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Mobile Networks Python RFCs & Standards Software

Dr StrangeEncoding or: How I learned to stop worrying and love ASN.1

Australia is a strange country; As a kid I was scared of dogs, and in response, our family got a dog.

This year started off with adventures working with ASN.1 encoded data, and after a week of banging my head against the table, I was scared of ASN.1 encoding.

But now I love dogs, and slowly, I’m learning to embrace ASN.1 encoding.

What is ASN.1?

ASN.1 is an encoding scheme.

The best analogy I can give is to image a sheet of paper with a form on it, the form has fields for all the different bits of data it needs,

Each of the fields on the form has a data type, and the box is sized to restrict input, and some fields are mandatory.

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Now imagine you take this form and cut a hole where each of the text boxes would be.

We’ve made a key that can be laid on top of a blank sheet of paper, then we can fill the details through the key onto the blank paper and reuse the key over and over again to fill the data out many times.

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When we remove the key off the top of our paper, and what we have left on the paper below is the data from the form. Without the key on top this data doesn’t make much sense, but we can always add the key back and presto it’s back to making sense.

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While this may seem kind of pointless let’s look at the advantages of this method;

The data is validated by the key – People can’t put a name wherever, and country code anywhere, it’s got to be structured as per our requirements. And if we tried to enter a birthday through the key form onto the paper below, we couldn’t.

The data is as small as can be – Without all the metadata on the key above, such as the name of the field, the paper below contains only the pertinent information, and if a field is left blank it doesn’t take up any space at all on the paper.

It’s these two things, rigidly defined data structures (no room for errors or misinterpretation) and the minimal size on the wire (saves bandwidth), that led to 3GPP selecting ASN.1 encoding for many of it’s protocols, such as S1, NAS, SBc, X2, etc.

It’s also these two things that make ASN.1 kind of a jerk; If the data structure you’re feeding into your ASN.1 compiler does not match it will flat-out refuse to compile, and there’s no way to make sense of the data in its raw form.

I wrote a post covering the very basics of working with ASN.1 in Python here.

But working with a super simple ASN.1 definition you’ve created is one thing, using the 3GPP defined ASN.1 definitions is another,

With the aid of the fantastic PyCrate library, which is where the real magic happens, and this was the nut I cracked this week, compiling a 3GPP ASN.1 definition and communicating a standards-based protocol with it.

Watch this space for more fun with ASN.1!

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