100G QSFP28 CWDM4 vs. 100G CLR4 Optical Transceiver

For the 100G Ethernet, although there are CFP, CFP2, CFP4 and QSFP28 optical modules for options, and QSFP28 transceivers undoubtedly are preferred due to its small form factor, higher transmission rate, longer transmission distance, etc. QSFP28 optics consist of QSFP28 100GBASE-CWDM4, QSFP28 100GBASE-PSM4, QSFP28 100GBASE-SR4, QSFP28 100GBASE-LR4, QSFP28 CLR4 and so on, among of which QSFP28 CWDM4 and QSFP28 CLR4 are the most suitable solutions for one who demands long distance, high transmission rate and low cost. However, what are differences between QSFP28 CWDM4 and QSFP28 CLR4? In this post, the answer will be gotten.

What Is CWDM4 Optical Transceiver?
The 100G QSFP28 CWDM4 optical transceiver is a full duplex, photonic-integrated optical transceiver module that provides a high-speed link with a maximum transmission distance of 2km for 100G Ethernet. CWDM4 interfaces with LC duplex connectors. It uses 4×25Gbps to achieve 100Gbps. Specifically speaking, four lanes with center wavelengths of 1270nm, 1290nm, 1310nm and 1330nm are controlled on the transmit side. On the receiving side, four lanes of optical data streams are optically de-multiplexed by an integrated optical demultiplexer. With an optical multiplexer and de-multiplexer, one just uses a duplex single-mode fiber to connect two 100G CWDM4 optical transceivers.



Why to Choose CWDM4?
CWDM is defined by CWDM4 MSA(Multi-Source Agreement), who targets a common specification for low-cost 100G optical interfaces that run up to 2km in data center applications. It’s with low power consumption, high compatibility, Digital Diagnostic Monitoring (DDM) support, high transmission rate, long transmission distance. In addition to these advantages, its most attractive benefits are with 2km long transmission distance and cost-saving in fibers. It’s exactly what ones want who are troubled by these two issues.

What Is CLR4 Optical Module?
100G QSFP28 CLR4 is an optical module standard defined by 100G LR4 Alliance,  similar to 100G CWDM4 QSFP28 optical module to large degree. It also utilizes single-mode CWDM （Coarse Wavelength Division Multiplexing）technology, and adopts duplex LC interface. It is with 4 channels, each running at 25Gbps to aggregate 100Gbps, and is with 20nm CWDM wavelength spacing between channels (same as 40G LR4). The transmission distance is 2km used with single-mode optical fiber. In the meantime, it’s with and without FEC enabled in the switch to support low‐latency applications.

Why  to Choose CLR4
The motivation for the 100G-CLR4 specification is to provide open, multi‐vendor specification for a cost‐effective, low power, 100Gbps (4x25.78GBd), CWDM optical transceiver with a reach of up to 2km over duplex single‐mode fiber and duplex LC optical connectors. On account of  its various benefits, it satisfies the demands on data rate, transmission distance, connectors, power consumption, etc. Then the advantages of CLR4 are as below:

• Low power-consumption.
• Compact transceiver form factor: The 100G-CLR4 Specification is transceiver form-factor agnostic, with QSFP28 as the initial form factor.
• Compatibility with SMF connectors and cable infrastructure for present and future datacenters,
• as well as telecom client-side interfaces.
• Application supports operation with and without Forward-Error-Correction (FEC):
• Enables use with FEC for increased link margin or operation without FEC for lower
• power consumption and lower latency.
• A variety of opto-electronic implementation approaches and technologies.
• Reach up to 2 km based on practical cabling design such as structured cabling or point to-point cabling.

100G CWDM4 vs. 100G CLR4 Optical Module: What Are Differences?
According to the above introductions to them, the differences between QSFP28 CWDM4 and QSFP28 CLR4 can be concluded(by comparison in the form of table shown as below). Generally speaking, their main difference is that the 100G CLR4 optical module can be used for both fiber links with forward error correction (FEC) functions and fiber links that do not have forward error correction (FEC) functionality. For the fiber link that have forward error correction (FEC) functions, 100G CWDM4 QSFP28 and 100G CLR4 QSFP28 optical module can be used together; while in the fiber links without forward error correction (FEC) functions, 100G CLR4 optical module can also maintain performance of low latency and low power, which makes it more popular in high-performance computing, high-frequency transactions and other areas.

 
Summary
It seems that the similarities are more than differences between 100G QSFP28 CWDM4 and 100G CLR4. But anyway, both of them are the most cost-effective solution for 2km transmission application in data center.