Guide to Two 40GBASE-LR4 QSFP+ Links - CWDM and PSM

The common 40GBASE-LR4 QSFP+ optical transceivers that are available on the market are QSFP-40GE-LR4, QSFP-40G-LR4, QSFP-40G-LR4-S and WSP-Q40GLR4L. They are all compatible with 40GBASE-LR4 standard, but differ with each other. Since I have already explained the difference between the above 4 optical transceivers, I will not go further about these topic today. Except for the different 40GBASE-LR4 QSFP+ transceivers types, there are also two links for 40GBASE-LR4 standards. One is coarse wavelength division multiplexing (CWDM). The other is parallel single-mode fiber (PSM). In this article, these two links of 40GBASE-LR4 QSFP+ transceivers will be introduced to you.

40GBASE-LR4 CWDM QSFP+ Transceiver

The 40GBASE-LR4 CWDM QSFP+ transceiver (like QSFP-40GE-LR4) is compliant to IEEE P802.3ba 40GBASE-LR4 standard. This QSFP module supports link lengths of up to 10km over single-mode fiber (SMF) with duplex LC connectors. This transceiver converts 4 inputs channels of 10G electrical data to 4 CWDM optical signals by a driven 4-wavelength distributed feedback (DFB) laser array, and then multiplexes them into a single channel for 40G optical transmission, propagating out of the transmitter module from the SMF. Reversely, the receiver module accepts the 40G CWDM optical signals input, and demultiplexes it into 4 individual 10G channels with different wavelengths. The central wavelengths of the 4 CWDM channels are 1271, 1291, 1311 and 1331 nm (defined as members of the CWDM wavelength grid in ITU-T G694.2). Each wavelength channel is collected by a discrete photo diode and output as electric data after being amplified by a transimpedance amplifier (TIA).

40GBASE-LR4 PSM QSFP+ Transceiver

Unlike CWDM QSFP+ transceiver using a LC connector, PSM QSFP+ is a parallel single-mode optical transceiver with an MTP/MPO fiber ribbon connector. It also offers 4 independent transmit and receive channels, each capable of 10G operation for an aggregate data rate of 40G on 10km of single-mode fiber. Proper alignment is ensured by the guide pins inside the receptacle. The cable usually cannot be twisted for proper channel to channel alignment. In terms of a PSM QSFP+, the transmitter module accepts electrical input signals compatible with common mode logic (CML) levels. All input data signals are differential and internally terminated. The receiver module converts parallel optical input signals via a photo detector array into parallel electrical output signals. The receiver module outputs electrical signals are also voltage compatible with CML levels. All data signals are differential and support a data rates up to 10.3G per channel.

Compare 40GBASE-LR4 CWDM QSFP+ With 40GBASE-LR4 PSM QSFP+ Transceiver

As noted before, 40GBASE-LR4 CWDM QSFP+ transceivers use a duplex LC connector via 2 optical single-mode fibers to achieve 40G without making any changes to the previous 10G fiber cable plant. However, 40GBASE-LR4 PSM QSFP+ transceivers use an MTP/MPO fiber ribbon connector via 8 optical single-mode fibers to reach 40G. Obviously, CWDM QSFP+ is a more cost-effective solution for 40G connectivity.

What’s more, in terms of the inner structure of an optical transceiver module, PSM QSFP+ uses a single uncooled CW laser that splits its output power into four integrated silicon modulators, which is much inexpensive than CWDM QSFP+. Besides, its array-fiber coupling to an MTP connector is relatively simple. A picture comparing the key differences between CWDM and PSM is shown below:

Additionally, the caveat is that the entire optical fiber infrastructure within a data center, including patch panels, has to be changed to accommodate MTP connectors and ribbon cables, which are more expensive than conventional LC connectors and regular SMF cables. Not to mention that cleaning MTP connectors is not a straightforward task.

Conclusion

PSM and CWDM are the two links of 40GBASE-LR4 QSFP+ transceivers. Both of them can support a link distance of 10km. However, 40GBASE-LR4 CWDM QSFP+ are more common than 40GBASE-LR4 PSM QSFP+ because of its performance and low cost. Fiberstore offers a wide brand compatible 40G CWDM QSFP+ transceivers. Each of our fiber optic transceivers has been tested to ensure its compatibility and interoperability. For more information or quotation, please contact us directly.