What is the OTUk_TT_So Atomic Function?
We formally call the OTUk_TT_So Atomic Function the OTUk Trail Termination Source Function.
Introduction
The OTUk_TT_So function is any function that accepts data from upstream circuitry (usually the OTUk/ODUk_A_So function). It uses the data (within this data stream) along with signals from a collocated OTUk_TT_Sk function to compute/generate and insert the OTUk Section Monitoring Overhead (SMOH) into the OTUk signal.
We have an extensive discussion of the OTUk_TT_So Atomic Function in Lesson 9 within THE BEST DARN OTN TRAINING PRESENTATION…PERIOD!!!
I show how we can connect these atomic functions below in Figure 1.
Figure 1, Drawing of a Bidirectional Network (consisting of various Atomic Functions) with the OTUk/ODUk_A_So, the OTUk_TT_So, and its collocated OTUk_TT_Sk functions highlighted
So What Does this Atomic Function Do?
If you recall, from our discussion of the OTUk/ODUk_A_So function, that function will only generate default values for the OTUk-SMOH within the OTUk signal it transmits. The OTUk/ODUk_A_So function creates this default SMOH as a place-holder for a future (and actual) SMOH.
Computes and Inserts the Real SMOH Values into the Outbound OTUk Frames
Well, the purpose of the OTUk_TT_So function is to calculate and replace these default SMOH values with actual SMOH values.
More specifically, this function will compute and replace the following SMOH fields with actual Overhead data.
- SM-BIP-8 field, within the Section Monitoring field
- SM-BEI/BIAE nibble field within the Section Monitoring Byte
- SM-BDI bit-field within the Section Monitoring Byte
- IAE bit-field within the Section Monitoring Byte
- SM-TTI (Trail Trace Identification) byte within the Section Monitoring field.
Afterward, the OTUk_TT_So function will transmit this OTUk data stream to either the OTSi/OTUk-a_A_S0 function (for OTU1/2 applications) or the OTSiG/OTUk-a_A_S0 function (for OTU3/4 applications).
These functions will condition the OTUk data stream for transmission over Optical Fiber.
Why Do We Care about the SMOH from the OTUk_TT_So Function?
The SMOH that the OTUk_TT_So function computes and inserts into the OTUk data stream serves as the basis of comparison for the OTUk_TT_Sk function (at the remote Network Element).
The OTUk_TT_Sk function (at the remote end of our OTUk connection) will use this SMOH data to determine:
- if it should declare any defect conditions, or
- if errors have occurred during transmission between the near-end OTUk_TT_So and the remote OTUk_TT_Sk functions.
I show an illustration where both the OTUk_TT_So and OTUk_TT_Sk functions “fit into the big picture” below in Figure 2.
Figure 2, Drawing of Unidirectional Connection between a Source STE and a Sink STE with the OTUk_TT_So and OTUk_TT_Sk functions highlighted.
Some Details about the OTUk_TT_So Function
Figure 3 presents a drawing of the ITU-T G.798 symbol for the OTUk_TT_So function.
Figure 3, Simple Drawing of the OTUk_TT_So function
The OTUk_TT_So function accepts a basic OTUk data stream from the upstream OTUk/ODUk_A_So function via the OTUk_AP Interface.
The data that is output from the OTUk/ODUk_A_So function includes the Clock Signal (AI_CK), Frame Synchronization Signal (AI_FS), the Multi-Frame Synchronization Signal (AI_MFS), the OTUk data-stream (AI_D) and the IAE (Input Alignment Error) indicator (via the AI_IAE signal).
The OTUk_TT_So function is responsible for accepting data from its various interfaces and then computing and inserting the correct SMOH data into the OTUk data stream.
Figure 2 shows that this function consists of the following four different interfaces.
- OTUk_AP
- OTUk_CP
- OTUk_RP and
- OTUk_TT_So_MP
We will discuss each of these interfaces below.
Figure 4 presents a functional block diagram of the OTUk_TT_So function.
