A look at the difference between the speed of light and the speed of electricity and how this has affected communications from telegraphs to the optical backplane in hyperscale datacenters.

“...how wonderful it was that these various signs should be made to cleave the air with such precision as to convey to the distance of three hundred leagues the ideas and wishes of a man sitting at a table at one end of the line to another man similarly placed at the opposite extremity...”

-- An optical telegraph, as described in The Count Of Monte Cristo (1844)


"Electricity is really just organized lightning."

-- George Carlin

The optical telegraph

In 1792, a Frenchman named Claude Chappe invented a semaphore relay system, using pivoting rods to send messages over long distances. The two rods, connected together by a cross-beam above a relay station, were moved into any one of 196 positions, and those positions were seen and interpreted by the operators of another station, who could then repeat the signal. Chappe’s system is now sometimes called an “optical telegraph.” Rick Beyer, of A&E’s The Greatest Stories Never Told, dubbed Beyer the inventor of the “mechanical internet.”

Two years later, in 1794, a Swede named Abraham Niclas Edelcrantz set up a rival optical telegraph system. He used it to send a message from Stockholm to Drottningholm, wishing King Charles IV John a happy birthday. Edelcrantz’s system would eventually send messages ten times faster than Chappe’s.

Auspiciously, the idea of optical telegraphs taking their rightful place in the advancement of communications technology was used to great effect by science-fiction author Terry Pratchett. In his 2004 novel Going Postal, he describes “clacks,” semaphore relay machines that allow for the sending of “c-mail.”

Optic signals are faster than electric ones

Alas, in reality, it was not to be. The optical telegraph was eventually replaced by the electrical telegraph, even though, all things being equal, optical signals travel faster than electric ones. But in the 18th century, all things were not equal. Chappe and Edelcrantz, like Iron Man’s father, were limited by the technology of their time. Optical telegraph stations could be only, at most, 25 kilometers apart. They didn’t work at night or in bad weather. It took time and effort to change the position of the rods. In modern networking parlance, one might say that the packet forwarding mechanism was inefficient. But on a clear afternoon, waves of light could carry the position of one station’s rods to the watching eyes of the next station’s signalman faster than the same information would be able to travel across an electric telegraph wire fifty years later.

Here’s why. Electrical signals, in the way we typically think of them, are not made of light. After all, light does not travel through solid objects, such as copper wires. (Try holding a penny in front of a light bulb, and see how much light gets through.) Electrical signals are actually electromagnetic waves. These waves comprise the transfer of energy between a series of electromagnetic fields around the electrons in the wire. (The movement of the electrons themselves is not what makes up the signal.)

For example, the electromagnetic signal in RG-6 coax cable, which is used with cable modems, moves at about 75 percent of light speed. And that does not take into consideration the need for the signal to move through any components other than the wire, nor does it take into consideration operational processes that would interpret the signal and generate a new one, such as the packet routing process that happens in a typical network.

The speed of light in hyperscale datacenters

In most cases, the difference between the speed of light and the speed of electricity is impossible for a human observer to detect. But in hyperscale datacenters utilizing Intel® Rack Scale Design (RSD), it matters.

Intel® RSD is an architecture for datacenter infrastructures that enables a system to use components that are not connected to the same motherboard, or even located on the same rack. But if a CPU is connected to a hard drive fifty meters away, using only an electrical cable, then the communication between them is going to be slow—relative, that is, to the communication between components on the same motherboard.

Combining packet and optical switching

Ericsson Hyperscale Datacenter System 8000, the world's first complete system based on Intel® RSD, is unique in that it uses an optical backplane: a fiber optic connection between components that connects to the backs of the circuit boards, effectively doing an end run around the circuitry that would slow down the signal communication.

Just like the codes of the optical telegraph sent in the 1790s, signals sent through the optics travel at the speed of light. Optical signals still need to go through a switching process, which is analogous to the 18th century task of decoding the telegraph signals, deciding whether they needed to be passed on further and, if so, setting up the mechanism to send the message to the next station.

Fortunately, that process has gotten a lot faster, and Ericsson has gotten very good at combining packet and optical switching. So communication at the speed of light can finally take its rightful place in the enterprise.

To explore our thoughts on this further, download our technical article: "Combining packet and optical switching for optimum performance":

Download the paper

For more information

Image by Ernest Damé, Georges Farcy (Le Petit Journal, 14 juillet 1893, p.1-2) [Public domain], via Wikimedia Commons

Cloud Infrastructure

Michael Bennett Cohn

Michael Bennett Cohn was head of digital product and revenue operations at Condé Nast, where he created the company's first dynamic system for digital audience cross-pollination. At a traditional boutique ad agency, he founded and ran the digital media buying team, during which time he planned and executed the digital ad campaign that launched the first Amazon Kindle. At Federated Media, where he was the first head of east coast operations, he developed and managed conversational marketing campaigns for top clients including Dell, American Express, and Kraft. He also has a master's degree in cinema-television from the University of Southern California. He lives in Brooklyn.

Michael Bennett Cohn