Disadvantages of Fiber-Optic Cabling

With all of its advantages, many people use fiber-optic cabling. However, fiber-optic cabling does have a couple of disadvantages, including higher cost and a potentially more difficult installation in some cases.

Cost

It's ironic, but the higher cost of fiber-optic cabling has little to do with the cable these days. Increases in available fiber-optic cable–manufacturing capacity have lowered cable prices to levels comparable to high-end UTP on a per-foot basis, and the cables are no harder to pull. Modern fiber-optic connector systems have greatly reduced the time and labor required to terminate fiber. At the same time, the cost of connectors and the time it takes to terminate UTP have increased because Category 5e and Category 6 require greater diligence and can be harder to work with than Category 5. This is even more of a concern for Category 6A and STP cabling. So the installed cost of the basic link, patch panel to wall outlet, is roughly the same for fiber and UTP.

Here's where the costs diverge. Ethernet hubs, switches, routers, NICs, and patch cords for UTP are relatively (no, not relatively, very) inexpensive. A good-quality UTP-based 10/100/1000 autosensing Ethernet NIC for a PC can be purchased for less than $25. A fiber-optic NIC for a PC costs at least four times as much. Similar price differences exist for hubs, routers, and switches. For an IT manager who has several hundred workstations to deploy and support, that translates to megabucks and keeps UTP a viable solution. The cost of network electronics keeps the total system cost of fiber-based networks higher than UTP, and ultimately, it is preventing a mass stampede to fiber-to-the-desk. This is why hierarchical star, the typical topology in a commercial building, involves running fiber backbone cabling between equipment and telecommunications rooms or enclosures, and copper UTP horizontal cabling between telecommunications rooms or enclosures and telecommunications outlets near workstations. However, optical fiber offers some options in network topologies that can make the overall network cost lower than a traditional hierarchical star network wired with more copper cabling (also see TIA's Fiber Optics LAN Section: www.fols.org).

Installation

Depending on the connector system you select, the other main disadvantage of fiber-optic cabling is that it can be more difficult to install. Copper-cable ends simply need a mechanical connection, and those connections don't have to be perfect. Most often, the plug connectors for copper cables are crimped on and are punched down in an insulation displacement connector (IDC) connection on the jack and patch-panel ends.

Fiber-optic cables can be much trickier to make connections for, mainly because of the nature of the glass or plastic core of the fiber cable. When you cut or cleave (in fiber-optic terms) the fiber, the unpolished end consists of an irregular finish of glass that diffuses the light signal and prevents it from guiding into the receiver correctly. The end of the fiber must be polished with a special polishing tool to make it perfectly flat so that the light will shine through correctly. Figure 1 illustrates the difference between a polished and an unpolished fiber-optic cable end. The polishing step adds extra time to the installation of cable ends and amounts to a longer, and thus more expensive, cabling-plant installation.

 
Figure 1: The difference between a freshly cut and a polished end

Connector systems are available for multimode fiber-optic cables that don't require the polishing step. Using specially designed guillotine cleavers, you can make a sufficiently clean cleave in the fiber to allow a good end-to-end mate when the connector is plugged in. And, instead of using epoxy or some other method to hold the fiber in place, you can position the fibers in the connector so that dynamic tension holds them in the proper position. Using an index-matching gel in such connectors further improves the quality of the connection. Such systems greatly reduce the installation time and labor required to terminate fiber cables.

Advantages of Fiber-Optic Cabling

The following advantages of fiber over other cabling systems explain why fiber is becoming the preferred network cabling medium for high-bandwidth, long-distance applications:

• Immunity to electromagnetic interference (EMI)
• Higher data rates
• Longer maximum distances
• Better security

Immunity to Electromagnetic Interference (EMI)

All copper-cable network media share one common problem: they are susceptible to EMI. EMI is stray electromagnetism that interferes with electrical data transmission. All electrical cables generate a magnetic field around their central axis. If you pass a metal conductor through a magnetic field, an electrical current is generated in that conductor.

When you place two copper communication cables next to each other, EMI will cause crosstalk; signals from one cable will be picked up on the other. The longer a particular copper cable is, the more chance for crosstalk.

Fiber-optic cabling is immune to crosstalk because optical fiber does not conduct electricity and uses light signals in a glass fiber, rather than electrical signals along a metallic conductor, to transmit data. So it cannot produce a magnetic field and thus is immune to EMI. Fiber-optic cables can therefore be run in areas considered to be "hostile" to regular copper cabling (such as elevator shafts, electrical transformers, in tight bundles with other electrical cables, and industrial machinery).

