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Verify Link and Transceivers with Pseudo Random Binary Sequence (PRBS) Test

Learn how to verify links and transceivers using the Pseudo Random Binary Sequence (PRBS) test and perform bidirectional diagnostics with remote loopback. Discover essential steps for clearing interface statistics effectively.

How to Use Pseudo Random Binary Sequence (PRBS) Test for Link and Transceivers Verification

The Pseudo Random Binary Sequence (PRBS) test enables you to verify link quality and transceiver operation. There are two diagnostic scenarios:

  • Bidirectional verification: The test sequence starts from the local end, and the remote end is configured with loopback. Analyze the test pattern at both ends.

  • Unidirectional verification: The test sequence starts from the local end, and the remote end analyzes the test pattern.

In the first case, bidirectional verification requires loopback support on the remote end.

Table 1: PRBS test on various MICs:

MIC Type

PRBS Test Engine

Description

JNP10003-LC2103

Packet Forwarding Engine (PFE)

The PFE for JNP10003-LC2103 is a specialized hardware module designed to handle high-performance packet forwarding, routing, and traffic management for the Juniper Networks JNP10003 system.

JNP-MIC1

Packet Forwarding Engine (PFE)

The PFE for JNP-MIC1 is a hardware module designed to manage and accelerate packet forwarding, enabling high-speed data processing and routing for the Juniper Networks JNP Series Modular Interface Cards (MIC).

JNP-MIC1-MACSEC

External physical layer (PHY) device

External physical layer (PHY) device is not capable of passing the PRBS pattern originating from the packet forwarding engine. Hence, the PRBS test is enabled on external physical layer (PHY) device.
Table 2: PRBS test details supported on various interfaces:

Interface Type

Interface Name

Interface Lane Characteristics

Description

10-Gigabit ethernet interface

xe

The interface is supported with 1 lane of 10Gbps speed

The PRBS test is executed on each lane supported. Hence, the show interfaces prbs-stats displays data for one lane.

40-Gigabit ethernet interface

et

The interface is supported with 4 lanes of 10Gbps speed.

The PRBS test is executed on each lane supported. Hence, the show interfaces prbs-stats displays data for four lanes.

100-Gigabit ethernet interface

et

The interface is supported with 4 lanes of 25Gbps speed.

The PRBS test is executed on each lane supported. Hence, the show interfaces prbs-stats displays data for four lanes.

Configure Unidirectional Diagnostics

  • Enable the transmission of pseudo-random binary sequence pattern on one end of the link (TX), and enable monitoring on the other end (RX).

  • The PRBS statistics report on the receiving end reflects link quality.

  • Collect and view the PRBS statistics.

    1. Start a TX (direction 0) by issuing the following command:

      Check the link status by executing show interfaces terse et-0/1/*:

    2. Start a RX (direction 1) by issuing the following command:

      Check the link status by executing show interfaces terse et-1/1/4:

    3. View the collected statistics at RX by issuing the following command:

      For example:

      Checking PRBS statistics at RX:

      The PRBS test is successful, if the state is pass with error count 0.

    4. Stop the PRBS statistics collection by issuing the following command:

      For example:

      To stop the PRBS at RX:

      Verify the statistics at TX by executing the following command:

      To stop the PRBS at TX:

      After executing the command, you can check the link status at RX by executing show interfaces terse et-1/1/4:

      Check the link status at TX by executing show interfaces terse et-0/1/2:

      This command only disables the statistics collection and does not clear the statistics collected. To clear the collected statistics, issue the clear interfaces statistics command.

Configure Bidirectional Diagnostics with Remote Loopback

Configure loopback on the remote end of the link. Test pattern generation and analysis is performed on the local end.

The PRBS statistics report on the receiving end reflects link quality.

Following are the steps to collect and view the PRBS statistics:

  1. Enable remote loopback.

    You must disable Forward Error Correction (FEC), if loopback is configured in the router with JNP-MIC1 MIC at the remote end.

