How To Generate/Modify and Monitor OTN TCMi Overhead Bytes?

Some VeEX test sets offer dedicated TCM-specific functions, such as the TCM Tasks. TCMi Bytes are part of the OTN Overhead, however, each TCMi channel contains its own Parity, Errors and Alarms, based on the different segments of the link under test


The OTN Tandem Connection Monitoring (TCMi, i = 1 to 6) channels are used to monitor the status of the different segments that make an end-to-end path, allowing operators to monitor and pinpoint problematic segments during testing or troubleshooting. It becomes very useful when multiple carriers or service providers are involved in the delivery of a service. Note that every level of ODUk, within the OTN map/mux structure, has its own TCMi channels.


OTN_Overhead-TCMi_Overhead_BytesThe TCMi functionality is part of the OTN overhead and it is used to independently monitor up to six tandem connections within a link (end-to-end path). The i index refers to the individual sections being monitored. There are six TCMi fields in the ODUk section of OTN overhead, each one with three bytes. The information is carried in a multi-frame format.

The TCMi bytes contain the following information:

  • TTI - Trail Trace Identifiers
    • SAPI - Source Access Point Identifier
    • DAPI - Destination Service Point Identifier
    • Operator Specific Information
  • BIP-8 - Bit Interleaved Parity
  • BEI - Backward Error Indication
  • BIAE - Backward Incoming Alignment Error
  • BDI - Backward Defect Indication
  • STAT - TCM Monitoring Status bits 

1. Testing OTN Links with TCMi (Handheld test sets)

This Quick Guide focuses on VeEX portable instruments. The TCMi functionality turned OFF by default. Users must manually turn ON individual TCMi channels when required. The following procedures may vary slightly from product to product, by OTN rates, or for older software versions.

To get started, select the Test Port Group >Test Port, pick the OTN or OTN/SDH/SONET tab from the technology menu, and pick the desired OTN test feature, for example, OTN SDH/SONET Mapping. Press OK to load and launch the test application.

Use the Setup menu to configure the test set's OTN test interface to match the network/link port or system under test. Use a clean and inspected patch cord to connect the test set to the link under test.

Knowledge of OTN, TCM theory, link architecture, OTN Errors and Alarms are required, to avoid unwanted responses from the far end or intermediate nodes and to get meaningful results (and interpretations).

2. Enabling Tandem Connection Monitoring

All required active TCMi (1 to 6), for each ODUk level needs to be enabled, to match the monitoring structure of the link or port under test. In normal conditions, active TCMi channels transmit STAT=001 and disabled ones transmit STAT=000.

The graphical user interface (GUI) for OTN functions may be a bit different for native Ethernet over OTN (direct mappings) and OTN with legacy TDM payloads (SDH/SONET). 

2.1 TCM in OTN with Legacy SDH/SONET Mappings Test Modes

To enable individual TCMi channels, go to >OTN Tools >TCM Tasks and set the Enabled field to ON for each of the required TCMi channels, in each ODUk level, to be monitored and included in the test. There is one tab for each TCM channel.


When enabled, individual Alarm and Error results for each monitored segments will be shown on those tabs (not on the main OTN BERT Results or Summary screens).



ODU-LTC - Loss of Tandem Connection
• STAT = 000

TCM-OCI - Open Connection Indication
• Defect:  STAT = 110 for ≥ 3 consecutive frames
• Clear:    STAT ≠ 110

TCM-BDI - Backward Defect Indication
• Defect:  Set to 1 for ≥ 5 consecutive frames 
• Normal: Set to 0 for ≥ 5 consecutive frames

TCM-AIS - Alarm Indication Signal
• Defect:  STAT = 111 for ≥ 3 consecutive frames
• Clear:    STAT ≠ 111

TCM-LCK - Tandem Connection is Locked
• Defect:  STAT = 101 for ≥ 3 consecutive frames
• Clear:    STAT ≠ 101

TCM-BIAE - Backward Incoming Alignment Error (BEI/BIAE bits)
• B (1011) ≥ 3 consecutive frames

