Link budget loss in fibre channel connections


This Tip discusses the concept of link loss budget and how to calculate decible (dB) loss in configurations that use fibre channel connections.


Link loss budget
The term "link loss budget" is used to specify how much loss (attenuation) in signal strength can be tolerated and still allow the receiver to interpret an accurate signal. Many factors can reduce (attenuate) the signal strength on its journey through a network, such as:
  • Fiber distance (length)
  • Fiber patch panel connections
  • Dirty or damaged fiber connectors
  • Broken, bent, or crimped fiber cables
  • Fiber splices
  • Fiber switches
Fiber channels will not function properly if too many of these combined factors attenuate the signal. It is important that none of the fibers have a link loss that exceeds the link budget of the fiber channel specifications. The signal strength loss is measured in decibels (dB).

dB or dBm - A terminology primer
It's important to understand link budget terminology. The decibel (dB) is a convenient way of expressing an amount of signal loss or gain within a system, or the amount of loss or gain caused by some component of a system. When signal power is lost, you never lose a fixed amount of power. Instead, you lose a portion of power by one half, one quarter, and so on. This makes it difficult to add up the lost power along a signal’s path through the network if measuring signal loss in watts.

For example, suppose a signal loses half its power through a bad connection, and then loses another quarter of its power later on through a bent cable. To calculate the total loss, you cannot simply add 1/2 plus 1/4; instead, you must multiply 1/2 by 1/4. This makes calculating large network dB loss time-consuming and difficult.

Decibels, though, are logarithmic. This allows you to easily calculate the total loss/gain characteristics of a system just by adding them up. Keep in mind, though, that they scale logarithmically. That is, if your signal gains 3dB, the signal doubles in power. If your signal loses 3dB, the signal halves in power.

It's also important to remember that the decibel is a ratio of signal powers. Therefore, you must have a reference point. For example, you can say "there is a 5dB drop over that connection." But you cannot say "the signal is 5dB at the connection." A decibel is not a measure of signal strength, but a measure of signal power loss or gain.

A decibel milliwatt (dBm) is a measure of signal strength. People often confuse dBm with dB, but they are different: a dBm is the signal power in relation to one milliwatt. A signal power of 0 dBm is one milliwatt, a signal power of 3 dBm is 2milliwatts, 6 dBm is 4 milliwatts, and so on.

Also, do not be misled by minus signs; they have nothing to do with signal direction. The more negative the dBm goes, the closer the power level gets to zero. For example, -3 dBm is 0.5 milliwatts, -6 dBm is 0.25 milliwatts, and -9 dBm is 0.125
milliwatts. Thus, a signal of -30 dBm is very weak.

Calculating dB loss
In calculating the network dB losses, three general rules are applied:
  • 9-micrometer single-mode fiber at 1550nm (long wavelength laser) has an average loss of 0.3 dB per km.
  • Patch panels have a loss of approximately 0.5 dB per fiber connection.
  • Add a 10% margin to total dB loss for fiber/connector "end-of-life" and repair.
For example, assume a configuration with sites 10 km apart and two patch panel fiber connections at each site. The network dB loss calculations for such a configuration are as follows:
  • dB loss due to fiber length: 0.3 dB/km x 10 km = 3.0 dB
  • dB loss due to patch panel fiber connections: 0.5 dB x 4 = 2.0 dB
  • 10% margin = 0.5 dB
Therefore, the total estimated network dB loss = 5.5 dB.

Special Notices

This material has not been submitted to any formal IBM test and is published AS IS. It has not been the subject of rigorous review. IBM assumes no responsibility for its accuracy or completeness. The use of this information or the implementation of any of these techniques is a client responsibility and depends upon the client's ability to evaluate and integrate them into the client's operational environment.


Publish Date
08 November 2002

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