Linking A Repeater: Part 1 – The Basics

This article is intended for owners of repeaters affiliated with the Arkansas GMRS Repeater Group and covers the basics of what you need to give your repeater the ability to “link” to other repeaters in the Arkansas GMRS Repeater Group Network. The AGRG Network was created exclusively for the interconnection of GMRS repeaters in the State of Arkansas, and does not support or grant network access to simplex nodes, private nodes, out-of-state networks, or any other form of station that is not specifically a GMRS repeater in the State of Arkansas.

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What is “linking”?

In this context, “linking” refers to the interconnection of two-way radio repeater systems. The Arkansas GMRS Repeater Group has constructed a 100% self hosted, self contained, self managed network specifically designed to enable the interconnection of GMRS repeaters in the state of Arkansas.

How does it work?

There are several methods to connect two or more repeaters (RF links, etc.); however, the AGRG Network exclusively uses Voice over Internet Protocol (VoIP). This method involves encoding an audio stream into a digital format and transmitting it via an IP data connection. Importantly, this does not necessarily imply the use of the internet. The data connection could be a wireless microwave link between towers, a cellular data modem, or a broadband internet connection to name a few examples. The internet is the easiest and most used connection method in the AGRG Network.

Once this data connection has been established (see below for required interface hardware), your repeater will have the ability to establish a VoIP connection to the AGRG Network. The VoIP connection is disabled by default, and can be enabled/disabled on-demand as you wish via several methods such as DTMF control, web browser, scheduling, and more. It can also be configured to stay connected 24/7 if you desire.

Your repeater will never be required to be linked. You will always maintain full control and ownership of your repeater, and it will only be linked if/when you choose.

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Before We Get Started…

Linking your repeater to the AGRG Network requires proper planning, implementation, setup, and maintenance. There are a few things to note before moving on:

• Setting up, owning, and managing a linked repeater requires basic Linux computer skills. If you have no Linux experience, that is OK – but be willing to research and learn!
• Linking your repeater requires accurate fine tuning of transmit and receive audio levels, often requiring the use of an RF Service monitor. Ensure you have access to the proper tools and test equipment.
• The Network passes lots of voice traffic and will increase the transmit load on your hardware substantially if linked for long periods of time. If you plan on being linked more than intermittently, ensure proper cooling methods are in place for your transmitter.

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What do I need?

In order to give your repeater the ability to establish a VoIP connection to the AGRG Network, there are a few things you will need to obtain. We will cover what each of these required items are below.

• Host (node)
• Interface
• Cable(s)

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The Host

To connect your repeater to the AGRG Network using VoIP, you need to equip your repeater with a specific device that facilitates audio transmission between your repeater and the network. This device is known as a “host”, or “node”. You will often hear these terms interchanged. Technically, a host is a physical computer running the required VoIP software to create a virtual node. A single physical host is able to support multiple virtual nodes, although you only need one node in this context.

BONUS: The host also acts as a full-fledged programmable repeater controller, capable of full-duplex repeat audio, CTCSS encode/decode, hang time, courtesy tones, and much more. Usually, any existing built-in and/or external controller will be disabled/bypassed with the addition of the host.

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The Interface

Additionally, you will require an interface – a physical device that connects the repeater to the host. Think of this as a “radio-to-computer adapter”. This is typically a USB sound card, although other options like Radio Over IP (RoIP) devices are available.

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The Cable

IMPORTANT: The following is an example of only the most common repeater interfacing. Required signals and connections methods will vary greatly depending on your specific equipment and setup. Read, read, and read again the following sections before getting started.

At the most basic level, there are four signals that the interface must be able to access to/from your repeater through the cable:

• PTT: Push To Talk – keys the transmitter
• RX Audio: Audio from the receiver
• TX Audio: Audio sent to the transmitter
• COR: Carrier Operated Relay – tells the interface/host that there is a valid signal currently being received by the receiver
  • Also referred to as COS, TOR, DET, etc.

IMPORTANT: Not all COR outputs are created equal. Some COR outputs activate any time there is a signal on the receiver frequency above squelch threshold, regardless of whether or not the correct CTCSS/CDCSS is present on the received signal. Make sure your COR does what you need!

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Configurations

There are many different configurations possible when interfacing a repeater to the host. The configuration you choose will depend on the capabilities of your repeater hardware, and will influence the wiring of your interface cable. If at all possible, use a full duplex configuration (example 1 or 2). Here are some examples:

1. Full Duplex – USBRADIO Driver (Recommended)
   • Description: The host fully controls the repeater, turning it into a “dumb” full duplex base radio. All built-in repeater functions are disabled.
   • Host Responsibilities: Manages the entire RX/TX repeat audio path, CTCSS encode/decode, IDs, hang time, courtesy tone, and more.
   • Advantages:
     • Smooth user experience.
     • Precise audio levels.
     • No need for COR connection; only TX, RX, and PTT are required.
     • Audio enhancements and multiple CTCSS configurations.
     • Remote control of squelch, audio levels, etc.
   • Disadvantages:
     • Higher CPU demand (Pi 3B+ or better, with CPU cooling).
     • Needs RF service monitor for setup.
     • Direct connections to the receiver discriminator and transmitter modulator required.
     • Repeater non-functional without host.
2. Full Duplex – SimpleUSB Driver (Second Best)
   • Description: Similar to DSP Driver, but the repeater manages CTCSS/CDCSS encode/decode.
   • Host Responsibilities: Handles RX/TX repeat audio path, IDs, hang time, courtesy tone, etc.
   • Advantages:
     • Smooth user experience.
     • Accurate audio levels.
     • Less demanding on CPU.
     • Possible setup without a service monitor (not recommended).
   • Disadvantages:
     • Requires tone-filtered COR connection.
     • Repeater non-functional without host.
3. Half Duplex – SimpleUSB Driver (Not Recommended)
   • Description: Limited control over repeater functions; the host primarily passes network audio to/from repeater.
   • Host Responsibilities: Limited to passing network audio.
   • Advantages:
     • Simple setup.
     • Repeater operates independently if the host fails.
   • Disadvantages:
     • Issues with network audio not being heard.
     • Limited customizability.
     • Potential conflicts between repeater controller and host.
     • Requires tone-filtered COR connection.

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Putting it Together

Once you have your host, interface, and cable connected to your repeater – you are ready to begin setup!

Click Here to continue to Part 2 – Setup & Configuration