What is the Difference Between 100G CLR4 and CWDM4

Since the existing 100G Ethernet links with QSFP28 SR4 or LR4 are either too short or too expensive, 100G modules that can cover this enormous middle ground between 100m and 10km are expected to come to the market. 100G CLR4 and CWDM4 are two types of 100G interface to deliver distance up to 2km over duplex single-mode fiber. As CLR4 and CWDM4 are similar, so what’s the difference between them?

CLR4 Alliance

100G CLR4 is a new, open, multi-vendor 100G optics specification created by 100G CLR4 Alliance who consists of end-customers, system companies, and optical companies. It is a low-power, cost-effective, 100G-CWDM solution that routes four 25Gbps optical transmissions down a single-mode fiber for reaches of up to 2km. CLR4 Alliance is designed to address market requirements of large data center customers. CLR4 specification supports FEC (Forward Error Correction) and non-FEC applications.

Clause & Players

CLR4 Alliance—Altera, Arista, Aurrion, Brocade, Ciena, ColorChip, Dell, ebay, Fabrinet, Fujitsu, Hewllett-Packard, Huawei, Intel, Juniper Networks, Kaiam, MACOM, Materials Magic, NeoPhotonics, Netronome, Oclaro, Oplink, Oracle, SAE Magnetics, Semtech, Skorpios, Source Photonics, TE Connectivity, VMware, 3ality Technica.


The CWDM4 MSA (Multi-Source Agreement) targets a common specification for low-cost 100G optical interfaces that run up to 2 km in data center applications. The MSA uses CWDM technology with 4 lanes of 25 Gbps optically multiplexed onto and demultiplexed from duplex single-mode fiber. CWDM4 MSA targets the broad data center 100G interconnects that support FEC applications.

Clause & Players

CWDM4 MSA—Avago Technologies, Brocade, ColorChip, Finisar, HiLight Semiconductor, Hitachi Metals, II-IV Inc, JDSU, Juniper Networks, Kaiam, Mitsubishi Electric, NeoPhotonics, Oclaro, Oplink, SiFotonics, Skorpios, Sumitomo Electric.

100G CLR4 vs CWDM4

As mentioned above, the specifications of the CLR4 and CWDM4 are very similar. Both include FEC, not in the module but as part of the system design, but whereas FEC is fundamental to the CWDM4, it is optional with the CLR4. The 100G CLR4 specification is fully interoperable with the 100G CWDM4 specification for links using FEC. But as FEC only as an option for CLR4, the CLR4 Alliance’s interface avoids the delay associated with FEC, so that it is more suitable to use in applications such as high-frequency trading where latency is an issue. The following table shows us some details about CLR4 vs CWDM4:


100G CLR4 and CWDM4 meet the demands on 2km span of large data center. They are similar enough to be interoperable—the CLR4 is fully interoperable with the CWDM4 for links using FEC.

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