Comparison of 100G QSFP28 CLR4 and 100G CWDM optical modules

With the advent of the 5G era and the rapid development of the artificial intelligence industry, HPC data centers that support intelligent algorithms and deep learning will be more used in the commercial field.

The 100G QSFP28 CWDM4 optical module is the most widely used optical module in the current 100G data center. Data centers including Facebook, Alibaba, and Tencent use it as the preferred solution for optical interconnection within a distance of 500m to 2km. Due to the low cost of the lasers and Mux/Demux devices used in CWDM4, this solution is more competitive in the 2km transmission scenario than 100GABSE-LR4.

However, in the field of high-performance computing, the application of CWDM4 is often stretched. HPC data centers not only need optical modules to meet the requirements of large bandwidth, but also low latency, strict signal transmission quality and other characteristics are more important. The advantage of low cost is no longer convincing, and the new CLR4 alliance came into being.

100G CLR4 is a 100G optical transceiver standard formulated by the 100G CLR4 Alliance. Enterprise organizations including Altera, Fujitsu, Macom, Neophotonics, Oclaro, etc. have joined the 100G CLR4 Alliance. The 100G CLR4 Alliance, led by Intel and Arista Network, believes that 100G QSFP28 CLR4 optical modules are very suitable for connecting ultra-large-scale data centers with a span of 100 meters to 2 kilometers.

In digital communication, in order to prevent external signal interference and protect the invariance of the signal, multiple error correction code settings are required. During the decoding process of digital signals, they are very sensitive to error signals. As long as there is a small bit error per second, decoding cannot be performed. Using the forward error correction FEC method can effectively reduce the bit error rate of digital signals and improve the reliability of signal transmission. It is worth noting that the optical module itself does not have the FEC forward error correction function, and the FEC is only aimed at the device side, not the module side.

Comparison between 100G QSFP28 CWDM4 and 100G QSFP28 CLR4

The same:

1. Both interface types are duplex LC;

2. Both adopt CWDM wavelength division multiplexing technology, and the working wavelengths are 1270, 1290, 1310, 1330nm;

3. Both are single-mode optical modules, and the transmission distance is 2KM.

the difference:

100G QSFP28 CWDM4 optical modules must be used for fiber optic links with Forward Error Correction (FEC).

The 100G QSFP28 CLR4 optical module can be used for both optical fiber links with forward error correction (FEC) function and optical fiber links without forward error correction (FEC) function.

Different from the 100G QSFP28 CWDM4, the 100G QSFP28 CLR4 can be applied to the link with and without FEC enabled at the same time, which is more flexible. In terms of signal transmission quality, without enabling FEC forward error correction, the BER error rate of CLR4 is only 1E-12BER, which can still meet the error-free transmission of 10KM links. It is much lower than that under the condition of enabling forward error correction. The 100G QSFP28 CWDM4 bit error rate; in addition, the parameters such as sensitivity also have good performance. In addition, the CLR4 emission eye diagram mask index is stricter than that of CWDM4. This means that this product is more suitable for the internal interconnection of super-large data centers than CWDM4.