This document clarifies under what circumstances a Synchronous Optical NETwork (SONET) link needs an attenuator to reduce the signal strength and protect the receive-side optics. This document provides the context to help you understand recommended formulas in order to calculate power budgets. This document explains the terms attenuation, wavelength, dispersion, and power, as well as reviews the formulas.
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Attenuation is a measure of the decay of signal strength or loss of light power that occurs as light pulses propagate through a run of MultiMode Fiber (MMF) or Single-Mode Fiber (SMF). Typically measurements are defined in terms of decibels or dB/km.
Several intrinsic and extrinsic factors lead to attenuation. Extrinsic factors include cable manufacturing stresses, environmental effects, and physical bends in the fiber. Intrinsic factors are described in this table:
|Scattering||Microscopic non-uniformities in fiber. Scattering leads to attenuation of light energy.||Causes almost 90 percent of attenuation. Increases sharply with shorter wavelengths.|
|Absorption||Molecular structure of the material, impurities in the fiber such as metal ions, OH ions (water), and atomic defects such as unwanted oxidized elements in the glass composition. These impurities absorb the optical energy and dissipate the energy as a small amount of heat. As this energy dissipates, the light becomes dimmer.|
Attenuation introduced by the fiber itself varies with the length of the cable run and with the wavelength of the light. This section discusses wavelengths.
The term wavelength refers to the wave-like property of light. It is a measurement of the distance that a single cycle of an electromagnetic wave covers as it travels through a complete cycle. Wavelengths for fiber optics are measured in nanometers (the prefix "nano" means one-billionth) or microns (the prefix "micro" means one-millionth).
The electromagnetic spectrum consists of light that is visible and non-visible (near-infrared light) to the human eye. Visible light ranges in wavelengths from 400 to 700 nanometers (nm) and has very limited uses in fiber optic applications due to the high optical loss. Near-infrared wavelengths range from 700 to 1700 nm. Most modern fiber optic transmission takes place at wavelengths in the infrared region.
In a discussion of wavelengths, you need to understand these two important terms:
Peak or center wavelength—Wavelength at which the source emits the most power and experiences the least amount of loss.
Spectral width—A light-emitting diodes (LED) or laser emits all light ideally at the peak wavelength, where the least amount of attenuation takes place. However, in reality, the light is emitted in a range of wavelengths centered at the peak wavelength. This range is called the spectral width.
The most common peak wavelengths are 780 nm, 850 nm, 1310 nm, 1550 nm, and 1625 nm. The 850 nm region, referred to as the first window, was used initially because this region supports the original LED and detector technology. Today, the 1310 nm region is popular because at this region, there is dramatically lower loss and lower dispersion. The 1550 nm region also is used today and can avoid the need for repeaters. Generally, performance and cost increase as wavelength increases.
MMF and SMF use different fiber types or sizes. For example, SMF uses 9/125 um and MMF uses 62.5/125 or 50/125. The different size fibers have different optical loss dB/km values. Fiber loss depends heavily on the functional wavelength. The practical fibers have the lowest loss at 1550 nm and the highest loss at 780 nm with all physical fiber sizes (for example, 9/125 or 62.5/125).
Dispersion describes light pulses that spread as they travel down the optical fiber. The two key types of dispersion are chromatic dispersion and modal dispersion.
Power defines the relative amount of optical power that can be coupled into an optical fiber with a LED or laser. The power level of a transmitter must be neither too weak nor too strong. A weak source provides insufficient power to transmit the light signal through a usable length of optical fiber. A strong source overloads a receiver and distorts the signal.
A Power Budget (PB) defines the amount of light necessary to overcome attenuation in the optical link and meet the minimum power level of a receiving interface. The proper operation of an optical data link depends on modulated light that reaches the receiver with enough power to be correctly demodulated.
