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.

dLOS-P – Loss of Signal Defect-Path
dLOFLANE[j] – Loss of Frame of Logical Lane, for Logical Lane j
dLOL – Loss of Lane Alignment Defect
dAIS – Alarm Indication Signal Defect (e.g., OTUk-AIS for STEs)
dLOF – Loss of Frame Defect
dLOM – Loss of Multi-Frame Defect
dTIM – Trail Trace Identifier Mismatch Defect(*)
dIAE – Input Alignment Error Defect(*)
dBIAE – Backward Input Alignment Error Defect(*)
dBDI – Backward Defect Indicator Defect
dDEG – Signal Degrade Defect(*)

(*) – 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.

Because the optical components (upstream) are detecting too little optical signal energy (within the incoming signal) or
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.

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.

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.

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.

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.

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 Defect	AI_TSF-P State	cLOS-P State	Comment
Cleared	FALSE	0
Declared	FALSE	1	Sets cLOS-P to TRUE, because dLOS-P is declared.
Don't Care	TRUE	0	We 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 Condition	dLOS-P Defect Condition	dAIS Defect Condition	AI_TSF-P State	cLOF State	Comments
Cleared	Cleared	Cleared	FALSE	Cleared
Declared	Cleared	Cleared	FALSE	Declared	We assert cLOF because we are declaring the dLOF Defect
Don't Care	Declared	Cleared	FALSE	Cleared	We set cLOF = 0 whenever dLOS-P is declared.
Don't Care	Cleared	Declared	FALSE	Cleared	We set cLOF = 0 whenever dAIS is declared.
Don't Care	Cleared	Cleared	TRUE	Cleared	We 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.

The OTUk_TT_Sk function is declaring the dAIS defect condition, or
If the upstream OTSi/OTUk_A_Sk function is asserting the CI_SSF input to the OTUk_TT_Sk function.

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.

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What is an STE for OTN Applications?

This post defines and describes both a Section and Section Terminating Equipment for OTN applications. This post also defines the term: OTUk-SMOH (Section Monitoring Overhead).

What is Section Terminating Equipment (STE) for OTN Applications?

Whenever we discuss the OTN Digital Layers (e.g., the OPUk, ODUk, and OTUk layers), we can group Networking Circuits and Equipment into one of two broad categories.

STE – Section Terminating Equipment and
PTE – Path Terminating Equipment

I will be using these terms throughout various OTN-related posts within this blog. So, we must have a strong understanding of these terms.

I have devoted this blog post to STE (Section Terminating Equipment).

I have devoted another post to PTE (Path Terminating Equipment).

NOTE: I discuss STEs and PTEs extensively in Lesson 3 within THE BEST DARN OTN TRAINING PRESENTATION….PERIOD!!! I also discuss the differences between STEs and PTEs as well.

What is a Section?

Before we define the term Section Terminating Equipment (or STE), we must first define the word Section as it pertains to an Optical Transport Network (OTN).

For OTN applications, a Section is a single optical link (or span) between two adjacent pieces of networking equipment.

NOTE: For lower speed applications, one can realize a Section via a Copper Medium (such as CAT5 or CAT6 Cable).

Figure 1 presents a simple illustration of an Optical Transport Network with some boxes labeled PTE and others labeled STE.

Figure 1 illustrates STE (Section Terminating Equipment) and PTE (Path Terminating Equipment). Note: Figure 1 shows a total of five (5) different boxes.

Two of these boxes are labeled PTE, and three of these boxes are labeled STE.

However, in reality, all 5 of these boxes are STEs.

From a system standpoint, many PTEs are STEs. However, not all STEs are PTEs.

We can also define a Section as any optical connections connecting these boxes (in Figure 1).

Now, we will define the term Section Terminating Equipment.

What is an STE (Section Terminating Equipment)?

For OTN applications, the basic definition of a Section Terminating Equipment is any equipment that (1) transmits data into or receives data from the Section and (2) also monitors and manages the data transmission over this Section (e.g., the optical fiber link that exists between the Near-End and the adjacent Far-End Network Equipment).

