QSFP28 CWDM4 Vs. PSM4

With the popularity of 100 Gigabit Ethernet, more and more Ethernet users urge to update their transceivers for a higher bandwidth. Currently, there are several 100G transceivers available on the telecom market, they are CFP, CFP2, CFP4, CXP and QSFP28 transceivers. Among them, QSFP28 is highly favored by many people for its smallest form factor, lowest power consumption and etc. There are many transmitter and receiver types supporting the QSFP28 transceiver, including SR4, SR10, LR4 and ER4, which are defined by IEEE group. Besides, 100G CWDM4 and 100G PSM4 are also defined by MSA group. It seems that these two types of 100G transceivers are less common than the other types. So what are they? And do they have any difference? We would provide a satisfied answer in the following passage.
Introduction to QSFP28 CWDM4 Transceiver
The 100G QSFP28 CWDM4 optical transceiver is a full duplex, photonic-integrated optic transceiver that provides a 100G Ethernet high-speed link with a maximum transmission distance of 2 km. QSFP28 CWDM4 interfaces with LC duplex connectors. It uses 4×25 Gbps to achieve 100 Gbps. More specifically, four lanes with center wavelengths of 1271 nm, 1291 nm, 1311 nm and 1331 nm are controlled on the transmit side. On the receive side, four lanes of optical data streams are optically de-multiplexed by an integrated optical demultiplexer. With an optical multiplexer and de-multiplexer, we only need to use a duplex single-mode fiber to connect two 100G CWDM4 optical transceivers. QSFP28 CWDM4 has many advantages, such as low power consumption, high compatibility, Digital Diagnostic Monitoring (DDM) support...Nowadays, it has been widely applied to Local Area Network (LAN), Wide Area Network (WAN), and Ethernet switches and router application.
Information About QSFP28 PSM4 Transceiver
PSM4 is short for parallel single-mode 4 lane, which is targeted to data centers that based on a parallel single-mode infrastructure. 100G QSFP28 PSM4 is a type of 100G optical transceiver that provides a low-cost solution to data center optical interconnects. The 100GBASE-PASM4 transceiver operates over PSM4 parallel single-mode ribbon fiber, using a nominal wavelength of 1310 nm. It is interfaced with standard MTP/MPO connector. The link length can be up to 500 m. Different from QSFP28 CWDM4, QSFP28 PSM4 uses eight parallel single-mode fibers (4 transmit and 4 receive) with point-to-point 100 GB/s link, which is defined by 100G PSM4 MSA standard. To have a further explanation, PSM4 uses four identical lanes per direction. Each lane carries a 25G optical transmission. QSFP28 transceiver is of high performance and durability.
The below figure shows a 100G CWDM4 and a 100G PSM4 transceiver.

Similarities
  • Media type—Both QSFP28 CWDM4 and QSFP28 PSM4 use single-mode fiber to transmit.
  • Lane count—QSFP28 CWDM4 and QSFP28 PSM4 use 4 lanes—4×25 Gbps to achieve 100 Gbps.
  • Wavelength—The using wavelength of QSFP28 CWDM4 and QSFP28 PSM4 is around 1310 nm.
Differences
  • Fiber count—QSFP28 CWDM4 uses 2 single-mode fibers to transmit while QSFP28 PSM4 uses 8 single-mode fibers.
  • Transmission distance—QSFP28 CWDM4 is longer than QSFP28 PSM4 on transmission distance. The maximum of transmission distance is 2 km and 500 m respectively.
  • Connector type—QSFP28 CWDM4 interfaces with LC duplex connectors while QSFP28 PSM4 with MTP/MPO connectors.
  • Cost—QSFP28 CWDM4 is more expensive than QSFP28 PSM4. At FS.COM, QSFP28 CWDM4 is $1350 and QSFP28 PSM4 is $750.
Summary
After looking through this article, we figure out that difference between QSFP28 CWDM4 and QSFP PSM4 mainly lies in fiber count, transmission distance,connector type as well as cost. You can have your selection according to your real need. If you require a 100G optical module for short length transmission over single-mode fiber, QSFP28 PSM4 transceiver is highly recommended. When longer transmission is needed, QSFP28 CWDM4 is ideal.

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