What are Consequent Equations?

This post briefly defines and describes Consequent Equations that ITU-T G.798 uses for OTN Applications.

What are Consequent Equations, and How Should You Interpret Them?  

The purpose of this blog post is two-fold.

  • To describe the concept of Consequent Equations and
  • To discuss how we can use and interpret these Consequent Equations.

Introduction

Many ITU Standards (such as ITU-T G.798 for OTN Applications) will discuss many aspects of defects. These standards will define defects such as dAIS (Alarm Indication Signal) and dLOM (the Loss of Multi-frame).  

These same standards will also define the criteria that an OTN Network Element (be it an STE or PTE) should use to declare or clear a given defect.  

For example, ITU-T G.798 specifies all of the following defects that an OTN STE can declare and clear.

And that’s all well and good.  

NOTE: (*) – Indicates that you need to be a member of THE BEST DARN OTN TRAINING PRESENTATION…PERIOD!!! to access these links.

How should an STE/PTE Respond Whenever it Declares a Defect?  

However, what else should an STE do whenever it declares (for example) the dLOF defect condition?  

Does this STE have a responsibility to notify other STEs of this defect?  

Similarly, what else should a PTE do whenever it declares (for example) the dTIM (ODUk-TIM) defect condition?  

Again, does this PTE have a responsibility to notify other nearby PTEs of this defect?

The short answer to both of these questions is, “Yes, for those specific defects that I mentioned, they do have a responsibility to notify upstream and downstream equipment of the occurrence of those defect conditions.”

However, to confuse things, the PTE/STE must notify upstream and downstream PTE/STE whenever some defects occur, but not others.  

How Do We Sort out This Confusion?  

The Answer:  Consequent Equations.  

Let’s assume that a certain STE is declaring the dLOF Defect condition, as shown below in Figure 1.

Consequent Equation - OTSi/OTUk_A_Sk function declares the dLOF defect

Figure 1, Illustration of the STE (e.g., the OTSi/OTUk-a_A_Sk Atomic Function) declares the dLOF defect condition.

What happens next?

At this point, let’s write down the Consequent Equation that pertains to this STE (or the OTSi/OTUk-a_A_Sk function in this case):

aSSF <- dLOS-P or dAIS or dLOF or dLOM or AI_TSF-P

Where:

aSSF is the current state of the CI_SSF output pin (of the OTSi/OTUk-a_A_Sk function).

dLOS-P is the current state of the Loss of Signal-Path Defect condition

dAIS is the current state of the OTUk-AIS Defect Condition

dLOF is the current state of the Loss of Frame Defect Condition

dLOM is the current state of the Loss of Multi-Frame Defect Condition, and 

AI_TSF-P is the current (logical) state of the AI_TSF-P input pin (to the OTSi/OTUk-a_A_Sk atomic function).  

This consequent equation states that the STE (or OTSi/OTUk-a_A_Sk function) will assert its CI_SSF output pin anytime it declares any of the following defect conditions:

This consequent equation also states that the STE (the OTSi/OTUk-a_A_Sk function) will assert the CI_SSF output pin anytime the upstream atomic function asserts the AI_TSF input to this function.  

NOTE:  Please see the OTSi/OTUk_A_Sk Function Post for more information about this particular Atomic Function.  

So What Does All of This Mean?

In Figure 2, I show the OTSi/OTUk_A_Sk function now asserting its CI_SSF output pin because it is currently declaring the dLOF defect condition.  

Consequent Equation - OTSi/OTUk_A_Sk function asserts CI_SSF due to dLOF defect

Figure 2, Illustration of the OTSi/OTUk_A_Sk Atomic Function asserting its CI_SSF output because it is currently declaring the dLOF defect condition.  

Please note that the CI_SSF output (from the OTSi/OTUk_A_Sk function) is connected to the CI_SSF input of the (downstream) OTUk_TT_Sk function.  

OK, that’s great. The above Consequent Equation states that the STE (e.g., the OTSi/OTUk_A_Sk function will assert the CI_SSF output pin whenever it declares the dLOF defect.  

How does that alert any other STE/PTE of the OTSi/OTUk_A_Sk function declaring the dLOF defect?

