The Differences Between Singlemode and Multimode QSFP28

After the evolution of optical network from 10G-40G-100G, 100G optical transceivers emerge, including 100G CFP, CFP2, CFP4, CXP and QSFP28. The most outstanding one type among these 100G transceivers is QSFP28 optical transceiver, which prevails in the optical interconnect industry by virtue its high speed, low in power consumption and cost, high density port, longer transmission distance, etc. Optics usually can be divided into single-mode/multimode optical transceiver. Then this post will make an analyses to QSFP28 optical transceiver from perspectives of single mode and multimode.

A Brief Introduction to Singlemode QSFP28 Optics
It’s known that single mode fiber is small in size of core and narrow in laser wavelength. It is due to it that single mode fiber is available for signal transmission with higher bandwidth to realize data transmission at longer distance. Single-mode QSFP28 usually adopts single mode fiber, mainly works at 1310nm wavelength and for longer distance transmission. Different types of single mode QSFP28 can achieve different data transmission distances, such as 100GBASE-PSM4 QSFP28 available for 500 meters, 100GBASE-CWDM4 QSFP28 available for 2 kilometers and 100GBASE-LR4 QSFP28 available for 10 kilometers. Simultaneously, there still exits some other differences among them, shown as the below table:

	connector interface	cable
100GBASE-PSM4 QSFP28	12-fiber MTP connector interface	MTP cable
100GBASE-CWDM4	duplex LC connector interface	duplex LC patch cable
100GBASE-LR4 QSFP28	duplex LC connector interface	duplex LC patch cable

A Brief Introduction to Multimode QSFP28 Optics
Multimode fiber is bigger in the size of core and wider in the wavelength of light. It is available for connection with inexpensive LED light sources and alignment of the connectors with a coupler less critical than that of single mode fiber. After that, optics transmitting over multimode fiber have a higher capability to gather light from the laser at a lower cost. Multimode QSFP28 usually utilizes the multimode fiber, mostly works at 850mn wavelength. At the same time, it is available for short optical link, such as 70ms over OM3, and 100ms over OM4. Among QSFP28 optics, one belonging to multimode optics is only QSFP28 100GBASE-SR4 with 12-fiber MTP connector interface, connected with MTP cable(shown as the pic).



Singlemode QSFP28 vs. Multimode QSFP28 Optical Transceiver: What Are the Differences?
With respect to the differences between single mode QSFP28 and multimode QSFP28 optics are mainly shown in these aspects:

• Transmission distance: the transmission distances that single mode QSFP28 optical modules can reach include 500 meters, 2 kilometers and 10 kilometers; while transmission distance that multimode QSFP28 optic can achieve is 100 meters.
• Power consumption: multimode QSFP28 consumes less power than singlemode QSFP28, which is an important consideration especially when assessing the cost of powering and cooling in the data center.
• Cost: on account that single-mode fiber optics require more expensive laser sources, the cost of single mode QSFP28 is nearly two or three times higher than that of multimode QSFP28 optic.
• Transmission media: single mode QSFP28 is usually connected with single-mode fiber; while multimode QSFP28 is connected with multimode fiber. It is known that 100GBASE-PSM4 QSFP28 is used with single-mode MTP cable and 100GBASE-SR4 QSFP28 is used with multimode MTP cable; while 100GBASE-CWDM4 QSFP28 and 100GBASE-LR4 QSFP28 are used with single-mode duplex LC patch cable.

Summary
Generally speaking, the difference between single mode QSFP28 and multimode QSFP28 mainly focuses on cost, power consumption, transmission distance, etc. At the same time, the advantages of them are also reflected by comparison. However, as for this question: which one is the best solution for 100G Ethernet? It is up to one’s actual demands.