Fiber Optic Transmission
Fiber optic technology is a major reason for the feasibility of residential broadband. More than metal wires, fiber has the carrying capacity to supply high-speed services to millions of homes. Fiber optic cables are made from very clear glass, much more transparent than window glass. Fiber technology uses lightwave signals to transmit data, whereas metal wire uses electro-magnetic signals. The clarity of the glass permits the distribution of lightwave impulses for possibly hundreds of miles.
Coding uses simple On/Off keying. A transmitter indicates a binary 1 by turning on the light. If the light is off, it indicates a binary 0. Light is perceptible when the receptor senses photons. More complicated coding schemes, such as amplitude modulation, are possible, but simple OOK works with current technologies for digital transmission.
Description of Key Fiber Elements
This section examines three central aspects of the fiber system: the fiber itself, the light sources, and modulation techniques.
Fiber
Fibers are broadly categorized into two groups: single-mode fiber and multimode fiber. The term mode refers to the path a photon takes in going from one end of the fiber to another. In multimode, a photon careens off the fiber wall as it goes from one end to the other, thereby defining a path. Another photon (there are a lot of them) will probably take a different path. The number of possible paths is a function of the core diameter with the wider the diameter, the more the paths.
According to the recommendations in ANSI T1E1.2/93-020R3, the core of multimode fiber is 62.6 microns in diameter, and the cladding is 125 microns. This is compatible with the FDDI multimode fiber spec ISO/IEC 9314-3.
As each photon bounces off the fiber core, it loses some energy. This limits the distance over which multimode fiber is usable without amplification. A second effect is that because the paths of photons that make up a data bit are of different lengths, a given bit is spread in time. Figure 2-11 shows two photons following different paths in a multimode fiber; photon B will arrive at the destination before photon A. The time difference between the arrival of photon A and photon B is called the delay spread. Because photons A and B are part of the same symbol (binary bit), significant delay spread can cause interference between bits. This is called intersymbol interference (ISI) and is impairment for metallic and wireless communications as well.
For single-mode fiber, all photons take the same path down the center of the core. This is because the core of the fiber is very narrow. According to the recommendations in ANSI T1E1.2/93-020R3, the fiber has an 8.3-micron core with 125-micron cladding. Much less signal loss and less ISI occur. Therefore, single-mode fiber is capable of greater distances and higher bit rate. The fact that the diameter of the fiber optic core is narrower than that of multimode, however, means that other elements of the system, such as connectors, transmitters, and receivers, must operate with much smaller tolerances. This makes the other components more expensive and difficult to handle.
Figure 2-11. Fiber Cross-Section
Modulation Techniques
The fiber can be modulated using On/Off keying, frequency modulation, or amplitude modulation. Symbol times are very short, on the order of picoseconds. Computer folk tend to think of fiber as a digitally encoded medium, which it can be. But the cable folk required analog modulation.
When cable TV was implemented, it was intended to mimic over-the-air analog broadcast signals. This meant that cable signals were analog as well. If fibers were to be encoded digitally, then it would have been necessary to convert from analog to digital (for fiber transport) and back to analog (for display on analog TV sets). The cable industry wanted to avoid extra analog-to-digital (A/D) conversions, even though there would have been possible benefits of data compression and improved picture fidelity—it was just too costly. Amplitude-modulated fiber enables analog line coding, which was the key that propelled the cable industry to deploy fiber.
Benefits
As compared with metal, fiber has more bandwidth, can travel longer distances without amplification or regeneration, is safer to handle, weighs less, and has much lower material cost. Single-mode fiber can carry light for up to 70 km, whereas coaxial cable must be amplified every 0.5 km, and phone wire has a reach of maybe 5 km, or up to 18,000 feet.
Fiber does not rust, which means less outside plant maintenance and lower ongoing operational costs for local loop networks. Fiber also is impervious to electromagnetic interference and is secure. In addition, it is extremely difficult (though not impossible) to wiretap, which is either good or bad, depending on which side of the law you stand.
Fiber promotes high reliability. A service break caused by a fiber cut can be corrected by switching to a redundant network in approximately 50 milliseconds. This nearly instantaneous switch won't be noticed by consumers or most networking applications. Carriers are moving to ring and other redundant topologies to secure their fiber networks.
Impairments
Fiber is wonderful stuff, but it has its limitations. Like metal wire, it is subject to attenuation in certain circumstances. Plus, there are some impairments unique to fiber.
Attenuation
In fiber, attenuation refers to the loss of optical power as light travels through the fiber. Measured in decibels per kilometer (dB/km), attenuation ranges from more than 300 dB/km for plastic fibers to around 0.2 dB/km for 1550 nanometer (nm) single-mode fiber (a nanometer is a billionth of a meter). For 1310 nm fiber, loss is about 50 percent greater than for 1550 nm. Additionally, each fiber splice adds attenuation of about 1 dB per splice. Due to fiber age, chemical makeup, construction, installation care, and other factors, the fiber loss per kilometer may not be the same for every link.
