The thickness of the core optical fiber used in fiber optic cables is very small and is measured in millionths of a meter, called micrometers (µm), or microns. One type of multimode fiber optic cable that was popular in the past has a 62.5 µm fiber optic core and 125 µm outer cladding (62.5/125). Modern cabling systems are based on multimode cable with a 50 µm core and 125 µm cladding (50/125).

Single-mode fiber has a much smaller core with the same 125 µm thickness of outer cladding. Commercial single-mode fibers have core diameters that can vary from approximately 8–10 µm. They are collectively referred to as “10 micron fiber” in the standard. By way of comparison, a single sheet of copy paper has a thickness of roughly 100 µm.

A fiber optic “mode” is a path that light can follow in traveling down a fiber. As the name implies, multimode cable has a larger core designed to support multiple modes, or paths, of light propagation.

When an incoherent light source such as an LED is coupled to multimode fiber, multiple paths of light from the LED are transmitted over the cable, as shown in Figure 17-1. An advantage of the larger core is easier coupling of the light source to the cable. A disadvantage is that the wider corridor for light transmission allows the multiple paths of LED light to bounce off the sides of the fiber.

Figure 17-1. Modes of light

When this happens, these light paths arrive at the far end slightly out of phase, causing the light pulse to become dispersed, or spread out. This modal dispersion, or jitter, of the signal can cause problems with signal recovery at the far end. The longer the distance, the more signal dispersion there will be at a given signaling rate.

Single-mode fiber has a much smaller core, optimized to propagate a single mode, or path. When long-wavelength light (e.g., 1,300 nanometers) is injected into this fiber, only one mode will be active, and the rays of light will travel down the middle of the fiber. When a coherent light source from a laser is coupled to a single-mode fiber, the single beam of laser light is transmitted over the single mode of the cable.

With single-mode fiber, the signals don’t bounce against the cladding of the fiber, meaning there is no modal signal dispersion. Therefore, the light can travel a much longer distance without signal problems. The smaller core requires more precision to couple the light source to the cable, which is one reason why single-mode equipment is more expensive.

It is possible to couple a coherent laser light source to a multimode fiber. However, when this was first done for the Gigabit Ethernet media system, it was discovered that there can sometimes be a problem with signal propagation over the older cables that were common when Gigabit Ethernet was initally developed in 1999. This issue is called differential mode delay (DMD), and it is further described later in this chapter.

Since that time, newer versions of multimode cables have been developed that are designed to support laser light sources. In particular, these cables support the use of a less expensive laser technology called a Vertical Cavity Surface Emitting Laser (VCSEL). These lasers are well suited for low-cost transmission at the 850 nm wavelength, allowing for higher data rates over multimode fiber.

The distance over which an Ethernet signal will travel down a multimode fiber segment is primarily affected by signal strength and signal jitter, or dispersion. To help characterize the effects of dispersion, multimode fiber manufacturers specify their cables with a bandwidth rating, which is based on a figure of merit called the bandwidth-distance product, or simply bandwidth.

The bandwidth for multimode fiber optic cable is specified as the product of megahertz times kilometers, shown as either MHz-km or MHz*km. Single-mode cable does not have the modal dispersion issues of multimode fiber, and therefore is not provided with a bandwidth rating.

A 200 MHz-km fiber can move 200 MHz of data up to one kilometer or 100 MHz of data as far as two kilometers. The amount of modal dispersion is different at different frequencies of light; therefore, the bandwidth rating depends on the frequency of light being sent over the cable. When using this spec, you need to know both the bandwidth rating and the frequency of light for which it applies on a given cable.

There is no way to field-test a fiber optic cable to derive a bandwidth-distance product. Instead, the bandwidth ratings can be found in vendor spec sheets (assuming you know the vendor and part number of the fiber cable). Multimode fiber optic media is manufactured in a range of bandwidths. A common rating for the older 62.5/125 µm cable was 160 MHz-km modal bandwidth at a wavelength of 850 nm. Newer versions of multimode fiber optic cables have since been developed with better ratings. To help keep things straight, the versions of multimode fiber have been provided with a rating system in the ISO/IEC 11801 standard based on the letters “OM,” which stand for “optical multimode.”