Figure 4 Functional Block Diagram of the OTUk_TT_So function
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The OTUk_AP (OTUk Access Point) Interface
Figure 4 shows that the circuitry connected to (and driving) the OTUk_AP Interface (e.g., the OTUk/ODUk_A_So function) will supply the following signals to this interface.
- AI_D – Bare-bones OTUk data (with the default SMOH)
- AI_CK – The OTUk clock input signal
- AI_FS – The OTUk Frame Start Input
- AI_MFS – the OTUk Multi-Frame Start Input
- AI_IAE – The OTUk Input Alignment Error Indicator Signal
The OTUk_TT_So function will then perform the following operations on these signals.
- SM-BIP-8 Computation and Insertion – We discuss BIP-8 Computation and Insertion in another post.
- IAE (Indicator) Insertion (into the OTUk-SMOH) – We discuss IAE insertion below in another post. (*)
NOTE: (*) – Indicates that you need to be a member of THE BEST DARN OTN TRAINING PRESENTATION….PERIOD!!! to see this post.
Let’s move on to another port within this atomic function.
The OTUk_TT_So_RP (Remote Port) Interface
The OTUk_TT_So function will also accept data via the OTUk_TT_So_RP interface. This interface consists of the following inputs.
- RI_BEI – Remote Interface – Backward Error Indicator
- RI_BIAE – Remote Interface – Backward Input Alignment Error Indicator
- RI_BDI – Remote Interface – Backward Defect Indicator
The OTUk_TT_So function will operate in conjunction with a collocated Near-End OTUk_TT_Sk function and perform the following operations on these signals.
- BDI Insertion (into the OTUk-SMOH) – The OTUk_TT_So function will accept the BDI information from the Near-End Collocated OTUk_TT_Sk function via the RI_BDI input and insert this data into the SM-BDI bit-field (within the SMOH) of the very next outbound OTUk frame.
- BEI Insertion (into the OTUk-SMOH) – The OTUk_TT_So function will take the BEI information from the Near-End Collocated OTUk_TT_Sk function via the RI_BEI input and insert this data into the SM-BEI/BIAE nibble-field (within SMOH) of the very next outbound OTUk frame.
- BIAE Insertion (into the OTUk-SMOH) – The OTUk_TT_So function will accept the BIAE information from the Near-End Collocated OTUk_TT_Sk function via the RI_BIAE input and (if appropriate) will insert this information into the SM-BEI/BIAE nibble-field (within the SMOH) of the very next outbound OTUk frame.
I show a drawing of our OTUk_TT_So function that is electrically connected to its collocated, Near-End OTUk_TT_Sk function via the Remote Port below in Figure 5.
Figure 5, Illustration of our OTUk_TT_So Function, along with its collocated, Near-End OTUk_TT_Sk function
We discuss the operations through the RP Interface in another post.
Next, let’s move on and discuss the Management Port of this atomic function.
The OTUk_TT_So_MP (Management Port) Interface
Finally, the OTUk_TT_So function accepts data from the OTUk_TT_So_MP Interface. This particular interface consists of the following input pin.
- MI_TxTI – Trail Trace Identifier Input
The function user is expected to load the contents of the outbound Trail Trace Identifier Message (64 bytes) to this input port.
The OTUk_TT_So function will then take this message data, and it will proceed to use the TTI byte-field within the OTUk-SMOH to transmit the contents of this message, one byte at a time, to the OTUk_TT_Sk function within the remote Network Element, via the OTUk data-stream. Since the TTI Message is 64 bytes long, the OTUk_TT_So function will require 64 OTUk frames to transmit the complete TTI Message.
We will discuss these processes in greater detail in the Trail Trace Identifier post.
How the OTUk_TT_So function sources each of the various Overhead Fields within the SMOH
As we mentioned earlier, the primary responsibility of the OTUk_TT_So function is to compute/source the correct values for multiple fields within the SMOH and insert those values into the SMOH within each outbound OTUk frame.
Table 1 presents a list of Overhead-fields that the OTUk_TT_So function computes and sources. This table also shows where this function gets its data for these Overhead fields.
