OTN – Lesson 9 – Video 11 – OTU Layer Defect Handling Requirements and Scenarios

In this video, we presume that some OTUk-Layer circuitry is declaring a certain defect condition. We then determine how OTU (and in some cases) ODU-layer circuitry is expected to respond.

OTN – Lesson 9 – Video 11 – OTU Layer Defect Scenarios

This blog post presents a video that discusses the various defects that OTN equipment can declare at the OTU Layer. Additionally, this video will describe how OTU Layer circuitry should respond to and handle these defect conditions.

This video will state whether OTU Layer circuitry should:

Transmit the SM-BDI Indicator upstream, or
Transmit the ODU-AIS Maintenance Signal downstream,
Increment Certain Performance Monitoring parameters, or
Halt Incrementing Certain Performance Monitoring parameters.

In response to each type of defect that OTU Layer circuitry can declare.

This video covers both Single-Lane (e.g., OTSi/OTUk_A_Sk) and Multi-Lane (e.g., OTSiG/OTUk_A_Sk) applications.

Continue reading “OTN – Lesson 9 – Video 11 – OTU Layer Defect Handling Requirements and Scenarios”

OTN – Lesson 9 – Video 6 – OTU Layer Sink Direction Circuitry – OTSi/OTUk_A_Sk Function

This post presents the 6th of the 11 Videos that covers training on Performance Monitoring at the OTU Layer. This post focuses on the Sink Direction OTU-Layer Atomic Functions.

OTN – Lesson 9 – Video 6 – OTU Layer Sink Direction Circuitry/Functionality – Part 4

This blog post contains a video that focuses on the OTSi/OTUk_A_Sk Atomic Function.

The OTSi/OTUk_A_Sk Function has many of the same features/functionality as the OTSiG/OTUk_A_Sk Function.

However, the OTSi/OTUk_A_Sk function supports the transport/reception of an OTU signal over a Single-Lane Connection (instead of 4 electrical lanes – such as the OTL3.4 or OTL4.4 Interface). Hence, this Atomic Function applies to OTU1 and OTU2 applications.

The main difference between the OTSi/OTUk_A_Sk function and the OTSiG/OTUk_A_Sk function is that the OTSi/OTUk_A_Sk function will detect and declare the OTUk-AIS defect.

Continue reading “OTN – Lesson 9 – Video 6 – OTU Layer Sink Direction Circuitry – OTSi/OTUk_A_Sk Function”

OTN – Lesson 9 – Video 3 – OTU Layer Sink Direction Circuitry – Part 1

This post presents the 3rd of 11 Videos that covers training on Performance Monitoring at the OTU-Layer. This post focuses on the Sink-Direction OTU-Layer Atomic Functions.

OTN – Lesson 9 – Video 3 – OTU Layer Sink Direction Circuitry/Functionality – Part 1

This blog post contains a video that begins our discussion of the Sink (Receive) Direction OTU-Layer Atomic Functions.

This Video starts with a brief re-cap of a portion of the OTSiG/OTUk_A_Sk Atomic Function (aka OTL4.4 Sink Terminal).

In this case, I specifically call out and identify portions of this Atomic Function that we discussed in Lessons 6 or 7 and those portions that we did not.

In Lesson 7, we discussed that part of the OTSiG/OTUk_A_Sk function handled OTL4.4 or OTL4.20 logical lane signals.
Lesson 6 also discussed part of the OTSiG/OTUk_A_Sk function that handled the OTL3.4 lane signal.

I also show you the differences between the OTU3 version of the OTSiG/OTUk_A_Sk function and that for OTU4.

Late in this Video, we start to discuss that portion of the OTSiG/OTUk_A_Sk function that handles the combined (composite) OTU3/OTU4 signal. This portion includes

Discussing how the OTSiG/OTUk_A_Sk function declares and clears the dLOF defect by reviewing the Frame Alignment Block – dLOF/In-Frame State Machine Diagram, and
Descrambling of the OTU3/OTU4 data stream.

Continue reading “OTN – Lesson 9 – Video 3 – OTU Layer Sink Direction Circuitry – Part 1”

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.

dAIS – (OTUk-AIS)
dLOS-P (Loss of Signal-Path Defect)
dLOF – Loss of Frame Defect
dLOM – Loss of Multi-Frame Defect
dLOFLANE – Loss of Frame-Lane Signal (OTL3.4/OTL4.4/OTL4.10 Applications Only) (*)
dLOL – Loss of Lane Alignment (OTL3.4/OTL4.4/OTL4.10 Applications Only)(*)
dIAE – Input Alignment Error Defect (*)
dBIAE – Backward Input Alignment Error Defect (*)
dTIM – OTUk-Trace Identifier Mismatch Defect (*)
dBDI – OTUk-Backward Defect Indicator Defect
dDEG – OTUk-Signal Degrade Defect (*)

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:

dLOS-P – The Loss of Signal-Path Defect Condition
dAIS – The (OTUk-AIS) Defect Condition
dLOF – The Loss of Frame Defect Condition
dLOM – The Loss of Multi-Frame Defect Condition

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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Declaring/Clearing the dLOM Defect

This post briefly describes the dLOM (Loss of Multi-Frame) defect for OTN applications. This post describes how an OTN STE should declare and clear the dLOM defect condition.

How an OTN STE should declare and clear the dLOM (Loss of Multi-Frame) Defect Condition.

The purpose of this post is to describe how an OTN STE (Section Terminating Equipment) will declare and clear the dLOM (Loss of Multi-Frame) Defect Condition.

Suppose you’re analyzing this topic from an ITU-T G.798 Atomic Function standpoint. In that case, I will tell you that the two atomic functions that are responsible for declaring and clearing the dLOM defect condition are:

The OTSi/OTUk_A_Sk Function, and
The OTSiG/OTUk_A_Sk Function.

Each of these atomic functions includes the dLOM Detection circuit.

A Note about Terminology:

Throughout this blog post, I will refer to the entity containing the dLOM Detection circuit (and declares/clears the dLOM defect condition ) as the Sink STE.

I’m using this terminology because it is technically correct, and it is much simpler to use that word than to use: OTSi/OTUk_A_Sk or OTSiG/OTUk_A_Sk functions. However, I will use atomic function-related terms below in Table 1 (at the end of this post).

A Brief Review of the OTUk Frame Structure

In the OTUk Post, I stated that the OTUk frame consists of a single multi-frame alignment signal (MFAS) byte.

I show a drawing of the OTUk Frame Structure, with the MFAS-field highlighted below in Figure 1.

Figure 1, Drawing of the OTUk Frame Structure, with the MFAS-Field Highlighted

In the OTUk blog post, we mentioned that the Source STE Terminal would generate and transmit OTUk traffic in the form of back-to-back multi-frames. Each of these multi-frames consists of 256 consecutive OTUk frames.

The MFAS Byte Increments with each Frame

The S0urce STE Terminal will designate one of these OTUk frames as the first frame within a multi-frame by setting the MFAS byte (within that particular frame) to 0x00. When the Source STE generates and transmits the next OTUk frame, it will set the MFAS byte (within that OTUk frame) to 0x01.

The Source STE will continue incrementing the MFAS byte within each outbound OTUk frame until it has transmitted the 256th frame within this multi-frame. And it has set the MFAS byte to 0xFF (or 255 in decimal format).

At this point, the Source STE has completed its transmission of a given 256-frame OTUk multi-frame, and it will start to transmit the very next multi-frame.

The Source STE will show that it is transmitting the next 256-frame multi-frame by setting the MFAS byte to 0x00 (once again) and then incrementing the value that it writes into each MFAS byte (within each outbound OTUk frame) until it reaches 0xFF (255).

This process repeats indefinitely.

Now that we have re-acquainted ourselves with the MFAS byte, we can discuss how a Sink STE will declare and clear the dLOM (Loss of Multi-frame) defect condition.

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A Basic Requirement before We can Clear the dLOM Defect Condition

Before I can begin to discuss the dLOM-Related State Machines (and the actual mechanics of declaring and clearing the dLOM defect). I need to be very clear about one thing.

The Sink STE can’t clear the dLOM defect condition unless it has first cleared the dLOF defect condition.

Additionally, anytime the Sink STE declares the dLOF defect condition, it loses MFAS synchronization.

There are several reasons for this:

The Sink STE needs to find the FAS-field first

The MFAS byte-field resides in Row 1, Byte 7 (within the OTUk frame). It is adjacent to the FAS field. Therefore, once the Sink STE finds the FAS field, it can quickly locate the MFAS byte.

The FAS field is FAR more accessible for the Sink STE to find than the MFAS byte.

There are two reasons for this.

The FAS field consists of a defined/fixed pattern. In other words, the FAS fields never change in value (except for the 3rd OA2 byte – for OTL4.4 and OTL4.10 applications ONLY).

The Sink STE can locate the FAS field by only looking for these fixed patterns. On the other hand, the MFAS byte value changes with every OTUk frame.

Additionally, we NEVER scramble the FAS field. However, we do scramble the MFAS byte.

I show a drawing of an OTUk frame below that identifies the portions of the OTUk frame that the Remote Source STE has scrambled before transmitting this OTUk frame to the Near-end Sink STE.

Figure 2, Drawing of an OTUk Frame – Showing the portions of the OTUk Frame that we scrambled.

Finally, the MFAS byte-field was scrambled by a Frame-Synchronous Scrambler (at the remote Source STE)

Therefore, you will need to descramble the MFAS byte (along with the rest of the OTUk frame) with a Frame-Synchronous Descrambler.

This means that the Sink STE needs proper synchronization with the incoming OTUk frames (e.g., clearing the dLOF defect) before we can even use this Frame-Synchronous Descrambler.

Now that I’ve made that point, I will discuss how the Sink STE declares and clears the dLOM defect.

We will first discuss dLOM-Related State Machines.

The dLOM-Related State Machines

Once again, whenever the Sink STE first powers up and receives a stream of OTUk data, it will first need to acquire FAS-frame synchronization with this incoming data stream, as described in the dLOF – Loss of Frame Defect blog post.

After the Sink STE has cleared the dLOF defect condition (and can now reliably locate the FAS field within each incoming OTUk frame), it can proceed to find the MFAS byte.

Figure 1 (above) shows that the MFAS byte resides in row 1, byte-column 7 (immediately following the FAS field) within the OTUk frame.

Whenever the Sink STE has acquired FAS-frame synchronization with the incoming OTUk frame (and cleared the dLOF defect condition), it will continuously operate simultaneously per two sets of state machines.

The OTUk-MFAS OOM/IM Low-Level State Machine and
The OTUk-dLOM Framing Alignment/Maintenance State Machine

These two state machines are hierarchical. In other words, one of these state machines operates at the low level (e.g., the OTUk-MFAS OOM/IM Low-Level State Machine), and the other state machine operates at a higher layer (on top of the low-level state machine).

