QSFP28 PSM4 vs. QSFP28 SR4 vs.QSFP28 LR4 Optics: What Are the Differences?


Although QSFP28 optical transceiver has the same appearance as 40G QSFP+ transceiver, and QSFP28 optical transceiver module wins out by virtue of its transmission optical signals up to 100G. Now, QSFP28 optical transceiver has developed as the mainstream of 100G optics and also as the first solution for an option to upgrade 100G Ethernet. While the main types of QSFP28 optics in the current optical communication market include 100G QSFP28 PSM4, 100G QSFP28 SR4, and 100G QSFP28 LR4, etc. Then in this article, Gigalight will have an detailed introduction to the differences of them.

Differences of 100G QSFP28 PSM4, 100G QSFP28 SR4, and 100G QSFP28 LR4

qsfp28 lr4, psm4, and sr4 transceivers

The Difference in Definition

The QSFP28 PSM4 optical transceiver is with a high-speed, low-power-dissipation product with a hot-pluggable QSFP form factor and built-in digital diagnostics function. Besides, it is with eight optic fibers, each fiber with a data rate of 25Gbps.

The QSFP28 SR4 optical transceiver is a parallel 100G optical transceiver with the advantages such as high port density and low cost. If an optical module for the short-reach transmission is needed to upgrade the 100G network, the QSFP28 SR4 optical transceiver can be chosen.

The QSFP28 LR4 optical transceiver is one with transmission distance up to 2km. It provides an ideal solution for the super-large-scale data centers which has ever-increasing requirements for transmission distance. Simultaneously, it will lead to the use of single-mode optic fiber in the data center.

The Difference in Transmission Media and Distance

The three modules can support different transmission distances. 100G-SR4 QSFP28 module works over the wavelength of 850nm and is used with 12-fiber MTP OM3 or OM4 multimode fiber cables for short transmission distances up to 100m. 100GLR4 QSFP28 module is suggested to be used with single-mode fiber. It works over 1310nm wavelengths and can transmit 100G signals up to 2km. 100G-PSM4 QSFP28 is also used with 12-fiber MTP fiber cables but the fiber type is single-mode and the transmission distance is up to 500m.

The Difference in Transmission Mode

It is known that QSFP28 modules generally use four lanes to transmit 100G with each lane supporting 25G. Thus, the transmission method is just like 40G QSFP+ transceiver. 100G QSFP28 SR4, LR4 and PSM4 all use the 4*25 transmission mode. However, both the QSFP28 SR4 and QSFP28 PSM4 use a 12-fiber MTP interface which achieves dual-way 100G transmission over 8 fibers at the same time. QSFP28 LR4 uses an LC duplex fiber optic interface for 100G transmission on two directions at the same time. QSFP28 LR4 transmit optical signals over four different wavelengths around 1310nm with each wavelength carrying a 25G optical signal. The wavelength ranges of the four lanes are as following:





The Difference in Cabling Structure

Optical transceiver transmission has to play a very important role in the optical fiber routing. Because the QSFP28 SR4 optical transceiver and the QSFP28 LR4 optical transceiver are used for short-distance transmission and long-distance transmission respectively, their wiring structures are different. The former requires multi-fiber cabling based on a 12-way MMF MTP interface, while the latter requires only a traditional two-fiber SMF cabling. In this case, the conversion between multimode fiber and single-mode fiber is very complicated because they use a completely different wiring structure.

Although the QSFP28 PSM4 optical transceiver operates in single-mode fiber, its wiring structure is the same as that of the QSFP28 SR4 optical transceiver. Using the QSFP28 PSM4 optical transceiver saves conversion costs between multimode and single-mode without changing existing cabling structures.

The Difference in Working Principle

a. Working Principle of QSFP28 PSM4:

The QSFP28 PSM4 optical transceiver works in much the same way as the QSFP28 SR4 optical transceiver. The difference is that the QSFP28 PSM4 optical transceiver operates on single mode fiber while the QSFP28 SR4 optical transceiver operates on OM4 multimode fiber.

b. Working Principle of QSFP28 SR4:

QSFP28 SR4 optical transceiver transmits signals at the transmitting end; the electrical signals are converted into optical signals by the laser array and then transmitted in parallel on the ribbon multimode fiber. Upon reaching the receiving termination, the photodetector array converts the parallel optical signals into parallel Electrical signals.

c. Working Principle of QSFP28 LR4:

QSFP28 LR4 optical transceivers are typically used with LC single-mode fiber optic cables to convert 4 25Gbps electrical signals into 4 LAN WDM optical signals and then multiplexed into a single channel for 100G optical transmission. At the receiving end, the module demultiplexes the 100G optical inputs into 4 LAN WDM optical signals, which are then converted into 4 electrical signal output channels.


In general, the differences in QSFP28 PSM4, QSFP28 SR4, and QSFP28 LR4 optical transceiver are as above mentioned.

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