TCM-TIM - Trail Trace Identifier Mismatch
• Received and expected TTI strings do not match (are different). This is not an OTN alarm, but one warning indication generated by the local device (e.g., test set or network element)


TCM-BIP-8 - Bit Interleaved Parity-level 8 code error (mismatch)
• Received and calculated BIP are different

IAE- Incoming Alignment Error (OTUk, SM)

IEC - Incoming Error Count

TCM-BEI - Backward Error Indication (BEI/BIAE bits)
• 0..8 Number of BIP-8 violations detected
• 9..A No BIP-8 error detected
• B Refer to BIAE
• C..F No BIP-8 error detected

2.1.1 TCMi Trail Trace Identifiers (TTI)

Each of the TCM channels has their own set of identifies. The TTI byte is actually a 64-byte multi-frame string, consisting of:

  • SAPI - Source Access Point Identifier
  • DAPI - Destination Access Point Identifier
  • Operator specific trace

Transmitted TTIs

When TCM is enabled, test sets transmit pre-defined strings by default. However, each of those traces can be customized (edited) by going to the >OTN Tools >Trace Identifier page, selecting the TX tab and customizing all the required OTU, ODUs and TCMs to match all the identifiers expected by the network.


Use the Copy buttons on the Received fields, to copy the received string to the corresponding setting for the test set's transmitter. Users can edit the transmit fields after copying.

Received and Expected TTIs

On the >OTN Tools >Trace Identifier page, tap on the RX (Receive) tab and select the OTU, ODUs and TCMs required for the link under test. By default, the TIM validation fields are set to OFF, so the test equipment ignores the received traces.


Users have the option to turn the TIM validation ON, for the test set to compare the Received SAPI/DAPI traces against the (user programmed) Expected SAPI/DAI traces. When enabled, the test set will generate a Trace Identification Mismatch (TIM) warning indication if/when any of the received traces don't match the expected string.

2.1.2 TCM Fields in the Overhead Analyzer/Monitor

Go to >OTN Tools >Overhead Analyzer to view the received TCMi bytes in the OTN overhead. Users can tap on individual byte fields to get quick decode of the current value or text strings.

Note: The TCMi fields on the Overhead Generator function are marked xx, to indicate that they can't be generated or modified from that page. Use the TCM Tasks feature instead.

2.2 TCM in OTN with Ethernet (EoOTN) Test Modes

Test sets with OTU4 and OTU3 with Ethernet & Bulk Payloads test modes may offer a slightly different user interface. All the TCM features are conveniently consolidated within the Overhead function. Go to >OTN Tools >Overhead and select the TCM tab. Use the small arrow buttons near the bottom of the screen to navigate through the six TCMi pages.

To activate a particular TCMi channel for monitoring, Enable the Rx Monitor field on each of the required TCMs.

Transmit and Expected traces can be programmed right on the same screen, using the Tx Generation and Rx Expectation fields.

To be alerted of Trace Identification Mismatches, Enable the TIM Monitor field for each of the TCMi channels to be monitored.

The Overhead Generation tab also allows users to modify certain bytes in real time. For the TCMi fields, users can modify the TTI text strings and the BEI bytes by tapping on the fields. Modified values (bytes) will be displayed in red color font, as a reminder. If critical bytes are modified, an OH (overhead) warning icon is displayed on the top of the screen, to remind users that non-standard values are being transmitted. Use the Reset OH button to bring the original values back.

The Overhead Analysis tab behaves in a similar fashion, allowing users to decode the different bytes being received, by tapping on the desired fields. 


Some test sets may also allow to capture up to 256 values (consecutive frames) for a selected overhead byte. The capture can get started as soon as users tap on a field (byte), or by looking for specific values with the Pre, Mid and Post trigger functions for advanced troubleshooting purposes. To enable this, change the BYTE field from DECODING to CAPTURE mode.


Once the capture mode has been enabled, users can select the Trigger Type to program when the acquisition stars. Use Multi Frame #0 to start at the beginning of the multi-frame or Pre, Mid and Post to start capturing after, before-and-after or before the specified Byte Value trigger is detected.