I show the relationship between these two-state machines below in Figure 3.

Figure 3, Illustration of the relationship between the two dLOM-related State Machines

We will discuss these two state machines below.

The OTUk-MFAS OOM/IM Low-Level State Machine

We will discuss the OTUk-MFAS OOM/IM Low-Level State Machine first, and then we will discuss the OTUk-dLOM Framing Alignment/Maintenance State Machine later.

I show a drawing of the OTUk-MFAS OOM/IM Low-Level State Machine Diagram below in Figure 4.

Figure 4, Drawing of the OTUk-MFAS OOM/IM Low-Level State Machine Diagram

Figure 4 shows that the OTUk-MFAS OOM/IM Low-Level State Machine contains the following two states.

The LL-OOM (Low-Level – Out of Multi-frame) state and
The LL-IM (Low-Level – In Multi-frame) state

When the System Operator powers up the Sink STE circuitry, feeds an OTUk data stream and clears the dLOF defect, it will continually operate in one of these states.

The Sink STE will (on occasion) need to transition from one state to another.

We will discuss these two states below.

The LL-OOM State

Whenever the Sink STE first clears the dLOF defect condition, it will (initially) be operating in the LL-OOM (Low Level – Out of Multi-Frame) state.

At this point, the Sink STE either has not located the MFAS byte or is not yet in sync with the incrementing MFAS byte value within the incoming OTUk data stream.

While the Sink STE is operating in this state, it will begin to evaluate bytes (that it “believes” to be the MFAS byte-field) within each incoming OTUk frame. More specifically, the Sink STE will locate a given Candidate MFAS byte and read in its value.

The value of this MFAS byte will be between 0x00 and 0xFF (255) inclusively. The Sink STE will note this value, and it will then perform the following computation.

Next_Expected_MFAS_Value = MOD(Candidate_MFAS_Value + 1, 256);

In other words, the Sink STE will take the byte value (of the Candidate MFAS byte that it has read in) and (internally) increment this value by 1.

I hope you understand why we run this incremented Candidate_MFAS_Value through a Modulus Equation with a divisor of 256.

The Sink STE will assign this newly incremented value to the variable Next_Expected_MFAS_Value. We will use this “Next _Expected_MFAS_Value” to evaluate the next incoming OTUk frame.

The Sink STE will then wait through 16,320-byte periods (or one OTUk frame period) and then read in another Candidate MFAS value (from this next OTUk frame), and it will compare that byte value with the “Next_Expected_MFAS_Value” that it has computed.

If the Sink STE determines that this new “Candidate MFAS Value” does not match the “Next_Expected_MFAS_Value,” then it will go back and parse through the incoming OTUk data-stream and look for another byte-field (within row 1, column 7 of the incoming OTUk frame).

The Sink STE will continue to operate in the LL-OOM state.

On the other hand, if the Sink STE does (indeed) determine that the expression “Candidate MFAS value ” matches that of the “Next_Expected_MFAS_Value,” then it will transition into the LL-IM (Low-Level – In-Multi-Frame) state.

The LL-IM State

Once the Sink STE enters the LL-IM state, then it will continue to check for the presence of the correct (properly incrementing byte values within the MFAS byte) at OTUk frame intervals.

If the Sink STE can consistently locate these MFAS bytes at each OTUk frame interval (with the correct and properly incrementing values), it will remain in the LL-IM state.

However, if the Sink STE lost synchronization with the MFAS field (of each incoming OTUk frame), it could not locate the MFAS field for five (5) consecutive OTUk frame periods, then the Sink STE will transition back into the LL-OOM state.

NOTE: The OTUk-MFAS OOM/IM Low-Level State Machine algorithm is tolerant of bit errors. In other words, the presence of occasional bit-errors or a burst of bit-errors is not enough to cause the Sink STE to transition from the LL-IM back to the LL-OOM state.

Now that we have discussed the OTUk-MFAS OOM/IM Low-Level State Machine, let’s move on and describe the OTUk-dLOM Framing Alignment/Maintenance State Machine.

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The OTUk-dLOM Framing Alignment/Maintenance State Machine

I show a drawing of the OTUk-dLOM Framing Alignment/Maintenance State Machine Diagram below in Figure 5.

Figure 5, Drawing of the OTUk-dLOM Framing Alignment/Maintenance State Machine Diagram

Hence, Figure 5 shows that the OTUk-dLOM Framing Alignment/Maintenance State Machine diagram consists of four states.

dLOM/LL-OOM State
dLOM/LL-IM State
In-Multiframe/LL-IM State
In-Multiframe/LL-OOM State

The OTUk-dLOM Framing Alignment/Maintenance State Machine rides on top of the OTUk-MFAS OOM/IM State Machine. Therefore, we can think of the OTUk-dLOM Framing Alignment/Maintenance State Machine as an extension of this Low-Level State Machine.

Hence, to illustrate that point, I have redrawn the OTUk-dLOM Framing Alignment/Maintenance State Machine diagram (that I show in Figure 5) to show the conditions that cause us to transition from one state to the next within the OTUk-MFAS OOF/IF State Machine. I show this redrawn figure below in Figure 6.

Figure 6, Illustration of the OTUk-dLOM Framing-Alignment/Maintenance State Machine Diagram (with OTUk-MFAS OOF/IF State Machine state change information shown).

The Sink STE will transition through each of the four states (within the OTUk-dLOM Framing Alignment/Maintenance State Machine) as it also transitions through the two states within the OTUk-MFAS OOM/IM Low-Level State Machine.

Whenever the System Operator powers up the Sink STE and starts receiving an OTUk data stream (once it has cleared the dLOF defect), it will continually operate in one of these four states. On occasion, the Sink STE will transition from one state to another. As it does this, it will declare or clear the dLOM defect, as shown in Figures 5 and 6.

We will now walk through the OTUk-dLOM Framing Alignment/Maintenace State Machine.

The dLOM/LL-OOM State

When the System Operator first powers up the Sink STE and is just starting to receive an OTUk data stream, the Sink STE will first clear the dLOF defect condition. Afterward, it will operate in the dLOM/LL-OOM State, as shown below in Figure 7.

Figure 7, Illustration of the OTUk-dLOM Framing Alignment/Maintenance State Machine Diagram with the dLOM/LL-OOM State Highlighted.

In the expression dLOM/LL-OOM, the reader should already know where the LL-OOM portion (of this state’s name) originates.

When we were discussing the OTUk-MFAS OOF/IF Low-Level State Machine (up above), we stated that whenever we first power up the Sink STE and it is just starting to receive an OTUk data-stream (and has cleared the dLOF defect condition), it will be operating in the LL-OOM state.

What does it mean to be in the dLOM/LL-OOM State?

The dLOM portion (of the expression dLOM/LL-OOM) means that the Sink STE is currently declaring the dLOM defect condition while operating in this particular state.

In summary, whenever the Sink STE is operating in the dLOM/LL-OOM state (within the OTUk-dLOM Framing Alignment/Maintenance State Machine), then we can state the following as fact:

The Sink STE is operating in the LL-OOM State (within the Lower-Level state machine, as we discussed earlier) and
The Sink STE is also declaring the dLOM defect condition (as the name of this state suggests).

Whenever the Sink STE operates in this state, it has NOT located the MFAS byte-fields within the incoming OTUk data stream. As far as the Sink STE is concerned, it only receives some stream of back-to-back OTUk frames. It cannot make sense of anything else within this data stream.

While the Sink STE operates in this state, it will parse through the incoming OTUk data stream and look for the MFAS byte-field.

Whenever the Sink STE has received (what it believes to be the MFAS byte because it immediately follows the FAS field), it will read in the contents of this “Candidate MFAS field.”

Next, the Sink STE will take that “Candidate MFAS field,” and it will insert that value into the following equation and compute a value for Next_Expected_MFAS_Value:

Next_Expected_MFAS_Value = MOD(Candidate_MFAS_Field + 1, 256)

Afterward, the Sink STE will wait an entire OTUk frame period (e.g., 16,320 bytes) later, and it will read out the contents of (what it believes is the MFAS byte). We will call this new byte value the “New_Candidate_MFAS_Value.”

Next, the Sink STE will compare that “New_Candidate_MFAS_Value” with its locally computed “Next_Expected_MFAS_Value,” by testing the following equation:

Next_Expected_MFAS_Value == New_Candidate_MFAS_Value;

If the New_Candidate_MFAS_Value fails the Test

If the Sink STE determines that the New_Candidate_MFAS_Value does NOT match the Next_Expected_MFAS_Value, then it has NOT located the MFAS byte. In this case, the Sink STE will continue to parse through (and search) the incoming OTUk data stream for another candidate MFAS byte.

It will also remain in the dLOM/LL-OOM state.

If the New_Candidate_MFAS_Value passes the Test

On the other hand, if the New_Candidate_MFAS_Value does (indeed) match the value for the Next_Expected_MFAS_Value, then the Sink STE will conclude that it has located the MFAS byte value. In this case, the Sink STE will transition from the LL-OOM state to the LL-IM state within the OTUk-MFAS OOM/IM State Machine.

As the Sink STE makes this transition within the low-level state machine, it will also transition from the dLOM/LL-OOM to the dLOM/LL-IM state within the OTUk-dLOM Framing Alignment/Maintenance State Machine.

The dLOM/LL-IM State

I illustrate the OTUk-dLOM Frame Alignment/Maintenance State-Machine diagram with the dLOM/LL-IM State highlighted below in Figure 8.

Figure 8, Illustration of the OTUk-dLOM Frame Alignment/Maintenance State-Machine Diagram, with the dLOM/LL-IM State highlighted

Whenever the Sink STE has transitioned into the dLOM/LL-IM State, we can state that the Sink STE has located (what appears to be) the MFAS-byte-field within the incoming OTUk data stream.

NOTES:

We refer to this state as the dLOM/LL-IM state because the Sink STE is still declaring the dLOM defect condition (just as it was while operating in the dLOM/LL-OOM state). However, the “LL-IM” portion of the state’s name (e.g., dLOM/LL-IM) reflects the fact that the Sink STE has transitioned into the LL-IM (Low-Level In-Multi-frame) state within the lower-level state machine.
I realize that calling this state the dLOM (Loss of Multi-Frame)/LL-IM (In-Multi-Frame) state is a bit of an oxymoron. In this case, the Sink STE is in-multi-frame because it has located the MFAS byte-field. However, it is still declaring the dLOM defect condition.

While the Sink STE is still operating in the dLOM/LL-IM state, it will perform the task of continuing to confirm whether or not it has located the MFAS byte (within the incoming OTUk data stream).

Whenever the Sink STE is operating in this state, two things can happen from here.

It can eventually transition (or advance) into the “In-Multi-Frame/LL-IM” state, or
It can transition (or regress) back into the dLOM/LL-OOM state.

We will discuss each of these possible events below.

