Light It Up
Fiber Transmissions and Applications
Several grades, speeds and applications exist for fiber optic network cabling plants. Two factors will govern the speeds for your fiber: the grade of fiber and the light source used to launch the data onto the fiber. This can also increase depending on which type of multiplexing is used to carry multiple signals across the same fiber strands.
Fiber applications include fiber backbone operations, fiber to the carrier options and fiber to the desktop operations. There is also a growing number of fiber to the premise (FTTP) applications including fiber-distributed video. Fiber based storage and network interfaces are readily available. The fibre channel industry association has been instrumental in setting operating parameters for fiber-attached storage in data centers. Regardless of your intended use for fiber, the characteristics, losses and bandwidth are critical to the success of your fiber network.
Transmitting on Fiber
The combination of fiber light source, grade of fiber and core diameter of the fiber will determine not only the functional distance of the channel, but also the speed of the communications. In the recent past, multimode fiber has seen a transition from 62.5/125 (core/cladding) micron fiber in various grades to 50/125 micron fiber, also available in various grades, including the highest performing laser optimized fiber. Multimode fiber is typically the choice for shorter distances. The light is carried is several paths or modes. A mode is the transmission of light in a steady state. Singlemode fiber allows for a single path/mode and is suitable for longer-range applications. The active electronics supporting multimode fiber are less costly than for singlemode.
The speed of light in a fiber is measured by its index of refraction. This is an equation that compares the speed of light in one medium to its fastest speed in a vacuum. This light travels in the non-visible spectrum, typically 850nm, 1300nm or 1550nm. The visible spectrum ends at roughly 750nm. This spectrum can carry both analog and digital signals. The devices that launch the light source convert those signals to light pulses within the spectrum required for transmission. At the other end, another device detects the light pulses within each mode and converts them back to analog or digital signals to be understood by its attached device.
The speed of fiber is measured in hertz or cycles per second. Each cycle equates to one pulse or lightwave. One hertz equals one cycle. As fiber transmits at a very high rates of speed, we measure the pulses/cycles that a device launches onto the fiber in megahertz (millions of hertz per second). Because light does not degrade over distance as rapidly as electrical signals, the attenuation or loss of the signal is lower than it's copper counterparts. This allows the light waves to travel greater distances.
Fiber Loss and Signal Degradation
Like copper, fiber signal loss or attenuation is measured in decibels (dB). Fiber attenuation will increase with each connector or splice. Typically the splice loss is around 0.2 dB per splice. However, poor terminations can increase this number. A fiber loss budget compares actual loss to the budget (or acceptable loss) based on the number of splices in the channel. In order to measure this loss, a power meter and light source must be used. Testing a fiber strictly with an Optical Time Domain Reflectometer (OTDR) provides characterization of the fiber segment, but does not provide definitive performance.
There are two types of attenuation that are encountered in optical fiber cables: intrinsic and extrinsic. Intrinsic attenuation is inherent to the fiber and is introduced during the manufacturing process. An example of this would be impurities or inhomogineities within the glass. These causes the light signals to either be absorbed or scattered and is the reason that some fibers support greater application distances than others. Advancements in manufacturing have introduced a new grade of multimode fiber known as laser-optimized fiber.
This fiber incorporates two essential manufacturing improvements. First is the elimination of the anomalies listed above by reducing impurities in the fiber core. Second is increased control of the index of refraction which reduces modal dispersion ensuring that all modes arrive at the receiver at essentially the same time. The combination of these improvements greatly increases the bandwidth capacity of the fiber which results in the support of higher speed applications including 10Gb/s transmissions as well as increased transmission distances. TIA refers to this as laser-optimized fiber while ISO/IEC refers to this as OM3 grade fiber.
Extrinsic attenuation is introduced during cable handling. Examples would be small mechanical stresses (microbends) or bend radius violations (macrobends) and result is light being refracted out of the core. In all fiber installations, the manufacturer-specified bend radii limits must be respected.
In fiber, the signals and pulses must be understood by the receiver at each end. Running too long a fiber channel can cause errors, as can excessive splices, poor fiber quality and poor installation. Any anomaly that keeps a receiver from registering the pulses will equate to a bit error.
Launching the Light
In order to send a signal over a fiber optic cable you need a light source. This can be an LED (Light Emitting Diode), a Laser (Light Amplification by Stimulated Emission of Radiation) or a VCSEL (Vertical Cavity Surface Emitting Laser). Lasers and VCSELs provide a stronger, more focused light source and therefore can transmit greater distances than their LED counterparts. Equipment that generates the signal via the latter two technologies is more expensive than an LED source.
Regardless of the type of fiber, the act of placing the light pulses onto the fiber is called the "launch". The launch method can vary from an overfilled launch to restricted mode launch. As discussed previously, the light path is called the mode. In an overfilled launch, the light introduces a signal that is greater in size than the actual core of the fiber. This allows all modes to become excited. In a restricted mode launch, a smaller core of light is introduced which excites only certain modes on the fiber. In singlemode, only a single path or mode is excited.
