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Quick note on Pre-FEC , Post-FEC ,BER and Q relation.

Pre-FEC BER corresponding to Q.

  • BER and Q can be calculated from one another according to the following relationship:

  • Or, in Excel: dBQ = 20*LOG10(-NORMSINV(BER))
  • dBQ = 20Log(Q)


  • Post-FEC BER of <1e-15 essentially means no post-FEC errors
  • This is equivalent to ~18dBQ, which is about as high as can be measured


FEC Limit

  • This is the lowest Q (or highest BER) that can be corrected by the FEC
  • Beyond this post-FEC errors will occur


 FEC Limit: 8.53dBQ or a BER of 3.8e-3
 FEC Limit: 5.23dBQ or a BER of 3.4e-2 (<97% of bits have to be correct!)

Pre-FEC Calculation eg.




219499456 : Bit Errors

0 : Uncorrectable words

6.4577198E-6 : PRE-FEC BER

Assume the Time at this instant of performance was 12:05:04 which means : 304 Seconds since the last time interval.

Assume The FEC settings was : STANDARD FEC so the Rate used for 100 G transponder is : 1.1181 * 10^11
General Formula to calculate PRE_FEC: 
PRE_FEC BER =    TotalErrors 

                (secsFromLast * (rate)) 

TotalErrors =((bitErrorCorrected count + (9 * (uncorrected Words count))

So Substituting the Values     
219499456 / (304*1.1181 * 10^11) = 6.4577198E-6   



Quick note on Physical Coding Sublayer (PCS) error in Optical System

What is PCS ?

The Physical Coding Sublayer (PCS) is a networking protocol sublayer in the  Ethernet standards. This layer resides at the top of the physical layer (PHY) which provides an interface between the Physical Medium Attachment (PMA) sublayer and the media-independent interface (MII). This layer is responsible for coding and decoding data streams flowing to and from the MAC layer , scrambling and descrambling it, block and symbol redistribution, alignment marker insertion and removal, and lane block synchronisation .Currently most of the optical client ports supports PCS lane to enable high data rate .

Where actually PCS layer lies ?




How to troubleshoot PCS errors issues in Optical Network links?

If you see PCS errors on the interfaces; it may cause the link to flap or you can see errors on the client interfaces of the optical or router ports. PCS block reports signal fail/signal degrade too based on pre-set thresholds.


  • Sometime you may see bit errors or error block in the performance of the interface.Bit errors can also be converted to PCS errors.PCS errors are generally due to physical component degrade or failure  issues like problem in  physical interface mapper, damaged or attenuated fiber, issue on patch panel,ODF or due to faulty or damaged optic pluggables.  The higher rate we go the complexity of the internal mapper/components increases so performance becomes more sensitive to optical path perturberation .
  • PCS errors are also visible on the interfaces if there is some activity involving manual fiber pull ,device reboots ,optics replacement etc.  During link bring-up or bring down or flapping kind of situation, it is expected to see PCS errors increase for a short interval of time; which  is because of the  initial synchronization or skew-deskew process  of the two Ethernet end points. PCS errors are always counted from the incoming direction on the receiving node.
  • The other reasons to see PCS errors could be damaged or bad fiber, faulty optical pluggable (sfp/xfp/cfp etc).
  • Low receive power on the interface can also result in this kind of error so it is always recommended to troubleshoot or investigate on physical fiber as well as physical port on the devices(router/optical client ports).

For PCS lane based modules like SR4,LR4 ,LR10  or multi lane pluggables ,it is recommended to see errors on the lanes of the pluggable. if only few lanes are having issue ,it is better to suspect the connector or the optical XFP/CFP.

Also there is a limit for max difference in receive/transmit power between any two lanes .If the difference is greater than the threshold it may also result in issues.

Max Difference in receive power between any two lanes

100GBASE-LR4  5.5dB
200GBASE-FR4  4.1dB
200GBASE-LR4  4.2dB
400GBASE-FR8  4.1dB
400GBASE-LR8  4.5dB

Max Difference in transmit power between any two lanes

100GBASE-LR4  5dB
200GBASE-FR4  3.6dB
200GBASE-LR4  4 dB
400GBASE-FR8  4dB
400GBASE-LR8  4.5dB


Consider the case of using an LR4 CFP for the optical transceiver; each of the 4 wavelengths used on the link will be carrying 5 PCS lanes. In the case of 5 of the PCS lanes being in errors this may indicate the errors being specific to that wavelength and so areas of investigation should include the individual transmitters and receivers within the CFP.