Advancing to the In-Multi-Frame/LL-IM State

ITU-T G.798 requires that the Sink STE remain in the LL-IM state (at the low level) for at least 3ms before it can transition into the next state.

If the Sink STE remains in the dLOM/LL-IM state for at least 3ms (and continues to locate the MFAS byte accurately), then it will do all of the following:

It will transition into the “In-Multi-Frame/LL-IM” state within the OTUk-dLOM Frame Alignment/Maintenance State Machine, and
It will clear the dLOM defect condition (e.g., dLOM ⇒ 0).

I will discuss the In-Multi-Frame/LL-IM State later in this blog post.

Regressing to the dLOM/LL-OOM State

On the other hand, if the Sink STE (while in the dLOM/LL-IM state) were to lose synchronization with the MFAS byte, such that it cannot locate the MFAS byte for at least five (5) consecutive OTUk frame periods, then the Sink STE will transition back into the dLOM/LL-OOM state.

This means that the Sink STE will transition from the LL-IM state back into the LL-OOM state (at the Lower-Level State Machine).

Additionally, the Sink STE will continue to declare the dLOM defect condition.

The In-Multi-Frame/LL-IM State

Once the Sink STE has reached the In-Multi-frame/LL-IM state, we can say that it operates in the NORMAL (or intended) state. We expect the Sink STE to spend most of its operating lifetime in this state.

I show a drawing of the OTUk-dLOM Framing Alignment/Maintenance State Machine Diagram, with the In-Multi-frame/LL-IM State highlighted below in Figure 9.

Figure 9, Illustration of the OTUk-dLOM Framing Alignment/Maintenance State Machine Diagram, with the In-Multi-Frame/LL-IM State highlighted.

Whenever the Sink STE operates in the In-Multi-frame/LL-IM state, it can locate the MFAS byte-fields within each incoming OTUk frame.

What Does Clearing the dLOM Defect Mean?

Clearing the dLOM defect also means that the Sink STE should be ready to evaluate other aspects of this incoming OTUk data stream (which requires multi-frame alignment). This includes extracting the following types of data from the incoming OTUk/ODUk data stream.

Within the OTUk Overhead
- SM-TTI (Trail Trace Identification) Messages
Within the ODUk Overhead
- PM-TTI Messages
- APS/PCC Messages
Within the OPUk Overhead
- PSI Messages

The MFAS byte is also critical for those applications where we are mapping and multiplexing lower-speed ODUj tributary signals into higher-speed ODUk server signals (where k > j).

Of course, the Sink STE is telling the whole world this fact by clearing the dLOM defect condition.

Whenever the Sink STE operates in the In-Multi-frame/LL-IM state, it is pretty tolerant of the occurrences of bit errors. In other words, if the Sink STE were to receive an OTUk frame, such that there was (for example) a single-bit error or even a burst of errors that affects an MFAS byte, the Sink STE would remain in the “In-Multi-frame/LL-IM” state.

On the other hand, if the Sink STE were to lose synchronization with the incoming OTUk data stream, such that it cannot locate valid MFAS bytes for five consecutive OTUk frames, then the Sink STE will transition from the LL-IM state into the LL-OOM state (within the Lower-Level State Machine).

Consequently, the Sink STE will transition from the In-Multi-frame/LL-IM state into the In-Multi-frame/LL-OOM state.

We discuss the In-Multi-frame/LL-OOM State below.

The In-Multi-frame/LL-OOM State

I show a drawing of the OTUk-dLOM Frame Alignment/Maintenance State Machine diagram with the In-Multi-frame/LL-OOM State highlighted below in Figure 10.

Figure 10, Drawing of the OTUk-dLOM Multi Frame Alignment/Maintenance State Machine diagram, with the In-Multiframe/LL-OOM State highlighted.

If the Sink STE transitions into the In-Multi-frame/LL-OOM state, this means the following things.

The Sink STE has transitioned from the LL-IM state into the LL-OOM state within the OTUk-MFAS OOM/IM Low-Level State Machine.
The Sink STE is still NOT declaring the dLOM (Loss of Multi-frame) defect condition (e.g., dLOM = 0).

NOTE: We refer to this state as the “In-Multi-frame/LL-OOM” state because the Sink STE is still NOT declaring the dLOM defect condition (just as it was NOT while operating in the In-Multi-frame/LL-IM state).

However, the LL-OOM portion of the state’s name (e.g., In-Multi-frame/LL-OOM) reflects that the Sink STE has transitioned into the LL-OOM state (within the Low-Level State Machine).

Only one of two possible things will happen whenever the Sink STE enters the In-Multi-frame/LL-OOM state.

The Sink STE will eventually re-acquire synchronization with the incoming MFAS bytes (within the OTUk data-stream), and it will advance back into the In-Multi-frame/LL-IM state or
The Sink STE does not re-acquire synchronization with the incoming MFAS bytes (within the OTUk data-stream), and it eventually regresses into the dLOM/LL-OOM state.

We will briefly discuss these two possible events below.

Advancing Back into the In-Multi-Frame/LL-IM State

If the Sink STE can locate and re-acquire the MFAS bytes (within the incoming OTUk data-stream), it will transition back into the In-Multi-Frame/LL-IM State. In this case, the Sink STE will continue to clear the dLOM defect condition (dLOM = 0).

Regressing to the dLOM/LL-OOM State

ITU-T G.798 requires that the Sink STE reside in the In-Multi-Frame/LL-OOM state for 3ms before it can transition into the dLOM/LL-OOM state and declare the dLOM defect condition.

This means that if the Sink STE cannot locate the MFAS-field (for 3ms after transitioning into the In-Multi-Frame/LL-OOM state), it will transition into the dLOM/LL-OOM state.

Whenever the Sink STE transitions into the dLOM/LL-OOM state, it will declare the dLOM defect condition (dLOM = 1).

In Summary

ITU-T G.798 requires that the Sink STE be able to locate the MFAS byte and consistently remain in the LL-IM state for at least 3ms before it can clear the dLOM defect condition (e.g., dLOM ⇒ CLEARED).

Further, ITU-T G.798 also requires that the Sink STE NOT be able to locate the MFAS byte and remain in this condition (e.g., the LL-OOM state) for at least 3ms before it can declare the dLOM defect condition (e.g., dLOM ⇒ DECLARED).

ITU-T G.798 imposes these 3ms persistency requirements (for declaring and clearing the dLOM defect) to prevent intermittent transitions between the LL-OOM and LL-IM states from causing sporadic changes (or chattering) in the dLOM state.

Table 1 summarizes the dLOM Defect Condition and how its State affects an OTN STE’s Operation.

Table 1, Summary of the dLOM Defect Condition and How its State affects an OTN STE’s Operation

Item	Description
Meaning of the dLOM Defect Condition	The Sink STE (or OTSi/OTUk_A_Sk or OTSiG/OTUk_A_Sk atomic function) will declare the dLOM defect, if it has cleared the dAIS and the dLOF defect conditions, but it is not able to reliably locate the MFAS bytes and OTUk Multi-frame boundaries, within the incoming OTUk data-stream. In other words, the Sink STE will declare the dLOM defect if it is able to find the FAS fields (within each incoming OTUk frame) but it still not able to reliably locate the MFAS bytes and align itself with each incoming 256-frame OTUk multi-frame.
Requirements to declare dLOM	The Sink STE (or OTSi/OTUk_A_Sk or OTSiG/OTUk_A_Sk atomic function) will declare the dLOM defect if it loses synchronization with the incoming MFAS bytes (and their increment value) for at least 3ms.
Requirements to clear dLOM	The Sink STE (or OTSi/OTUk_A_Sk or OTSiG/OTUk_A_Sk atomic function) will clear the dLOM defect if it is able to maintain synchronization with the incoming (and incrementing) MFAS byte-fields (within the OTUk data-stream) for at least 3ms.
Any defects that will suppress the dLOM defect?	Yes, the dAIS (OTUk-AIS) and dLOF defects. Or if upstream circuitry is declaring the Trail Signal Fail defect condition. If the Sink STE (or OTSi/OTUk_A_Sk or OTSiG/OTUk_A_Sk atomic function) declares any of these defect conditions, then it cannot declare the dLOM defect.
Impact of declaring the dLOF defect on other defect conditions within the Sink STE	The Sink STE (or OTSi/OTUk_A_Sk or OTSiG/OTUk_A_Sk atomic function) should not declare any of the following defects, whenever it is also declaring the dLOM defect. - dTIM, and - dDEG
Impact of declaring the dLOM defect to Performance Monitoring.	None

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Declaring/Clearing the dLOF Defect

This post briefly describes the dLOF (Loss of Frame) Defect Condition. It describes when an OTN STE should declare and clear the dLOF defect condition.

How an OTN STE should declare and clear the dLOF (Loss of Frame) Defect Condition

The purpose of this post is to describe how an OTN STE (Section Terminating Equipment) will declare and clear the dLOF (Loss of Frame) Defect Condition.

The OTSi/OTUk_A_Sk Function, and
The OTSiG/OTUk_A_Sk Function.

Each of these atomic functions includes the dLOF Detection circuit. However, if you look closely at the OTSiG/OTUk_A_Sk function post, you will see that I do show that this particular dLOF Detection circuit is optional.

A Note About Terminology:

Throughout this blog post, I will refer to the entity containing the dLOF Detection circuit (and declares/clears the dLOF defect condition) as the Sink STE.

I’m using this terminology because it is technically correct, and it is much simpler to use that word than to use the words: OTSi/OTUk_A_Sk or OTSiG/OTUk_A_Sk functions. However, I will use atomic function-related terms below in Table 1 (at the bottom of this post).

A Brief Review of the OTUk Frame Structure

In the OTUk Post, we mentioned that the OTUk frame consists of six framing alignment signal (FAS) bytes.

I present an illustration of the OTUk Frame Structure, with the FAS field highlighted below in Figure 1.

Figure 1, Illustration of the OTUk Frame Structure, with the FAS-Field Highlighted

Figure 1 shows that the FAS field consists of 3-byte fields (which I’ve labeled OA1) and another set of 3-byte fields, which I’ve labeled OA2.

The OA1 byte fields are each set to the fixed value of 0xF6. Similarly, the OA2 byte fields are each set to the specified value of 0x28.

Hence, this 6-byte FAS field will (for each OTUk frame) always have the fixed pattern of 0xF6, 0xF6, 0xF6, 0x28, 0x28, 0x28.

Since each OTUk frame is a 4080-byte column x 4-row structure, each frame contains 16,320 bytes. Therefore, we know that a Sink STE (or OTSi/OTUk_A_Sk function) will always receive this fixed pattern of six bytes (e.g., the FAS field) every 16,320 byte-periods.

Now that we are armed with this information, we can discuss how the Sink STE will declare and clear the dLOF defect.

