Radio Hackers: Packet Radio
Packet fuses radios with computers and has several foundational skills for learning.
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If you’ve started your journey to explore the radio spectrum it probably hasn’t taken you long to realise that if you’re starting from the absolute beginning, there’s quite a bit of information to take in. You might also find that some of these skills can come with physical components that tend to require a bit of practice to get right or alternately, you might decide to look beyond the license-free ISM bands with the intention of transmitting with more power.
A great way of building some of these foundational skills can be achieved by obtaining an amateur class radio license. With entry-level licenses requiring little more than a weekend's study in most places, you’ll find that the world of amateur radio offers much to the potential RF hacker. You’ll learn about antennas, electrical and system design, transmitter essentials and of course, large amounts of information about the radio spectrum in general. This is all pretty useful stuff for the beginner.
However, in today’s article, we’ll be exploring another concept that has much to offer for educational purposes. Known as packet radio, it fuses computers and radio transmitters, providing digital communications over great distances.
You might be reading this article wondering just what exactly an amateur protocol has to offer to the world of radio hacking but the truth is packet offers users the chance to interface computers and radios, teaches the importance of working in the terminal, helps to learn about packet switching technology and allows us to provide fully independent digital point to point links using nothing more than a transmitter and antenna. Let’s check it out!
What The Heck is Packet?
Typically, before digital modes radio would often be used in one of two formats. We’d have continuous wave (CW) transmissions using Morse code to travel long distances, often at low power levels. And we’d see voice (aka Phone) transmissions, which would require little more than a push-to-talk (PTT) button on a microphone.
While voice made radios more accessible for everyone, the fact is that these voice transmissions weren’t like what you’d expect to find on a telephone or similar voice communication systems. Due to these systems using a lot of High Frequency (HF) setups for long-range communications, these transmissions were often poor quality, subject to atmospheric variables and other forms of interference.
Usually, when conditions were bad, you’d often need an experienced user to assist in sending and receiving messages and while Morse code would typically offer a way around these restrictions, these were low-bandwidth transmissions that offered little in the way of data transmission. Packet aimed to fix a lot of these issues by offering an independent yet standardised modulation type that was based on the X.25 protocol.
History & Development
It might surprise you to learn that when we look at the development of packet radio the timeline aligns quite closely with the internet. This is because, despite the internet using TCP/ IP protocols, there are still actually quite a few similarities between the two. The amateur radio community, for instance, had a dedicated IP address block available for amateurs to use within the conditions of their license.
Part of the reason for the success of packet radio though, was due to the fact that before the internet, X.25 packet was THE communications mode for many many pieces of technology. Not only would the telephone companies spend decades using it for their digital data transmissions, but it would also spend decades as a mainstay format for the aviation industry. It would also eventually be responsible for sending billions of dollars worldwide due to its leading role in early credit card and financial systems.
Packet Radio would even find a home in the pay-as-you-go world, being used in France as the backbone for the subscription-based Minitel service.
While much of this success would erode with the implementation of the internet, you’ll typically still find some systems using X.25-based systems even today.
In terms of its development though, Packet would come to life in the 70’s and find its niche through the 80’s electronics boom and development of the internet. While its data speeds and bandwidth would prove to be eventual limitations, even the hardware was similar to the early internet systems. Instead of using a modem though, early packet-based systems would use something called a Terminal Node Controller, a similar type of system that would provide the electronic modulation and demodulation of both incoming and outgoing transmissions.
Some amateur radio operators (known as hams) would even build their own hardware, focusing on simplicity and ease of use. The relatively new format meant that many would take time to adjust to these newfangled computers talking to transmitters. And some weren’t interested in learning at all.
Some Specifications
One of the biggest differences between packet radio and the internet is the transmission rates for data. While the internet has joined forces with fibre in most places to provide genuine, high-speed internet access, the truth is that packet systems haven’t really changed that much over time and for the most part, face limitations inherent to the protocol. However, the data rates that we can get are typically pretty reliable for most purposes.
