Fiber Optic Cable

Over the last 20 years or so, fiber optic lines have taken over and transformed the long distance telephone industry. Optical fibers are also a huge part of making the Internet available around the world. When fiber replaces copper for long distance calls and Internet traffic, it dramatically lowers costs. To understand how a fiber optic cable works, imagine an immensely long drinking straw or flexible plastic pipe.

For example, imagine a pipe that is several miles long. Now imagine that the inside surface of the pipe has been coated with a perfect mirror. Now imagine that you are looking into one end of the pipe.

Several miles away at the other end, a friend turns on a flashlight and shines it into the pipe. Because the interior of the pipe is a perfect mirror, the flashlight's light will reflect off the sides of the pipe (even though the pipemay curve and twist) and you will see it at the other end. If your friend were to turn the flashlight on and off in a morse code fashion, your friend could communicate with you through the pipe. That is the essence of a fiber optic cable.

Making a cable out of a mirrored tube would work, but it would be bulky and it would also be hard to coat the interior of the tube with a perfect mirror. A real fiber optic cable is therefore made out of glass. The glass is incredibly pure so that, even though it is several miles long, light can still make it through (imagine glass so transparent that a window several miles thick still looks clear). The glass is drawn into a very thin strand, with a thickness comparable to that of a human hair.

The glass strand is then coated in two layers of plastic. By coating the glass in plastic, you get the equivalent of a mirror around the glass strand. This mirror creates total internal reflection, just like a perfect mirror coating on the inside of a tube does.

You can experience this sort of reflection with a flashlight and a window in a dark room. If you direct the flashlight through the window at a 90 degree angle, it passes straight through the glass. However, if you shine the flashlight at a very shallow angle (nearly parallel to the glass), the glass will act as a mirror and you will see the beam reflect off the window and hit the wall inside the room. Light traveling through the fiber bounces at shallow angles like this and stays completely within the fiber.

To send telephone conversations through a fiber optic cable, analog voice signals are translated into digital signals (see How analog and digital recording works for details). A laser at one end of the pipe switches on and off to send each bit. Modern fiber systems with a single laser can transmit billions of bits per second -- the laser can turn on and off several billions of times per second. The newest systems use multiple lasers with different colors to fit multiple signals into the same fiber.

Modern fiber optic cables can carry a signal quite a distance -- perhaps 60 miles (100 km). On a long distance line, there is an equipment hut every 40 to 60 miles. The hut contains equipment that picks up and retransmits the signal down the next segment at full strength.

FTTH, or Fiber To The Home, refers to fiber optic cable that replaces the standard copper wire of the local Telco. FTTH is desirable because it can carry high-speed broadband services integrating voice, data and video, and runs directly to the junction box at the home or building. For this reason it is sometimes called Fiber To The Building, or FTTB.

Traditional copper telephone wires carry analog signals generated by telephone equipment, including fax machines. Analog technology is by nature a less precise signaling technology than digital technology. Though multiplexing has allowed digital signals to be transmitted across multiple channels over copper lines, fiber optic cable is superior for relaying these signals and allows for faster transfer rates and virtually unlimited bandwidth. This opens the door to better Internet speed, streaming video, and other demanding applications.

The Internet utilizes a backbone of fiber optic cables capable of delivering incredible bandwidth. This inherent ability makes the Internet a prime source for advancing network technologies that can be brought to the home or business. Most subscribers, however, log on to this network through copper lines with limited capacity. This creates a bottleneck for advancing technologies that increasingly require greater bandwidth. FTTH bridges this gap.

Fiber optic cables are made of glass fiber that can carry data at speeds exceeding 2.5 gigabits per second (gbps). FTTH services commonly offer a fleet of plans with differing speeds that are price dependent. At the lower end of the scale, a service plan might offer speeds of 10 megabits per second (mbps), while typical DSL (Digital Subscriber Line) service running on existing copper lines is 1.5 mbps. A more expensive FTTH plan might offer data transfer speeds of over 100 mbps –- that’s about 66 times faster than typical DSL.

FTTH is cost-prohibitive in many cases. Installing FTTH can be expensive, and the monthly charge for broadband services thereafter can also be off-putting, though these figures vary widely. Expense is likely to drop with time as FTTH becomes more common.

Because of the cost involved and the logistic difficulty in replacing existing copper lines in some neighborhoods, FTTH is more often being installed in newly built communities as an added selling feature. Installing FTTH raises the value of existing property.

FTTH can be installed as a point-to-point architecture, or as a passive optical network (PON). The former requires that the provider have an optical receiver for each customer in the field. PON FTTH utilizes a central transceiver and splitter to accommodate up to 32 clients. Optical electric converters, or OECs, are used to convert the signals to interface with copper wiring where necessary.

FTTH differs from Fiber To The Curb (FTTC) in that FTTC does not run directly to the home or building. Instead it runs to the curb, and the last leg of wiring to individual buildings remains copper wire.