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What is the difference between 40G QSFP+ CWDM and PSM?
Knowledge Base + 2024.01.05

QSFP+ transceiver, the updated version of QSFP (Quad Small Form-Factor Pluggable), is known by providing four 10-Gigabit transmit and receive channels in a single pluggable optical module for an aggregate bandwidth of 40 Gbps. It’s nowadays widely adopted to provide high-density and low-power 40 Gigabit Ethernet connectivity. Based on the root of QSFP+ transceiver, types of 40G transceivers have been bred, such as QSFP+ 40G LR4 CWDM and 40G PSM LR4. In this post, a comparison between QSFP 40G LR4 CWDM and 40G LR4 PSM will be explored.

CWDM and PSM Basics

Before the comparison, it’s necessary to go through some basics of CWDM and PSM:

CWDM (Coarse Wavelength Division Multiplexing): As one type of WDM, CWDM is used to combine multiple signals on laser beams at various wavelengths for transmission along with fiber optic cables. It is typically deployed in point-to-point topology in enterprise networks and telecom access networks.

PSM (Parallel Single Mode Fiber): It is a technology that enables signals transmitted or received over four parallel fibers in each direction rather than over multiple channels on a single fiber.

40GBASE-LR4 PSM QSFP+ Transceiver

PSM QSFP+ transceiver is a parallel single-mode optical transceiver with an MTP/MPO fiber ribbon connector. Moreover, it offers four independent transmit and receive channels and each channel is capable of 10G operation for an aggregate data rate of 40G on 10 km of single-mode fiber. The guide pins inside the receptacle could ensure proper alignment. Usually, the cable cannot be twisted for proper channel to channel alignment. For a PSM QSFP+ transceiver, 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.


40GBASE-LR4 CWDM QSFP+ Transceiver

The 40GBASE-LR4 CWDM QSFP+ transceiver like QSFP-40GE-LR4 is compliant to 40GBASE-LR4 of the IEEE P802.3ba standard. It has a duplex LC connector for the optical interface and its maximum transmission distance is 10 kilometers. Single-mode fiber (SMF) has to be used to minimize 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 four individual 10G channels with different wavelengths. The central wavelengths of the four CWDM channels are 1271, 1291, 1311 and 1331 nm as members of the CWDM wavelength grid defined in ITU-T G694.2. Moreover, each wavelength channel is collected by a discrete photo diode and output as electric data after being amplified by a transimpedance amplifier (TIA).


Differences Between the Two

From the perspective of an optical transceiver module structure, PSM seems more cost effective as it uses a single uncooled CW laser splitting its output power into four integrated silicon modulators. Moreover, its array-fiber coupling to an MTP connector is relatively simple.

 However, from the perspective of an infrastructure, PSM would be more expensive when the link distance is long, because it uses 8 optical single-mode fibers while CWDM only uses 2 optical single-mode fibers. The following table illustrates the main differences between CWDM and PSM.


In addition, 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. Moreover, it is not a straightforward tack to clean MTP connectors. So CWDM is a more profitable and popular 40G QSFP link.