Troubleshooting 40G and 100G Transceivers

Transceivers are the backbones of optical networks. As data demands have exploded globally, optical transceiver data rates have gone up to keep pace with these rising demands. 40G, 100G and 400G transceivers are becoming more commonplace in optical networks worldwide.

Standard 4 Lanes Transceiver Design

This article covers the basics of troubleshooting a 40G, 100G or 400G transceiver. For that, first we must understand the basic architecture of a 4-lane transceiver.

For example, a 100G transceiver converts 4 input channels of 25Gb/s electrical data to 4 channels of optical signals and then multiplexes them into a single channel for 100Gb/s optical transmission. Reversely on the receiver side, the module de-multiplexes a 100Gb/s optical input into 4 channels of optical signals and then converts them to 4 output channels of electrical data.

The diagram below is of a standard CDR that is used in transmitters and receivers. The interface of the 40G is 4 x 10.3Gbps and 100G is 4 x 25.78126 Gbps.

De-muxing 100G QSFP28-CW4

In a 4-lane transceiver, troubleshooting becomes easier when you can isolate the channels. This is achieved by de-muxing.

The eye diagram below represents the transmitter eye when it is de-muxed. Each eye represents one lane or channel .

To de-mux the lanes , you have to access the registers at page 0, 56 hex. If the value is 00, all 4 transmitters are on. If it’s ff, all are off. If it’s 0E, only lane 0 is on, 0D is lane 1, 0B, lane 2, 07 lane 3.

A0(00).0x56Tx Description
0x00All on
0xFFAll off
0X0ELane 0 on only
00X0DLane 1 on only
0X0BLane 2 on only
0X07Lane 3 on only

This is how you can control and de-mux the four channels one by one. The register values come from MSA standards and you are allowed to make changes.

Examples of Bad Lanes (100G-CW4)

The above eye diagram was that of a good transceiver. Now, let us look at a 100G CWDM-4 transceiver that is not working correctly. The first thing to do is to determine which channel or lane is problematic.

For this you can run an optical spectrum scan with an OSA (Optical Spectrum Analyzer). After scanning , you can see that lane 2 is bad. A resolution of 0.05nm was used to see the finer details.

This is what we call an SMSR (Side Mode Suppression Ratio) issue. The SNR (Signal to Noise Ratio) value for lane 2 is 13dB while the others are around 42dB. In this instance, the side mode becomes very bad.

This is also confirmed by measuring the optical eye. Lane 2 has no eye because the optical power has dropped.

Another thing to note is the peak power of the lanes when measured with an optical spectrum analyzer (OSA). On the optical spectrum, the peak power shows the true peak power, and it does not capture the area underneath it. On lane 2, it is about 4 to 5 dBm lower than the rest of the lanes. OSA is an ideal tool for testing transceivers with 4-lane architectures. When you measure power using an optical power meter (OPM), the power depends on the filter bandwidth that you have. OPM may not give the most accurate power readings in many cases. For example, from the table below, it is difficult to judge the bad lane.

Lasers degrade over time. They are also prone to damage from extreme temperatures. Stringent testing and correct operation are essential for prolonging the life of a laser.

Vitex offers a five-year warranty on most of our transceivers. Contact us at if you need assistance with troubleshooting transceivers or choosing the right product.

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