The dLOF-Related State Machines

Anytime the Sink STE is powered-up and receives a stream of OTUk data, it will continually operate per two sets of state machines simultaneously.

The OTUk-FAS OOF/IF Low-Level State Machine and
The OTUk-dLOF Framing Alignment/Maintenance State Machine

These two state machines are hierarchical. In other words, one of these state machines operates at the low level (e.g., the OTUk-FAS OOF/IF Low-Level State Machine), and the other state machine (e.g., the OTUk-dLOF Framing Alignment/Maintenance State Machine) operates at a higher level (on top of the lower-level state machine).

I show the relationship between these two state machines below in Figure 2.

Figure 2, Illustration of the relationship between the two dLOF-Related State Machines

We will discuss each of these two state machines below.

The OTUk-FAS OOF/IF Low-Level State Machine

We will discuss the OTUk-FAS OOF/IF Low-Level State Machine first, and then we will discuss the OTUk-dLOF Framing Alignment/Maintenance State Machine later.

I present an illustration of the OTUk-FAS OOF/IF Low-Level State Machine Diagram below in Figure 3.

Figure 3, Illustration of the OTUk-FAS OOF/IF Low-Level State Machine Diagram

Figure 3 shows that the OTUk-FAS OOF/IF Low-Level State Machine consists of the following two states:

The LL-OOF (Low-Level Out-of-Frame) State and
The LL-IF (Low-Level In-Frame) State

From the moment the System Operator powers up the Sink STE circuitry and feeds an OTUk data stream to it (from the remote Source STE), it will continuously operate in one of these two states.

The Sink STE will (on occasion) need to transition from one state to another.

We will discuss each of these two states below.

The LL-OOF State

Whenever the System Operator first powers up a Sink STE and starts receiving an OTUk data stream (from the remote Source STE), it will operate in the LL-OOF state.

The Sink STE has not located the FAS-bytes (and the OTUk frame boundaries). Therefore, the Sink STE cannot “make sense” of this data.

While the Sink STE operates in this state, it will begin to parse through the incoming OTUk data stream. It will search for the occurrence of the FAS pattern within this data stream.

Earlier, I mentioned that this FAS-field is a 6-byte field that consists of the following fixed pattern: 0xF6, 0xF6, 0xF6, 0x28, 0x28, 0x28.

ITU-T G.798’s Requirement for Searching for the FAS field

ITU-T G.798 states that the Sink STE when operating the LL-OOF state, MUST look for a 4-byte subset of this 6-byte FAS pattern. Therefore, this 4-byte FAS pattern could be any one of the following patterns.

0xF6, 0xF6, 0xF6, 0x28
0xF6, 0xF6, 0x28, 0x28
or 0xF6, ox28, ox28, ox28

Whenever the Sink STE has detected a set of four consecutive bytes that resembles a four-byte subset of the FAS field (as we’ve listed above), then the Sink STE will then wait an entire OTUk frame period (e.g., 16,320 bytes later) and it will check and see if that same FAS pattern is present then again.

If the Sink STE does not find the FAS pattern (one OTUk frame period later), then it will continue to parse through the incoming OTUk data stream and search for a 4-byte subset of the FAS pattern.

The Sink STE will also continue to operate in the LL-OOF state.

On the other hand, if the Sink STE does (indeed) find the FAS pattern (one OTUk frame period later), then the Sink STE will transition into the LL-IF state.

We will discuss the LL-IF State below.

The LL-IF State

Once the Sink STE enters the LL-IF state, it will continue to check for the presence of the FAS field at OTUk-frame intervals.

NOTE: Once the Sink STE enters the LL-IF state, ITU-T G.798 only requires it to check for a 3-byte subset of the FAS-field. Specifically, the standard states that the Sink STE must continue to check for the presence of the OA1, OA2, and OA2 (e.g., 0xF6, 0x28, 0x28) patterns at each OTUk frame interval.

As long as the Sink STE can consistently locate these FAS bytes at each OTUk frame interval, it will remain in the LL-IF state.

However, suppose the Sink STE were to lose synchronization with the FAS-field (of each incoming OTUk frame) such that it could not locate the FAS-field for five (5) consecutive OTUk frame periods. In that case, the Sink STE function will transition back into the LL-OOF state.

NOTE: The OTUk-FAS OOF/IF Low-Level State Machine algorithm tolerates bit errors. In other words, the presence of occasional bit errors is not enough to cause the Sink STE to transition from the LL-IF state back into the LL-OOF state.

Now that we have discussed the OTUk-FAS OOF/IF Low-Level State Machine, let’s move on and discuss the OTUk-dLOF Framing Alignment/Maintenance State Machine.

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The OTUk-dLOF Framing Alignment/Maintenance State Machine

Figure 4 illustrates the OTUk-dLOF Framing Alignment/Maintenance State-Machine Diagram.

Figure 4, Illustration of the OTUk-dLOF Framing-Alignment/Maintenance State Machine Diagram (with State Machine Transition Criteria shown)

Hence, Figure 4 shows that the OTUk-dLOF Framing Alignment/Maintenance State Machine Diagram consists of the following four states.

dLOF/LL-OOF State
dLOF/LL-IF State
In-Frame/LL-IF State
In-Frame/LL-OOF State

The OTUk-dLOF Framing Alignment/Maintenance State Machine “rides” on top of the OTUk-FAS OOF/IF State Machine. Therefore, we can think of the OTUk-dLOF Framing Alignment/Maintenance State Machine as an extension of this Low-Level State Machine.

Hence, to illustrate that point, I have redrawn the OTUk-dLOF Framing Alignment/Maintenance State Machine diagram (that I show in Figure 4) to also show the underlying state changes within the OTUk-FAS OOF/IF State Machine. I show this redrawn figure below in Figure 5.

Figure 5, Illustration of the OTUk-dLOF Framing-Alignment/Maintenance State Machine Diagram (with OTUk-FAS OOF/IF State Machine state change information shown).

The Sink STE will transition through each of the four states (within the OTUk-dLOF Framing Alignment/Maintenance State Machine) as it also transitions between the two states within the OTUk-FAS OOF/IF Low-Level State Machine.

Whenever the System-Operator powers up the Sink STE and starts receiving an OTUk data stream, it will continually operate in one of these four states. On occasion, the Sink STE will transition from one state to another. As it does, it will either declare or clear the dLOF defect, as shown in Figures 4 and 5.

We will now “walk” through the OTUk-dLOF Framing Alignment/Maintenance State Machine.

The dLOF/LL-OOF State

Whenever the System Operator first powers up the Sink STE and is just starting to receive an OTUk data stream, it will initially operate in the dLOF/LL-OOF State, as I show below in Figure 6.

Figure 6, Illustration of the OTUk-dLOF Framing Alignment/Maintenance State Machine Diagram with the dLOF/LL-OOF State Highlighted.

In the expression dLOF/LL-OOF, the reader should already know where the LL-OOF portion (of this state’s name) originates.

When we were discussing the OTUk-FAS OOF/IF Low-Level State Machine (up above), we stated that whenever we power up the Sink STE, it is just starting to receive an OTUk data stream. It will be operating in the LL-OOF state.

What does it mean to be in the dLOF/LL-OOF State?

The dLOF portion (of the expression dLOF/LL-OOF) means that the Sink STE is currently declaring the dLOF defect condition while operating in this particular state.

In summary, whenever the Sink STE is operating in the dLOF/LL-OOF state (within the OTUk-dLOF Framing Alignment/Maintenance State Machine), then we can state the following as fact:

The Sink STE is operating in the LL-OOF state (within the lower-level state machine, as we discussed earlier), and
The Sink STE also declares the dLOF defect condition (as the name of this state suggests).

Whenever the Sink STE operates in this state, it has NOT located the FAS fields nor the boundaries of any OTUk frames within the incoming OTUk data stream. As far as the Sink STE is concerned, it receives nonsensical data.

While the Sink STE is operating in this state, it will parse through the incoming OTUk data stream and look for the four-byte subset of the FAS field (as we discussed earlier).

Whenever the Sink STE has detected a set of four consecutive bytes that resembles a four-byte subset of the FAS field, the Sink STE will then wait an entire OTUk frame period (e.g., 16,320 bytes) later, and it will check and see if that same FAS pattern is again present, within the OTUk data-stream.

If the Sink STE Fails to Find the FAS field

If the Sink STE does not find the FAS pattern (one OTUk frame period later), it will continue to parse through (and search) the incoming OTUk data stream for that 4-byte subset of the FAS pattern.

It will also remain in the dLOF/LL-OOF state.

If the Sink STE Successfully Locates the FAS field

On the other hand, if the Sink STE does (indeed) find the FAS pattern (one OTUk frame period, later), then the Sink STE will transition from the LL-OOF state to the LL-IF state within the OTUk-FAS OOF/IF State Machine.

As the Sink STE makes this transition within the low-level state machine, it will also transition from the dLOF/LL-OOF to the dLOF/LL-IF state within the OTUk-dLOF Framing Alignment/Maintenance State Machine.

The dLOF/LL-IF State

I illustrate the OTUk-dLOF Frame Alignment/Maintenance State-Machine diagram with the dLOF/LL-IF State highlighted below in Figure 7.

Figure 7, Illustration of the OTUk-dLOF Frame Alignment/Maintenance State-Machine Diagram, with the dLOF/LL-IF State highlighted.

Whenever the Sink STE has transitioned into the dLOF/LL-IF state, we can state that the Sink STE has located (what appears to be) the FAS fields within the incoming OTUk data stream.

NOTES:

We refer to this state as the dLOF/LL-IF state because the Sink STE is still declaring the dLOF defect condition (just as it was while operating in the dLOF/LL-OOF state). However, the “LL-IF” portion of the state’s name (e.g., dLOF/LL-IF) reflects the fact that the Sink STE has transitioned into the LL-IF (Low-Level In-Frame) state within the lower-level state machine.
I realize that calling this state the dLOF (Loss-of-Frame)/LL-IF (In-Frame) state is a bit of an oxymoron. In this case, the Sink STE is in-frame because it has located the FAS fields. However, it is still declaring the dLOF defect condition.

While the Sink STE is operating in the dLOF/LL-IF state, it will perform the task of continuing to confirm whether or not it has located the FAS bytes (within the incoming OTUk data stream).

Two possible things can happen from here whenever the Sink STE is operating in this state.

It can eventually transition (or advance) into the “In-Frame/LL-IF” state, or
It can transition (or regress) back into the dLOF/LL-OOF state.

We will discuss each of these possible events below.

Advancing to the In-Frame/LL-IF State

ITU-T G.798 requires that the Sink STE remain in the LL-IF state (at the low level) for at least 3ms before it can transition into the next state.

If the Sink STE remains in the dLOF/LL-IF state for at least 3ms (and continues to locate the FAS bytes accurately), then it will do all of the following:

It will transition into the “In-Frame/LL-IF” state within the OTUk-dLOF Frame Alignment/Maintenance State Machine, and
It will clear the dLOF defect condition (e;g., dLOF ⇒ 0).

