What is the OTSiG/OTUk_A_So Atomic Function?

The expression: OTSiG/OTUk_A_So is an abbreviation for the term: Optical Tributary Signal Group to OTUk Adaptation Source Function.

This blog post will briefly describe the OTSiG/OTUk_A_So set of atomic functions.

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 OPSM/OTUk_A_So function.

However, the standards committee has recently decided to change the wording from using the term OPSM (Optical Physical Section Multilane) to using the word OTSiG (for Optical Tributary Signal Group).

For completeness, I will tell you that ITU-T G.709 defines the term OTSiG as:

“The set of OTSi signals that supports an OTU.”

Therefore, where we used the OTSi/OTUk_A_So and OTSi/OTUk_A_Sk functions for “single-lane” applications, we will use the OTSiG/OTUk_A_So and OTSiG/OTUk_A_Sk functions for “multi-lane” applications.

In summary, to “speak the same language,” as does the standard committee, we will call this atomic function the OTSiG/OTUk_A_So atomic function.

Likewise, in another post, we will now call (what we used to call the OPSM/OTUk_A_Sk function) the OTSiG/OTUk_A_Sk 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.

A Little More about Multi-Lane Interfaces for OTU3 and OTU4 Applications

First, we only use the OTSiG/OTUk_A_So and OTSiG/OTUk_A_Sk functions for OTU3 and OTU4 Multi-Lane applications.

We will use the OTSiG/OTUk_A_So function for OTU3 applications to model circuitry that demultiplexes an OTU3 signal into a four-lane signal (e.g., an OTL3.4 signal). ITU-T G.709 defines the OTL3.4 signal as an approach to transporting OTU3 traffic over 4-lanes in parallel.

Likewise, we will use the OTSiG/OTUk_A_So function for OTU4 applications to model circuitry that demultiplexes (and multiplexes) an OTU4 signal into a four-lane signal (e.g., an OTL4.4 signal). ITU-T G.709 also defines the OTL4.4 signal as an approach to transporting OTU4 traffic over 4-lanes in parallel.

Please see the blog posts for the OTL3.4 and OTL4.4 Interfaces for more information on these topics.

The OTSiG/OTUk_A_So Function

The OTSiG/OTUk_A_So function is any circuit that takes an OTUk data-stream, clock, frame-start, and multi-frame start signals and:

converts this data into a combined, scrambled data stream, which (in some cases) contains a FEC (Forward-Error-Correction) field, and
demultiplexes this data into four electrical lanes, each of which can be readily converted into the optical format (at the output pins of this function).

Once again, ITU-T G.798 states that the system designer can use this function only for the OTU3 or OTU4 rates.

For OTU1 and OTU2 rates, we recommend that the system designer use the OTSi/OTUk_A_So function instead.

We discuss the OTSi/OTUk_A_So atomic function in another post.

Figure 1 presents a simple illustration of the OTSiG/OTUk_A_So function.

Figure 1, Simple Illustration of the OTSiG/OTUk_A_So function.

ITU-T G.798 defines two versions of this particular function. Additionally, there are other versions of this function that are not specified by ITU-T G.798. I list some popular versions of this function below in Table 1.

Table 1, List of Some Popular Versions for the OTSiG/OTUk_A_So function.

Function Name	Description	Comments
OTSiG/OTUk-a_A_So	OTSiG to OTUk Adaptation Source Function with ITU-T G.709 Standard FEC.	Can be used for OTU3 and OTU4 applications ONLY.
OTSiG/OTUk-b_A_So	OTSiG to OTUk Adaptation Source Function with No FEC	Can only be used for OTU3 Applications. Cannot be used for OTU4 applications.
OTSiG/OTUk-v_A_So	OTSiG to OTUk Adaptation Source Function with Vendor-Specific FEC	Can be used for OTU3 and OTU4 Applications. Not specified by ITU-T G.798.

Table 1 shows that the OTSiG/OTUk-a_A_So and the OTSiG/OTUk-v_A_So functions will compute and append some sort of FEC field to the back-end of each outbound OTUk frame.

