8.4. Fibre Channel Cabling and Cable Concepts

The Fibre Channel specification allows for two types of cabling: copper fiber and optical fiber.

Copper fiber is typically used for shorter distances measured in meters (as opposed to kilometers). Copper fiber uses electrical pulses rather than light and is less expensive than optical fiber.

! Important

HP does not support copper cable. Current HP Fibre Channel implementations use fiber optic cabling.

Optical fiber cable is composed of two active elements, the core and the cladding, as shown in Figure 8-9.

Figure 8-9. Optical fiber components.

The core is the inner, light-carrying member. It carries about 80% of the light. The cladding is the middle layer. It confines the light to the core because it has a higher reflective index than the core.

The outer layer of the optical fiber is known as the buffer or coating. It serves as a shock absorber to protect the core and cladding from damage.

Optical fiber can be either single-mode or multimode. The main distinction between single-mode fiber and multimode fiber is the physical diameter of the core.

8.4.1 Single-Mode Optic Fiber

Single-mode fiber has the highest bandwidth and lowest loss performance. The core is so small that only a single mode of light can enter it. Therefore, the chromatic and modal dispersion are greatly reduced or eliminated. Single-mode fiber is shown in Figure 8-10.

Figure 8-10. Single-mode fiber.

The information-carrying capabilities of the single-mode fiber are infinite. Single-mode fiber supports speed of tens of gigabits per second and can carry many gigabit channels simultaneously. Each channel carries a different wavelength of light without any interference.

Single-mode fiber is the preferred medium for long-distance telecommunications. It is also useful in networks for interbuilding runs and high-speed backbones. Applications for single-mode fiber to the desk are not anticipated.

Networking and data communications use a single-mode step-index fiber to carry communications.

8.4.2 Multimode Fiber

The diameter of multimode fiber is large enough to allow multiple streams of light to travel different paths from transmitter to receiver. Shortwave lasers are used with multimode fiber for transmitting over medium distances. Multimode fiber is shown in Figure 8-11.

Figure 8-11. Multimode fiber.

The following are the most popular multimode fibers for networking, FDDI, and Ethernet:

• 50mm/125mm (primarily used in Europe and most common worldwide)

• 62.5mm/125mm (primarily used in the United States)

Multimode fiber carries information at 100MB/s (1062.5Mb/s) and up to 500m.

The two types of multimode fiber are step-index fiber and graded-index fiber.

8.4.2.1 MULTIMODE STEP-INDEX OPTIC FIBER

Step-index fiber has a high modal dispersion. See Figure 8-12 to understand how step-index fiber works.

Figure 8-12. How multimode step-index fiber works.

The core material (n1) is not graded, so it does not focus the light beam to follow the axis (axial transmission) of the core. Different light rays leaving the source simultaneously reach the destination at significantly different times, reducing the bandwidth of the fiber and its distance.

Step-index fiber has the following characteristics:

• It is relatively inexpensive.
  
  
• It decreases bandwidth and distance.
  
  
• It has a higher modal dispersion than graded-index fiber.
  
  
• It is seldom used in networking and data communications.
  
  

8.4.2.2 MULTIMODE GRADED-INDEX FIBER

Grading of the fiber-optic core focuses the light beam closer to the axis of the core. The light rays travel closer to the axis, which reduces travel distance and synchronizes arrival rates.

Figure 8-13 shows multimode graded-index fiber.

Figure 8-13. How multimode graded-index fiber works.

Grading is achieved by varying the chemical composition of the core material (n1).

Key characteristics of graded-index fiber include the following:

• It is more expensive than step-index fiber. Manufacturing advances and wide adoption of fiber-optic cabling have significantly reduced its manufacturing costs.

• It increases bandwidth and distance.

• It has a lower modal dispersion than step-index fiber because it provides more accurate signal transmission.

• It is frequently used in networking and data communications. (Nearly all multimode fibers used in networking have a graded index.)

With graded-index fiber, data input slows down when the data enters the cable and accelerates when it exits the cable.

Additional information about graded-index fiber can be found at http://www.fibrechannel.org.

8.4.3 Cable Safety Requirements

Because the light produced from the laser has the potential to damage the eye, there are established regulations for the amount of optical power that can be used. For example, the Class 1 laser safety standard mandates the amount of allowable safety levels.

The Fibre Channel industry has adopted the Open Fiber Control technology to meet these safety requirements. Open Fiber Control is the ability of line driver components to reduce the laser power output when they recognize that their light circuits are broken.

Note

Because of their signaling differences, Open Fiber Control cables and non-Open Fiber Control cables are not compatible.

8.4.4 Attenuation

Attenuation is the loss of power as a signal travels over a distance. It is measured in decibels per kilometer (dB/km).

For commercially available fibers, attenuation ranges from approximately 0.5dB/km for single-mode fibers to 1000dB/km for large-core plastic fibers.

Attenuation is lessened with higher-quality, more-expensive, single-mode fibers, and it is greater with lower quality, less expensive, multimode fibers.

Power loss can result from any of the following conditions:

• Light absorption caused by material impurities

• Light scattering caused by material impurities, defects at the core/cladding interface, or scattering of the molecules of the medium (silica)

• Macro bends (cable bends past the specified radius) and micro bends (cable wrapping or squeezing)

• Scattering and reflection at cable splices

Attenuation varies with the wavelength of light. There are three low-loss windows, measured in nanometers:

• 780 to 850nm (HP recommends 790nm) Most widely used because 780 to 850nm devices are inexpensive (shortwave)

• 1200 to 1300nm (HP recommends 1250nm) Lower loss, but a modest increase in cost for LEDs (longwave)

• 1550nm Mainly of interest to very long-distance (VLD) telecommunications applications

Attenuation in optical fiber is mainly the result of scattering and absorption within the core material. Dirty or poorly made connections will also diminish the light intensity. Therefore, loss occurs over long distances and at connections. Any connection, regardless of quality, will induce an insertion loss.

8.4.5 Macro Bending

Macro bending is the physical bending of the fiber cable past the specified radius (1.25 to 1.50 inches). Figure 8-14 illustrates this concept.

Figure 8-14. Macro bending.

As the fiber exceeds the specified radius, the light loses some particles and attenuation increases.

8.4.6 Micro Bending

Micro bending losses occur when the beam does not follow an entirely linear path, such as when a cable is wrapped with a tie or the cladding is squeezed, as illustrated in Figure 8-15.

Figure 8-15. Micro bending.

Micro bends in the axis of an optical fiber can dramatically reduce the transmission of light through it.