Table 1, A List of the SMOH Overhead-fields that the OTUk_TT_So function computes/sources and how/where this function gets/derives this data.
| Overhead Field | Location within OTUk-SMOH | Source of Data/How Derived? |
|---|---|---|
| BIP-8 Byte | BIP-8 Byte within the Section Monitoring Field | The OTUk_TT_So function locally computes the BIP-8 value based upon the contents within the OPUk portion of the OTUk frame. |
| IAE Bit-Field | The IAE Bit-field within the Section Monitoring Byte | Based upon the AI_IAE input to the function (at the OTUk_AP Interface) |
| BDI Bit-Field | The BDI bit-field within the Section Monitoring byte | Based upon the RI_BDI input to this function (at the Remote Port Interface). |
| BEI Nibble-Field | The BEI/BIAE Nibble-field within the Section Monitoring Byte | Based upon the RI_BEI and RI_BIAE inputs to this function (at the Remote Port Interface). |
| BIAE Nibble-Field | The BEI/BIAE bit-fields within the Section Monitoring byte. | Based upon the RI_BIAE and RI_BEI inputs to this function (at the Remote Port Interface). |
| TTI Byte-Field | TTI Byte within the Section Monitoring Field. | The user is expected to load in the contents of the 64-byte Trace Identifier Message (TIM) into a buffer via the MI_TxTI input to this function. The function will proceed to transmit the contents of this TIM, one byte at a time, via the TTI byte-field within each outbound OTUk Frame. The OTUk_TT_So function will transport the entire TIM over 64 consecutive OTUk frames. |
List of Input and Output Signals for the OTUk_TT_So Atomic Function
Table 2 presents a Pin Description for each of the Input/Output signals of the OTUk_TT_So Atomic Function.
Table 2, Input/Output Pin Description of the OTUk_TT_So Atomic Function
| Signal Name | Type | Description |
|---|---|---|
| OTUk_AP Interface | ||
| AI_D | Input | OTUk Adapted Interface - OTUk Data Input: The function user is expected to apply a bare-bones OTUk signal (which is output from the AI_D output of the OTUk/ODUk_A_So function) to this input pin. NOTE: This OTUk data will contain the following fields - Default OTUk SMOH data, - The contents of the APS/PCC channel and - The rest of the OTUk frame. This OTUk data-stream will not include the FAS, MFAS or FEC field. This function will compute, generate and insert the appropriate OTUk-SMOH into this OTUk data-stream The OTUk_TT_So function will sample this input signal on one of the edges of the AI_CK input clock signal. |
| AI_CK | Input | OTUk Adapted Interface - Clock Input: The OTUk_TT_So function will sample all data and signals (that the user applies to the OTUk_AP Interface) upon one of the edges of this input clock signal. This statement applies to the following signals: AI_D, AI_FS, AI_MFS and AI_IAE. This clock signal will also function as the timing source for this function as well. |
| AI_FS | Input | OTUk Adapted Information - Frame Start Input: The upstream OTUk/ODUk_A_So function should pulse this input signal HIGH whenever the OTUk_AP Interface accepts the very first bit (or byte) of a new OTUk frame, via the AI_D inputs. The upstream OTUk/ODUk_A_So function should drive this input HIGH once for each OTUk frame. |
| AI_MFS | Input | OTUk Adapted Information - Multiframe Start Output: The upstream OTUk/ODUk_A_So function should pulse this input signal HIGH whenever the OTUk_AP interface accepts the very first bit (or byte) of a new OTUk superframe, via the AI_D input. The upstream OTUk/ODUk_A_So function should drive this input HIGH once for each OTUk superframe. |
| AI_IAE | Input | OTUk Adapted Information - Input Alignment Error Input: the OTUk/ODUk_A_So function (upstream from this function) will drive this input pin HIGH whenever it detects a frame-slip (or IAE event). Anytime the OTUk_TT_So function detects this input pin, going from LOW to HIGH, then it should respond by setting the IAE bit-field to "1" for 16 consecutive Superframes (or 4096 consecutive OTUk frames). Please see the blog on IAE for more information on this feature. |
| OTUk_CP Interface | ||