I will discuss the In-Frame/LL-IF State later in this blog post.

Regressing to the dLOF/LL-OOF State

On the other hand, if the Sink STE (while operating in the dLOF/LL-IF state) were to lose synchronization with the FAS byte-fields, it can no longer locate the FAS bytes for at least five (5) consecutive OTUk frame periods. The Sink STE will transition back into the dLOF/LL-OOF state.

This means that the Sink STE will transition from the LL-IF state back into the LL-OOF state (within the Lower-Level State Machine).

Additionally, the Sink STE will continue to declare the dLOF defect condition.

The In-Frame/LL-IF State

Once the Sink STE has reached the In-Frame/LL-IF state, we can say that it operates in the NORMAL (or intended) state. We expect the Sink STE to spend most of its operating lifetime in this state.

I illustrate the OTUk-dLOF Framing Alignment/Maintenance State Machine Diagram with the In-Frame/LL-IF State highlighted below in Figure 8.

Figure 8, Illustration of the OTUk-dLOF Framing Alignment/Maintenance State Machine Diagram, with the In-Frame/LL-IF State highlighted.

Whenever the Sink STE is operating in the In-Frame/LL-IF state, it can now locate the boundaries of each incoming OTUk frame. This also means that the Sink STE should be ready to start making sense of the data within the incoming OTUk data stream. It can now begin to evaluate the OTUk data stream for other defects or error conditions.

Of course, the Sink STE is telling the whole world this fact by clearing the dLOF defect condition.

Whenever the Sink STE operates in the In-Frame/LL-IF state, it is pretty tolerant of the occurrences of bit errors. In other words, if the Sink STE were to receive an OTUk frame, such that there was (for example) a single bit-error or even a burst of errors that affects one set of FAS bytes, the Sink STE would remain in the “In-Frame/LL-IF” state.

On the other hand, if the Sink STE were to lose synchronization with the incoming OTUk data stream, such that it cannot locate valid FAS bytes for five consecutive OTUk frames, then the Sink STE will transition from the LL-IF state into the LL-OOF state (within the Low-Level State Machine).

Consequently, the Sink STE will transition from the In-Frame/LL-IF state into the In-Frame/LL-OOF state.

We discuss the In-Frame/LL-OOF State below.

The In-Frame/LL-OOF State

I illustrate the OTUk-dLOF Frame Alignment/Maintenance State Machine diagram with the In-Frame/LL-OOF State highlighted below in Figure 9.

Figure 9, Illustration of the OTUk-dLOF Frame Alignment/Maintenance State Machine diagram, with the In-Frame/LL-OOF State highlighted.

If the Sink STE transitions into the In-Frame/LL-OOF state, then it means the following things:

The Sink STE has transitioned from the LL-IF state into the LL-OOF state within the OTUk-FAS OOF/IF Low-Level State Machine.
The Sink STE is still NOT declaring the dLOF (Loss of Frame) defect condition (e.g., dLOF = 0).

NOTE: We refer to this state as the “In-Frame/LL-OOF” state because the Sink STE is still NOT declaring the dLOF defect condition (just as it was NOT while operating in the In-Frame/LL-IF state).

However, the LL-OOF portion of the state’s name (e.g., In-Frame/LL-OOF) reflects that the Sink STE has transitioned into the LL-OOF state (within the Low-Level State Machine).

Only one of two possible things will happen whenever the Sink STE enters the In-Frame/LL-OOF state.

The Sink STE will eventually re-acquire synchronization with the incoming FAS frames, and it will advance back into the In-Frame/LL-OOF state, or
The Sink STE does not re-acquire synchronization with the incoming FAS frames and eventually regresses into the dLOF/LL-OOF state.

We will briefly discuss each of these two possible events below.

Advancing Back into the In-Frame/LL-IF State

If the Sink STE can locate and re-acquire the 4-byte subset of the FAS-field, then it will transition back into the In-Frame/LL-IF state. In this case, the Sink STE will continue to clear the dLOF defect condition (dLOF = 0).

Regressing to the dLOF/LL-OOF State

ITU-T G.798 requires that the Sink STE reside in the In-Frame/LL-OOF state for 3ms before it can transition into the dLOF/LL-OOF state and declare the dLOF defect condition.

This means that if the Sink STE cannot locate the FAS field (for 3ms after transitioning into the In-Frame/LL-OOF state), it will transition into the dLOF/LL-OOF state.

Whenever the Sink STE transitions into the dLOF/LL-OOF state, it will declare the dLOF defect condition (dLOF = 1).

NOTE: If the Sink STE enters the dLOF/LL-OOF state from the In-Frame/LL-IF (by way of the In-Frame/LL-OOF state), it will operate with the exact frame-start location that it had when it was running in the In-Frame/LL-IF state.

In other words, the Sink STE will continue to look for the FAS-field in the exact location (e.g., N x 16,320-byte periods later, where N is an integer) in which it was locating the FAS-field while it was operating normally in the In-Frame/LL-IF state.

In Summary

ITU-T G.798 requires that the Sink STE be able to locate the FAS field and remain in the LL-IF state for at least 3ms before it can clear the dLOF defect condition (e.g., dLOF ⇒ CLEARED).

Further, ITU-T G.798 also requires that the Sink STE NOT be able to locate the FAS field and remain in this condition (e.g., the LL-OOF state) for at least 3ms before it can declare the dLOF defect condition (e.g., dLOF ⇒ DECLARED).

ITU-T G.798 imposes these 3ms persistency requirements (for declaring and clearing the dLOF defect) to prevent intermittent transitions between the LL-OOF and LL-IF states from causing rapid changes (or chattering) in the dLOF state.

Table 1 presents a summary of the dLOF Defect Condition and how its State affects an OTN STE’s operation when handling an OTUk signal over a Single Lane (e.g., the OTSi/OTUk_A_Sk function).

Table 1, Summary of the dLOF Defect Condition and How its State affects an OTN STE’s Operation when handling an OTUk signal over a Single Lane (e.g., the OTSi/OTUk_A_Sk function).

Item	Description
Meaning of dLOF	The Sink STE (or OTSi/OTUk_A_Sk function) will declare the dLOF defect if it is not able to reliably locate the FAS field (and in-turn, the boundaries) of the incoming OTUk frames within the incoming data-stream. In short, if the Sink STE (OTSi/OTUk_A_Sk function) is declaring the dLOF defect condition, then it is NOT able to make any sense of the data within its OTUk data-stream. The Sink STE (and downstream circuitry) cannot perform any useful functions on this data-stream until it clears this defect.
Requirements to declare dLOF	The Sink STE (or OTSi/OTUk_A_Sk function) will declare the dLOF defect if it loses synchronization with the incoming FAS fields (within the incoming OTUk datastream) for at least 3ms. Additionally, the Sink STE (or OTSi/OTUk_A_Sk function) must not be declaring the dAIS (OTUk-AIS) defect condition.
Requirements to clear dLOF	The Sink STE (or OTSi/OTUk_A_Sk function) will clear the dLOF defect if it is able to maintain synchronization with the incoming FAS fields (within the incoming OTUk data-stream) for at least 3ms.
Any defects that can suppress the dLOF defect?	Yes, the Sink STE (or OTSi/OTUk_A_Sk function) cannot declare the dLOF defect, if it is currently declaring either of the following defect. - dAIS (OTUk-AIS), - dLOS-P or - TSF (Trail Signal Failure) - if upstream (optical circuitry) asserts the AI_TSF input. NOTE: Please see the blog post on the OTSi/OTUk_A_Sk function for more information about the AI_TSF signal.
Impact of declaring the dLOF defect on other defect conditions within the Sink STE (or OTSi/OTUk_A_Sk or OTUk_TT_Sk functions)	The Sink STE (or OTSi/OTUk_A_Sk and OTUk_TT_Sk functions) should NOT declare any of the following defects, whenever it is declaring the dLOF defect condition. - dLOM - dTIM - dDEG NOTE: Please see the blog post for the OTUk_TT_Sk Atomic Function for more information about the dTIM, dDEG defects and why the dLOF defect will affect these defect conditions.
Impact of declaring the dLOF defect on Downstream Circuitry (e.g., the OTUk_TT_Sk function).	The Sink STE (or OTSi/OTUk_A_Sk function) should automatically assert the CI_SSF (Server Signal Fail) signals whenever it is declaring the dLOF defect condition. This indication will notify the downstream OTUk_TT_Sk function that there is a service-affecting defect upstream. NOTE: Please see the blog post for the OTSi/OTUk_A_Sk atomic function for more information about the CI_SSF signal.
Impact of declaring the dLOF defect to Performance Monitoring.	The Sink STE (or OTSi/OTUk_A_Sk function) must inhibit Performance Monitor tallying of the pFECcorrErr (Number of Symbol Errors Corrected by FEC) for the duration that it is declaring the dLOF defect condition. NOTE: Whenever the Sink STE or OTSi/OTUk_A_Sk function declares the dLOF defect condition, it will also affect Performance Monitoring activities within the OTUk_TT_Sk atomic function.

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Declaring/Clearing dAIS Defect (OTUk)

This post describes how an OTN STE (Section Terminating Equipment) should declare and clear the dAIS Defect Condition (OTUk-AIS).

How Should an OTN STE Declare and Clear the dAIS Defect Condition (OTUk-AIS)?

In another post, we describe the OTUk-AIS Maintenance Signal.

Further, in that post, I stated that ITU-T G.709 does not require that an OTN STE be able to generate and transmit the OTUk-AIS Maintenance Signal.

However, I also stated that ITU-T G.709 DOES require that an OTN STE be capable of receiving and processing the OTUk-AIS Maintenance signal, such that it can declare and clear the OTUk-AIS defect condition.

What about this Post?

This post will discuss how an STE should declare and clear the dAIS (OTUk-AIS) defect.

NOTE: Please do not confuse this particular dAIS Defect (in response to the detection of the OTUk-AIS Maintenance signal) with the other AIS Defect (in response to receipt of the ODUk-AIS Maintenance Signal).

Although their names are similar, they are two very different maintenance signals and defects.

The OTUk-AIS Maintenance Signal Post states that the OTUk-AIS Maintenance Signal is an Unframed PN-11 Pattern. More specifically, ITU-T G.709 defines this PN-11 sequence by the generating polynomial: 1 + x⁹ + x¹¹.

How to Detect the PN-11 Pattern?

If we want to detect, declare, and clear the dAIS condition, then we need to have some ability to detect this unframed PN-11 pattern.

Fortunately, the ITU-T Standard Committee did much of the work for us and defined such a circuit within ITU-T G.798.

I show this Inverse PN-11 Circuit below in Figure 1.

Figure 1, Illustration of the Inverse PN-11 Circuit

How Does this Inverse PN-11 Circuit Work?

