Power Semaphore Towers

Vance was idly oiling his dagger in the Mercenary's Guildhall when a younger mercenary, one of the new Class Cs, came barging into the armory. She spotted him and waved frantically.
"Semaphore message for you, Randell! Get to the tower across the street!"
A semaphore message? Who would bother trying to get his attention like this? He left his dagger on the bench and jogged out of the armory, though the stone facade of the Mercenary's Guild, and towards the Air Post station across the crowded boulevard, its semaphore tower squat and ugly next to the comparatively graceful clacking pulley of the Air Post tower. He was met at the door by a frowning man, wearing the badge of the Semaphore service on his vest.
"Vance Randell? The message is 'Respond affirmatively if class A Mercenary Vance Randell is at Guildhall'." The functionary gave him a look over. "You look like the guy, so I'll send the response back." Vance followed him up a twisting staircase into the semaphore transmission room, a loudly whirring room with a number of colored lanterns resting on the tables. With a deft hand, the semaphore operator pulled and arranged the lanterns in a specific order, and hung the arrangement up on the pulley, causing them to whisk away towards the transmission port. As the lanterns came back down on the other side, the operator disassembled the sequence, and gave him a look.
"What are you waiting for? Go up the stairs to the receiver's end." With a frown, Vance climbed the indicated staircase, and another semaphore operator was there, her eyes watching the other tower across the way with some disinterest. Next to her was a huge, rectangular block of glass, machined in a strange way, and a whirring mechanism that kept a long scroll of paper spinning behind the glass. 
A flash lit up the room briefly, and the operator sprang into action, double checking the receiver alignment as the colors began to light up the room. The receiver lens drew a pattern on the paper that the operator observed, marking her hand blackboard. When the second flash hit the room, she stood, pulling the clutch lever of the paper scroll, and tearing off the relevant encoding. She turned to Vance. "Take this downstairs."
The semaphore decoder was a bored looking man in a desk in the corner of the air post office, who seemed only mildly interested in the code pattern Vance handed him. With a quick touch, he scrawled out the encoding.
"Randell, you owe me a dinner. From Rathnait.". The man glanced up at Vance from under his spectacles. "Pretty public forum for a lover's quarrel, Mister Randell."
Vance had his hand firmly planted on his forehead. "She's by no means my lover. The noble scion of House Numio is making fun of me." This was his punishment for having to pull a double duty shift instead of accompanying Rathnait to that dinner she wanted at the new Stagonid-themed restaurant near the upper quays. And now the entire semaphore network knew about it. 

Utility

Once set-up, a powered semaphore network can reliably send messages out from a central hub to all distribution points across the network, vastly improving communication time. This comes into play when, for example, sending out an alert to look for specific people, sending news of accidents and blockages, and the like - short, pithy messages under roughly twenty five words. Larger messages take progressively longer to send, and their transmission bottles up the network much longer, resulting in higher costs; those types of messages are more easily sent through courier.
While messages are optimally transmitted in the evenings, when color semaphore lights are much easier to pick out, daytime transmission is remarkably successful, as long as there's enough tallow oil in the lanterns to cast a bright enough glow.

Manufacturing

Colored lanterns and light-sensitive paper are the principal manufactured items required to support a powered semaphore system. The paper is exotic-sounding, but is actually easily produced on site using ordinary paper soaked in a custom alchemic lime solution to make burn slightly under focused light. The recent development of Wizard's Undying Ember promises to replace the ordinary oil lantern in this role, as the light cast by an unquenched Ember is far brighter. Ember-driven semaphore lighting can likely be seen from over twice the distance as ordinary semaphore lights today.

Social Impact

The presence of semaphore towers in a city, next to the presence of Air Post, is a major indicator of a city's advancement and Progress. Even if the ordinary Etoilean will never broadcast or receive a message by color semaphore, the mere fact that it is available is indicative of their city as on the leading edge of Power technology.
This is most apparent in the Etoile Capital City itself, which has nearly a hundred semaphore towers, as well as the largest semaphore routing tower in existence, right on top of the Hall of the Princeps, linked to at least twenty other towers.

Parent Technologies

• Power

Access & Availability

Most cities with power distribution systems have powered semaphore towers spaced throughout the city, typically either with the towers used to deliver Air Post or built over guardhouses. Messages are typically only sent by those on official Principality of Etoile business; while private citizens are permitted to send messages this way, the fees charged are cost prohibitive compared to simply using a courier.

Complexity

Assuming Power is available, semaphore towers are not particularly complex in concept or execution. Most semaphore towers are laid out in a bidirectional pattern, similar to the public tram - each tower has one tower 'behind' it and one tower 'ahead' of it, with the exception of the central 'routing' tower, which has a larger staff and can be linked to any number of towers. A 'link' between two towers consists of a powered cable assembly on one end, for transmission of a semaphore signal, and a special focusing lens that serves as a prism, against a steadily rotating scroll of light-sensitive paper, the 'receiver'. Two towers bidirectionally linked will have both sender and receiver arrangements on both sides, and workers managing each; a fully equipped and manned tower has four operators handling the inputs and outputs in both directions.
When a signal is transmitted, a series of colored lanterns (coming in four varieties, red, yellow, green, and blue) are attached to the semaphore transmission cable, and raised at a specific speed and order, preceded by a mirrored piece of glass to cause a flash at the receiver, an indicator of an incoming message. After the flash, the various colored lanterns shine at a distance, and their light is refracted through the receiver lens, focusing light at different physical parts of the light sensitive paper behind the lens. The paper sears when light is focused at it, producing an encoded pattern corresponding to the colors used to produce the signal. As long as transmitter and receiver are on the same power network, this is a foolproof and errorproof way of transmission, and the receiving operator can simply hand the encoding to the transmission specialist, who can replicate the color array and transmit it to the next station over. The encoding of a message in ordinary Etoilean into a color pattern is through a complex series of mappings and encodings, of interest only to semaphore tower operators, who continually iterate on the design to minimize the color-length of any messages propagated through the network.

Discovery

Transmission of signals over long distances has always been a recurring opportunity for military and commercial application. The ability to reliably send detailed information at speeds faster than that of a courier's delivery would herald a sea change in the speed of execution of orders. Historical attempts at this practice were always imperfect - colored smoke signals and the like were unreliable and dependent on weather, and using systems of flags was error prone and limited to a very basic 'language' of what certain flags meant. In addition, if signals had to be repeated from point to point, they would often be unintelligible after only three or four re-transmissions.
Power technology changed this by introducing the concept of timing and synchronization. Power distribution is famously synchronized; when one reciprocating rod completes one rotation on the rooftops, so do all of the others, in perfect harmony; desynchronization is due to a step-down coupling or a major fault somewhere in the transmission network. What this means, effectively, is that two stations hooked up to the same power network are timing-synchronized by mains power delivery. Power Semaphore Towers were first built in the year 646, after a proof of concept was demonstrated in The Academy of Etoile, newly built at the time.