Higher Possible Data Rates

Because light is immune to interference, can be modulated at very high frequencies, and travels almost instantaneously to its destination, much higher data rates are possible with fiber-optic cabling technologies than with traditional copper systems. Data rates far exceeding the gigabit per second (Gbps) range and higher are possible, and the latest IEEE standards body is working on 100Gbps fiber-based applications over much longer distances than copper cabling. Multimode is the preferred fiber-optic type for 100–550 meters seen in LAN networks, and since single-mode fiber-optic cables are capable of transmitting at these multi-gigabit data rates over very long distances, they are the preferred media for transcontinental and oceanic applications.

You will often encounter the word "bandwidth" when describing fiber-optic data rates. With optical fiber, bandwidth does not refer to channels or frequency, but rather just the bit-throughput rate.

Longer Maximum Distances

Typical copper data-transmission media are subject to distance limitations of maximum segment lengths no longer than 100 meters. Because they don't suffer from the EMI problems of traditional copper cabling and because they don't use electrical signals that can degrade substantially over long distances, single-mode fiber-optic cables can span distances up to 75 kilometers (about 46.6 miles) without using signal-boosting repeaters.

Better Security

Copper-cable transmission media are susceptible to eavesdropping through taps. A tap (short for wiretap) is a device that punctures through the outer jacket of a copper cable and touches the inner conductor. The tap intercepts signals sent on a LAN and sends them to another (unwanted) location. Electromagnetic (EM) taps are similar devices, but rather than puncturing the cable, they use the cable's magnetic fields, which are similar to the pattern of electrical signals. If you'll remember, simply placing a conductor next to a copper conductor with an electrical signal in it will produce a duplicate (albeit lower-power) version of the same signal. The EM tap then simply amplifies that signal and sends it on to the person who initiated the tap.

Because fiber-optic cabling uses light instead of electrical signals, it is immune to most types of eavesdropping. Traditional taps won't work because any intrusion on the cable will cause the light to be blocked and the connection simply won't function. EM taps won't work because no magnetic field is generated. Because of its immunity to traditional eavesdropping tactics, fiber-optic cabling is used in networks that must remain secure, such as government and research networks.

Introducing Fiber-Optic Transmission

Fiber-optic technology is different in its operation than standard copper media because the transmissions are "digital" light pulses instead of electrical voltage transitions. Very simply, fiber-optic transmissions encode the ones and zeroes of a digital network transmission by turning on and off the light pulses of a laser light source, of a given wavelength, at very high frequencies. The light source is usually either a laser or some kind of light-emitting diode (LED). The light from the light source is flashed on and off in the pattern of the data being encoded. The light travels inside the fiber until the light signal gets to its intended destination and is read by an optical detector, as shown in Figure 1.

 
Figure 1: Reflection of a light signal within a fiber-optic cable

Fiber-optic cables are optimized for one or more wavelengths of light. The wavelength of a particular light source is the length, measured in nanometers (billionths of a meter, abbreviated nm), between wave peaks in a typical light wave from that light source (as shown in Figure 2). You can think of a wavelength as the color of the light, and it is equal to the speed of light divided by the frequency. In the case of single-mode fiber, many different wavelengths of light can be transmitted over the same optical fiber at any one time. This is useful for increasing the transmission capacity of the fiber-optic cable since each wavelength of light is a distinct signal. Therefore, many signals can be carried over the same strand of optical fiber. This requires multiple lasers and detectors and is referred to as wavelength-division multiplexing (WDM).

 
Figure 2: A typical light wave

Typically, optical fibers use wavelengths between 850 and 1550nm, depending on the light source. Specifically, multimode fiber is used at 850 or 1300nm and single-mode fiber is typically used at 1310, 1490, and 1550nm (and, in WDM systems, in wavelengths around these primary wavelengths). The latest technology is extending this to 1625nm for single-mode fiber that is being used for next-generation passive optical networks (PON) for FTTH (fiber-to-the-home) applications. Silica-based glass is most transparent at these wavelengths, and therefore the transmission is more efficient (there is less attenuation of the signal) in this range. For a reference, visible light (the light that you can see) has wavelengths in the range between 400 and 700nm. Most fiber-optic light sources operate within the near infrared range (between 750 and 2500nm). You can't see infrared light, but it is a very effective fiber-optic light source.

Note

Most traditional light sources can only operate within the visible wavelength spectrum and over a range of wavelengths, not at one specific wavelength. Lasers (light amplification by stimulated emission of radiation) and LEDs produce light in a more limited, even single-wavelength, spectrum.

Figure 3 shows the typical attenuation of single-mode and multimode fibers as a function of wavelength in this range. As you can see, the attenuation of these fibers is lower at longer wavelengths. As a result, longer distance communications tends to occur at 1310 and 1550nm wavelengths over single-mode fibers.