This Inverse PN-11 Circuit makes up a big part of our dAIS Detection Circuit (that we also mention in the post on the OTSi/OTUk_A_Sk atomic function).

The user should apply the Recovered OTUk Data and Clock Signal at the IN and Clock inputs of our Inverse PN-11 Circuit, respectively.

If our OTUk data-stream is carrying the OTUk-AIS Maintenance signal (e.g., an Unframed PN-11 signal) and if we are applying this data to the IN input (of our circuit), then our Inverse PN-11 circuit will generate an All-Zeros Pattern at the Node, that I’ve labeled OUT.

I show our Inverse PN-11 Circuit, again, below in Figure 2. However, in this figure, I also highlight these two reference points.

Figure 2, Illustration of the Inverse PN-11 Circuit – with the Locations of the OTUk-AIS Maintenance Signal and the Resulting All-Zeros Pattern Highlighted.

Before we get too excited, we need to recognize that two conditions will cause our Inverse PN-11 circuit to generate an All-Zeros Pattern at the OUT node.

Our Inverse PN-11 Circuit will generate the All-Zeros pattern at the OUT Node whenever the OTUk-AIS Maintenance Signal is present at the IN input (to this circuit), and
Our Inverse PN-11 Circuit will also generate the AIl-Zeros pattern (at the OUT Node) whenever someone applies an All-Zeros Pattern at the IN Input.

OTUk-AIS Maintenance Signal or All-Zeros Pattern Signal at the IN input?

Hence, whenever we use the Inverse PN-11 Circuit to check for the OTUk-AIS Maintenance signal, we (of course) need to check the OUT Node (or our Inverse PN-11 Circuit).

However, we also need to check and ensure that we are NOT receiving an All-Zeros pattern at the IN input.

If we are TRULY receiving the OTUk-AIS Maintenance Signal, we will see an All-Zeros pattern at the OUT Node, while the signal at the IN input is NOT an All-Zeros pattern.

I summarize how the Inverse PN-11 Detector circuit works for various signals (at the IN input) below in Table 1.

Table 1, A Truth Table presenting How the Inverse PN-11 Detector Circuit responds to Various Signals (at the IN input)

IN Input	OUT Node	Comments
All-Zeros Signal	All-Zeros Signal	An All-Zeros pattern at the IN Input results in an All-Zeros pattern at the OUT Node. No OTUk-AIS.
Ordinary OTUk Traffic	Non All-Zeros Pattern Signal	Normal Traffic Situation
OTUk-AIS Maintenance Signal	All-Zeros Signal	The Presence of an All-Zeros Signal at the OUT Node, and the Non All-Zeros pattern at the IN input indicates OTUk-AIS.

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Criteria for Declaring the dAIS Defect?

OK, we now have a basic understanding of how the Inverse PN-11 Detector circuit works. We also know what signals to look for to determine if the Inverse PN-11 Circuit detects the OTUk-AIS Maintenance signal.

Let’s now move on to the full criteria for declaring the dAIS defect.

When checking for dAIS, ITU-T G.798 recommends that we continuously monitor both of the signals at the IN input signal and the OUT Node of our Inverse PN-11 Circuit.

ITU-T G.798 goes on to (effectively) state that we should continuously check these signals over a rolling 8192 bit-interval (or sliding window, if you will).

If our Inverse PN-11 circuit detects a set of three (3) consecutive strings each of 8192-bit periods (in length), such that BOTH of the following conditions are TRUE for each of these three 8192 bit-periods, then we MUST declare the dAIS defect condition.

The number of 1s bits at the OUT Node is less than 256; AND
the number of 1s bits at the IN Input is 256 or more.

I show an illustration of the dAIS Defect Declaration Criteria below in Figure 3.

Figure 3, Illustration of the dAIS (OTUk-AIS) Defect Declaration Criteria

Criteria for Clearing the dAIS Defect Condition

On the other hand, while we are declaring the dAIS defect, if our Inverse PN-11 circuit detects a set of three (3) consecutive strings, each of 8192-bit periods (in length) such that EITHER of the following conditions is TRUE for each of these three 8192 bit periods, then we MUST clear the dAIS defect condition.

If the number of 1s bits at the OUT Node is 256 or more, OR
If the number of 1s bits at the IN input is less than 256 in three consecutive 8192-bit intervals.

I show an illustration of the dAIS Defect Clearance Criteria below in Figure 4.

Figure 4, Illustration of the dAIS (OTUk-AIS) Defect Clearance Criteria

What Entities or Atomic Functions declare and clear the dAIS (OTUk-AIS) defect condition?

The OTSi/OTUk_A_Sk function is the only atomic function that contains an Inverse PN-11 Detector circuit. Hence, it is the one atomic function that will declare or clear the OTUk-dAIS Defect condition.

NOTE: For Multi-Lane Applications, the OTSiG/OTUk_A_Sk function does not contain an Inverse PN-11 Detector circuit nor declare or clear the dAIS Defect condition.

If for some reason, an OTL3.4 or OTL4.4 signal were carrying the OTUk-AIS Maintenance Signal (which, again, is an Unframed PN-11 Pattern), then the OTSiG/OTUk_A_Sk function (that is receiving this signal) would instead, continuously declare the dLOFLANE defect condition(*) within each of the 4 or 20 Logical Lanes.

This atomic function would also declare the dLOL defect(*) as well.

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What is the OTSi/OTUk_A_Sk Function?

This blog post briefly describes the OTSi/OTUk_A_Sk (OTSi to OTUk Adaptation Sink) Atomic Function. This post also describes how this atomic function declares and clears the dLOF, dLOM, and dAIS defects.

What is the OTSi/OTUk_A_Sk Atomic Function?

The expression: OTSi/OTUk_A_Sk is an abbreviation for the term: Optical Tributary Signal to OTUk Adaptation Sink Function.

This blog post will briefly describe the OTSi/OTUk_A_Sk set of atomic functions.

We discuss the OTSi/OTUk_A_Sk Atomic Function in detail in Lesson 9, within THE BEST DARN OTN TRAINING PRESENTATION…PERIOD!!!

Changes in Terminology

Before we proceed on with this post, we need to cover some recent changes in terminology. Before the June 2016 Version of ITU-T G.709, the standard documents referred to this particular atomic function as the OCh/OTUk_A_Sk function.

However, the standards committee has recently decided to change the wording from using the term OCh (for Optical Channel) to OTSi (for Optical Tributary Signal).

For completeness, I will tell you that ITU-T G.959.1 defines the term OTSi as:

“Optical signal that is placed within a network media channel for transport across the optical network. This may consist of a single modulated optical carrier or a group of modulated optical carriers or subcarriers”.

Hence, to “speak the same language” as the standard committee, we will call this atomic function the OTSi/OTUk_A_Sk atomic function.

Likewise, in another post, we will now call (what we used to call the OCh/OTUk_A_So function) the OTSi/OTUk_A_So function.

I have created another post that provides documentation of the relationships between some old (now obsolete) terms and the new (and approved) ones that our standard committee is currently using.

The OTSi/OTUk_A_Sk Function

The OTSi/OTUk_A_Sk function is any circuit that takes an OTSi electrical signal and converts this data back into the OTUk signal.

More specifically, the System-Designer will apply an OTSi signal (which will be a fully-framed and scrambled OTUk electrical signal that often includes Forward-Error-Correction) to the OTSi_AP input interface.

This function will convert this signal into OTUk data, clock, frame start, and multi-frame start signals.

This function will also decode the Forward-Error-Correction field (if available) and output these signals to downstream circuitry (such as the OTUk_TT_Sk function).

ITU-T G.798 states that the system designer can use this function for all OTUk rates (e.g., from OTU1 through OTU4).

However, in most cases, we will typically use the OTSi/OTUk_A_Sk function for OTU1 and OTU2 applications. We will usually use the OTSiG/OTUk_A_Sk atomic function for OTU3 and OTU4 applications.

We discuss the OTSiG/OTUk_A_Sk atomic function in another post.

Figure 1 presents a simple illustration of the OTSi/OTUk_A_Sk function.

Figure 1, Simple Illustration of the OTSi/OTUk_A_Sk function

ITU-T G.798 defines three versions of this particular function. I have listed these versions below in Table 1.

Table 1, List of the ITU-T G.798 -specified Versions for the OTSi/OTUk_A_Sk functions

Function Name	Description	Comments
OTSi/OTUk-a_A_Sk	OTSi to OTUk Adaptation Sink Function with ITU-T G.709 Standard FEC	Can be used for OTU1 through OTU4 applications.
OTSi/OTUk-b_A_Sk	OTSi to OTUk Adaptation Sink Function with No FEC	Cannot be used for OTU4 applications
OTSi/OTUk-v_A_Sk	OTSi to OTUk Adaptation Sink Function with Vendor-Specific FEC	Can be used for OTU1 through OTU4 applications.

Table 1 shows that the OTSi/OTUk-a_A_Sk and the OTSi/OTUk-v_A_Sk functions will compute and decode some sort of FEC field within the backend of each incoming OTUk frame.

However, this table also shows that the OTSi/OTUk-b_A_Sk version does not support FEC decoding.

Therefore, ITU-T G.798 states that one can use the OTSi/OTUk-a_A_Sk and OTSi/OTUk-v_A_Sk functions for OTU1 through OTU4 applications. Further, the standard recommends that the user NOT use the OTSi/OTUk-b_A_Sk function for OTU4 applications.

Network Terminals operating at the OTU4 rate are required to use Forward-Error-Correction.

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What Version (of the OTSi/OTUk_A_Sk function) will we Discuss Throughout this Post?

Throughout this post, we will be discussing the OTSi/OTUk-a_A_Sk version of this atomic function.

The OTSi/OTUk-b_A_Sk and OTSi/OTUk-v_A_Sk atomic functions do everything that the OTSi/OTUk-a_A_So does, except that the -b version does NO FEC Decoding and the -v version does FEC Decoding differently than what I describe here.

So What All Does this Atomic Function Do?

The OTSi/OTUk-a_A_Sk function will accept an OTSi data stream from the upstream Optical-to-Electrical Conversion circuitry. This function will perform the following tasks on this incoming data stream.

Descrambling – It will descramble this incoming data stream.
FEC Decoding – The function will decode the FEC field (within the backend of each incoming OTUk frame) and detect and correct most symbol errors within this data stream.
Extract the Frame-Start and Multi-Frame Start signals from this incoming data stream.
Detect and Flag the following service-affecting defect conditions
- dLOS-P – Loss of Signal – Path
- dAIS – Alarm Indication Signal (OTUk-AIS)
- dLOF – Loss of Frame
- dLOM – Loss of Multiframe
Assert the CI_SSF (Server Signal Fail Indicator) output signal (towards the downstream OTUk_TT_Sk function) anytime it declares any service-affecting defect conditions.
Output the remaining OTUk data stream, the OTUk clock signal, the Frame-Start, and Multi-Frame Start signals to downstream circuitry (e.g., typically the OTUk_TT_Sk atomic function).

