

2.1.4.2 TDMA for Dimetra IP Radios

2.1.4.2 TDMA for Dimetra IP Radios

Radios only operate on a single channel at any one time to send or receive traffic. Since the radios do not have true full-duplex capability, the radio switches between the transmit and receive channels to simulate full-duplex capability for telephone interconnect calls and full-duplex individual calls. The transmit and receive time slots are offset by two slots to accommodate this switching and to allow the radio to both send and receive traffic within each time frame.

Figure 2-5 Time Division Multiple Access - Radio (Full-Duplex Operation)

2.2 Network Technology

The Dimetra IP system is based on various networking technologies and architectures.

2.2.1 Local Area Network

A Local Area Network (LAN) is a data communications system designed to link computers and peripheral devices such as printers and modems. LAN cabling has a limited usable distance of up to 100 m (329 ft) and is best used within a building or campus environment. The advantage of using a LAN is that users can share peripheral devices connected to the LAN instead of having those devices attached to each computer. Network users can also share information stored in the network server, such as databases and programs. In addition, network users can communicate with each other through messaging or email.

2.2.1.1 Ethernet Technology

Ethernet technology refers to a LAN used to connect computers and peripheral devices (such as printers, modems) so they can be shared by users of the network. Originally developed to run at 10 Mbps, Ethernet networks can now run at 100 Mbps. Ethernet can use twisted pair, coaxial, or fiber optic cabling with BNC, RJ-45, or fiber optic connectors.

The Institute of Electrical and Electronic Engineers (IEEE) created the 802.3 standard for the operation of 10 Mbps networks. There are various types of 802.3 standard, based on the type of cabling used:

• 10Base-5 - Thick Ethernet • 10Base-2 - Thin Ethernet • 10Base-T - Twisted pair Ethernet • 10Base-FL - Fiber Optics

Ethernet accesses data using Carrier Sense Multiple Access with Collision Detection (CSMA/CD). This method allows multiple users to access the network through a common cable. All devices attached to the network check for transmissions in progress, signals are checked at the start of transmission and during transmission. Signals are sent if no other transmission is detected; otherwise, the transmission is delayed. Collision detection is applied when two or more devices transmit at the same time. A device knows if a collision occurred when it does not receive its own transmission back. Each device stops transmission and attempts to retransmit after waiting a certain amount of time, which is different for each device and determined by an algorithm.

2.2.1.2 Star Topology

The LAN topology most frequently used in Dimetra IP System is the star topology, where the end points on a network are connected to a common central device by point-to-point links. The information arriving at the common device is broadcast to all the end point devices; each device is responsible for determining whether the information is intended for it or not. Characteristics of the star topology include:

• Twisted pair cable is used for the links between the central and end devices.

• Link isolation is used-if a fault occurs on one link, the other links remain unaffected. • A switch serves as the central device.

• The end devices share the available bandwidth.