In this blog post, we’ll explore the evolution of internet technology from ADSL to fiber-optic internet and the principles behind fiber-optic internet’s high transmission speeds.
Around 1995, people used to connect modems to telephone lines to access the internet at speeds that were extremely slow by today’s standards. This setup meant you couldn’t make or receive phone calls while using the internet—a situation that’s hard to imagine now. In 1999, singer Yoo Seung-jun even introduced ADSL in a TV commercial with the tagline, “ISDN runs, but I’m ADSL.” Both ISDN and ADSL are electrical wired communication methods that use copper wires. Later, as fiber-optic communication using optical cables became widespread, data transmission speeds improved dramatically—to a degree incomparable to those of the past.
It is well known that light travels very fast. Compared to the copper-wire-based communication of the past, today’s fiber-optic-based communication offers much higher transmission speeds. For example, 1 Gbps means that 1 Gbit of data can be transmitted per second—a speed fast enough to download a movie of about 2 GB in roughly 16 seconds. Today, even residential internet services offer speeds in the gigabit range, while data centers and backbone networks utilize transmission technologies ranging from hundreds of Gbps to terabits per second. Thanks to the high transmission speeds of fiber-optic communication, we can enjoy a much smoother internet experience. What is commonly referred to as “fiber-optic broadband” refers precisely to this optical communication method.
However, just because it’s called optical communication doesn’t mean that only light is used throughout the entire process. Electricity also plays a crucial role. Let’s imagine two people chatting online. When a message is typed on Computer A, that message is first converted into an electrical signal. This electrical signal is then converted into a light signal via an electrical-to-optical converter at the sending end and transmitted as light along the optical cable. It is then converted back into an electrical signal by the optical-to-electrical converter at the receiving end and displayed as a text message on Computer B. Even if A and B are in different countries, the message is delivered almost instantly because the signal travels at a very high speed—close to the speed of light—along the optical cable.
How are light signals transmitted inside an optical cable? The principle behind this lies in total internal reflection, which occurs due to differences in the refractive index. The refractive index is a value that indicates how much slower light travels in a specific medium compared to its speed in a vacuum. Inside an optical cable, there is a core with a high refractive index, surrounded by a cladding with a lower refractive index. Total internal reflection is a phenomenon in which light is completely reflected at the interface when it travels from a medium with a higher refractive index to one with a lower refractive index, and the angle of incidence exceeds the critical angle. Since light travels relatively slower in the core and faster in the cladding, total internal reflection occurs; optical cables utilize this principle to transmit light signals without allowing them to escape to the outside.
This may raise a question: even if all light is reflected by total internal reflection, isn’t some of it actually absorbed, causing the signal to weaken? Just as light weakens as depth increases in the ocean, the intensity of light in an optical fiber gradually decreases as the distance increases. Therefore, the performance of an optical fiber is evaluated by how little the signal attenuates over a given distance. In the early days of fiber-optic technology, signals would weaken significantly after just a few hundred meters; however, today, optical fibers with extremely low loss have been commercialized, enabling efficient transmission over distances of tens of kilometers or more. Furthermore, in long-distance communications, optical amplifiers are installed to amplify weakened signals, ensuring stable international communications across continents and across the seabed.
In the late 1990s, electrical wired communication methods using copper wires—such as modems, ISDN, and ADSL—were primarily used. Today, optical communication using fiber-optic cables has become the core technology of the Internet, and both transmission speed and stability have improved significantly compared to the past. The reason optical communication offers such high performance is that signals are transmitted in the form of light through optical cables, and the principle of total internal reflection is used to minimize signal loss even over long distances. Optical communication technology continues to advance, and it is expected to provide an even faster and more stable Internet environment in the future.