If a 10 lane CFP (SR10 or LR10) is being used then each wavelength (in the case of the LR10) or fibre (in the case of the SR10) would each be carrying two PCS lanes. In this case then if two PCS lanes within the same CAUI lane are found to contain errors or defects then as well as investigating the CAUI the two lanes would also be carried on the same wavelength or fibre. In this case once again  the optical components should be investigated at both ends of the link. In the case of an SR10 based link the multi-fibre cable should also be checked as it may be possible that one of the individual fibres has been damaged within the cable.


CAUI -(Chip to) 100Gb/s Attachment Unit Interface

CFP :Centum Form factor Pluggable



Understanding Optical Return Loss (ORL) in Optical Fiber system

Optical fiber communication professionals might have heard  about ORL (Optical Return Loss ) during design and operation on an Optical Fiber Network.Intend of this article is to share the information on this topic which could help optical fiber  engineers and professionals understanding the concept and they can utilize this knowledge to understand a network in a better way.


In this article we will discuss

-What is ORL?

-What are the major sources of ORL ?

-What are the implications of ORL?

-How to test and rectify ORL?

-Methods to help improve ORL.

-Standards and references 

What is Optical Return Loss (ORL)?

Let me share few definitions so that it will be easy for every stage of engineers ;it could be student, beginner, professional or expert.

         1).  When light passes through an optical component most of it travels in the intended direction, but some light is reflected or scattered. In many applications these reflections are unwanted, because they can affect the emission characteristics of any laser in the system. In such applications, it is important to measure the reflections for the components of the system. The Return Loss is defined as the light reflected back into the input path. It is caused by scattering and reflection from optical surfaces like mirrors, lenses, and connectors or from defects, such as cracks and scratches. The backreflection is equal to the return loss with a negative quantity.


          2).  ORL is  defined  as the ratio (in dB) of the optical power (Pinc) traveling downstream at a system interface to the optical power reflected back upstream to the same interface. 

This includes the reflected power contributions from all system components downstream from the interface.


 To clarify :


Reflectance (dB) = P reflected (dBm) – P incident(dBm)


A discrete reflection will always be a negative quantity as the reflected power cannot be greater than the incident power.


By convention ORL is defined as:   


ORL(dB) = P incident (dBm) – P reflected (dBm)


This means that ORL will always be a positive number.The fact that we want all power to move forward and none to be reflected means that the higher the positive number, the better.




            3).  The reflection factor for a component is a measure of how much light the component reflects. It is a ratio of the power reflected by the device to the power incident on the  device. More normally we talk about the return loss of a component. The return loss has units of dB. Return loss is given by:


Return Loss(dB) = –10log(Reflection Factor) (dB)

ORL(dB) = P incident (dBm) – P reflected (dBm)


               4).  Optical return loss is the ratio of the output power of the light source to the total amount of back-reflected power 

(reflections and scattering). It is defined as a positive quantity.

PT: Output power of the light source
PAPC: Back-reflected power of APC connector

PPC: Back-reflected power of PC connector

PBS: Backscattered power of fiber

PR: Total amount of back-reflected power


ORL is measured in dB and is a positive value.Reflectance (dB) is the ratio of reflected power to incident power due to a single interface. It is defined as a negative quantity 

The higher the number, the smaller the reflection - yielding the desired result.    





What are the major sources of ORL ?

System components such as

  • connectors,
  • mechanical splices,
  • attenuators,
  • patch cords
  • glass/air terminations

All create a change in index of refraction as seen by an optical signal. The components are reflective in nature and can contribute to system ORL.

The fiber optic cable itself creates backscatter as light propagates through it.  The amount of reflected power due to backscatter cannot be eliminated but is magnitudes smaller than the power from discrete reflections


Sources of loss include reflections and scattering along the fiber network. A typical Return Loss value for an Angled Physical Contact (APC) connector is about -55dB, while the RL from an open flat polish to air is typically about -14dB. High RL is a large concern in high bitrate digital or analog single mode systems and is also an indication of a potential failure point, or compromise, in any optical network.




What are the implications of ORL?


The main effects of back-reflection  due to ORL include the following:


  • Less light is transmitted from the transmitter.
  • Increase in light source interference
  • Increasing the BER in digital transmission systems
  • Multi path distortion can also occur. 
  • Reducing the OSNR in  transmission
  • Reflections can distort the optical signal as reflections travel back and forth between reflective components.
  • Strong fluctuations in the laser output power.
  • Increase in transmitter noise.
  • Changes central wavelength and output power.
  • Permanent damage to the laser.

How does reflected power affect laser stability ?