Answer:  There is more to this, and it involves more Consequent Equations.  

Let’s Take a Look at the Downstream Circuitry

Let’s now look at the OTUk_TT_Sk and OTUk/ODUk_A_Sk Atomic Functions (which are both downstream from the OTSi/OTUk_A_Sk function).  

In Figure 3, I show the OTUk_TT_Sk and the OTUk/ODUk_A_Sk Atomic Functions. 

I also show that the upstream (OTSi/OTUk_A_Sk function) circuitry is now asserting the CI_SSF input (to the OTUk_TT_Sk function) – as we described above.  

Consequent Equations - Upstream Circuitry asserts CI_SSF input to the OTUk_TT_Sk function

Figure 3, Illustration of the OTUk_TT_Sk and OTUk/ODUk_A_Sk functions – with upstream circuitry asserting the CI_SSF input pin.

Now, the OTUk_TT_Sk Atomic Function happens to have two sets of Consequent Equations:

I will list each of these equations below.

  • aTSF <- CI_SSF or dAIS or (dTIM and (NOT TIMActDis))
  • aBDI <- CI_SSF or dAIS or dTIM

I will explain each of these equations below.  

The First Consequent Equation – OTUk_TT_Sk Function

Let’s start with the first Consequent Equation for the OTUk_TT_Sk function.

aTSF <- CI_SSF or dAIS or (dTIM and (NOT TIMActDis))

Where:

aTSF is the current state of the AI_TSF output of the OTUk_TT_Sk Atomic Function.

CI_SSF is the current state of the CI_SSF input of the OTUk_TT_Sk Atomic Function.  

dAIS is the current state of the dAIS defect condition, and 

dTIM is the current state of the Trail Trace Identifier Mismatch Defect Condition.  

This Consequent Equation states that the OTUk_TT_Sk Atomic Function should assert the AI_TSF output signal anytime it declares any of the following defect conditions.

  • dTIM – Trail Trace Identifier Mismatch Defect
  • dAIS – AIS Defect

This Consequent Equation also states that the OTUk_TT_Sk function should assert the AI_TSF output whenever the upstream circuitry (e.g., the OTSi/OTUk_A_Sk function) asserts the CI_SSF input pin.  

In Figure 4, I show the OTUk_TT_Sk function asserting its AI_TSF output pin because the upstream OTSi/OTUk_A_Sk function is asserting the CI_SSF input pin (to this function).  

Consequent Equation - OTUk_TT_Sk Atomic Function asserts AI_TSF due to CI_SSF being asserted

Figure 4, The OTUk_TT_Sk Atomic Function asserts the AI_TSF output pin because the upstream OTSi/OTUk_A_Sk function is asserting its CI_SSF input pin.  

OK, now let’s look at the second Consequent Equation for the OTUk_TT_Sk Function.

The Second Consequent Equation – OTUk_TT_Sk Function

aBDI <- CI_SSF or dAIS or dTIM

Where:  

aBDI is the state of the RI_BDI output (of the Remote Port Interface) of the OTUk_TT_Sk function.  

Earlier in this post, we have defined CI_SSF, dAIS, and dTIM.  

Therefore, this Consequent Equation states that the OTUk_TT_Sk function will assert the RI_BDI output pin anytime it declares the dAIS or dTIM defect conditions.

This equation also states that the OTUk_TT_Sk function will also assert the RI_BDI output pin anytime the upstream circuitry asserts the CI_SSF input (to the OTUk_TT_Sk function).  

If you recall from the OTUk_TT_So and OTUk_TT_Sk posts, I state that anytime the OTUk_TT_Sk function asserts the RI_BDI output pin, it will command its collocated OTUk_TT_So function to transmit the OTUk-BDI indicator back out to the remote end.

I show this phenomenon below in Figure 5.  

Consequent Equation - OTUk_TT_Sk function asserts RI_BDI due to CI_SSF

Figure 5, The OTUk_TT_Sk function asserts its RI_BDI output pin, commanding its Collocated OTUk_TT_So function to transmit the BDI (Backward Defect Indicator) back to the upstream STE because its CI_SSF is being driven HIGH.

