Signs and Signals

A recent discussion about nautical language (footloose, cut-of-your-jib etc) started a train of thought about words that jump from one usage to another over the years. Sometimes words become obsolete in their original context, and computing and communications are full of examples of words that have been shifted to new meanings.

For example “semaphore” is a word that has mainly lost currency in its original context: a messaging system involving two operators waving flags at each other to spell out the alphabet. This line-of-sight signaling system had a good technological run before electricity came along, and a number of European countries had towers in the 18th century with wooden signaling arms atop that could send messages across the country. In the UK, a chain of towers was used to send messages from the admiralty in London down to the naval dockyard at Portsmouth. It was even possible to synchronize clocks at the two sites to within a quarter of a minute: important, since correct navigation relied on precise track of time on board ship.

Some types of railway signal (with a moving arm) are still called semaphore, but pretty much the term is archaic, except in computing. In operating systems, software flags called semaphores are used to synchronize multiple processes or threads when they access a shared, limited resource. A semaphore allows a program to indivisibly reduce a counter and block if the resource is exhausted.  Once the program is finished with the resource it “signals” the semaphore, which again indivisibly increase the count again. These operations go on constantly in operating systems, and multi-tasking systems could not work without them.

Incidentally, the phrase “multi-tasking” once only related to computers and operating systems, but has been absorbed back into mainstream English, and  teenagers now tell us that they can do their homework, while simultaneously operating Facebook and texting, via their “multi-tasking” skills.

In communications, “signaling” is a big deal. Telephony systems have a “signaling layer” (e.g. SIP, SS7 or Q.931) responsible for call set-up and tear-down.  We use the term “digital signal processing” to describe algorithms we apply to audio/voice streams in order to encode, decode, compress or filter them for digital systems. We talk about analog signals (i.e. a representation of audio) that can be sampled and quantized into digital signals that can then be held, manipulated or transmitted using media like IP networks. Computers also have “signals”, which are software messages that tell a process to stop what it’s doing and do something else.

The provenance of the word “signal” is interesting. Signe in Latin really meant a sign, and the word “signal” comes to us via French or Italian (signale/segnale). Late Roman period Emperor Constantine was said to have seen an early Christian symbol in the sky, and was told “in hoc signo vinces”, or “in this sign you will conquer”. A sign in the sky is certainly a form of communication, pretty rarely used, and the more powerful for it.

“Signals” have been used in a military context for a long time. In the navy, sequences of flags were run up the mast, e.g. Nelson’s 1805 “England expects every man to do his duty”, or waving semaphore flags for a more interactive communication. Incidentally, Nelson originally said “confides”, but the word “expects” could be done with fewer flags.  In modern communications, the same concerns still exist: we change the representation of messages or compress them in order to more efficiently use the medium; to optimize transmission time, bandwidth or storage space. Military communication is still today referred to as “signals”, and there are specialist “signals” divisions.

Arguably, the use of the word signal in digital communication is quite divergent from the original meaning. Technology has the ability to bring new meaning to language as well as to hijack and re-purpose existing words. Computing and communication shapes the world as profoundly as ships did from the 15th to the 20th century, and much of that specialized vocabulary will no doubt shape the spoken language of the future in the way that nautical language already has.

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