For OTN applications, the OTUk Layer is the protocol layer responsible for managing and monitoring the transmission/reception of data across a Section.

More specifically, an OTN Source (or Transmitting) STE is any equipment that performs ALL the following functions.

The Source STE Operation In the Transmit Direction

It will accept data from upstream circuitry (typically in the form of ODUk frames).
It electrically preconditions all data (that it is about to transmit to the remote Sink STE via an optical connection) by computing and attaching the OTUk (or OTUkV) overhead to this data stream. This data will typically (but not always) include the FEC.
Once the Source STE has finished preconditioning this data, it will convert this electrical data into the optical format and transmit it over optical fiber to the remote Sink STE.

Sink STE Operation In the Receive Direction

The Sink (Receiving) STE performs all of the following operations.

It receives data (from a remote Source STE) in the optical format.
The Sink STE then converts this optical data into the electrical format, where it can check and process these newly received OTUk/OTUkV frames.
As the Sink STE checks and processes this data, it will check for the following items.
- FEC errors (correctable and uncorrectable)
- Defects or Failures (e.g., dLOF, dLOM, dTIM, dAIS, dIAE, dBIAE, dDEG, dLOL, dLOFLANE[j], dLOS-P and SM-BDI)
- Other errors (e.g., SM-BIP-8, SM-BEI)
It will then pass this data to the downstream circuitry as an ODUk data stream (for further processing at the ODUk-layer).

Therefore, if we were to combine our simple definition of the word Section with the description of a Section Terminating Equipment, we can say the following.

Summarizing our Definitions of Section and STE

An STE begins at the point where the Network Equipment (or the Source STE) will precondition and process electrical data in preparation for transmission over an Optical link.

Afterward, the Source STE will convert this signal into the Optical Format, transmitting this optical signal to the remote Sink (or Receiving) STE.

A Section ends (or is terminated) at the point where the Sink STE (that receives this optical signal) converts it back into the electrical format, processes this data, and sends it to downstream equipment.

How the STE Operates in the Optical Transport Network (OTN)

A Source STE will manage and monitor the transmission of this data (across a Section) by encapsulating this data into OTUk/OTUkV frames.

This Source STE will encapsulate this (ODUk) data by generating and inserting some overhead data (that we call the OTUk-SMOH [Section Monitoring Overhead]) into these OTUk/OTUkV frames.

NOTE: In some of my other posts, I refer to this Source (or Transmitting) STE as the OTUk/ODUk_A_So, OTUk_TT_So, and OTSi/OTUk_A_S0 or OTSiG/OTUk_A_So atomic functions.

The Sink (or Receiving) STE will use this OTUk-SMOH to manage data reception across the Section.

NOTE: In my other posts, I also refer to this Sink (or Receiving) STE as the OTUk/ODUk_A_Sk, OTUk_TT_Sk, and OTSi/OTUk_A_Sk or OTSiG/OTUk_A_Sk atomic functions.

The STE STE will manage the reception of data across the Section by using this OTUk-SMOH to check for data transmission errors and service-affecting defects.

What is the OTUk-SMOH (Section Monitoring Overhead)?

But when we say “OTUk-SMOH,” what exactly do we mean?

Figure 2 illustrates the OTUk Overhead data (within an OTUk frame) that the Section Terminating Equipment will process and terminate as it manages data transmission across a Section.

This figure also highlights a particular field (regarding Section Monitoring). This figure highlights the Section Monitoring field.

Figure 2, Illustration of an OTUk Frame with the OTUk SMOH Fields highlighted

I highlight the SM (or Section Monitoring) field because the actual OTUk-SMOH (that the Sink STE will use to check for the presence of defects or errors) resides within the Section Monitoring (or SM) field (within the OTUk Overhead).

In Figure 3, I focus on the Section Monitoring field and illustrate the byte format of this 3-byte field.