Figure 5 shows that because the STE (OTSi/OTUk_A_Sk function) declared the dLOF defect, the downstream OTUk_TT_Sk function responded by commanding its collocated OTUk_TT_So function to transmit the OTUk-BDI indicator back to the upstream STE (the source of the defective OTUk signal).  

However, we’re not done yet.  

Since the OTUk_TT_Sk function is also asserting its AI_TSF output, it is also asserting the AI_TSF input to the (down-stream) OTUk/ODUk_A_Sk atomic function.  

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Let’s move on to the OTUk/ODUk_A_Sk Atomic Function

As you can see in Figure 5, the OTUk_TT_Sk function, by asserting its AI_TSF output pin, is also asserting the AI_TSF input pin to the OTUk/ODUk_A_Sk function.  

And the OTUk/ODUk_A_Sk function comes with several Consequent Equations of its own.  

  • aSSF <- AI_TSF and (not MI_AdminState = LOCKED), and
  • aAIS <- AI_TSF and (not MI_AdminState = LOCKED)

Let’s take each of these equations, one at a time.

The First Consequent Equation – OTUk/ODUk_A_Sk Function

aSSF <- AI_TSF and (not MI_AdminState = LOCKED)

Where:  

aSSF is the current state of the CI_SSF output pin (from the OTUk/ODUk_A_Sk function).

AI_TSF is the current state of the AI_TSF input pin to the OTUk/ODUk_A_Sk function, and

MI_AdminState reflects the current state of the MI_AdminState input signal (which the System Operator can set).

This Consequent Equation states that the OTUk/ODUk_A_Sk function will automatically assert its CI_SSF output signal whenever the upstream circuitry asserts its AI_TSF input, provided that the System Operator has not put the OTUk/ODUk_A_Sk function into the LOCKED state.  

In Figure 6, I show the OTUk/ODUk_A_Sk function asserting its CI_SSF output pin because the upstream circuitry is asserting its AI_TSF input pin.  

Consequent Equations - OTUk/ODUk_A_Sk function asserts its CI_SSF output pin

Figure 6, The OTUk/ODUk_A_Sk function asserts its CI_SSF output pin because the upstream circuitry (e.g., the OTUk_TT_Sk and OTSi/OTUk_A_Sk functions) is asserting its AI_TSF input pin.

I should also point out that APS (Automatic Protection Switching) systems often trigger (and start protection switching) whenever the OTUk/ODUk_A_Sk function asserts its CI_SSF output pin.  

Now, let’s move on to the next Consequent Equation.  

The Second Consequent Equation – OTUk/ODUk_A_Sk Function

aAIS <- AI_TSF and (not MI_AdminState = LOCKED)

Where:

aAIS is the current state of the ODUk-AIS Maintenance Signal

If aAIS = TRUE, then the OTUk/ODUk_A_Sk function is overwriting its output signal with the ODUk-AIS Maintenance signal.

Conversely, if aAIS is FALSE, then the OTUk/ODUk_A_Sk function transmits an ODUk data stream, carrying regular client traffic.  

Therefore, this Consequent Equation states that the OTUk/ODUk_A_Sk function will transmit the ODUk-AIS Maintenance Signal anytime the upstream circuitry pulls its AI_TSF input TRUE; provided that the System Operator has NOT put the OTUk/ODUk_A_Sk function into the LOCKED State.

In Figure 7, I illustrate the OTUk/ODUk_A_Sk function transmitting the ODUk-AIS Maintenance Signal because upstream circuitry (e.g., the OTUk_TT_Sk and OTSi/OTUk_A_Sk functions) is asserting its AI_TSF  input.  

Consequent Equation - OTUk/ODUk_A_Sk function transmits ODUk-AIS downstream

Figure 7, The OTUk/ODUk_A_Sk function replaces the ODUk signal (carrying client data) with the ODUk-AIS Maintenance Signal whenever upstream circuitry asserts its AI_TSF input.  

So What Does All This Mean?

If we were to combine the OTSi/OTUk_A_Sk function, the OTUk_TT_Sk function, its Collocated OTUk_TT_So function, and the OTUk/ODUk_A_Sk function into a single box, that we call OTN STE.

