What is a loopback test?

The loopback test is a common testing method used in transmission systems. It is generally used to verify the optical modules’ interface circuit, internal logic, and hardware implementation of the transmission line are correct. In simple terms, in a loopback test, the signal is sent from the tested device and then returned (loopback) to the original place, to determine whether the device is running normally or locate the failed node in the network. Loopback test doesn’t need specific testing devices and software, so it is considered an effective solution to quickly check and locate the failing nodes in a transmission network. 

Types of loopback test

Hard loopback and soft loopback

Based on different loopback ways, loopback tests can be divided into two types, hard loopback, and soft loopback. 

• Hard loopback: A loopback test is performed on physical ports (optical or electrical interfaces) via connection devices or cables. That is, data can be directly looped back to receivers through their own transmitters.

• Soft loopback: A loopback test performed via network management software.

Internal loopback and external loopback

• Internal loopback: A loopback test performed between the transmitter and receiver of a module itself. Put simply, internal loopback refers to data looped internally from Tx to Rx through the internal chip. It is used to test if the port hardware of the module is working properly in the switch. Internal loopback can be performed without access to a remote site.

• External loopback: A loopback test to verify whether the interface correctly sending and receiving packets. To create a loop, the external loopback requires a cable to physically loop the transmitter signal into the receiver port. External loopback must be performed at the site.

How to perform a loopback test on an optical module

Internal loopback guide on the optical module:

Internal loopback test is generally configured on the Ethernet switches. Run loopback internal command to place the interface in loop internal mode to

take all locally generated transmit data and return it to the receive data path. 

External loopback guide on the optical module:

Optical transceiver loopback test is performed by connecting the transmitter and receiver of the same optical transceiver, to verify if the hardware of the transceiver interface is working properly.

First of all, before performing a loopback test on an optical transceiver, we need a loopback cable or a single strand of fiber. The single strand of fiber can get via a duplex fiber, as the following steps show.

Step 1: Isolate a single strand of fiber from a duplex fiber cable. Pull apart the duplex connectors of both cable ends and remove the collars.

Step 2: Insert an optical transceiver into the Ethernet switch. 

Step 3: Creating a loopback line. Install the isolated ends of the single strand of fiber or loopback cable to the Tx and Rx ports of the optical transceiver.

Step 4: Checking the indicator light of the interface. If the indicator light keeps on, the port of hardware functions correctly, if not, further troubleshooting is required.

Note: 

1. Optical attenuator is necessary when testing a long-haul optical transceiver. 

2. When performing the external loopback test, prevent the received optical power is too high to affect the module. Generally, the short-range optical module and multimode optical fiber are selected to perform the loopback test, and the received optical power needs to be checked through the optical module information query command to ensure that the received optical power is lower than the maximum threshold value of the received light.

Conclusion

Loopback test is a useful and effective testing method to troubleshoot an optical module and Ethernet switch interface. In this post, we made a demonstration of how to perform a loopback test on modules.