40GBASE-LR4 CWDM vs. 40GBASE-LR4 PSM QSFP+ Transceiver

In response to the increasing bandwidth demands, the IEEE certificated 802.3ba Ethernet standard paving the way for the introduction of 40G and 100G Ethernet operations. Whether you believe it or not, the era of 40G Ethernet is upon us. 40GBASE QSFP+ (quad small form factor pluggable) portfolio offers customers a wide variety of high-density and low-power 40 Gigabit Ethernet connectivity options for data center, high-performance computing networks, enterprise core and distribution layers, etc. For instance, 40GBASE-SR4 and 40GBASE-LR4 QSFP+ transceiver are the common 40 Gigabit Ethernet connectivity options. 40GBASE-SR4 transceiver usually uses multimode fiber for short-reach application, while 40GBASE-LR4 QSFP+ is mostly favored by users for long-hual application. However, there are two links for 40GBASE-LR4 standards. One is coarse wavelength division multiplexing (CWDM). The other is parallel single-mode fiber (PSM). What’s the difference between them? In this article, a brief contrast between them will be introduced to you.

40GBASE-LR4 CWDM QSFP+ Transceiver

The 40GBASE-LR4 CWDM QSFP+ transceiver 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).

CWDM-QSFP

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.

40G-PSM-QSFP

40GBASE-LR4 QSFP+—CWDM vs. PSM 

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:

two links of QSFP+ 40GBASE-LR4

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.

Summary

To sum up, 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.

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