Reflected light can provide unwanted feedback to the laser cavity which will effect:

  • Frequency Modulation Response changes
  • Relative Intensity Noise (RIN)
  • Optical frequency variations
  • Laser line-width variations

Reflection induced degradation increases with system bit-rate !The end result is higher bit error rates (BER)



How to test  ORL?


The measurement of ORL is becoming more important in the characterization of optical networks as the use of wavelength-division multiplexing increases. These systems use lasers that have a lower tolerance for ORL, and introduce elements into the network that are located in close proximity to the laser


The two major test methods:

   Optical Continuous Wave Reflectometry (OCWR)

            A laser source and a power meter, using the same test port, are connected to the fiber under test.

  Optical Time Domain Reflectometry (OTDR)

          The OTDR is able to measure not only the total ORL of the link but also section ORL.



To measure the ORL of a fiber span, an optical continuous wave reflectometer (OCWR) is used. The OCWR is an instrument designed to specifically measure system  and component ORL  reflectance. The OCWR launches a stable, continuous wave signal into the optical fiber and measures the strength of the time-integrated return signal. The ORL meter will return a single negative value which is the total reflectance from all reflective components seen from the point of test.  On fiber spans with multiple reflective components, discrete reflectance values cannot be determined unless component isolation is performed. The measured reflectance value is a directional value so tests should be performed on both ends of a fiber span.


The ORL reference measures background reflection of the fiber under test.  The reference procedure is performed each time a new test setup is required.A mandrel wrap is applied to the fiber test jumper before the point of measurement to isolate and attenuate any reflectance generators. The glass to air interface on the test connector end will be isolated from the OCWR.  The ORL zero function on the OCWR provides storage of the background reflectance level to provide the total optical return loss of the fiber test jumper. Once the mandrel wrap is taken out, the displayed ORL value represents the total ORL of the system from the point of termination.




















OTDR method is explained at 


Typical OTDR report snapshot for reference:-




Methods to help improve ORL are as follows:

  1. Use ultra polish connectors that have low reflectance such as UPC type. APC type connectors have even better reflectance values but are not compatible with other non-APC connectors. Connection to a non-APC connector can damage the APC connector.

  2. Use fusion splices instead of mechanical connectors or mechanical splices where possible.

  3. Re-do fusion splices that are shown to have reflectance. A good fusion splice should have no reflectance.

  4. Install optical isolators at the laser to reduce back reflectance.


Typical Reflectance for few connectors:-


     PC connecters: -30dB to –40 dB

    UPC connectors: -40dB to –50dB

    APC  connectors: -60dB  

    Fiber to air interface on a PC connector: -14.7 dB

    Rayleigh backscatter for telecom fiber:  -70 dB/meter


The angle reduces the back-reflection of the connection.

Typical good ORL measurements range from 30-35 dB. Read more at



Standards and references 


 -Telcordia document GR-1312

R7-79 [361] The discrete reflectance seen from any ONE optical port shall be less than -27 dB.

O7-80 [362] The discrete reflectance seen from any ONE optical port should be less than –40dB.


-Telcordia Document GR-2918

R7-38 [35] The individual channel Optical Return Loss, ORL as defined above, shall be 24dB or more for all wavelengths used in the DWDM system.


All equipment and component manufactures are required to design their systems  to meet reflectance specifications set out by the ITU-T which are adopted by bodies such as Telcordia (formerly Bellcore).Their specifications are intended to minimize system degradation due to reflections and they propose:

1)Enforce reflectance requirements on individual components in a fiber span.

  R7-79 and O7-80 relate to system components.

  Taken from GR-1312, Issue 3, April 1999 

  Generic Requirements for OFAs and Proprietary DWDM systems.


2)Ensure system performance to have a tolerance to specified reflection values.

  R7-38[35] relates to system ORL.

  Taken from GR-2918-CORE, Issue 4, December 1999 



Note :This article is sourced from multiple informations available on internet and books.







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Its is little hard to understand could u please elaborate with more example

Hi Pankaj,

Thanks for reading this article.I am glad that it is helpful.

Pt(Transmit) you can get from Transmit power from 6500 Amplifier modules.I believe for reflected power there is a photodiode which is attached by coupler on the Tx module.This Photodiode detects the reflected power and ORL calculation is done. This value is not visible in management interfaces like Site Manager but you can definitely see it via debug commands .Recommending to contact some Ciena Engineer who can guide you to get the reflected value.

This link will be helpful to visualize the context


Thank you for this descriptive ORL article. It's really helpful

Just one query:
How can we identify the Pt and Pr (reflected power) in any optical system?
E.g. Ciena 6500

When you say passband, does that equal to MC, MEADIA CHANNEL


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