| CI_D | Output | OTUk Characteristic Information - OTUk Data Output: The OTUk_TT_So function will compute, generate and insert the SMOH (Section Monitoring Overhead) into the outbound OTUk data-stream. It will then output this OTUk data-stream via this output signal. NOTE: This OTUk data will contain the following fields. - The newly computed, inserted BIP-8 value - The newly received and inserted BEI-nibble value or BIAE indicator. - The newly received and inserted BDI bit-value. - the newly received and inserted IAE value. - The contents of the APS/PCC channel and - The rest of the OTUk frame. This OTUk data-stream will not include the FAS, MFAS or FEC field. The OTUk_TT_So function will update this output signal on one of the edges of the CI_CK output clock signal. |
| CI_CK | Output | OTUk Adapted Information - Clock Output: The OTUk_TT_So function will output all data and signals (via the OTUk_CP interface) upon one of the edges of this output clock signal. This statement applies to the following signals: CI_D, CI_FS and CI_MFS. |
| CI_FS | Output | OTUk Characteristic Information - Frame Start Output: This function will drive this output pin HIGH whenever the OTUk_CP interface outputs the very first bit (or byte) of a new OTUk frame, via the CI_D output. The OTUk_TT_So function should only pulse this output pin HIGH once for each outbound OTUk frame. |
| CI_MFS | Output | OTUk Characteristic Information - Multiframe Start Output: This function will drive this output pin HIGH whenever the OTUk _CP Interface outputs the very first bit (or byte) of a new OTUk Superframe via the CI_D. The OTUk_TT_So function will drive this output pin HIGH once for each OTUk Superframe. |
| OTUk_TT_So_RP Interface | ||
| REI_BEI | Input | Remote Port Interface - BEI (Backward Error Indicator) Input: The OTUk_TT_So function will accept one nibble of data (for each outbound OTUk frame) via this input signal and it will insert this data into the BEI/BIAE nibble-field within the Section Monitor field within each outbound OTUk frame. The BEI value will reflect the number of BIP-8 errors that the collocated OTUk_TT_Sk function has detected and flagged within its most recently recevied and verified OTUk frame. NOTE: If the OTUk_TT_So function receives a BIAE = 1 (via the RI_BIAE input) then it will overwrite the BEI/BIAE nibble-field with the value "1011" to denote a BIAE event. Please see the BEI post for more information about Backward Error Indication. |
| RI_BIAE | Input | Remote Port Interface - BIAE (Backward Input Alignment Error) Input: The OTUk_TT_So function will accept one bit of data (for each outbound OTUk frame) via this input signal and it will do either of the following, depending on the value of this single bit-field. If BIAE = 0 Then the OTUk_TT_So function will write the BEI value that it has received via the RI_BEI input, into the BEI nibble-field within the Section Monitor byte of the next outbound OTUk frame. If BIAE = 1 Then the OTUk_TT_So function will not write the BEI value (that it has received from the collocated OTUk_TT_Sk function). It will instead, write the value "1011" into the BEI/BIAE nibble-field, within the Section Monitor byte of the next outbound OTUk frame. |
| RI_BDI | Input | Remote Port Interface - Backward Defect Indicator Input: The OTUk_TT_So function will accept one bit of data (for each outbound OTUk frame) via this input pin and it will write the contents of this value into the RDI bit-field (within the Section Monitor byte) of the next outbound OTUk frame. If RI_BDI = 0 The the OTUk_TT_So function will set the BDI bit-field to "0" within the next outbound OTUk frame. If RI_BDI = 1 Then the OTUk_TT_So function will set the BDI-bit-field to "1" within the next outbound OTUk frame. |
| OTUk_TT_So_MP Interface | ||
| MI_TxTI | Input | Management Interface - Trail Trace Identifier Input: The function user is expected to load in the 64-byte TTI Message into the OTUk_TT_So circuitry via this input. The OTUk_TT_So function will then transmit the message to the remote Network Element, one byte-at-a-time, over 64 consecutive outbound OTUk frames. |
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