Then, we could state that if the OTN STE declares the dLOF defect (as we discussed earlier in this post), then that same OTN STE will do all of the following:

  • It will transmit the OTUk-BDI indicator back towards the upstream STE.
  • The OTN STE will also replace the missing (or defective) ODUk data stream (carrying client traffic) with the ODUk-AIS Maintenance Signal, and
  • It will also trigger APS (Automatic Protection Switching) activities by asserting the CI_SSF output (from the OTUk/ODUk_A_Sk function).  

I show a drawing of these actions below in Figure 8.

Consequent Equations - Overall OTN STE's response to it declaring the dLOF Defect Condition

Figure 8, Illustration of the OTN STE responding to the dLOF Defect Condition

Conclusion

Thanks to Consequent Equations, we can define and predict how an OTN STE or PTE will respond to certain defects.

I have presented Consequent Equations in each post pertaining to OTN Atomic Functions.  

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What is Defect Correlation?

This post briefly defines and explains what Defect Correlation means. In short, the Defect Correlation equations will specify how we expect a system to respond to a specific defect condition.

What is Defect Correlation, and How Should You Interpret It?

The purpose of this blog post is two-fold.

  • To describe the concept of Defect Correlation and
  • To discuss how to interpret the meaning of Defect Correlation and their Equations.

Introduction

Numerous ITU Standards (such as ITU-T G.798 for OTN applications) will define various aspects of defects. These standards will define a defect, such as dLOS (the Loss of Signal) and dLOF (the Loss of Frame).

These standards will (sometimes) describe the conditions that an OTN Network Element (be it an STE or PTE) should use to declare or clear a given defect.

For instance, ITU-T G.798 specifies all of the following defects that an OTN STE can declare and clear.

(*) – Requires membership to THE BEST DARN OTN TRAINING PRESENTATION…PERIOD!!! to see these links.  

And it is excellent that the ITU-T standard committee does this for us.

But let’s now take a closer look at these defects from a System-Level standpoint.

Should One Defect Lead to Many Other Defects?

Suppose an OTN STE declares the dLOS-P (Loss of Signal-Path) defect condition with its incoming optical lanes or signal.

This STE will declare the dLOS-P condition for one of two reasons.

  1.  Because the optical components (upstream) are detecting too little optical signal energy (within the incoming signal) or
  2. the Clock and Data Recovery circuitry (within the STE electronics) is detecting an absence of recovered (data) signal activity for an extended period.

In Figure 1, I illustrate the OTSi/OTUk-a_A_Sk function, declaring the dLOS-P defect.

OTSi/OTUk-a_A_Sk Function declares dLOS Defect - Defect Correlation

Figure 1, The OTSi/OTUk-a_A_Sk Atomic Function, declares the dLOS Defect Condition.  

In either of these cases, it is clear that this OTN STE should declare the dLOS-P defect condition.

How about the dLOF Condition?

However, if that same OTN STE is not receiving any discernable signal from the remote STE, it is safe to say that it will not be receiving the FAS fields (within this now non-existent incoming data stream).

Should this OTN STE also declare the dLOF defect as well?

In Figure 2, I illustrate the OTSi/OTUk-a_A_Sk function, declaring the dLOF defect condition and the dLOS-P defect condition.

OTSi/OTUk-a_A_Sk Function Declares both the dLOS and dLOF Defects

Figure 2, The OTSi/OTUk-a_A_Sk function declaring the dLOF and dLOS-P Defect Conditions

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What about the dLOM Condition?

And since the OTN STE is not receiving any FAS field bytes, it cannot locate the MFAS bytes.

Should this OTN STE also declare the dLOM defect too?

In Figure 3, I illustrate the OTSi/OTUk-a_A_Sk function, declaring the dLOM, dLOF, and dLOS-P Defect conditions.

OTSi/OTUk-a_A_Sk Function declares dLOS-P, dLOF and dLOM Defects

Figure 3, The OTSi/OTUk-a_A_Sk Atomic Function declaring the dLOM, dLOF, and dLOS-P Defect Conditions

How about the dTIM Condition?

