Intro to PACKRAT (Part 0/5) 🐀

Paul Tagliamonte 2021-12-02

Hello! Welcome. I’m so thrilled you’re here.

Some of you may know this (as I’ve written about in the past), but if you’re new to my RF travels, I’ve spent nights and weekends over the last two years doing some self directed learning on how radios work. I’ve gone from a very basic understanding of wireless communications, all the way through the process of learning about and implementing a set of libraries to modulate and demodulate data using my now formidable stash of SDRs. I’ve been implementing all of the RF processing code from first principals and purely based on other primitives I’ve written myself to prove to myself that I understand each concept before moving on.

I’ve just finished a large personal milestone – I was able to successfully send a cURL HTTP request through a network interface into my stack of libraries, through my own BPSK implementation, framed in my own artisanal hand crafted Layer 2 framing scheme, demodulated by my code on the other end, and sent into a Linux network interface. The combination of the Layer 1 PHY and Layer 2 Data Link is something that I’ve been calling “PACKRAT”.

$ curl http://44.127.0.8:8000/
* Connected to 44.127.0.8 (44.127.0.8) port 8000 (#0)
> GET / HTTP/1.1
> Host: localhost:1313
> User-Agent: curl/7.79.1
> Accept: */*
>
* Mark bundle as not supporting multiuse
* HTTP/1.0, assume close after body
< HTTP/1.0 200 OK
< Content-Length: 236
<



 ____   _    ____ _  ______      _  _____ 
|  _ \ / \  / ___| |/ /  _ \    / \|_   _|
| |_) / _ \| |   | ' /| |_) |  / _ \ | |  
|  __/ ___ \ |___| . \|  _ <  / ___ \| |  
|_| /_/   \_\____|_|\_\_| \_\/_/   \_\_|  
                                          


* Closing connection 0

In an effort to “pay it forward” to thank my friends for their time walking me through huge chunks of this, and those who publish their work, I’m now spending some time documenting how I was able to implement this protocol. I would never have gotten as far as I did without the incredible patience and kindness of friends spending time working with me, and educators publishing their hard work for the world to learn from. Please accept my deepest thanks and appreciation.

The PACKRAT posts are written from the perspective of a novice radio engineer, but experienced software engineer. I’ll be leaving out a lot of the technical details on the software end and specific software implementation, focusing on the general gist of the implementation in the radio critical components exclusively. The idea here is this is intended to be a framework – a jumping off point – for those who are interested in doing this themselves. I hope that this series of blog posts will come to be useful to those who embark on this incredibly rewarding journey after me.

This is the first post in the series, and it will contain links to all the posts to follow. This is going to be the landing page I link others to – as I publish additional posts, I’ll be updating the links on this page. The posts will also grow a tag, which you can check back on, or follow along with here.

Tau

Tau (𝜏) is a much more natural expression of the mathematical constant used for circles which I use rather than Pi (π). You may see me use Tau in code or text – Tau is the same as 2π, so if you see a Tau and don’t know what to do, feel free to mentally or textually replace it with 2π. I just hate always writing 2π everywhere – and only using π (or worse yet – 2π/2) .when I mean 1/2 of a circle (or, 𝜏/2).

Psuedo-code

Basicaly none of the code contained in this series is valid on its own. It’s very lightly basically Go, and only meant to express concepts in term of software. The examples in the post shouldn’t be taken on their own as working snippits to process IQ data, but rather, be used to guide implementations to process the data in question. I’d love to invite all readers to try to “play at home” with the examples, and try and work through the example data captures!

Captures

Speaking of captures, I’ve included live on-the-air captures of PACKRAT packets, as transmitted from my implementation, in different parts of these posts. This means you can go through the process of building code to parse and receive PACKRAT packets, and then build a transmitter that is validated by your receiver. It’s my hope folks will follow along at home and experiment with software to process RF data on their own!

Posts in this series