How to Integrate a Telephone PBX with a Radio Network

Introduction

Integrating professional radio systems with telephone PBXs (Private Branch Exchanges) represents a significant technological challenge for many organizations. The convergence of these two worlds requires an in-depth understanding of both traditional telecommunication systems and modern radio protocols.

The Ecosystem of Radio Technologies

The current landscape of professional radio communications is based on various technologies, each with specific technical characteristics:

1. DMR (Digital Mobile Radio) : Operates on 12.5 kHz channels using 4FSK (Four-Level Frequency Shift Keying) modulation, ensuring two simultaneous communications through TDMA (Time Division Multiple Access).
2. TETRA (Terrestrial Trunked Radio) : A more sophisticated technology that supports encrypted communications and offers greater bandwidth, making it ideal for emergency services and public safety.
3. VHF (Very High Frequency) : Networks operating in the 136-174 MHz band, representing a reliable solution for long-range communications.
4. LMR (Land Mobile Radio) and PMR (Private Mobile Radio) : Constitute the foundation of professional radio communications.

Telephone Gateways

Telephone gateways function as interfaces between radio systems and telephone networks, allowing radio and telephone users to communicate with each other. In the specific case of DMR networks, the DMR Network Gateway supports direct communications between DMR users and telephone users. The system also allows communications between radios and private telephone extensions through a PBX, either automatically or with operator assistance.

How the DMR Gateway Works

The DMR Network Gateway plays a fundamental role in interfacing the radio network with the telephone system, converting the audio from DMR packets into a format compatible with the telephone system. The voice path passes through the gateway for transcoding. For each PBX voice path, a network gateway is required to handle the translation between DMR AMBE+2 voice streams and PBX G.711 streams.

Network Architecture

Integration with traditional telephone systems requires one or more telephone gateways to interface the PBX or telephone exchange with the DMR network. The telephone gateway converts the voice traffic from the DMR Controller into the signaling format required by the PBX/PSTN and vice versa.

Call Process

The call process works bidirectionally:

1. From Phone to Radio:
   • The telephone user dials the number of a dedicated line.
   • Subsequently, they enter the individual or group radio number.
   • The DMR Controller receives the dialed string and uses the "in-phone" table rules to find a match.
   • It replaces the dialed digits with a number provided by the table.
   • Sets up the DMR call to the resulting radio or group number.

1. From Radio to Phone:
   • The radio user dials a string of numbers.
   • The DMR Controller receives the string and uses the "out-phone" table rules to find a match.
   • It replaces the string with a telephone number.
   • Sets up the call to that telephone number, routing it to an appropriate FXO group.

4-Wire E&M Interface Gateway

The 4-Wire E&M (Ear & Mouth) interface is a well-established standard for interconnection with telephone exchanges. The system uses four wires for bidirectional audio transmission—two for transmission (TX) and two for reception (RX)—plus E&M control signals. This configuration ensures physical separation of audio paths, minimizing interference and ensuring professional audio quality with latency below 100 ms.

VoIP Gateways for DMR and TETRA Radio Networks

With the advent of IP networks and VoIP (Voice over IP) technologies, integrating telephone PBXs with digital radio networks like DMR and TETRA has become even more efficient. VoIP Gateways for DMR and TETRA allow voice traffic to be carried over IP networks, facilitating interconnection and offering advanced new functionalities.

Advantages of Using VoIP Gateways

• Integration Flexibility : Enable connection of different types of radio and telephone systems, regardless of the protocols used.
• Cost Reduction : By utilizing existing IP networks, cabling and maintenance costs are reduced.
• Centralized Management : Allow administration and monitoring of the entire communication system from a single interface.
• Reliability and Security : Support advanced security protocols to protect communications and ensure system reliability.

Implementing VoIP Gateways

Implementing a VoIP Gateway requires:

• Needs Analysis : Evaluate specific requirements such as the number of users, required services, and existing network infrastructure.
• Hardware Selection : Choose a gateway compatible with radio technologies (DMR or TETRA) and the telephone system in use.
• System Configuration : Set network parameters, voice codecs, call routing rules, and security settings.
• Testing and Validation : Perform tests to ensure call quality and correct integration between systems.

Design Considerations

For proper integration, it's necessary to conduct a thorough analysis of system requirements, considering parameters such as:

• Number of Simultaneous Channels Needed : Determines the system's capacity to handle multiple calls concurrently.
• Type of Supported Voice Codecs : Ensures compatibility between different systems and audio quality.
• Latency and Audio Quality Requirements : Important to guarantee smooth communications without perceptible delays.
• Need for Redundancy : To increase system reliability in case of failures or interruptions.
• Bandwidth Sizing : Essential to ensure the network can support additional traffic without degrading performance.