Figure 2 illustrates a Unidirectional Connection where the OTSi/OTUk-a_A_Sk function “fits in” a system.

Figure 2, Illustration of an STE, transmitting an OTUk signal (over optical fiber) to another STE – the OTSi/OTUk-a_A_Sk function is highlighted.

Functional Description of this Atomic Function

Let’s now take a closer look at this function.

Figure 3 presents the Functional Block Diagram of the OTSi/OTUk-a_A_Sk Atomic Function.

Figure 3, Illustration of the Functional Block Diagram of the OTSi/OTUk-a_A_Sk Atomic Function

Therefore, Figure 3 shows that this function contains the following functional blocks

Clock Recovery and dLOS Detection Blocks
dAIS Detection Block
Frame Alignment and dLOF Detection Blocks
Descrambler Block
FEC Decoder Block
Multi-Frame Alignment and dLOM Detection Blocks
Consequent Actions Block

I will briefly discuss each of these functional blocks below.

The Clock Recovery and dLOS (Loss of Signal Defect) Detection Blocks

The Clock Recovery block is responsible for recovering the clock signal and data content within the incoming OTSi signal via the AI_PLD input pin.

To that end, I illustrate the OTSi/OTUk-a_A_Sk Functional Block Diagram with the Clock Recovery and dLOS Detection Blocks highlighted below in Figure 4.

Figure 4, Illustration of the OTSi/OTUk-a_A_Sk Functional Block Diagram, with the Clock Recovery and dLOS Detection Blocks highlighted.

Since the OTSi/OTUk-a_A_So atomic function (within the remote STE) should have scrambled this data stream, there should always be good timing content (or transitions) within the incoming OTSi signal so that this Clock Recovery block can acquire and extract out both a recovered clock signal and data-stream.

Suppose the Clock Recovery and dLOS Detection blocks determine a lengthy absence in signal transitions (within the incoming OTSi data-stream). It will declare the dLOS-P (Loss of Signal-Path) defect condition in that case.

Please check out the dLOS blog post for more information about the dLOS-P defect condition.

The OTSi/OTUk-a_A_Sk function will route this recovered clock and data signal to the dAIS Detector and Frame Alignment blocks for further processing.

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The dAIS (Alarm Indication State Defect) Detector Block

As the newly recovered clock and data signal travel to the Frame Alignment block, the dAIS Detector block will also parse through this data stream to see if it should declare or clear the dAIS (Alarm Indication Signal Defect) condition or not.

To make things more convenient, I present an illustration of the OTSi/OTUk-a_A_Sk Functional Block Diagram, with the AIS Detector block highlighted below in Figure 5.

Figure 5, Illustration of the OTSi/OTUk-a_A_Sk Functional Block Diagram, with the AIS Detector block highlighted.

In this case, the dAIS Detector block will check to see if the incoming data stream matches an OTUk-AIS maintenance signal.

ITU-T G.709 further states that the OTUk-AIS maintenance signal is an unframed PN-11 repeating pattern.

The standard defines the PN-11 sequence by the generating polynomial of 1 + x⁹ + x¹¹.

Please see the blog post on the OTUk-AIS Maintenance signal for more information about this type of signal.

Additionally, please see the dAIS post for more information on how the AIS Detection circuit declares and clears the dAIS defect condition.

The Frame Alignment and dLOF (Loss of Frame Defect) Detection Blocks

As long as the dAIS Detector block is NOT declaring the dAIS defect condition, then the Frame Alignment block will process the incoming recovered block and data stream.

To make things more convenient for you, I present an illustration of the OTSi/OTUk-a_A_Sk Functional Block Diagram. This block diagram highlights the Frame Alignment and dLOF Detection below in Figure 6.

Figure 6, Illustration of the OTSi/OTUk-a_A_Sk Functional Block Diagram with the Frame Alignment and dLOF Detection circuitry highlighted.

The incoming recovered data stream should be a full, scrambled OTUk frame. However, the FAS field (e.g., the three OA1 and OA2 byte fields) should NOT be scrambled.

The Frame Alignment block will parse through the FAS fields within the incoming OTUk data stream. This block and the dLOF (Loss of Frame) Detection Block will declare and clear the dLOF defect as appropriate.

Please see the blog post on the dLOF defect for more information about how the Frame Alignment and dLOF Detection blocks declare and clear the dLOF defect condition.

Descrambler Block

In the OTSi/OTUk-a_A_So blog post, we mentioned that the OTSi/OTUk-a_A_So function would scramble the content of each OTUk frame.

That function will scramble all bytes (within each OTUk frame) except for the FAS fields. This function will even scramble the MFAS field as well.

The purpose of the Descrambler block is to restore the content of each OTUk frame to its original state before being scrambled at the remote STE.

To that end, I illustrate the OTSi/OTUk-a_A_Sk Functional Block Diagram with the Descrambler block highlighted below in Figure 7.

Figure 7, Illustration of the OTSi/OTUk-a_A_Sk Functional Block Diagram, with the Descrambler block highlighted.

In the OTSi/OTUk-a_A_So function, we scrambled the contents of each OTUk frame, using the polynomial generating equation of 1 + x + x³ + x¹² + x¹⁶.

Therefore, the Descrambler block (within this function) will descramble the incoming OTUk data-stream (again) using the polynomial generating equation of 1 + x + x³ + x¹² + x¹⁶.

I show a simple diagram of how one can implement the Descrambler within their OTSi/OTUk-a_A_Sk function design below in Figure 8.

Figure 8, High-Level Block Diagram of the Frame Synchronous Descrambler

I discuss the Descrambler function and requirements in greater detail in another post.

Next, the OTUk signal will proceed to the FEC Decoder block for further processing.

FEC (Forward-Error-Correction) Decoder Block

The OTSi/OTUk-a_A_So function (at the remote STE) is responsible for performing FEC (Forward Error Correction) Encoding.

This means that this function computed a FEC Code and inserted that code into a 4-row x 128-byte column field at the backend of each OTUk frame, as shown below in Figure 9.

Figure 9, Illustration of the OTUk Frame Format with the FEC Field Highlighted

The purpose of the FEC Decoder (within the OTSi/OTUk-a_A_Sk function) is to parse through the incoming OTUk data stream and (by using the contents of the FEC-field) detect and correct most symbols errors within this data stream.

The FEC Decoder block will tally any occurrences of Symbol errors (within the incoming OTUk data stream). It will report this information to System Management via the MI_pFECcorrErr output (via the OTSi/OTUk-a_A_Sk_MP Interface).

I discuss this Forward-Error-Correction scheme in much greater detail in another post.

Multi-Frame Alignment and dLOM (Loss of Multi-Frame Defect) Detection Blocks

Once the incoming OTUk data stream passes through the FEC Decoder block, the OTSi/OTUk-a_A_Sk function will route this signal to the Multi-Frame Alignment and dLOM Detection blocks.

The Multi-Frame Alignment block will parse through and check the contents of the MFAS field within the incoming OTUk data stream. The Multi-Frame Alignment block will check the contents of this data stream to see if it (and the dLOM Detection Block) should declare or clear the dLOM defect condition.

Please see the blog post on the dLOM Defect for more information on how the Multi-Frame Alignment block will declare and clear the dLOM defect condition.

Removal of the FAS, MFAS, and FEC Fields from the incoming OTUk Data-stream

The Frame-Alignment block will drive the CI_FS (Frame-Start) output of the OTUk_CP Interface, HIGH for one CI_CK (Clock Signal) period, each time it detects the FAS field within its incoming OTUk data-stream.

Likewise, the Multi-Frame Alignment block will drive the CI_MFS (Multi-Frame Start) output of the OTUk_CP Interface, HIGH, for one CI_CK (Clock Signal) period each time it receives an MFAS byte with the value of 0x00.

The Frame-Alignment and Multi-Frame Alignment block will also remove the FAS and MFAS fields from the OTUk data stream (before it outputs this data stream via the CI_D output of the OTUk_CP Interface).

From this point on, the CI_FS and CI_MFS signals will now carry the framing and multi-framing alignment information downstream toward the OTUk_TT_Sk atomic function.

The FEC Decoder block will also remove the contents of the FEC field from the OTUk data stream before it outputs this data via the CI_D output pin.

Consequent Actions Block

In most cases, the Consequent Actions block will consist of digital logic circuitry that will assert the CI_SSF (Server Signal Fail) Output (of the OTUk_CP Interface) anytime the OTSi/OTUk-a_A_Sk function declares any of the following defect conditions.

dLOS-P – Loss of Signal- Path Defect Condition
dAIS – OTUk-AIS (Alarm Indication Signal) Defect Condition
dLOF – Loss of Frame Defect Condition
dLOM – Loss of Multi-Frame Defect Condition.
AI_TSF – If the upstream circuitry asserts the AI_TSF-P Input pin to this function.

Consequent Equation

ITU-T G.798 has the following Consequent Equation for the OTSi/OTUk_A_Sk function.

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

This Consequent Equation states that the OTSi/OTUk_A_Sk function MUST set aSSF to “1” (or drive the CI_SSF output pin to HIGH) if any of the following conditions are true:

The OTSi/OTUk_A_Sk function declares any of the following defect conditions.
- dLOS-P – Loss of Signal – Path Defect
- dAIS – OTUk-AIS Defect
- dLOF – Loss of Frame Defect
- dLOM – Loss of Multi-Frame Alignment Defect
Or, if the upstream Optical Circuitry drives the AI_TSF (Trail Signal Fail) input pin HIGH.

NOTE: Whenever this function asserts the CI_SSF output signal, it also asserts the CI_SSF input to the downstream OTUk_TT_Sk function.

Defect Correlation

If you wish to learn more about Defect Correlation and how you should interpret it, please see the Defect Correlation Post.

ITU-T G.798 specifies the following correlation equations for each OTSi/OTUk-a_A_Sk function-related defect.

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)

I will briefly explain what each of these equations means below.

cLOS-P ⇐ dLOS-P and (NOT AI_TSF-P)

This equation means that the OTSi/OTUk-a_A_Sk function will ONLY declare the dLOS defect (and assert the cLOS-P output pin) if:

The Clock Recovery and LOS Detection circuitry is declaring the dLOS-P defect condition, and
The upstream circuitry is NOT asserting the AI_TSF-P input of this function.

In other words, the OTSi/OTUk-a_A_Sk function should only declare the dLOS defect (and assert the cLOS-P output pin) if it is internally declaring the dLOS-P defect condition.

