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.

Inverse PN-11 Detector - for dAIS (OTUk-AIS) Detection

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.

OTUk-AIS is applied to Inverse PN-11 Detector

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.

  1. 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
  2. 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 InputOUT NodeComments
All-Zeros SignalAll-Zeros SignalAn All-Zeros pattern at the IN Input results in an All-Zeros pattern at the OUT Node.

No OTUk-AIS.
Ordinary OTUk TrafficNon All-Zeros Pattern SignalNormal Traffic Situation
OTUk-AIS Maintenance SignalAll-Zeros SignalThe 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.

dAIS Defect Declaration Criteria

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.

OTUk-AIS Defect Clearance Criteria

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.

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

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