Within the fiber, light pulses can spread over distance, called dispersion. When the pulses overlap, they can limit the receiver's ability to register distinct pulses, thereby limiting the bandwidth on a fiber. Light travels at different speeds in different colors as well. In order to counteract some dispersion loss, the light source can provide what is called a restricted launch, typically used for higher speed applications. Rather than filling all modes within a fiber with light, only certain modes are excited thereby restricting the range of pulses and the affects of dispersion.
At longer wavelengths for gigabit speeds running on older grades of 62.5/125 micron multimode fiber, the restricted launch causes differential mode delay (the signals do not arrive at the receiver at the same time). For these applications, mode conditioning patch cords must be used. These cords provide an offset so that the light does not enter directly in the center of the fiber core. By offsetting the beam to an area outside the center of the core, dispersion is minimized. One set of restricted launch cords must be used at each end of the system.
The bandwidth of the fiber is the information transmission capacity of the fiber. It is inversely proportional to the amount of dispersion. Thus, the extent to which dispersion can be controlled essentially determines the usable bandwidth of the fiber.
ISO/IEC 11801 Ed2.0 defines three optical types of multimode fiber. OM1 primarily comprises historic 62.5/125 micron fiber. OM2 has an effective bandwidth of 500 MHz·km at both wavelengths and represents standard grade 50/125 micron fiber. OM3 has 1500/500 MHz·km bandwidth for overfilled launches and 2000 MHz·km with a restricted mode launch and is referred to by TIA/EIA as "laser optimized" 50/125 micron fiber.
ISO Multimode Bandwidth Specifications
|Minimum Modal Bandwidth
MHz . km
|Optical Fiber Type||Core Diameter (μm)||Wavelength (nm)||Overfilled Launch (OFL)||Restricted Mode Launch (RML) 850 nm|
|OM1||50 or 62.5||850
|OM2||50 or 62.5||850
Note: Effective laser launch bandwidth is assured using differential mode delay (DMD) as specified in IEC/PAS 6073-1-49.
TIA Multimode Bandwidth Specifications
|Minimum Modal Bandwidth
MHz . km
|Optical Fiber Type||Wavelength (nm)||Overfilled Launch (OFL)||Restricted Mode Launch (RML)|
|50/125μm Multimode||1300||500||Not Required|
Understanding the Equipment Side
Each piece of active electronics will have a variety of light sources used to transmit over the various types of fiber. The distance and bandwidth will vary with light source and quality of fiber. In most networks, fiber is used for uplink/backbone operations and connecting various buildings together on a campus. The speed and distance are a function of the core, modal bandwidth, grade of fiber and the light source, all discussed previously. For gigabit transmissions, the IEEE approved distances are shown in the following table.
|GBIC||Wavelength (nm)||Fiber Type||Core Size (micron)||Modal Bandwidth (MHZ/km)||Cable Distance|
|1000BASE-SX||850||MMF||62.5||160||722 ft. (220m)|
|62.5||200||920 ft. (275 m)|
|50||400||1640 ft. (500m)|
|1000BASE-LX||1300||MMF¹||62.5||500||1804 ft. (550m)|
|50||400||1804 ft. (550m)|
|9/10||N/A||6.2 miles (10km)|
|1000BASE-EX||1550||SMF||9/10||N/A||43.4 to 62 miles
(70 to 100km)
¹ A mode conditioning patch cord is required.
As you can see, depending on the type of fiber and the type of light source, the distances supported vary from 722 feet to multiple kilometers. Some are singlemode only as noted. The cost of each option increases with distance due to the type of light source. Each of these distances are maximums based on the type, quality and installation of the fibers. Additional loss introduced can vary the distances greatly. The GBIC (Gigabit Interface Converters) listed in column one are listed as SX (short haul), LX (long haul) and EX (extended haul). It is also important to note that using singlemode fiber for short distances can cause the receiver to be overwhelmed and an inline attenuator may be needed to introduce attenuation into the channel.
*These interfaces are also seen noted with an X in place of the R but can be used interchangeably.
The interface for 10Gb/s fiber is called a Xenpack, as opposed to GBIC for gigabit. Both 10GBASE-SR and 10GBASE-LR have a counterpart for wide area communications to allow them to connect to SONET networks at 9.584640 Gb/s (OC-192) through its wide area interface. These interfaces are known as 10GBASE-SW and 10GBASE-LW respectively.
With gigabit to the desktop becoming commonplace, 10Gb/s backbones are also becoming more common. The SR interfaces are also becoming common in data center applications and even some desktop applications. As you can see, the higher quality fiber (or laser optimized fiber) provides for greater flexibility for a fiber plant installation. Although some variations (10GBASE-LRM and 10GBASE-LX4) support older grades of fiber to distances 220m or greater, the equipment is more costly. In many cases, it is less expensive to upgrade fiber than to purchase the more costly components that also carry increased maintenance costs over time.
Part of Siemon's 10G ip™ solution, the XGLO® fiber system is ideal for next generation backbone or fiber-tothe- desk applications. XGLO cable assemblies feature premium fiber meeting IEEE 802.3 10 Gigabit Ethernet Standard as well as IEC-60793-2-10 and TIA-492AAAC specifications for laser bandwidth Differential Mode Delay (DMD) specifications. XGLO utilizes laser-optimized fiber for superior transmission performance for 1G or 10G Ethernet applications.