100G Optical Transceivers Links: PSM4 vs CWDM4

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In the past few years, 100G optical transceivers are become more popular and widely used than before. The most common 100G optical transceivers we use today are CFP, CFP2, CFP4 and QSFP28, especially the QSFP28. Besides 100G optical transceivers links (-SR10, SR4, -LR4) defined by IEEE standard, Multi-Source Agreement (MSA) also defines two 100G optical transceivers links: PSM4 and CWDM4. Both architectures take the 100GE signal and carry it over 4 separate channels. So, what’s the difference between them? PSM4 vs CWDM4–difference between them will be introduced in this blog.

PSM4
The 100G PSM4 Specification defines requirements for a point-to-point 100 Gb/s link over eight single mode fibers (4 transmit and 4 receive) up to at least 500 m, each transmitting at 25Gbps. Four identical and independent lanes are used for each signal direction (as shown in figure below). Therefore, two transceivers communicate usually over 8-fiber MTP/MPO single mode patch cords. PSM4 is limited to 500 m, and it is usually used in 100G QSFP28 optical transceivers.

100G Optical Transceivers Links PSM4

CWDM4
Similar to PSM4, CWDM4 also uses 4 x 25 Gbps to achieve 100 Gbps. But unlike it, CWDM4 uses an optical multiplexer and de-multiplexer to reduce the number of fibers to 2 (as shown in figure below). Therefore, we only need to use a duplex single mode fibers to connect two 100G CWDM4 optical transceivers modules. CWDM4 is limited to 2 km. At present, CWDM4 links are used in both 100G CFP4 or the QSFP28 optical transceivers.

100G Optical Transceivers Links CWDM4

100G Optical Transceivers Links: PSM4 vs CWDM4
A summary table comparing the key differences between the two 100G transceivers is shown below. From an optical transceiver module structure viewpoint, PSM4 can be more cost effective because it uses a single uncooled CW laser which splits its output power into four integrated silicon modulators. However, from an infrastructure viewpoint, this transceiver would be more expensive when the link distance is long, mainly due to the fact that it uses 8 optical single-mode-fibers while CWDM4 uses only 2 optical single-mode-fibers.

100G Optical Transceivers Links PSM4 vs CWDM4
When considering the above two factors, a total cost comparison can be qualitatively shown in the figure below. As can be seen in the figure, PSM4 starts with a lower cost due to its lower transceiver cost, but as the link distance increases, its total cost climbs up very fast due to the fact that it uses 8 optical fibers.

100G links PSM4 vs CWDM4

Conclusion
Different companies have different opinions on what the link distance is at the crossing point, and what the transceiver cost difference is at zero distance. But based on the specifications of PSM4 MSA, the technology has to be limited to 500 meters, which can actually cover the majority of today’s data center needs. FS.COM provides both 100G PSM4 QSFP28 ($ 750.00) and 100G CWDM4 QSFP28 ($ 1350.00) optical transceivers for your options.

Related Article: http://www.cables-solutions.com/100g-qsfp28-pam4-coherent-cfp.html