12 de abr de 2010

Informação basica sobre redes de fibra optica

Fiber optic LAN and WAN cable basics pop quiz answers: "

1. Which of the following is a critical safety concern when working with or around LAN or WAN fiber optic cables?

  • Hearing loss
  • Burns to the hands
  • Permanent eye damage (Correct)
  • Exposure to noxious fumes

There are two significant safety concerns when working with fiber optic cable–the glass shards often generated during fiber termination (unless you’re working with plastic optical fiber) and permanent eye damage. Whether you’re using a laser to transmit data or make a point during a presentation, you should never point the beam at someone’s eyes. The wavelength of light used by most networking equipment falls outside the spectrum of light visible to the human eye making it difficult to determine that a cable is active with a mere visual inspection. Therefore, you should never look into the end of a fiber cable.

When working with fiber optic cabling, you should always disconnect the cable at both ends, wear safety glasses rated for the wavelength of light you’re working with, check each fiber with an optical power meter.

2. What are the two general types of fiber optic cable found in current networking environments?

  • Red and Green
  • Singlemode and Multimode (Correct)
  • High-frequency and Low-frequency
  • Narrowband and Wideband

There are two general types of fiber optic cable found in LAN and WAN networking equipment–singlemode and multimode. The fiber core used in singlemode cable is significantly smaller (typically between 8 and 10 microns) than the core used is multimode cable (usually 50 or 62.5 microns but sometimes larger). The smaller core size allows only a single ray of light (or mode) to travel through the cable.

Multimode cable can use either an LED or laser light source and operate at wavelengths of 850 nm and 1,300 nm. Singlemode cable uses laser sources exclusively and operates at wavelengths of 1310 nm or 1550 nm. Singlemode cabling is less susceptible to attenuation than multimode cable, and is used to transmit data at extremely high bandwidth or over great distances. Most campus and single-building fiber networks use multimode cable.

Note: While the cores of singlemode and multimode cables have traditionally been made from glass and in some cases quartz, multimode cables can also have a plastic core–usually made from acrylic or a perfluorinated polymer. Plastic optical fiber (POF) has a much larger core (100, 200, or often 980 microns) than traditional glass optical fiber (GOF). POF is more susceptible to attenuation than glass fiber and is often used for short runs, less than 50 meters. POF is designed to work with light sources operating at wavelengths visible to the human eye, typically 650nm. The tools and equipment need to install and use POF is typically much cheaper than GOF. The installation process is also much easier.

3. True or False: All fiber optic cables have the same type of physical connector?

  • True
  • False (Correct)

Ethernet equipment manufacturers long ago standardized around the 8P8C connector–referred to as an RJ45 connector by most IT professionals. Unfortunately, the same cannot be said for the makers of fiber optic networking equipment. Fiber optic cables can be terminated with a variety of connectors, such as:

  • FC (Ferrule Connector)
  • LC (SFF Lucent Connector)
  • MIC FDDI (Media Interface Connector - Fiber Distributed Data Interface)
  • MPO (Multi-fiber Push-on)
  • MRRJ (Mechanical Transfer Registered Jack)
  • ST (Straight Tip)
  • SC (Subscriber Connector or Standard Connector)
  • SFF (Small Form Factor)

4. Which of the following is NOT a light source used with LAN or WAN fiber cable?

  • LED
  • GSM (Correct)
  • FP lasers

There are basically two categories of light sources used with fiber optic networking equipment–LED (light emitting diode) and laser. LED Within these two broad categories, there are several specific types of emitters, such as:

  • Surface-emitting LED
  • Edge-emitting LED
  • FP laser (Fabry Perot laser)
  • DFB laser (distributed feedback laser)
  • VCSEL (vertical cavity surface emitting laser)

5. Attenuation, or transmission loss, in fiber optic cables, beyond the intrinsic absorption of the fiber material, can be due to which of the following?

  • Density fluctuations on impurities in the cable core
  • Misaligned connections
  • Bends in the cable
  • All of the above (Correct)

Several factors can cause transmission loss, or attenuation, in fiber optic cables. The glass molecules in the fiber core may absorb a very small amount of light energy. The manufacturing process may cause density fluctuations or leave impurities or air bubbles in the fiber core. When light travels across a fiber connection, loss can occur because of the differences in the refractive index between the core and the air in the gap between the cores being coupled. Loss can occur with connections that aren’t properly aligned or when the core diameters are mismatched. If fiber cables are bent too sharply, loss can occur.

6. What type of coupling loss occurs when each fiber core is perfectly seated and aligned and has the same diameter, but the transmitting fiber has a light cone larger than the receiving fiber can take?