Figure 3, Illustration of the Byte-Format of the Section Monitoring field.

Figure 3 shows that the Section Monitoring field contains the following three byte-fields.

The BIP-8 Byte
The TTI Byte and
The Section Monitoring (or SM) Byte

In Figure 4, I further focus on the SM Byte and show the bit format of that particular byte field.

Figure 4, Bit-Format of the SM (Section Monitoring) Byte – within the Section Monitoring field

If you have seen the OTUk Frame post, Figures 2 through 4 should look familiar.

All of the overheads fields that the Sink STE will need to check for OTUk-related defects and errors (not including FEC) reside within the SM field.

Hence, the OTUk-SMOH is the Section Monitoring field within the OTUk Overhead.

NOTE: For “nuts and bolts” details on the Source and Sink STEs handling and processing the OTUk-SMOH, check out the posts on the following Atomic Functions.

For the Source STE
- OTUk/ODUk_A_So – OTUk to ODUk Adaptation Source Function
- OTUk_TT_So – OTUk Trace Termination Source Function
- OTSi/OTUk_A_So – OTSi to OTUk Adaptation Source Function (Single-Lane Applications)
- OTSiG/OTUk_A_So – OTSiG to OTUk Adaptation Source Function (Multi-Lane OTU3/OTU4 Applications)
For the Sink STE
- OTUk_TT_Sk – OTUk Trace Terminating Sink Function
- OTUk/ODUk_A_Sk – OTUk to ODUk Adaptation Sink Function
- OTSi/OTUk_A_Sk – OTSi to OTUk Adaptation Sink Function (Single-Lane Applications)
- OTSiG/OTUk_A_Sk – OTSiG to OTUk Adaptation Sink Function (Multi-Lane OTU3/OTU4 Applications)

Now let’s proceed to show an example of STE and its Section.

AN EXAMPLE OF AN STE AND ITS SECTION

Figure 5 illustrates an STE and Section within a typical OTN network connection.

Figure 5, Illustration of the STE and Section (from End to End) in a Typical OTN System

Finally, Figure 5 shows that the Section and STE begin (and end) before and after the OTUk Framer Block.

Please note that the STE also includes the OTUk Framer blocks in this Figure.

The OTUk Framer Blocks (and, in some cases, the OTUk Transceiver Blocks) are responsible for generating and inserting the OTUk-SMOH into the outbound OTUk data stream.

These same functional blocks are also responsible for processing and terminating the OTUk-SMOH within the incoming OTUk data stream.

Throughout numerous blog posts, we discuss the generation and processing of the OTUk-SMOH in detail.

Examples of STE

The following is a list of examples of the various types of OTN STE that are being deployed into the network infrastructure today.

Any 3R type of repeater.
Any network element that takes electrical data and maps it into an OTUk signal for transport to another terminal over an optical (or copper) connection (e.g., equipment that transmits data through sub-marine links, etc.).
CFP Optical Modules that also contains the DSP Transceiver.
Line Cards that include CFP2/CFP4 Optical Modules and OTN Framers.

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Introduction

Should One Defect Lead to Many Other Defects?

How about the dLOF Condition?

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What about the dLOM Condition?

How about the dTIM Condition?

Many Defects, all due to the dLOS-P Condition

Hence the Purpose of Defect Correlation

For the OTSi/OTUk_A_Sk Atomic Function

For the OTUk_TT_Sk Function

So Now Let’s Study some of these 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)

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

Summary

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What is Section Terminating Equipment (STE) for OTN Applications?

What is a Section?

What is an STE (Section Terminating Equipment)?

The Source STE Operation In the Transmit Direction

Sink STE Operation In the Receive Direction

Summarizing our Definitions of Section and STE

How the STE Operates in the Optical Transport Network (OTN)

What is the OTUk-SMOH (Section Monitoring Overhead)?

AN EXAMPLE OF AN STE AND ITS SECTION

Examples of STE

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