Table 17-1 shows the OM ratings for multimode fiber, showing the minimum modal bandwidth for two frequencies of operation.

Fiber manufacturers have improved the design and manufacturing of their cables since the older OM1 and OM2 cables were sold, including the development of laser-optimized multimode fiber (LOMF). The lower-cost light emitting diodes (LEDs) used in older OM1/OM2 fiber optic media systems for Ethernet have a maximum signaling rate of roughly 600 Mb/s. The VCSELs that are used to transmit signals over LOMF are capable of signaling at rates of over 10 Gb/s, which makes them ideal for use in high-speed networks.

The optical power losses on a fiber optic link must be small enough to allow the signal to be received accurately. The link power loss budget is the total optical power loss allocated for all fiber cables and patch cords and associated connectors on the segment, as well as all of the power penalties allocated in the standard to account for dynamic signal impairment factors. The power loss caused by the cable and connectors alone is called either static power loss or channel insertion loss.

The power penalties allocated for dynamic signal impairment cannot be measured in the field with static power loss testers. Instead, the dynamic power penalties are allocated in the standard to account for signaling issues such as modal noise, relative intensity noise, and intersymbol interference. The combined set of cable and connector losses and the power penalties allocated for signaling losses make up the total optical power budget for the link.

The static loss, or channel insertion loss, includes the entire set of cables, patch cords, and connectors in the link. Optical power loss for a given type of fiber optic cable is expressed in dB/km (decibels per kilometer) at a specified wavelength. The “km” portion is assumed, and you will often see fiber optic loss measurements expressed using only the “dB” portion.

The channel insertion loss for a given link segment can be measured in the field with fiber optic test instruments that can tell you exactly how much optical loss there may be over a given segment at a given wavelength of light. The more connectors you have, and the longer your fiber link cable is, the higher the channel insertion loss will be. If the connectors or fiber splices are poorly made, or if there is finger oil and/or dust on the connector ends, then there will be higher optical loss on the segment.

When working with fiber optic cables, it is very important to keep the ends of the cables extremely clean. In addition, dust caps should be provided for any unused connectors to avoid any accumulation of dust and oil on the fiber optic equipment and cables. Fiber optic cleaning devices are available for cleaning the ends of fiber optic jumpers, cables, and transceiver ports before installation.

Note that fiber optic loss meters may use LED light sources operating at a typical wavelength of 850 nm. There is an issue when using such testers for faster Ethernet types, because they could produce an attenuation reading that would cause an otherwise acceptable link to be rejected. The faster Ethernet links (1 Gb/s and up) use laser light, which propagates more efficiently than LED light in most cases. Therefore, a loss reading performed with an LED-based tester can report a higher loss value than a tester with a laser light source.

The ST (“straight tip”) fiber optic connector is a registered trademark of AT&T Corp., formerly known as the American Telephone and Telegraph Company. The formal name of the ST connector in the ISO/IEC international standards is BFOC/2.5. ST connectors were once a popular choice for cabling system termination panels.

Figure 17-2 shows a pair of fiber optic cables equipped with ST plug connectors. The ST connector is a spring-loaded bayonet connector whose outer ring locks onto the connection. The ST connector has a key on an inner sleeve along with the outer bayonet ring.

To make a connection, you line up the key on the inner sleeve of the ST plug with a corresponding slot on the ST receptacle. Then you push the connector in and lock it in place by twisting the outer bayonet ring. This provides a tight connection with precise alignment between the two pieces of fiber optic cable being joined. The ST connector provides a very reliable connection that does not easily loosen or pop out of place.

Figure 17-2. ST connectors

SC, meaning “subscriber connector,” is a registered trademark of Nippon Telegraph and Telephone (NTT). The SC connector is widely used by vendors for Ethernet interfaces, including long-range interfaces for 10, 40, and 100 Gb/s Ethernet.

The duplex SC connector shown in Figure 17-3 is also a recommended connector in the 100BASE-FX and 1000BASE-X standards.