However, this table also shows that the OTSiG/OTUk-b_A_So version does not generate the FEC field.

Consequently, ITU-T G.798 states that one can use the OTSiG/OTUk-a_A_So and OTSiG/OTUk-v_A_So functions for OTU3 and OTU4 applications. The standard also recommends that the user NOT use the OTSiG/OTUk-b_A_So function for OTU4 applications.

The OTU4 rate requires the use of Forward-Error-Correction.

What Version will we Discuss Throughout this Post?

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

The OTSiG/OTUk-b_A_So and OTSiG/OTUk-v_A_So atomic functions do everything that the OTSiG/OTUk-a_A_So does, except that the -b version does NO FEC Encoding, and the -v version does FEC Encoding differently than what I describe here.

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So What All Does this Atomic Function Do?

The OTSiG/OTUk-a_A_So function will accept the OTUk data-stream, OTUk clock signal, frame-start signal, and multi-frame start signals via the CI_D, CI_CK, CI_FS, and CI_MFS inputs (of this function) respectively, and it will perform the following tasks.

It will insert the FAS and MFAS fields into the outbound OTUk data stream (coincident with whenever the upstream OTUk_TT_So function asserts the CI_FS and CI_MFS inputs, respectively).
This function will compute and append a FEC field to the back-end of each outbound OTUk frame.
It will scramble this “combined” OTUk data stream (consisting of the FAS, MFAS, and FEC fields).
Next, this function will demultiplex and combine this data stream into four parallel lanes of OTL3.4 traffic (for OTU3 applications) and OTL4.4 traffic (for OTU4 applications).
This function will transmit these four lanes to external Electrical-to-Optical Conversion circuitry (which will convert our OTL3.4 or OTL4.4 traffic into the optical format).

I illustrate a Unidirectional Connection that shows where the OTSiG/OTUk-a_a_So function “fits in” within a system below in Figure 2.

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

Figure 2 shows that the OTSiG/OTUk-a_A_So function will accept traffic from the upstream OTUk_TT_So function. The OTSiG/OTUk-a_A_So function will then perform additional processing on this data before it sends it over to optical circuitry for conversion and transport.

We will discuss this additional processing below.

Functional Description of this Atomic Function

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

The OTSiG/OTUk-a_A_So functional blocks are different for OTU3 applications than for OTU4 applications. Therefore, we will first walk through the Functional Block Diagram for OTU3 applications.

Afterward, we will do the same for OTU4 applications.

Review of the OTSiG/OTU3-a_A_So Functional Block Diagram – OTU3 Applications

Figure 3 presents the Functional Block Diagram of the OTSiG/OTUk-a_A_So Atomic Function for OTU3 Applications.

Figure 3, Illustration of the Functional Block Diagram of the OTSiG/OTU3-a_A_So Atomic Function (for OTU3 Applications).

Hence, Figure 3 shows that the OTU3-version of this function contains the following functional blocks.

FAS/MFAS Insertion Block
FEC Encoder Block
Scrambler Block
16-Byte Block Distributor and Rotator Block

I will briefly discuss each of these functional blocks below.

The FAS/MFAS Insertion Block

The FAS/MFAS Insertion Block is the first block the OTU3 signal will pass through after entering this atomic function. I illustrate the OTSiG/OTU3-a_A_So function with the FAS/MFAS Insertion block highlighted below in Figure 4.

Figure 4, Illustration of the OTSiG/OTU3-a_A_So Functional Block Diagram with the FAS/MFAS Insertion Block highlighted.

The FAS/MFAS Insertion block will insert the FAS and MFAS fields into the outbound OTU3 data stream each time the upstream OTUk_TT_So function asserts the CI_FS input pin.

Likewise, the FAS/MFAS Insertion block will initialize the MFAS byte-field to 0x00 within the outbound OTU3 data stream each time the upstream OTUk_TT_So function asserts the CI_MFS input pin.

The FAS/MFAS Insertion block will proceed to increment the contents of the MFAS field within each OTU3 frame it generates.