Finally, since our OTN STE is not receiving any discernable signal (from the remote STE), and it cannot locate the boundaries of each incoming OTUk frame, it will certainly not obtain a Trail Trace Identification Message that matches that of the “Expected Trail Trace Identification” Message.

Should this OTN STE also declare the dTIM defect as well?

In Figure 4, I illustrate the OTUk_TT_Sk function declaring the dTIM defect, while the upstream OTSi/OTUk-a_A_Sk function reports the dLOS-P, dLOF, and dLOM defect conditions.

OTUk_TT_Sk Function declares dTIM defect - due to No Defect Correlation

Figure 4, The OTUk_TT_Sk Atomic Function (downstream from the OTSi/OTUk-a_A_Sk Function) declares the dTIM defect.

Many Defects, all due to the dLOS-P Condition

In this scenario, a Loss of Signal event would cause the OTN STE to declare the dLOS, dLOF, dLOM, and dTIM defect conditions.

The OTN STE will accurately declare all four defect conditions because conditions warrant that the STE declare each of these defects.

However, allowing an STE to declare multiple defects (e.g., dLOS, dLOF, dLOM, and dTIM) can be confusing to both System-Management and the System Operator.

Confused Guy - Too Many Defects

I could take this exercise even further and include some of the PTE/ODUk-related defects that an OTN PTE would declare (e.g., ODUk-AIS), all because of the dLOS-P condition. But I think that you get my point.

Whenever a service-affecting defect occurs, the OTN STE needs to alert System Management of a concise description of the problem (just dLOS-P in this case).

The intent should be to help the System Operator isolate the root cause of these problems.

We should not be bombarding the System Operator with a whole slew of defects, which are just artifacts of a single defect.

If the OTN STE declares the dTIM, dLOM, dLOF, and dLOS-P defects, the root cause of this problem has nothing to do with a mismatch in the Trail-Trace Identification Message.

Hence the Purpose of Defect Correlation

The purpose of Defect Correlation and Defect Correlation equations is to establish and report ONLY the root cause of problems to System Management.

The Defect Correlation Equations accomplishes this by creating a hierarchy of defects.

I’ll explain this.

Let’s list some Defect Correlation Equations for the OTSi/OTUk_A_Sk and OTUk_TT_Sk Atomic Functions.

For the OTSi/OTUk_A_Sk Atomic Function

The OTSi/OTUk_A_Sk function has the following Defect Correlation equations:

  • cLOS-P ⇐ dLOS-P and (NOT AI_TSF-P)
  • cLOF ⇐ dLOF and (NOT dLOS-P) and (NOT dAIS) and (NOT AI_TSF-P)
  • cLOM ⇐ dLOM and (NOT dLOS-P) and (NOT dLOF) and (NOT dAIS) and (NOT AI_TSF-P)

Let’s also include the following Consequent Equation to bridge the OTUk_TT_Sk function to the OTSi/OTUk_A_Sk function.

aSSF ⇐ dLOS-P or dAIS or dLOF or dLOM or AI_TSF-P

For the OTUk_TT_Sk Function

In this case, we will focus on the Defect Correlation equation that pertains to the dTIM defect condition.

  • cTIM ⇐ dTIM and (NOT CI_SSF) and (NOT dAIS)

So Now Let’s Study some of these Defect Correlation Equations

Let’s start with the first equation for the OTSi/OTUk-a_A_Sk function.

  • cLOS-P ⇐ dLOS-P and (NOT AI_TSF-P)

Where: 

cLOS-P is the correlated defect value of the dLOS-P defect state.

dLOS-P is the current state of the dLOS-P defect condition that the OTSi/OTUk_A_Sk function will declare or clear.

AI_TSF-P is the current state of the AI_TSF-P (Trail Signal Fail – Path Indicator) Input to the OTSi/OTUk-A_Sk function.

In this equation, the parameter that begins with the letter “c” is the correlated defect parameter (or defect) state that we ultimately report to System Management.

This equation states that we should only set the variable cLOS-P to TRUE if dLOS-P is TRUE.