Low-Speed packet data is usually transmitted at 1200 baud while high-speed packet is less common and usually runs at around 9600 baud. While this is a significant gain over the low-speed setup even the so-called “high speed” bandwidth looks positively archaic when compared to even dial-up internet speed.
Despite this, you’ll typically find packet to function pretty well within these design limitations. For instance, a single position packet filled with identifiers about the user and their location can be sent in a single 12000 baud packet using the APRS format. This position can then be received by an internet-equipped station and “gated’ to the internet, providing a live position including track worldwide where Igates are in range. Check it out at this website and see if you can identify transmissions in your local area.
Effectively though, to set up or experiment with a packet station you’ll need:
Radio (VHF or UHF system)
Antenna (Vertical or Directional)
Terminal Node Controller ( Virtual or Hardware)
Coaxial Cables
Computer with Terminal Capability (Laptop, Raspberry Pi etc)
Somewhere to connect to (your own, or someone else's).
Links to RF & Spectrum hacking
Despite this being a purely amateur protocol you’ll still find many valuable skills to be acquired by experimenting with this type of project. While you can choose to acquire pre-built systems (although these are a bit rarer in today's world) most of the skills you’ll acquire and hone by constructing your station yourself.
While for some, this may entail building components to make the whole thing work, for others this might simply be taking a plug-and-play approach, similar to how we’d assemble a modern computer.
A project like this is a goldmine for learning experiences though. We’ll look at configuring our Linux station and ensure our transmitter talks to our computer in both directions (transmit and receive). Then, we’ll need to ensure we’ve paired our transmitter to a tuned antenna and finally, we’ll use our terminal to issue commands and send and receive data. Throughout all of this, we’ll also need to ensure our transmitter is stable and transmitting and receiving on the correct frequencies needed to establish a link.
It’s important to note that a configured terminal will act in a similar fashion to bulletin board systems used in the early days of the Internet. This means that they can be used in a “live chat” mode, or operate as a “store and forward” system, saving a message to the TNC’s onboard memory for you to review later on.
So, as we can see, there are some pretty essential skills that can be acquired and used later on. In future articles, we’ll be releasing tutorials detailing how you might set up a cheap system based on the Raspberry Pi.
Listen to AX-25 Packet, Live or Recorded
To listen to samples of AX-25 packet, there are a few options. We can use live signals or pre-recorded ones.
If you’d like to use a live signal but don't have a receiver, you can use a web browser and the free WebSDR system to browse the APRS frequency for your location. Check it out via this link.
If you’d like a real challenge, the International Space Station often transmits APRS packets as it flies overhead in orbit. You’ll also need frequency information and orbital information for your location.
If you’d like to keep it simple, you can find pre-recorded samples at the Signal Identification Wiki, a great resource for identifying transmissions.
Or if you’d rather look at an embed so you don’t forget later on then we’ve got you covered as well. Take a look below.
Don’t forget, if you’re tackling this project with the RTL-SDR then you’ll need to acquire a plugin for decoding APRS with SDR #. If this is you, you may find this blog post helpful when you’re trying to configure it.
Future Radio Hackers Tutorials
While that’s all for today's article, as we mentioned earlier, there’s a tutorial in the works using Raspberry Pi to create a KISS-based terminal node controller. We’ll also be exploring some more of the terminal, using our onboard Wi-Fi and Bluetooth cards to both identify and interact with devices in our environment.
Do you hold an amateur radio license? Do you have a cool project in the works? Or maybe you’d like to see a tutorial focusing on a specific topic? Whatever it is, we want to hear about it so, let us know in the comments!
The amateur radio spectrum is governed by various agencies worldwide, and unlike the ISM bands a license will be required to transmit. To obtain a license, contact the spectrum regulator in your country for the most up-to-date information.
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