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

This equation means that the OTSi/OTUk-a_A_Sk function will ONLY declare the dLOF defect (and assert the cLOF output pin) if:

The Frame Alignment and dLOF Detection circuitry declare the dLOF defect condition, and
The Optical upstream circuitry is NOT asserting the AI_TSF-P input of this function, and
The Clock Recovery and dLOS Detection circuitry is NOT currently declaring the dLOS-P defect condition, and
The dAIS Detection circuitry is NOT also declaring the dAIS defect condition.

In other words, the OTSi/OTUk-a_A_Sk function should only declare the dLOF defect (and assert the cLOF output pin) if it internally declares the dLOF defect condition.

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

This equation means that the OTSi/OTUk-a_A_Sk function will ONLY declare the dLOM defect (and assert the cLOM output pin) if:

The Multi-Frame Alignment and dLOM Detection circuitry declare the dLOM defect condition, and
The Optical upstream circuitry is NOT asserting the AI_TSF-P input of this function, and
The Clock Recovery and dLOS Circuitry is NOT currently declaring the dLOS-P defect condition, and
The dAIS Detection circuitry is NOT also declaring the dAIS defect condition,
The Frame Alignment and dLOF Detection circuitry are not currently declaring the dLOF defect condition.

Performance Monitoring

ITU-T G.798 requires that the OTSi/OTUk-a_A_Sk or OTSi/OTUk-v_A_Sk Functions tally and report the following Performance Monitoring parameter to System Management:

pFECcorrErr ⇐ ∑nFECcorrErr

In other words, the OTSi/OTUk-a_A_Sk or OTSi/OTUk-v_A_Sk functions are expected to tally and report each instant that the FEC Decoder block corrects an errored symbol within the incoming OTUk data stream.

Pin Description

I list the Input/Output Pin Description for the OTSi/OTUk-a_A_Sk Atomic Function below in Table 2.

Table 2, Pin Description for the OTSi/OTUk-a_A_Sk Atomic Function

Signal Name	Type	Description
OTSi_AP Interface
AI_PLD	Input	OTUk Adaptation Information - OTUk Payload Input: The user is expected to apply a fully-framed and scrambled OTUk signal (with FEC) to this input port. NOTE: In most cases, this data will be received data that has just been converted back into the electrical format (from the optical format). The OTSi/OTUk-a_A_Sk function will accept and descramble this data and extract out all of the following data from this signal. - FEC - It will decode the FEC and it will correct most symbol errors that this function detects within this incoming data stream. - FAS - The Framing Alignment Signal. The Framing Alignment signal information will be output via the CI_FS output of this function. - MFAS - The Multiframe Alignment Signal. The Multiframe Alignment signal information will be output via the CI_MFS output of this function. - OTUk Data - The content of the rest of the unscrambled OTUk data-stream. This remaining OTUk data-stream will be output via the CI_D output of this function. - OTUk Clock signal. The resulting OTUk clock signal will be output via the CI_CK output of this function.
AI_TSF-P	Input	Adapted Information - Trail Signal Fail - Path: This signal indicates whether the upstream circuitry is declaring a service-affecting defect condition (within the signal path) with the data that is being applied to the AI_PLD input. This signal has (essentially) the same meaning as AIS. If this signal is TRUE, then the OTSi/OTUk-a_A_Sk function will automatically set the CI_SSF output TRUE.
AI_TSF-O	Input	Adapted Information - Trail Signal Fail - Overhead: This signal indicates whether upstream circuitry is declaring a service-affecting defect condition within the signal overhead. NOTE: This signal does not reflect the health of the signal-path.
OTUk_CP Interface
CI_D	Output	OTUk Characteristic Information - Data Output: The OTSi/OTUk-a_A_Sk function will output the OTUk data-stream via this output pin. This OTUk data-stream will be unscrambled and it will contain all of the following portions of the OTUk frame. - OTUk SMOH (Section Monitoring Overhead) data - All remaining OTUk payload data (e.g., the ODUk/OPUk data). This data will not include the FAS, MFAS nor FEC fields. Data that is output via this signal, will be aligned with one of the edges of the CI_CK clock output signal. The system designer will typically route this signal to the CI_D input to the downstream OTUk_TT_Sk function.
CI_CK	Output	OTUk Characteristic Information - Clock Output: As the OTUk_CP interface outputs data via the CI_D, CI_FS, CI_MFS and CI_SSF outputs; all of this data will be updated on one of the clock-edges of this clock output signal.
CI_FS	Output	OTUk Characteristic Information - Frame Start Output: The OTUk_CP Interface will pulse this output signal HIGH (for one CI_CK clock period) whenever the OTUk_CP interface outputs the very first bit (or byte) of a new OTUk frame, via this CI_D output. This output signal will pulse HIGH once for each OTUk frame.
CI_MFS	Output	OTUk Characteristic Information - Multiframe Start Output: The OTUk_CP Interface will pulse this output signal HIGH (for one CI_CK period) whenever the OTUk_CP Interface outputs the very first bit (or byte) or a new OTUk multi-frame via the CI_D output. This output signal will pulse HIGH once for each OTUk Multi-frame (or one for every 256 OTUk frames).
CI_SSF	Output	OTUk Characteristic Information - Server Signal Failure Output: The OTUk_CP Interface will assert this signal anytime the OTSi/OTUk-a_A_Sk function is declaring a service-affecting defect with the data that it is receiving via the AI_D input). The OTUk_CP Interface will assert this output signal, whenever the OTSi/OTUk-a_A_Sk function is declaring any of the following defects. - dLOF - dLOM - dAIS - AI_TSF (if the upstream circuitry is driving the AI_TSF-P input pin, to this function, HIGH).
OTSi/OTUk-a_A_Sk_MP Interface
MI_FECEn	Input	Management Interface - OTSi/OTUk-a_A_Sk FEC Decoding Enable/Disable Input: This input pin permits the function user to either enable or disable FEC Decoding within the OTSi/OTUk-a_A_Sk function. Setting this input HIGH enables FEC Decoding. Setting this input LOW disables FEC Decoding. If the FEC Decoder is enabled, then it will use the FEC field to correct most symbol errors within the incoming OTUk data-stream (via the AI_PLD input).
MI_pFECcorrErr	Output	Management Interface - FEC Corrected Symbol Count Output: This output port reflects the number of symbol errors that the OTSi/OTUk-a_A_Sk function has corrected via the FEC Decoder. This is a Performance Monitoring feature within the OTSi/OTUk-a_A_Sk function. NOTE: This output pin is INACTIVE if the MI_FECEn input pin is set low (to disable the FEC Decoder).
MI_cLOM	Output	Management Interface - Loss of Multiframe (Correlated) Output Indicator: This output pin indicates if the OTSi/OTUk-a_A_Sk function is currently declaring the dLOM defect. If this input pin is LOW, then it indicates that the function is NOT currently declaring the dLOM defect condition. Conversely, if this input pin is HIGH, then it indicates that the function is currently declaring the dLOM defect condition. Please see the dLOM defect post for more information on this topic.
MI_cLOF	Output	Management Interface - Loss of Frame (Correlated) Output Indicator: This output pin indicates if the OTSi/OTUk-a_A_Sk function is currently declaring the dLOF defect. If this output pin is LOW, then it indicates that the function is NOT currently declaring the dLOF defect condition. Conversely, if this output pin is HIGH, then it indicates that the function is currently declaring the dLOF defect condition. Please see the blog post on the dLOF defect for more information on this topic.
MI_cLOS	Output	Management Interface - Loss of Signal (Correlated) Output Indicator: This output pin indicates if the OTSi/OTUk-a_A_Sk function is currently declaring the dLOS defect. If this output pin is LOW, then it indicates that the function is NOT currently declaring the dLOS defect. Conversely, if this output pin is HIGH, then it indicates that the function is currently declaring the dLOS defect condition. Please see the blog post on the dLOS defect, for more information about this topic.

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OTN – Lesson 9 – Video 11 – OTU Layer Defect Scenarios

OTN – Lesson 9 – Video 6 – OTU Layer Sink Direction Circuitry/Functionality – Part 4

OTN – Lesson 9 – Video 3 – OTU Layer Sink Direction Circuitry/Functionality – Part 1

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

Introduction

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

How Do We Sort out This Confusion?

So What Does All of This Mean?

Let’s Take a Look at the Downstream Circuitry

The First Consequent Equation – OTUk_TT_Sk Function

The Second Consequent Equation – OTUk_TT_Sk Function

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

The First Consequent Equation – OTUk/ODUk_A_Sk Function

The Second Consequent Equation – OTUk/ODUk_A_Sk Function

So What Does All This Mean?

Conclusion

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How an OTN STE should declare and clear the dLOM (Loss of Multi-Frame) Defect Condition.

A Note about Terminology:

A Brief Review of the OTUk Frame Structure

The MFAS Byte Increments with each Frame

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A Basic Requirement before We can Clear the dLOM Defect Condition

The Sink STE needs to find the FAS-field first

The FAS field is FAR more accessible for the Sink STE to find than the MFAS byte.

Finally, the MFAS byte-field was scrambled by a Frame-Synchronous Scrambler (at the remote Source STE)

The dLOM-Related State Machines

The OTUk-MFAS OOM/IM Low-Level State Machine

The LL-OOM State

The LL-IM State

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The OTUk-dLOM Framing Alignment/Maintenance State Machine

The dLOM/LL-OOM State

What does it mean to be in the dLOM/LL-OOM State?

If the New_Candidate_MFAS_Value fails the Test

If the New_Candidate_MFAS_Value passes the Test

The dLOM/LL-IM State

Advancing to the In-Multi-Frame/LL-IM State

Regressing to the dLOM/LL-OOM State

The In-Multi-Frame/LL-IM State

What Does Clearing the dLOM Defect Mean?

The In-Multi-frame/LL-OOM State

Advancing Back into the In-Multi-Frame/LL-IM State

Regressing to the dLOM/LL-OOM State

In Summary

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A Note About Terminology:

A Brief Review of the OTUk Frame Structure

The dLOF-Related State Machines

The OTUk-FAS OOF/IF Low-Level State Machine

The LL-OOF State

ITU-T G.798’s Requirement for Searching for the FAS field

The LL-IF State

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The OTUk-dLOF Framing Alignment/Maintenance State Machine

The dLOF/LL-OOF State

What does it mean to be in the dLOF/LL-OOF State?

If the Sink STE Fails to Find the FAS field

If the Sink STE Successfully Locates the FAS field

The dLOF/LL-IF State

Advancing to the In-Frame/LL-IF State

Regressing to the dLOF/LL-OOF State

The In-Frame/LL-IF State

The In-Frame/LL-OOF State

Advancing Back into the In-Frame/LL-IF State

Regressing to the dLOF/LL-OOF State

In Summary

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How Should an OTN STE Declare and Clear the dAIS Defect Condition (OTUk-AIS)?

How an OTN STE should declare and clear the dLOF (Loss of Frame) Defect Condition

What is the OTSi/OTUk_A_Sk Atomic Function?