Understanding Fiber Link Loss budgets

Hi Guys!

Thanks to a co-worker of mine I now have a pretty good understanding of fiber link loss budgets and how to calculate them (Or do I? Post below if you think i have made an error in some calculations or understanding, I love hearing from you guys!)

Let's briefly chat about what we are trying to calculate here and why it is important.


As you may know, fiber optic cable works by using light from a transmit and receive pair on a optic cable (although in some topologies this is done on a single strand of fibre using a TDM method but we will leave that out of this discussion)

As light travels over longer distances it dissipates and becomes weaker, sometimes it is so "dull" that the photosensitive receiver at the other end cannot see the signal, conversely sometimes the light source is so bright the receiver is overloaded, like staring into a bright light or the sun.

So, when choosing fiber optics and deciding if you need an attenuator (which is like sun glasses for the SFP) or a power booster, you need to take into account the distance of the fibre, and from there you can calculate the link loss, then by using the cisco data sheets you can calculate the exact SFP you will need.

Your best bet when performing this is to speak to your fiber optics cable installer and ask for the OTDR report, this report contains information from the installer as to his measured span loss, the loss is normally listed in DB.

If you don't have this report, you can only really guess based on the average loss per km over typically installed fiber, note that diffirent installers produce varying quality when installing fibre so this is not the preferred method, seek out an accurate OTDR report if you can! The average loss per KM for a fibre run is between 0.2 to 0.5 per KM, although higher has been reported! Splices on the fibre also introduce a certain amount of loss between 0.2 to 0.5, and finally patch panels can also add similiar levels of loss, hence your always better off with an OTDR report!

Let's talk a little more about the OTDR report, first of all you should check to make sure that the wavelength that the installer used when he ran the test is the same wavelength that your SFP is going to use, lower wavelength has less power therefore it travels shorter distances.

Once you have this info, let's perform our calculations!

So first of all, you need to know the maximum transmit and receive power of your SFP:

Let's take a typical example which is the ER module: (SFP-10GB-ER)
(all values are in the datasheet) http://www.cisco.com/en/US/prod/collateral/modules/ps5455/data_sheet_c78-455693.html


Transmit Power

Maximum: 4.0
Minimum: -4.7

Receive power:
Maximum: -1
Minimum: -15.8


If you compare these values to a shorter range such as the LR: (SFP-10GB-LR)

Transmit Power

Maximum:0.5
Minimum:-8.2

Receive power:
Maximum:0.5
Minimum:-14.4

(all measurements in dBm)
 You can see from the above that the ER has signifcantly more transmit power, interestingly the receive power on the LR as a maximum is quite high, I suspect this might be to prevent people accidentally burning out short-range LR modules, I would be keen to hear more from someone on this.


So how do we calculate if our SFP is going to be suitable? Let's assume you have an OTDR report and the report says that the loss is going to be 5.6 dBm, do the following:

Maximum transmit power minus Loss in OTDR Report = X
Minimum Transmit power minus Loss in OTDR Report = Y

As long as X and Y are below the maximum receive power but ABOVE the minimum receive power, you will be fine, lets work this out:

0.5 - 5.6 = -5.1
-8.2 - 5.6 = 13.8

As you can see we are safely in the margins.

You can view at any time on a SFP with DOM support the current details of its transmit and receive power:


show int eth1/50 transceiver details
Ethernet1/50
    transceiver is present
    type is 10Gbase-LR
    name is CISCO-FINISAR

           SFP Detail Diagnostics Information (internal calibration)
  ----------------------------------------------------------------------------
                Current              Alarms                  Warnings
                Measurement     High        Low         High          Low
  ----------------------------------------------------------------------------
  Temperature   33.25 C        75.00 C     -5.00 C     70.00 C        0.00 C
  Voltage        3.34 V         3.63 V      2.97 V      3.46 V        3.13 V
  Current       32.75 mA       70.00 mA     1.00 mA    68.00 mA       2.00 mA
  Tx Power        0.46 dBm       3.49 dBm  -12.21 dBm    0.49 dBm     -8.21 dBm
  Rx Power       -1.24 dBm       3.49 dBm  -18.53 dBm    0.49 dBm    -14.43 dBm  ----------------------------------------------------------------------------
  Note: ++  high-alarm; +  high-warning; --  low-alarm; -  low-warning
As you can see the warning values match up nicely with the information available in the data sheet, curiously the Alarms are actually quite high values, but when it comes to optics you probably want to stay well within the warning levels and not push towards alarm levels!