A core part of the hacker mentality is the desire to test limits: trying out ideas to see if something interesting, informative, and/or entertaining comes out of it. Some employees of Andrews & Arnold (a UK network provider) applied this mentality towards connecting their ADSL test equipment to some unlikely materials. The verdict of experiment: yes, ADSL works over wet string.
ADSL itself is something of an ingenious hack, carrying data over decades-old telephone wires designed only for voice. ADSL accomplished this in part through robust error correction measures keeping the bytes flowing through lines that were not originally designed for ADSL frequencies. The flow of bytes may slow over bad lines, but they will keep moving.
How bad? In this case, a pair of strings dampened with salty water. But there are limits: the same type of string dampened with just plain water was not enough to carry ADSL.
The pictures of the test setup also spoke volumes. They ran the wet string across a space that looked much like every hacker workspace, salt water dripping on the industrial carpet. Experimenting and learning right where you are, using what you have on hand, are hallmarks of hacker resourcefulness. Fancy laboratory not required.
Thanks to [chris] and [Spencer] for the tips.
Transferring data without error when there is a lot of background noise is a challenge. Dr. Claude E. Shannon in 1948 provided guidance with his theory addressing the capacity of a communications channel in the presence of noise. His work quickly spread beyond communications into other fields. Even other aspects of computer use were impacted. An example is the transfer of data from a storage medium, like a hard drive or CD-ROM. These media and their sensors are not 100% reliable so errors occur. Just as Shannon’s work defines communication channel capacity it defines the transfer rate from a media surface to the read head.
Shannon told us how much could be passed through a channel but didn’t say how. Since 1948 a lot of research effort went into accurately detecting errors and correcting them. Error correction codes (ECC) add extra bits to messages, but their cost pays off in their ability to work around errors. For instance, without ECC the two Voyager spacecraft, now leaving our solar system, would be unable to phone home with the information they’ve gathered because noise would overwhelm their signals. Typically in hardware, like memory, error correction is referred to as ECC. In communications, the term forward error correction (FEC) is used.
Robust communication, or data transfer, is a combination of fancy software and tricky signal processing. I’m going to focus on the software side in this article. You may find some of these techniques useful in communicating data among your devices. While I’ll be using the term communications keep in mind this is applicable to transferring data in general.
Continue reading “Error Detection And Correction: Reed-Solomon, Convolution And Trellis Diagrams”
Despite the title, there’s no religious content in this post. The Hell in question is the German inventor [Rudolph Hell]. Although he had an impressive career, what most people remember him for is the Hellschreiber–a device I often mention when I’m trying to illustrate engineering elegance. What’s a Hellschreiber? And why is it elegant?
The first question is easy to answer: the Hellschreiber is almost like a teletype machine. It sends printed messages over the radio, but it works differently than conventional teletype. That’s where the elegance comes into play. To understand how, though, you need a little background.
Continue reading “Messages From Hell: Human Signal Processing”
If there’s one downside to digital storage, it’s the short lifespan. Despite technology’s best efforts, digital storage beyond 50 years is extremely difficult. [Robert Grass, et al.], researchers from the Swiss Federal Institute of Technology in Zurich, decided to address the issue with DNA. The same stuff that makes you “You” can also be used to store your entire library, and then some.
As the existence of cancer shows, DNA is not always replicated perfectly. A single mismatch, addition, or omission of a base pair can wreak havoc on an organism. [Grass, et al.] realized that for long-term storage capability, error-correction was necessary. They decided to use Reed-Solomon codes, which have been utilized in error-correction for many storage formats from CDs to QR codes to satellite communication. Starting with uncompressed digital text files of the Swiss Federal Charter from 1291 and the English translation of the Archimedes Palimpsest, they mapped every two bytes to three elements in a Galois field. Each element was then encoded to a specific codon, a triplet of nucleotides. In addition, two levels of redundancy were employed, creating outer- and inner- codes for error recovery. Since long DNA is very difficult to synthesize (and pricier), the final product was 4991 DNA segments of 158 nucleotides each (39 codons plus primers).
Continue reading “Store Digital Files For Eons In Silica-Encased DNA”