 
Figure 3: Attenuation of single-mode and multimode fibers

Notice that typical fibers have a larger attenuation at 1385nm. This water peak is a result of very small amounts (in the part-per-million range) of water incorporated during the manufacturing process. Specifically it is a terminal –OH (hydroxyl) molecule that happens to have its characteristic vibration at the 1385nm wavelength; thereby contributing to a high attenuation at this wavelength. Historically, communications systems operated on either side of this peak. However, in 1999 Lucent Technologies's optical fiber division (now OFS) created a zero water peak (ZWP) process whereby this water peak was eliminated by significantly reducing and then modifying the −OH molecule.

To help you understand, let's use a very simple spring and weight analogy: If you replace the hydrogen with deuterium (an isotope of hydrogen that weighs twice as much) the molecule would now have a characteristic vibration that is not at a frequency of 1385nm and therefore does not cause high attenuation—still there, but out of the way. This invention opened up this wavelength range to transmission systems and allowed the International Telecommunication Union (ITU) to create a new operating band referred to as the E-band. This type of fiber is commonly referred to as low water peak (LWP) and has been standardized in the industry as ITU-T G.652D fiber. Earlier fibers had much larger attenuations at 1385nm and are referred to as ITU-T G.652B fiber.

Warning

Laser light sources used with fiber-optic cables are extremely hazardous to your vision. Looking directly at the end of a live optical fiber can cause severe damage to your retinas. You could be made permanently blind. Never look at the end of a fiber-optic cable without first knowing that no light source is active.

When the light pulses reach the destination, a sensor picks up the presence or absence of the light signal and transforms the pulses of light back into electrical signals.

The more the light signal scatters or confronts boundaries, the greater the likelihood of signal loss (attenuation). Additionally, every fiber-optic connector between signal source and destination presents the possibility for signal loss. Thus, the connectors must be installed correctly at each connection.

Most LAN/WAN fiber transmission systems use one fiber for transmitting and one for reception. However, the latest technology allows a fiber-optic transmitter to transmit in two directions over the same fiber strand. The different wavelengths of light do not interfere with each other since the detectors are tuned to only read specific wavelengths. Therefore, the more wavelengths you send over a single strand of optical fiber, the more detectors you need.

Common Problems with Copper Cabling

Sophisticated testers may provide a reason for a failed test. Some of the problems you may encounter include:

• Length problems
• Wire-map problems
• NEXT and FEXT (crosstalk) problems
• Attenuation problems

Length Problems

If a cable tester indicates that you have length problems, the most likely cause is that the cable you have installed exceeds the maximum length. Length problems may also occur if the cable has an open or short. Another possible problem is that the cable tester's NVP (Nominal Velocity of Propagation) setting is configured incorrectly. To correct it, run the tester's NVP diagnostics or setup to make sure that the NVP value is set properly. The NVP value can be obtained from the cable manufacturer if it's not properly installed in your tester.

Wire-Map Problems

When the cable tester indicates a wire-map problem, pairs are usually transposed in the wire. This is often a problem when mixing equipment that supports the T568-A and T568-B wiring patterns; it can also occur if the installer has split the pairs (individual wires are terminated on incorrect pins). A wire-map problem may also indicate an open or short in the cable.

NEXT and FEXT (Crosstalk) Problems

If the cable tester indicates crosstalk problems, the signal in one pair of wires is "bleeding" over into another pair of wires; when the crosstalk values are strong enough, this can interfere with data transmission. NEXT problems indicate that the cable tester has measured too much crosstalk on the near end of the connection. FEXT problems indicate too much crosstalk on the opposite side of the cable. Crosstalk is often caused by the conductors of a pair being separated, or "split," too much when they are terminated. Crosstalk problems can also be caused by external interference from EMI sources and cable damage or when components (patch panels and connectors) that are only supported for lower categories of cabling are used.

NEXT failures reported on very short cable runs, 15 meters (50′) and less, require special consideration. Such failures are a function of signal harmonics, resulting from imbalance in either the cable or the connecting hardware or induced by poor-quality installation techniques. The hardware or installation (punch-down) technique is usually the culprit, and you can fix the problem by either re-terminating (taking care not to untwist the pairs) or by replacing the connecting hardware with a product that is better electrically balanced. It should be noted that most quality NICs are constructed to ignore the "short-link" phenomenon and may function just fine under these conditions.

Attenuation Problems

When the cable tester reports attenuation problems, the cable is losing too much signal across its length. This can be a result of the cabling being too long. Also check to make sure the cable is terminated properly. When running horizontal cable, make sure that you use solid-conductor cable; stranded cable has higher attenuation than solid cable and can contribute to attenuation problems over longer lengths. Other causes of attenuation problems include high temperatures, cable damage (stretching the conductors), and the wrong category of components (patch panels and connectors).