[SFDXA] Build a Long-Distance Data Network Using Ham Radio

[Bill]

 From Tony N2MFT:


  Build a Long-Distance Data Network Using Ham Radio


    Send data via IPv4 up to 300 kilometers with easy-to-assemble hardware

By F4HDK
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        Hands on: A Ham Radio for Makers

<https://spectrum.ieee.org/geek-life/hands-on/hands-on-a-ham-radio-for-makers>
A medium-range UHF band using a directional antenna.Photo: F4HDK Data 
links work over the medium-range UHF band, but for the best results, a 
directional antenna is used. 
<https://spectrum.ieee.org/geek-life/hands-on/hands-on-a-ham-radio-for-makers> 


I have been a hobbyist and maker for almost 15 years now. I like 
inventing things and diving into low-level things. In 2013, I was 
looking at a protocol called 
<https://spectrum.ieee.org/geek-life/hands-on/hands-on-a-ham-radio-for-makers>NBP 
<http://lea.hamradio.si/~s53mv/nbp/nbp.html>, used to create a data 
network over amateur radio links. NBP was developed in the 2000s as a 
potential replacement for the venerable AX.25 protocol 
<http://lea.hamradio.si/~s53mv/nbp/nbp/AX25V20.pdf> [PDF] that’s been in 
use for digital links since the mid-1980s. I believed it was possible to 
create an even better protocol with a modern design that would be easier 
to use and inexpensive to physically implement.

It took six years, but the result is New Packet Radio (NPR), which I 
chose to publish under my call sign, F4HDK, as a nom de plume. It 
supports today’s de facto universal standard of communication—the 
Internet’s IPv4—and allows data to be transmitted at up to 500 kilobits 
per second on the popular 70-centimeter UHF ham radio band 
<https://en.wikipedia.org/wiki/70-centimeter_band>. Admittedly, 500 kb/s 
is not as fast as the megabits per second that flow through amateur 
networks such as the European Hamnet <https://www.hamnet.eu/site/> or 
U.S. AREDN <https://www.arednmesh.org/>, which use gigahertz

frequencies like those of Wi-Fi. But it is still faster than the 1.2 
kb/s normally used by AX.25 links, and the 70-cm band permits 
long-distance links even when obstructions prevent line-of-sight 
transmissions.

Initially, I considered using different frequency bands for the uplink 
and downlink connections: Downlinks would have used the DVB-S standard 
<https://www.etsi.org/technologies/satellite/dvb-s-s2>, originally 
developed for digital satellite television. Uplinks would have used a 
variation of FSK (frequency-shift keying) to encode data. But the 
complexity involved in synchronizing the uplink and downlink was too 
high. Then I tried using a software-defined radio equipped with a 
field-programmable gate array (FPGA). I had some experience with FPGAs 
thanks to a previous project in which I had implemented a complete 
custom CPU using an Altera Cyclone 4 FPGA 
<https://hackaday.io/project/18206-a2z-computer>. The goal was to do all 
the modulation and demodulation using the FPGA, but again the method was 
too complex. I lost almost two years chasing these ideas to their dead ends.

imgThe modem is principally a microcontroller attached to a radio 
transceiver. imgAn amplifier designed for DMR radio links boosts the 
signal to required levels. imgThe transceiver is a shielded module built 
around the Si4463 ISM chip, which operates at 434 megahertz. Photos: F4HDK

Then, in one of those why-didn’t-I-think-of-this-earlier moments, I 
turned to ISM (industrial, scientific, and medical) chips. These are 
transceivers designed to operate in narrow radio frequency bands that 
were originally allocated for noncommunication purposes, such as RF 
heating. However, the ISM band has become popular for communications as 
well because typically a license is not required for its use 
<https://www.acma.gov.au/theACMA/spectrum-at-434-mhz-for-low-powered-devices>. 
In Africa, Europe, and North Asia, there is an ISM band lying inside the 
70-cm ham radio band at 434 megahertz, so commercial ISM chips are 
available for this frequency.

I chose to build my hardware around the Si4463 
<https://www.silabs.com/documents/public/data-sheets/Si4463-61-60-C.pdf> [PDF] 
ISM transceiver: It’s cheap, flexible, and available in many modules and 
breakout boards, and it can handle a raw data rate up to 1 megabyte per 
second. It’s designed for short-range applications, so the radio part of 
the chip is not optimal, but it works. In order to reach reasonable 
distances, you need an amplifier to provide more RF power. For my NPR 
plan, I needed an amplifier that can also switch very rapidly between 
transmitting and receiving. I found some widely available external 
20-watt amplifiers for handheld radios designed for the 
European-developed Digital Mobile Radio (DMR) standard 
<https://www.dmrassociation.org/>, which was ratified in 2005. In the 
DMR standard, radio equipment must be able to handle a complete 
transmit/receive cycle within 60 milliseconds. I established a minimum 
of an 80-ms-long cycle time for NPR with this bound in mind.

The ISM transceiver is connected to an Mbed Nucleo STM32 L432KC 
<https://os.mbed.com/platforms/ST-Nucleo-L432KC/>microcontroller, which 
uses an Arm Cortex CPU.

This microcontroller is in turn connected to an Ethernet interface, and 
it takes care of all the details of running the NPR protocol. Any 
connected PC or network sees the radio link as just another IPv4 
connection with no need to install specific NPR software. The NPR modem 
can be configured over this link or via a USB connection. The total cost 
of the hardware is about US $80, and a partner, Funtronics, will be 
making kits available for purchase online. If you want to build a modem 
yourself from scratch, detailed instructions and the NPR protocol 
software are available from my Hackaday project page 
<https://hackaday.io/project/164092>.

The NPR protocol is based on a hub-and-spoke model, in which a central 
modem links several client modems. Currently there can be as many as 
seven modems, although I plan to expand this to 15. The theoretical 
maximum distance between a client modem and the central modem is 300 
kilometers. This limit arises because NPR uses a managed time-division 
multiple access (TDMA) technique, in which the central modem and the 
clients each transmit on the same frequency but at different times, with 
the central modem dictating when each client can transmit, and making 
scheduling adjustments to account for time delays due to distance. The 
complete transmit/receive cycle is between 80 ms and 200 ms, depending 
on the exact type of modulation and data rate chosen.

The creation of the NPR protocol was a very fun part of the project for 
me: deciding how data should be packed and arranged inside radio frames 
and how the NPR modems should interact with each other. But after two 
more years it was time to stop working alone, so I shared NPR with my 
local ham radio community in France. By the end of 2018, we began 
testing it in real-world conditions. We have already achieved distances 
over 80 km, and I am now getting help from the global amateur community, 
especially in Germany. Currently, NPR is primarily being used to access 
existing local high-speed amateur radio networks from places that cannot 
have the line-of-sight radio links required for 2.4- and 5.6-gigahertz 
signals.

Although it’s usable, I would be the first to admit that NPR is a young 
technology and probably not totally mature. In addition to increasing 
the number of clients that can be supported by a central modem, I have a 
number of enhancements in mind, such as adding support for QoS (quality 
of service) prioritization, so that NPR could be used to transmit 
digital voice; allowing Ethernet frames to be transported directly; and 
separating downlink and uplink frequencies.

/This article appears in the November 2019 print issue as “Ham Radio 
Does Distant Data Networking.”/


https://spectrum.ieee.org/geek-life/hands-on/build-a-longdistance-data-network-using-ham-radio


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