PSM4 or CWDM4, Which One Is Better for QSFP28 Transceiver?

ponedjeljak , 27.02.2017.

As the most commonly used module in 100G Ethernet network, QSFP28 transceiver can be fitted with various of 100G interfaces to meet the requirements of different applications. For instance, the most basic 100G interfaces, 100GBASE-SR4 always used for short distance transmission, no longer than 100 m, and 100GBASE-LR4 highly recommendable for long-haul optical transmission, with the lengths up to 10 km, occupy the majority of the 100G market. Considering that, if we want to deploy the 100G Ethernet network with the transmission distance longer than 100 m but shorter than 10 km, the QSFP28 100GBASE-SR4 transceiver can’t satisfy the requirement and the QSFP28 100GBASE-LR4 transceiver is not the most cost effective one. In order to deal with the mentioned applications, the 100GBASE-PSM4 and 100GBASE-CWDM4 interfaces for QSFP28 transceivers came into market, expected to be the optimized, alternative solutions for 100G Ethernet network in data center.

Why PSM4 and CWDM4 Interfaces Are Used for QSFP28 Transceivers?
It is well known that 100GBASE-SR4 and 100GBASE-LR4 defined by IEEE are the most popular 100G interfaces, which can support the 100G network with too short or too long distance. However, for data center managers, they would like to choose the QSFP28 transceiver that can transmit the 100G signal within 500m or 2km maximum distance. Hence, the two basic 100G interfaces are not the most suitable ones for them.

Under this condition, MSA (Multi-Source Agreement) strategy brings a mid-reach solution to the 100G market, and the PSM4 and CWDM4 interfaces for the 100G QSFP28 transceivers are published in this revolution. Compared to the basic 100G interfaces, these two non-IEEE defined interfaces can not only support 100G network with longer distance than 100GBASE-SR4, but also cost less than 100GBASE-LR4. There is no doubt that these two interfaces can gradually become more and more popular due to these two advantages.

What are PSM4 and CWDM4 QSFP28 Transceivers?
QSFP28 transceiver with PSM4 interface can be also called PSM4 QSFP28 transceiver. It is designed with four lanes of parallel ribbon fiber, carrying serialized data through single mode fiber at a rate of 25 Gbps per lane, which totally achieves the 100G transmission, as shown in the following figure. With the use of 12 fiber MTP/MPO connectors, it can support the 100G network with the distance limited to 500 m. As for its light source, it is a single uncooled distributed feedback (DFB) laser with 1310nm wavelength. Hence, in its working process, there should be either a directly modulated DFB laser (DML) or an external modulator for each single mode fiber.

Instead of parallel technology, QSFP28 transceiver with CWDM4 interface specially uses the CWDM technology, which makes only one single mode fiber required for data transmission. Meanwhile, it can support the 100G transmission at lengths up to 2 km. In its working process, the optical multiplexer and de-multiplexer are required to separate and combine the four 25G signals which operate around 1310nm. Besides, the connector in the CWDM4 QSFP28 transceiver is also different, duplex LC connector. To better know the working principle of CWDM4 QSFP28 transceiver, you can learn it from the figure below.



PSM4 vs. CWDM4 Interfaces for QSFP28 Transceivers
After knowing the basic knowledge of PSM4 and CWDM4 QSFP28 transceiver, let’s compare these two kinds of 100G transceivers with different interfaces and find which one is more cost effective. As the components like multiplexer and de-multiplexer fitted in the CWDM4 QSFP28 transceiver are very expensive, the cost for QSFP28 transceiver with CWDM4 interface is much higher than that of the PSM4 one. But if the whole 100G connection cost is taken into account, the answer should depend on the link distance. As it can be seen in the following figure, PSM4 QSFP28 starts with a lower cost due to its lower transceiver cost, but as the connection distance increases, its total cost climbs up very fast due to the fact that it uses 8 optical fibers. Hence, if you want to deploy the 100G network with a short distance, you are highly suggested to choose the QSFP28 transceiver with PSM4 interface. Or the QSFP28 transceiver with CWDM4 interface is the best choice for you, as a cost effective transceiver solution. In addition, if you have any interest, the following article offers more information about these two kinds of interfaces for QSFP28 transceivers.