In other words, we should only report the Loss of Signal condition (e.g., setting cLOS-P to TRUE) if the STE circuitry declares the dLOS-P defect (due to a lack of signal activity within the Clock Recovery Block, for example).

This equation also states that we should NOT set cLOS-P to TRUE because the upstream Optical Circuitry is declaring some other defect condition and is then asserting its AI_TSF-P output – towards the OTSi/OTUk_A_Sk function).

I show a TRUTH TABLE for this Defect Correlation Equation below in Table 1.

Table 1, TRUTH TABLE for the Defect Correlation Equation, cLOS-P ⇐ dLOS-P AND (NOT AI_TSF-P)

dLOS-P DefectAI_TSF-P StatecLOS-P StateComment
ClearedFALSE0
DeclaredFALSE1Sets cLOS-P to TRUE, because dLOS-P is declared.
Don't CareTRUE0We set cLOS-P to 0 when AI_TSF-P is TRUE.

Let’s look at another Defect Correlation Equation.

  • cLOF ⇐ dLOF and (NOT dLOS-P) and (NOT dAIS) and (NOT AI_TSF-P)

Where:

cLOF is the correlated value of the dLOF defect state.

dAIS is the current state of the dAIS defect condition within the OTSi/OTUk_A_Sk function.

In this equation, we are stating that we should only set cLOF = TRUE (and report the Loss of Frame condition to System Management) if the STE circuitry declares the dLOF condition.

This equation also states that we should NOT be setting cLOF = TRUE (and report the Loss of Frame Condition to System Management) if:

  • The STE is also declaring the dLOS-P defect, or
  • declaring the dAIS (OTUk-AIS) defect, or
  • If the upstream Optical Components assert the AI_TSF-P input to the OTSi/OTUk_A_Sk function.

If any of the three items (above) are TRUE, then we must set cLOF = FALSE.

I show the TRUTH TABLE for this Defect Correlation Equation below in Table 2.

Table 2, The TRUTH TABLE for the Defect Correlation Equation, cLOF ⇐ dLOF AND (NOT dLOS-P) AND (NOT dAIS) AND (NOT AI_TSF-P)

dLOF Defect ConditiondLOS-P Defect ConditiondAIS Defect ConditionAI_TSF-P StatecLOF StateComments
ClearedClearedClearedFALSECleared
DeclaredClearedClearedFALSEDeclaredWe assert cLOF because we are declaring the dLOF Defect
Don't CareDeclaredClearedFALSEClearedWe set cLOF = 0 whenever dLOS-P is declared.
Don't CareClearedDeclaredFALSEClearedWe set cLOF = 0 whenever dAIS is declared.
Don't CareClearedClearedTRUEClearedWe set cLOF = 0 whenever AI_TSF-P is driven TRUE.

At the risk of “whipping a dead horse,” I will show one more example.

  • cTIM ⇐ dTIM and (NOT CI_SSF) and (NOT dAIS)

Where:

cTIM is the correlated value of the dTIM defect state.

CI_SSF is the current state of the CI_SSF (Server Signal Fail Indicator) input pin to the OTUk_TT_Sk function.

If the STE circuitry declares this defect, this equation states that we must only report the Trail Trace Identifier Mismatch Defect (and set cTIM = TRUE).

This equation also states that we MUST NOT set cTIM = TRUE if any of the following is true.

NOTE:  We have the following Consequent Equation for the CI_SSF signal (from the OTSi/OTUk_A_Sk function).

  • aSSF <- dLOS-P or dAIS or dLOF or dLOM or AI_TSF-P

This equation states that if the upstream OTSi/OTUk_A_Sk function declares any of the following defects, it will set aSSF = TRUE.

  • dLOS-P
  • dAIS (OTUk-AIS)
  • dLOF
  • dLOM, or
  • If the upstream Optical Components assert the AI_TSF-P input to the OTSi/OTUk_A_Sk function.

If aSSF = TRUE, then the OTSi/OTUk_A_Sk function will assert the CI_SSF output signal (towards the OTUk_TT_Sk function).

Finally, we get to the bottom line.

These equations state that the STE MUST NOT set cTIM = TRUE (and MUST NOT report the Trail Trace Identifier Mismatch defect to System Management) if any of the following defect conditions are TRUE.