  2. Start a TX (direction 0) by issuing the following command:

    After executing the command, you can check the link status by executing show interfaces terse et-0/1/*:

  3. Start a RX (direction 1) by issuing the following command on the same host.
  4. View the collected statistics at RX by issuing the following command:

    For example:

    Checking PRBS statistics at RX:

    The PRBS test is successful, if the state is pass with error count 0.

  5. Stop the PRBS statistics collection by issuing the following command:

    For example:

    To stop the PRBS at RX:

    Verify the statistics at TX by executing the following command:

    To stop the PRBS at TX:

    After executing the command, you can check the link status at RX by executing show interfaces terse et-0/1/2:

    This command only disables the statistics collection and does not clear the statistics collected. To clear the collected statistics, issue the clear interfaces statistics command.

Table 3: Interface Card Specific Differences

Behavior

JNP-MIC1-MACSEC MIC

JNP-MIC1 MIC

RX Latching to PRBS Signal

If RX is not latching to any PRBS signal, then the "state" in the show interfaces interface-name prbs-stats displays as “Disabled" with Error count as 0. 

user@host> test interface et-0/1/10 prbs-test-start pattern-type 31 direction 1 flip 0
user@host> show interfaces et-0/1/10 prbs-stats

    PRBS Statistics : Enabled
    Lane 0 : State : Disabled, Error count : 0
    Lane 1 : State : Disabled, Error count : 0
    Lane 2 : State : Disabled, Error count : 0
    Lane 3 : State : Disabled, Error count : 0

If RX is not latching to any PRBS signal, then the "state" in the show interfaces interface-name prbs-stats displays as failed with MAX error count.

user@host> test interface et-0/0/1 prbs-test-start pattern-type 31 direction 1 flip 0
user@host> show interfaces et-0/0/1 prbs-stats

    PRBS Statistics : Enabled
    Lane 0 : State : Fail, Error count : 4294967295
    Lane 1 : State : Fail, Error count : 4294967295
    Lane 2 : State : Fail, Error count : 4294967295
    Lane 3 : State : Fail, Error count : 4294967295

Pattern Mismatch

Displays a “disabled” state for pattern mismatches and flips between TX and RX (flip supported).

Maximum errors due to pattern mismatches between TX and RX flips (flip not supported).

TX Interruptions

RX shows “Disabled” with Error Count 0; no need to restart RX when TX restarts.

RX shows a failed state with error counts; RX must be restarted if TX restarts.

Consecutive Starts

Behaves differently from JNP-MIC1 MIC when starting TX or RX consecutively without stopping earlier runs.

Shows distinct behavior when starting TX or RX consecutively without stopping earlier runs.

DFE Tuning

Does not require Decision Feedback Equalization (DFE) tuning.

Requires DFE tuning; will show errors if PRBS restarts without stopping earlier runs.

Forward Error Correction (FEC

No specific mention of FEC settings.

FEC must be disabled if loopback is configured at the remote end.

user@host> show interfaces et-1/1/1 | display set 
set interfaces et-1/1/1 gigether-options loopback-remote   
set interfaces et-1/1/1 gigether-options fec none

Clear the Interface Statistics

The clear interface statistics command clears only the error counters and not the status, RX needs to be restarted to get the right status.

Following are the steps to clear the interface statistics:

  1. Check the statistics at RX by issuing the following command:

    Because only RX is started, there is no PRBS pattern and RX shows maximum error.

  2. To clear the interface statistics, execute the following command:

    Here the status still shows as fail, although the statistics displays the delta value. In this case, because both the current and previous vales are INT_MAX, the delta value 0 is displayed.

    Consider a scenario where the error count increments as the PRBS test is ongoing. In this case, the show interfaces interface-name prbs-stats shows the incremental error (delta value). Also after issuing clear interfaces statistics et-1/1/4 , if the error count is updated, the clear interfaces statistics displays the incremental error too.

    For Example:

    As shown above, in the second instance the show interfaces interface-name prbs-stats command displays the "change" in error count. So, the total error count is 640 + 52 = 692 for Lane 0.

See Also