This table lists the factors that contribute to link loss and the estimate of the link loss value attributable to those factors:
|Link Loss Factor||Estimate of Link Loss Value|
|Higher order mode losses||0.5 dB|
|Clock recovery module||1 dB|
|Modal and chromatic dispersion||Dependent on fiber and wavelength used|
|Fiber attenuation||1 dB/km for multimode (0.15-0.25 dB/km for single-mode)|
The LED used for a multimode transmission light source creates multiple propagation paths of light, each with a different path length and time requirement to cross the optical fiber which causes signal dispersion (smear). Higher Order Loss (HOL) results when light from the LED enters the fiber and radiates into the fiber cladding. A worst-case estimate of Power Margin (PM) for MMF transmissions assumes minimum Transmitter Power (PT), maximum Link Loss (LL), and minimum Receiver Sensitivity (PR). The worst-case analysis provides a margin of error; not all of the parts of an actual system operate at the worst-case levels.
The PB is the maximum possible amount of power transmitted. This equation lists the calculation of the power budget:
PB = PT - PR PB = -20 decibels per meter (dBm) - (-30 dBm) PB = 10 dB
The power margin calculation is derived from the PB and subtracts the link loss:
PM = PB - LL
If the power margin is positive, or greater than zero, the link usually works. It is possible that links whose results are less than zero have insufficient power to operate the receiver.
For a list of maximum transmit and receive dB levels for many Cisco optical hardware products, refer to the Fiber Loss Budgets document. If your particular hardware is not listed or to make sure you obtain the most accurate information, refer to the configuration guide for your particular interface. Apply the recommended formulas or use an optical meter.
Here is an example of multimode PB calculated based on these variables:
Length of multimode link = 3 kilometers (km) 4 connectors 3 splices HOL Clock Recovery Module (CRM) Estimate the PB as follows: PB = 11 dB - 3 km (1.0 dB/km) - 4 (0.5 dB) - 3 (0.5 dB) - 0.5 dB ( HOL) - 1 dB (CRM) PB = 11 dB - 3 dB - 2 dB - 1.5 dB - 0.5 dB - 1 dB PB = 3 dB
The positive value of 3 dB indicates that this link has sufficient power for transmission.
This example has the same parameters as the Sufficient Power for Transmission example, but with an MMF link distance of 4 km:
PB = 11 dB - 4 km (1.0 dB/km) - 4 (0.5 dB) - 3 (0.5 dB) - 0.5 dB (HOL) - 1 dB (CRM) PB = 11 dB - 4 dB - 2 dB - 1.5 dB - 0.5 dB - 1 dB PB = 2 dB
The value of 2 dB indicates that this link has sufficient power for transmission. Due to the dispersion limit on the link (4 km x 155.52 MHz > 500 MHz/km), this link does not work with MMF. In this case, SMF is the better choice.
This example of a SMF PB assumes two buildings, 8 km apart, are connected through a patch panel in an intervening building with a total of 12 connectors:
Length of single-mode link = 8 km 12 connectors Estimate the power margin as follows: PM = PB - LL PM = 13 dB - 8 km (0.5 dB/km) - 12 (0.5 dB) PM = 13 dB - 4 dB - 6 dB PM = 3 dB
The value of 3 dB indicates that this link has sufficient power for transmission and is not in excess of the maximum receiver input power.
Alternately, you can use an optical power meter to measure the signal strength. Make sure you set the wavelength to be the same as the interface and then do not go outside of the range given for that specific line card.
For more information, refer to these publications:
T1E1.2/92-020R2 ANSI, the Draft American National Standard for Telecommunications entitled Broadband ISDN Customer Installation Interfaces: Physical Layer Specification.
Power Margin Analysis, AT&T Technical Note, TN89-004LWP, May 1989.
You can connect SMF interfaces back-to-back within close proximity, such as in a lab environment or over an intra-Point-of-Presence (POP) link. However, take extra care not to overload a receiver, particularly with long-reach optics. Cisco advises you to insert at least a 10-dB attenuator between the two interfaces. Review the engineering specifications for the input optical receiver of the associated card to provide an input optical range window of the optical light level. Most vendors recommend that you attenuate to the mid-range of the receiver optical light level range.
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