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40GBASE-LR4 CWDM and PSM QSFP+ Transceiver Links

40GBASE-LR4 QSFP+ transceivers use two links: coarse wavelength division multiplexing (CWDM) and the other is parallel single-mode fiber (PSM). Do you know what their differences are? 

There are 40GBASE-SR4 QSFP+ transceivers and 40GBASE-LR4 QSFP+ transceivers. It’s known that 40GBASE-SR4 uses a parallel multimode fiber (MMF) link to achieve 40G. It offers 4 independent transmit and receive channels, each capable of 10G operation for an aggregate data rate of 40G over 100 meters of OM3 MMF or 150 meters of OM4 MMF. While for 40GBASE-LR4 QSFP+ transceivers, there are two kinds of links. One is coarse wavelength division multiplexing (CWDM) and the other is parallel single-mode fiber (PSM). What’s the difference?

40GBASE-LR4 CWDM QSFP+ Transceiver

QSFP-40GE-LR4, one of 40GBASE-LR4 CWDM QSFP+ transceivers, is compliant to 40GBASE-LR4 of the IEEE P802.3ba standard. The optical interface is a duplex LC connector. It can support transmission distance up to 10 km over single-mode fiber by minimizing the optical dispersion in the long-haul system.

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 as members of the CWDM wavelength grid defined 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

Different from CWDM QSFP+ transceiver which uses a LC connector, PSM QSFP+ is a parallel single-mode optical transceiver with an MTP/MPO fiber ribbon connector. It offers 4 independent transmit and receive channels, each capable of 10G operation for an aggregate data rate of 40G over single-mode fiber about 10 km. The cable can’t be twisted to keep proper channel to channel alignment.

In a PSM QSFP+, the transmitter module accepts electrical input signals and the receiver module converts parallel optical input signals via a photo detector array into parallel electrical output signals. The receiver module. All data signals are differential and support a data rates up to 10.3G per channel.

Difference of Two Links

What’s the difference of these two links? From the viewpoint of optical transceiver module structure, PSM seems more cost effective because it uses a single uncooled CW laser which splits its output power into four integrated silicon modulators. Besides, its array-fiber coupling to an MTP connector is relatively simple. From the viewpoint of infrastructure, PSM is more expensive, because when the link distance is long, PSM uses 8 optical single-mode fibers while CWDM uses only 2 optical single-mode fibers. For more about their differences, please see the following table:

What’s more, in the data center fiber infrastructure, the patch panel has to be changed to accommodate MTP cables. This would cost more than LC connectors and regular SMF cables. Besides, it’s not easy to clean MTP connectors. So CWDM is more profitable and popular for 40GBASE-LR4 QSFP+ link.

For 40GBASE-LR4 QSFP+ transceivers, both CWDM link and PSM link can reach the transmission distance of 10 km. 40GBASE-LR4 CWDM QSFP+ transceivers use a duplex LC connector via 2 optical single-mode fibers. And 40GBASE-LR4 PSM QSFP+ transceivers use an MTP/MPO fiber ribbon connector via 8 optical single-mode fibers. Thus, there is no need to make any changes to upgrade 10G fiber cable plant to 40G connectivity, which is more cost-effective. Fiberstore provides wide brand compatible 40G CWDM QSFP+ transceiversBusiness Management Articles, such as Juniper compatible JNP-QSFP-40G-LR4 and HP compatible JG661A. Each fiber optic transceiver has been tested to ensure its compatibility and interoperability.

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I'm currently working as a Telecom Engineer in Fiberstore, an innovative company providing fiber connectivity network solutions for carriers, ISPs, content providers and networks.

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