Figure 17-3. Duplex SC connector

The SC connector is designed for ease of use; the connector is pushed into place and automatically snaps into the connector housing to complete the connection. Make sure to seat the connector firmly, pushing until it has “clicked” into place. An SC connector might still work if it is not installed tightly, but you will encounter high error rates and eventually the link may fail completely.

The LC connector was developed by Lucent, hence the name (“Lucent connector”). An LC connector uses a retaining tab mechanism, similar to an RJ45 connector, while the connector body has a square shape similar to the SC connector but smaller in size. LC connectors are usually held together in a duplex configuration with a plastic clip, as shown in Figure 17-4. The ferrule of an LC connector is 1.25 mm.

Figure 17-4. LC connector

The LC connector provides two fiber optic connections in a smaller space. Because the LC connector takes up about half the space required by an SC connector, this allows vendors to provide more ports on a switch front panel or chassis module.

As its name implies, the multifiber push-on (MPO) connector provides multiple fibers in a connector that is both “push to connect” and “push to disconnect.” This connector is defined by IEC-61754-7, “Fibre optic interconnecting devices and passive components,” and TIA-604-5-D, “Fiber Optic Connector Intermateability Standard, Type MPO.” Both standards specify 12- and 24-fiber versions of the MPO connector.

You will also see the term MTP used for this connector type, which is a registered trademark of US Conec for a connector that is compliant with the MPO standards (meaning that the MTP connector is an MPO connector). However, the MTP connector has been enhanced by US Conec to provide several product features, including the ability to change gender or to repolish in the field, a floating ferrule for improved optical performance, and elliptical guide pins to provide for tighter tolerance in alignment. Some of these features are covered under patents.

Figure 17-5 shows two MPO connectors. One is a 12-fiber MPO plug, with the alignment pins sticking out. The other is a 24-fiber MPO jack, with alignment holes that mate with the alignment pins.

Figure 17-5. MPO connector

According to the TIA standard, unless the color coding is used for some other purpose, the connector plug body should be identified by the following colors where possible:

The strain relief on the cable end and the cable jacket are also identified by a color code, as outlined in Table 17-2.

Multifiber cables, also known as ribbon cables, have a color code for each of the fiber optic strands in the cable. As shown in Table 17-3, solid colors are used on the first 12 strands. If the ribbon cable has more than 12 strands, then the next 12 strands will have a solid color base and a “tracer” color, which is a second color that is marked with a dashed or continuous line, a dashed or spiral line, ring stripes, or hash marks.

Ribbon cables terminated with MPO connectors present some challenges when it comes to maintaining the correct signal crossover in a segment consisting of multiple fiber optic strands. The ANSI/TIA-568-C.3 standard provides a set of “Guidelines for Maintaining Polarity Using Array Connectors” that describe three types of MPO-to-MPO array cables, defined as Types A, B, and C. These cables are used to provide three different methods for maintaining a crossover connection. Method A is the preferred method, and is based on Type A MPO cables.

Figure 17-6 shows a Type A straight-through ribbon cable with 12 fibers terminated in MPO connectors. A Method A backbone link is cabled “straight through,” terminating in the cabling system patch panel. One end of the link will have a straight-through patch cable, connecting from the patch panel to the Ethernet interface. The other end of the link will have a crossover cable connecting to the Ethernet interface. The guidelines recommend keeping all of the crossover patch cables at one end of the link, to keep the system as simple as possible and help the installer to avoid connecting the wrong type of patch cable.

Figure 17-6. MPO Type A straight-through cable

The guidelines also show Method B and Method C, which are two methods for providing a crossover path built into the MPO backbone cables themselves. Given the complexity of these approaches and the difficulty of implementing them correctly, they are both rarely used.

As you can see, there are a variety of approaches to managing the signal crossover for the 12-fiber and 24-fiber systems needed to support 40 and 100 Gb/s Ethernet. For the best results, make sure you know which method your site is using in the cabling system, and order the correct MPO cable types to make the connections and achieve the signal crossover. Note that some vendors provide special MPO connectors that make it possible to change connector gender and polarity (crossover) in the field, which could be a handy way to resolve MPO-to-MPO connectivity issues.^[55]