Dr. Dobb's is part of the Informa Tech Division of Informa PLC

This site is operated by a business or businesses owned by Informa PLC and all copyright resides with them. Informa PLC's registered office is 5 Howick Place, London SW1P 1WG. Registered in England and Wales. Number 8860726.

Channels ▼


Transporting Binary Data In a Channel With a Few Reserved Codes

Baard Nossum is R&D Manager at Network Electronics AS. He can be reached at [email protected]

Several times I have needed to transport binary data through a channel where certain symbols are illegal. In one instance I had to make use of a RS232 channel to download a configuration file to an FPGA (via a microcontroller). A proprietary protocol layer on the RS232 channel made it imperative to avoid the characters CR, LF, '?' and '\0'. In another application I had to transport binary data in digital serial video (SD-SDI). SD-SDI is a stream of 10-bit video samples, but with in-line control so the binary codes 0b00000000xx and 0b11111111xx are illegal.

When devising a recoding scheme, I want a simple solution. For many years I used the IHEX format when downloading a configuration file. IHEX is straightforward, robust, well-understood, but inefficient. Each byte to transfer is coded as two hexadecimal numbers, giving a usable relative bandwidth of 0.5 (actually a lot less, IHEX has a considerable overhead). The maximum usable relative bandwidth of my RS232 channel (where four symbols were to be avoided) was log2(256 - 4) ≈ 7.98 per 8 transported bits, hence 7.98/8 = 0.997. A simple downloading solution is a good thing, but when the consequences is a two-fold increase in download time, I ran into problems. I decided to do better.

The ultimate recoding would be radix-conversion. In theory I could then reach the maximum efficiency. Radix-conversion is computationally intensive, requiring repeated divisions or multiplications. An interesting application is called Ascii85, which is used by Adobe (Ascii85 essentially encodes a 32-bit integer in 40 bits). I wanted to avoid multiplications in my algorithm, and examined other methods. It is easy to map 7-bit symbols into 8-bit symbols; the efficiency would increase to 7/8 = 0.875. While considerably more efficient than IHEX, I found I could do even better. In fact, I came very close to the theoretical maximum relative bandwidth, and this method I want to share with you.

Related Reading

More Insights

Currently we allow the following HTML tags in comments:

Single tags

These tags can be used alone and don't need an ending tag.

<br> Defines a single line break

<hr> Defines a horizontal line

Matching tags

These require an ending tag - e.g. <i>italic text</i>

<a> Defines an anchor

<b> Defines bold text

<big> Defines big text

<blockquote> Defines a long quotation

<caption> Defines a table caption

<cite> Defines a citation

<code> Defines computer code text

<em> Defines emphasized text

<fieldset> Defines a border around elements in a form

<h1> This is heading 1

<h2> This is heading 2

<h3> This is heading 3

<h4> This is heading 4

<h5> This is heading 5

<h6> This is heading 6

<i> Defines italic text

<p> Defines a paragraph

<pre> Defines preformatted text

<q> Defines a short quotation

<samp> Defines sample computer code text

<small> Defines small text

<span> Defines a section in a document

<s> Defines strikethrough text

<strike> Defines strikethrough text

<strong> Defines strong text

<sub> Defines subscripted text

<sup> Defines superscripted text

<u> Defines underlined text

Dr. Dobb's encourages readers to engage in spirited, healthy debate, including taking us to task. However, Dr. Dobb's moderates all comments posted to our site, and reserves the right to modify or remove any content that it determines to be derogatory, offensive, inflammatory, vulgar, irrelevant/off-topic, racist or obvious marketing or spam. Dr. Dobb's further reserves the right to disable the profile of any commenter participating in said activities.

Disqus Tips To upload an avatar photo, first complete your Disqus profile. | View the list of supported HTML tags you can use to style comments. | Please read our commenting policy.