The FEC Encoder Block

Once the OTU3 signal leaves the FAS/MFAS Insertion block, it will pass through the FEC Encoder block. I show an illustration of the OTSiG/OTU3-a_A_So Functional Block Diagram, with the FEC Encoder Block Highlighted below in Figure 5.

Figure 5, Illustration of the OTSiG/OTU3-a_A_So Functional Block Diagram with the FEC Encoder block highlighted.

The FEC Encoder block will compute the FEC field and append this field to the back-end of each outbound OTU3 frame.

ITU-T G.709 recommends that (for a Fully-Compliant OTUk Frame), one uses the Reed Solomon RS(255,239) Code for its Forward-Error-Correction scheme.

The standard also recommends that the System Designer use Symbol-Interleaving and that the user place the resulting FEC code into a 4-row x 256-byte column field at the back-end of each outbound OTUk frame.

I show the location (that the FEC Encoder block should insert the FEC Code) within the OTUk frame below in Figure 6.

Figure 6, Illustration of the OTUk Frame, with the Location of the FEC-field highlighted

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

Once this OTU3 data stream leaves the FEC Encoder block, it will proceed to the Scrambler block for further processing.

The Scrambler Block

I illustrate the OTSiG/OTU3-a_A_So Functional Block Diagram with the Scrambler Block highlighted below in Figure 7.

Figure 7, Illustration of the OTSiG/OTU3-a_A_So Functional Block Diagram with the Scrambler Block highlighted.

ITU-T G.709 requires that we scramble OTUk data before we transmit it over optical fiber.

The standard requires that we do this to ensure that this OTUk signal has sufficient bit-timing content for Clock and Data Recovery PLLs (Phase-Locked Loops) within the Sink STE (at the remote end).

ITU-T G.709 also states that the Scrambler must operate as a frame synchronous scrambler of sequence length 65,535 (e.g., 2¹⁶-1), running at the OTUk rate.

Finally, ITU-T G.709 states that the Scrambler must use the generating polynomial of 1 + x + x³ + x¹² + x¹⁶.

I show a simple diagram of how the user can implement the Scrambler within their OTSiG/OTUk-a_A_So function design below in Figure 8.

Figure 8, Illustration of the High-Level Block Diagram for the Frame-Synchronous Scrambler

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16-Byte Block Distributor and Rotator Block

Once the OTU3 signal leaves the Scrambler block, it will pass through the 16-Byte Block Distributor and Rotator Block. I illustrate the OTSiG/OTU3-a_A_So Functional Block Diagram with the 16-Byte Block Distributor and Rotator Block highlighted below in Figure 9.

Figure 9, Illustration of the OTSiG/OTU3-a_A_So Functional Block Diagram with the 16-Byte Block Distributor and Rotator Block highlighted.

The 16-Byte Block Distributor and Rotator block will convert a composite OTU3 signal (that is of the form of a single serial data stream) into the OTL3.4 format (over four lanes) that we describe in the OTL3.4 post.

In this case, the OTSiG/OTU3-a_A_So function has four outputs (one output for each of the four OTL3.4 lanes), which we labeled AI_PLD[1] through AI_PLD[4] in Figure 9.

The user is expected to connect these four output signals to optical circuitry (that will convert this electrical data into the optical format).

We have briefly covered the OTSiG/OTUk-a_A_So function block for OTU3 applications. Let’s move on and discuss this atomic function for OTU4 applications.

Review of the OTSiG/OTU4-a_A_So Functional Block Diagram – OTU4 Applications

I present the Functional Block Diagram of the OTSiG/OTUk-a_A_So Atomic Function for OTU4 Applications below in Figure 10.

Figure 10, Illustration of the Functional Block Diagram of the OTSiG/OTU4-a_A_So Atomic Function (for OTU4 Applications)

Hence, Figure 4 shows that the OTU4 version of this function contains the following functional blocks.

FAS/MFAS Insertion Block
FEC Encoder Block
Scrambler Block
LLM (Logical Lane Marker) Insertion Block
16 Byte Block Distributor and Rotator Block

I will discuss some of these Functional blocks below. Please note that some of these blocks are identical to what I’ve presented for OTU3 applications. I will note that whenever I come to those functional blocks.