  • Gap or space between the cable ends
  • Core size mismatch
  • Angular misalignment
  • Numerical aperture mismatch (Correct)

As noted above, there are many potential causes of transmission loss or attenuation. When two cables are spliced together or joined with mated connection, several types of coupling loss can occur, such as:

  • Core size mismatch: Occurs when the core of one cable is smaller/larger than the other.
  • Numerical aperture loss: Occurs when the transmitting fiber core emits a larger cone of light than the receive fiber core can accept. This can occur even when the cores are exactly the same size and perfectly aligned.
  • Angular misalignment: Happens when the cable ends aren’t completely flush, but one or both are tilted slight away from each other.
  • Axis/Lateral misalignment: Occurs when the fiber cores are completely flush and the same size, but not aligned on the same central axis.

Gap or space between the cable ends: Occurs when a connector is not completely seated and a space exists between the two cable ends.

7. Which of the following fiber optic testing devices uses light pulses to estimate a fiber cable’s length and measure attenuation?

  • Flashlight
  • Visual Fault Locator (VFL)
  • Optical Time Domain Reflectometer (OTDR) (Correct)
  • Time Domain Reflectometer (TDR)

A variety of devices can be used to test fiber optic cables, including:

  • Light sources: Flashlight, LED, or laser used to check the continuity and polarity of a cable
  • Visual Fault Locators (VFL): A very powerful light source, such as a Class 2 laser, used to check the continuity and polarity of a fiber
  • Fiber optical power meters: Devices that measure absolute light power in dBm
  • Optical Time Domain Reflectometers (OTDR): which sends a series of light pulses into a fiber and extracts, from the same end, light that is scattered and reflected back. The returning light is measured as a function of time, allowing the OTDR to determine optical return loss and locate events in the fiber (breaks, bends, connectors, splices). The OTDR can then determine the distance to each event from the end of the fiber to which it is connected.

8. Which of the following devices is often used in conjunction with an overfilled light source when testing for attenuation on multimode fiber cables?

  • VFL
  • Mandrel (Correct)
  • TSR
  • Flashlight

Using a mandrel when performing power through testing on multimode fiber cable produces more accurate results. When using an overfilled light source to fully flood the fiber, some of the light will travel straight down the cable’s fiber core and other high-order modes will enter the cladding. Over longer runs, these high-order modes will attenuate and not appear at the cable’s far end. But in shorter cables, such as the test jumper used during the reference step of an attenuation test, these high-order modes do not completely dissipate. A mandrel is to attenuate the high-order modes in the cladding when setting the reference power level, and remains in place for the actual cable testing.

Note: Many people marked VLF and TSR as the correct answers, 41 percent and 34 percent receptively. A visual fault locator (VFL) is basically a device (often a laser) that produces light in the visible spectrum and can be connected to the end of a fiber cable. A VFL can help you detect cable faults, such as breaks, but it can’t perform true attenuation testing–determining signal loss across a cable connection. The term TSR has nothing to do with fiber optic cable testing. Although the HellermannTyton company uses the TSR acronym when referring to the HellermannTyton surface raceway (a molded conduit through which cable can be run), I wouldn’t categorize it as related to cable testing.

9. An OTDR can locate which of the following characteristics within a fiber run?

  • Connectors
  • Splices
  • Bends
  • All of the above (Correct)

As I mention above, an OTDR or Optical Time Domain Reflectometer can be used to measure optical return loss and determine the presence and location of events in the fiber (breaks, bends, connectors, splices).

10. Which type of fiber cable splicing welds fibers together using a small electric arc?

  • Mechanical splicing
  • Fusion splicing (Correct)
  • Electric splicing
  • Hot splicing

Splicing is the process of permanently joining two fiber cables. There are two types of splices–mechanical and fusion. Mechanical splices use specially designed connectors that align the fiber cores and physically hold the cable ends securely in place. During fusion splicing, heat is applied to the fiber cores fusing them together. Fusion splicers typically use an electric arc to weld the two fiber cores, but they can also use CO2 lasers or gas flames.

The equipment required to make a mechanical splice (normally a cleaver, cable stripper, and the connector itself) is significantly cheaper (although the per connection cost is higher) than the hardware required for fusion splicing. Fusion splicers can cost several thousand dollars (US). Despite the high initial cost, fusion splices generally experience less signal loss (<0.1 dB) than mechanical splices (<0.5 dB).

Learn more about fiber optic cabling

For more information on fiber optic cabling, check out the following resources:


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