  • dLOS-P
  • dAIS
  • dLOF
  • dLOM
  • If the AI_TSF-P signal (from the upstream Optical Components) is HIGH.

Summary

I believe that you can see that using Defect Correlation Equations makes Defect Reporting and System-Management MUCH EASIER.

Happy due to Defect Correlation

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OTUk-Backward Defect Indicator

This post defines and describes the Backward Defect Indicator (dBDI) defect for the OTUk Layer


What is the dBDI (Backward Defect Indicator) defect at the OTUk Layer?

In short, the dBDI (or Backward Defect Indicator) signal is functionally equivalent to the RDI (Remote Defect Indicator) for OTN applications.

In OTN applications, Network Equipment can declare the dBDI defect at either the OTUk Layer or the ODUk Layer.

This post will discuss the dBDI defect for the OTUk Layer, which we can call the OTUk-BDI defect condition.

We address the dBDI defect for the ODUk Layer in another post.

In another post, I’ve also described the RDI (Remote Defect Indicator) signal or defect in generic terms.

In this post, we are going to describe the following items.

  • What conditions will cause an OTUk Network Element to transmit the dBDI indicator to the remote Network Element?
  • How does the OTUk Network Element transmit the dBDI indicator to the remote Network Equipment?
  • How does the OTUk Network Element receiving the dBDI signal detect and declare the dBDI defect condition?
  • And, how does the OTUk Network Element clear the dBDI defect condition?

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What conditions will cause an OTUk Network Element to transmit the dBDI indicator?

In Figure 1, we illustrate two Network Elements (consisting of OTUk Framers and OTUk Transceivers) exchanging OTUk traffic over Optical Fiber.

We will call one of these Network Elements NETWORK ELEMENT WEST and the other Network Element, NETWORK ELEMENT EAST.

NETWORK ELEMENT WEST contains the following pieces of hardware

  • OTUk Framer West
  • OTUk Transceiver East and
  • Optical I/F Circuitry (O->E)/(E->O)

Likewise, NETWORK ELEMENT EAST contains the following pieces of hardware.

  • OTUk Framer East
  • OTUk Transceiver East and
  • Optical I/F Circuitry (O -> E)/(E -> O)

Normal Condition - Network Element West and East

Figure 1, Illustration of two Network Elements that are connected over Optical Fiber

A Defect Condition

Now, let us imagine that some impairment occurs in the span of Optical Fiber carrying OTUk traffic from NETWORK ELEMENT WEST to NETWORK ELEMENT EAST.

This impairment will then cause NETWORK ELEMENT EAST to declare a service-affecting defect, as shown in Figure 2.

Network Element East declares Service Affecting Defect

Figure 2, Illustration of NETWORK ELEMENT EAST declaring a Service-Affecting Defect due to an impairment in Optical Fiber

NETWORK ELEMENT EAST might respond to this defect condition in several ways.  It might transmit the ODUk-AIS indicator towards downstream equipment (as a replacement signal).

NETWORK ELEMENT EAST might also invoke Protection Switching (if supported).

Sending the OTUk-BDI Indicator in Response

Finally, NETWORK ELEMENT EAST will also respond to this defect by transmitting the dBDI (or OTUk-BDI) indicator back towards the upstream Network Element (NETWORK ELEMENT WEST, in this case).

Figure 3 shows an illustration of NETWORK ELEMENT EAST, transmitting the OTUk-BDI indicator (back towards NETWORK ELEMENT WEST) in response to it declaring this service-affecting defect.

Network Element East sends OTUk-BDI signal to Network Element West

Figure 3, Illustration of NETWORK ELEMENT EAST responding to the Defect Condition by sending the OTUk-BDI indicator back towards NETWORK ELEMENT WEST

NETWORK ELEMENT EAST sends the OTUk-BDI indicator (back to NETWORK ELEMENT WEST) to alert it of this defect condition (between the two Network Elements).

In other words, NETWORK ELEMENT EAST is saying, “Hey, NETWORK ELEMENT WEST, I’m having problems with the data that you are sending me.  I’d just thought that I’d let you know”.