Also, over time, cracks occur in the fibers. In freezing conditions, fiber optic cable can be damaged when moisture inside imperfect insulation turns to ice. As ice crystallizes, it can exert pressure on the fiber cable inside the conduit. That pressure can cause fissures in the fiber and thereby degrades the signal, or it can break the fiber and kill the signal altogether
Dispersion
Dispersion causes light to spread as it travels down a fiber and limits bandwidth. Some photons will travel straight down the middle of the fiber. Other photons will bounce off the walls of the fiber and go careening through the fiber. The photons going down the middle will arrive earlier than the bouncing photons. So the affect of dispersion is loss of temporal integrity. The bit rate through fiber must be low enough to ensure that pulses do not overlap.
The question arises whether attenuation or dispersion is the more significant limiting factor in fiber transmission. The answer depends on speed and attenuation and optical loss budget. Most cable operators will operate with a maximum optical budget of around 37 dB. Because 1310 single-mode fiber attenuates at roughly 0.5 dB/km (fiber attenuation, splices, bends, kinks, and a safety margin), a distance of 74 km can be supported. For OC12 over 1310 nm single-mode fiber, the dispersion distance is roughly 40 km. For OC3, which is slower but more forgiving than OC12, distance is about 160 km.
Handling Problems
Fiber is difficult to handle because electrons are more forgiving than photons. As an example, the core of single-mode fiber is 9 microns in diameter. This makes splicing, as well as the construction of connectors and sockets, very difficult. The splices must be scrupulously clean and absolutely flush. Furthermore, the ends of the splices must be perfectly aligned, whereas there are no alignment problems with electricity. Any little contact, and electrons move—not so with fiber. Handling problems will become particularly acute for fiber to the home. Great care and fine tools are needed to align the glass cores on the respective segments. New devices are available to do splice automatically, but you need to be a telephone company to afford them.
Cuts and Other Damage
Fibers can be cut more easily than copper—and when they are cut, the effects are large. One industry rule of thumb points toward one fiber cut per year per half mile of fiber. Alcatel says that typical fiber breaks require 6 to 12 hours to locate and repair. Separately, Fiber Optics News reports 390 fiber cuts in the United States in the year ending June 1996. Reported fiber outages not caused by cuts—including storms, fire, vandalism, and rodent damage—were 247. The catastrophic effect of cuts means that redundancy is a must, thereby greatly increasing the number of fiber miles of a system.
Bending
If you bend copper wire, electricity flows through the bend without loss. If you bend fiber excessively, light escapes the core and ultimately the fiber may break. The amount of tolerable bend is a function of the refractive index of the core and surrounding cladding. Fibers specify a minimum bend radius; any tighter bending than the specified radius and transmission can stop altogether. Most single-mode fiber can tolerate a bend radius no less than an inch.
Clipping
Fibers transmit signals within an allowable dynamic range, and there cannot be too much or too little laser power. When excessive power is input into an amplitude modulated laser, the laser may shut off briefly. This is known as laser clipping. Clipping can occur when several light sources are input simultaneously. This primarily affects wide-area transmission and is a potential problem for hybrid fiber/coaxial cable systems. Furthermore, in cable systems, the symptoms of clipping can look very much like impulse noise on the coaxial cable.
An Improvement to Fiber
Even with the continued purchase and installation of fiber, more capacity will be needed. Fiber is expensive to install: Underground fiber installation in urban areas can be as high as $70,000 per mile. So carriers are looking for ways to better utilize the fiber they have.
Dense wavelength division multiplexing (DWDM) is a new technology that provides in fiber the equivalent of frequency division multiplexing in metallic wire. The idea is that separate parallel channels are transmitted on a single fiber, with one wavelength (or color) for each channel. Current products from Ciena Corporation enable 16 channels of 2.5 Gb each for a total of 40 Gb per fiber for a half mile. DWDM can operate over existing single-mode fiber and therefore substantially reduces upgrade costs for large carriers.
A single fiber bundle of 36 fibers can carry 15 million phone calls at 64 Kbps per call, enough to accommodate the entire voice traffic of the United States at peak hour.
The growth of RBB and the growth of fiber are linked. Fiber alone has the carrying capacity to move data across Core Networks. It will be key to scaling some Access Networks and may even find its way into the home.
Table 2-6 show fiber deployment worldwide in millions of kilometers, according to KMI Corp, a research firm that tracks fiber.
| Year | Fiber Deployment (millions km) |
|---|---|
| 1997 | 35 |
| 1998 | 41 |
| 1999 | 47 |
| 2000 | 56 |
| 2001 | 66 |