3. Alarm & Error Generation

During tests, users can inject different Alarms and Errors into the different levels of the OTN mapping and multiplexing structures. Use the Set Injection button, on the right side of the screen, to configure the type of impairments to be inserted into the transmitted test signal. There are two dedicated injection buttons for Alarm and Error.

Select the Mode (e.g., ODU2 TCM) or Layer (e.g., OTU4/ODU4), then the Error and/or Alarm TypeTCM # to be affected and the expected Behavior. Once set, the Error and Alarm buttons will behave as programmed by the user.

Alarm and/or error icons will be displayed on the top of the screen, to indicate that impairments are currently being injected into the test signal.

3.1 Alarm Behaviors

  • Continuous - The selected alarm is manually turned ON and OFF by pressing the button. 
  • Count - Upon pressing the Alarm button, the associated alarm will remain active for the number of seconds programmed and then it turns off automatically.
  • Single Burst - Upon pressing the Alarm button, the associated alarm will remain active for the number of ON Frames programmed and then it turns off automatically.
  • Continuous Burst - The selected alarm is generated affecting a number of ON Frames as programmed, followed by a number of OFF Frames without alarms, as programmed. The behavior repeats until the alarm button is pressed again.

3.2 Error Behaviors

  • Single - Generates a single error every time the button is pressed.
  • Count - Lets user program the exact number of errors to be generates each time the button is pressed.
  • Single Burst - The programmed number of errors are generated consecutively.
  • Continuous Burst - The selected error is generated affecting a number of ON Frames as programmed, followed by a number of error free OFF Frames, as programmed. The behavior repeats until the error button is pressed again.
  • Fixed Rate - Lets users select from a pre-defined list of common error rates.
  • Rate - Lets user set specific error rates in X.XXE-YY format.

3.3 How Bit Interleaved Parity Bits (BIP) are Generated

The BIP byte is calculated in real-time, based on the transmitted data and is therefore non-modifiable by users. However, TCM-BIP errors can be generated from the Alarm/Error page.

For OTU2, users would select Error Mode = ODU2 TCM, then select the TCM number (1-6), select OTN TCM Error Type = TCM BIP, select the error flow (behavior) and tap on the Error Injection button to enable its transmission.

3.4 How Bits in the BEI Fields are Generated

The BEI byte actually consists of several bit fields, namely BEI/BIAE, BDI and STAT. The values of these fields can be changed by sending Alarms/Errors generation as defined below:

Alarm Mode = ODU2 TCM, OTN TCM and Alarm Type = TCM BIAE:

  • The BEI/BIAE bit field will be transmitted with a value of 1011.

Error Mode = ODU2 TCM, OTN TCM and Error Type = TCM BEI:

  • The BEI/BIAE bit field will contain values related to the current TX BEI error rate.

Alarm Mode = ODU2 TCM, OTN TCM and Alarm Type =TCM BDI:

  • The BDI bit field will be transmitted with a value of 1 when this alarm is enabled, otherwise is set to 0 (zero).

Alarm Mode = ODU2 TCM, OTN TCM and Alarm Type = TCM OCI:

  • The STAT bit field will be transmitted with a value of 110 when this alarm is enabled, 001 otherwise.

Alarm Mode = ODU2 TCM, OTN TCM and Alarm Type = TCM AIS:

  • The STAT bit field will be transmitted with a value of 111 when this alarm is enabled, 001 otherwise.

Alarm Mode = ODU2 TCM, OTN TCM and Alarm Type = TCM LCK:

  • The STAT bit field will be transmitted with a value of 101 when this alarm is enabled, 001 otherwise.

Alarm Mode = ODU2 TCM, OTN TCM and Alarm Type = TCM IAE:

  • The STAT bit field will be transmitted with a value of 010 when this alarm is enabled, 001 otherwise.

Alarm Mode = ODU2 TCM, OTN TCM and Alarm Type = TCM LTC:

  • The STAT bit field will be transmitted with a value of 000 when TCM is disabled, 001 otherwise.

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Refer to the OTN Glossary for more details on OTN acronyms and basic TCMi Alarm/Error definitions.

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