There are many reasons why all of these notifications are useful.

This notification gives the Overall Network a clearer picture of exactly where the problem (or impairment) is.

It can also notify maintenance personnel of these problems and provide them with helpful information before they “roll trucks.”

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So What EXACTLY are those Defects that will cause a Network Element to transmit the OTUk-BDI indicator?

The Network Element will transmit the OTUk-BDI indicator anytime it declares any service-affecting defect conditions.

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The Network Element will continue to transmit the OTUk-BDI indicator for the duration it declares any of these defects.

Once the Network Element no longer declares these defect conditions, it will stop transmitting the OTUk-BDI indicator.

NOTE: ITU-T G.798 is the standards document that specifies the conditions and set of defects that will cause the Network Element to transmit the OTUk-BDI indicator to the remote terminal.

If you wish to see a detailed analysis of how ITU-T G.798 specifies these requirements, please look at the standards document itself or check out the OTUk-BDI – ITU-T G.798 Analysis post.

How does the OTUk Network Element transmit the dBDI indicator?

The Network Element will send the OTUk-BDI indicator by setting the BDI bit-field (Bit 5) within the SM (Section Monitoring) Byte, to  1, within each outbound OTUk frame.

The SM byte resides within the 3-byte SM (Section Monitoring) field of the OTUk Overhead.

Figures 4a, 4b, and 4c present the location of the BDI field.
Figure 4a presents an illustration of the SM-field within the OTUk Overhead.

OTUk Overhead with SM Field Identified

Figure 4a, The SM Field within the OTUk Overhead

Further, Figure 4b illustrates the SM byte’s location within the 3-byte SM Field (within the OTUk Overhead).

SM field with the SM Byte identified

Figure 4b, The SM-Byte within the SM Field

Finally, Figure 4c shows the location of the BDI-field within the SM-byte (within the SM-field of the OTUk Overhead).

SM Byte with OTUk-BDI field identified

Figure 4c, The Location of the BDI bit-field within the SM Byte, within the SM Field, within the OTUk Overhead

Likewise, the Network Element will end its transmission of the OTUk-BDI indicator by setting the BDI bit-field back to “0” within each outbound OTUk frame.

How does the OTUk Network Element detect and declare the dBDI indicator?

In the scenario that we described above (via Figure 3), NETWORK ELEMENT EAST will continue to transmit the OTUk-BDI signal to NETWORK ELEMENT WEST as long as it (NETWORK ELEMENT EAST) declares the service-affecting defect within its Ingress (Receive) signal.

If NETWORK ELEMENT WEST receives the OTUk-BDI indicator within at least five (5) consecutive OTUk frames, it will declare the dBDI defect condition.

In other words, if NETWORK ELEMENT WEST (or any Network Element) were to receive at least five (5) consecutive OTUk frames, in which the BDI bit-field is set to “1”, then it will declare the dBDI defect.

Figure 5 illustrates NETWORK ELEMENT WEST declaring the dBDI defect after receiving five consecutive OTUk Frames with the SM-BDI field set to “1”.

Network Element West declares the dBDI defect condition

Figure 5, Illustration of NETWORK ELEMENT WEST declaring the dBDI defect condition

How does the OTUk Network Element clear the dBDI defect condition?

Whenever NETWORK ELEMENT EAST has determined that the service-affecting defect (which caused it to transmit the dBDI signal in the first place) is cleared, it will stop sending the dBDI signal back out to NETWORK ELEMENT WEST.

NETWORK ELEMENT EAST will stop sending the dBDI signal by setting the BDI bit-field (within the SM field) to “0” within each outbound OTUk frame.

If NETWORK ELEMENT WEST (which is currently declaring the dBDI defect condition) were to receive at least five (5) consecutive OTUk frames, in which the BDI bit-field is set to “0”, then it will clear the dBDI defect.

Figure 6 illustrates NETWORK ELEMENT WEST clearing the dBDI defect after receiving five consecutive OTUk Frames with the SM-BDI field set to “0”.

Network Element East declares Service-Affecting Defect

Figure 6, Illustration of NETWORK ELEMENT WEST clearing the dBDI defect condition

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