400G Multimode Fiber: 400G SR4.2 vs 400G SR8

Cloud and AI applications are driving demand for data rates beyond 100 Gb/s, moving to high-speed and low-power 400 Gb/s interconnects. The optical fiber industry is responding by developing two IEEE 400G Ethernet standards, namely 400G SR4.2 and 400G SR8, to support the short-reach application space inside the data center. This article will elaborate on the two standards and their comparison.

400G SR4.2

400G SR4.2, also called 400G BD4.2, is a 4-pair, 2-wavelength multimode solution that supports reaches of 70m (OM3), 100m (OM4), and 150m (OM5). It is not only the first instance of an IEEE 802.3 solution that employs both multiple pairs of fibers and multiple wavelengths, but also the first Ethernet standard to use two short wavelengths to double multimode fiber capacity from 50 Gb/s to 100 Gb/s per fiber.

400GBASE-SR4.2 operates over the same type of cabling used to support 40G SR4, 100G SR4 and 200G SR4. It uses bidirectional transmission on each fiber, with each wavelength traveling in opposite directions. As such, each active position at the transceiver is both a transmitter and a receiver, which means 400G SR4.2 has eight optical transmitters and eight optical receivers in a bidirectional optical configuration.

The optical lane arrangement is shown as follows. The leftmost four positions labeled TR transmit wavelength λ1 (850nm) and receive wavelength λ2 (910nm). Conversely, the rightmost four positions labeled RT receive wavelength λ1 and transmit wavelength λ2.

FS 400G SR4.2 Transceivers:

FS QSFP-DD 400G SR4.2 facilitate seamless breakout to 4x QSFP28 100G SR1.2. The architecture not only utilizes the existing fiber optic cable infrastructure to achieve 400G connectivity, meeting the demand for consumer upgrade rates, but also optimizes cost-effectiveness.

400G SR8

400G SR8 is an 8-pair, 1-wavelength multimode solution that supports reaches of 70m (OM3), 100m (OM4 & OM5). It is the first IEEE fiber interface to use eight pairs of fibers. Unlike 400G SR4.2, it operates over a single wavelength (850nm) with each pair supporting 50 Gb/s transmission. In addition, it has two variants of optical lane arrangement. One variant uses the 24-fiber MPO, configured as two rows of 12 fibers, and the other interface variant uses a single-row MPO-16.

FS 400G SR8 Transceivers:

FS 400G SR8 transceivers provide the flexibility of fiber shuffling with 50G/100G/200G configurations. These transceivers also support breakout at various speeds for diverse applications, including compute, storage, flash, GPU, and TPU. FS 400G-SR8 QSFP-DD/OSFP transceivers support migration from 50G to 400G networks, functioning as 8x50G SR.

400G SR4.2 vs. 400G SR8

As multimode solutions for 400G Ethernet, 400G SR4.2 and 400G SR8 share some features, but they also differ in a number of ways as discussed in the previous section.

The following table shows a clear picture of their comparison.

	400G SR4.2	400G SR8
Alliance	IEEE 802.3cm	IEEE 802.3cm (breakout: 802.3cd)
Max reach	150m over OM5	100m over OM4/OM5
Fibers	8 fibers	16 fibers (ribbon patch cord)
Wavelength	2 wavelengths (850nm and 910nm)	1 wavelength (850nm)
BiDi technology	Support	/
Signal modulation format	PAM4 signaling	PAM4 signaling
Laser	VCSEL	VCSEL
Form factor	QSFP-DD	QSFP-DD, OSFP

400G SR8 is technically simple but requires a ribbon patch cord with 16 fibers. It is usually built with 8 VCSEL lasers and doesn’t include any gearbox, so the overall cost of modules and fibers remains low. By contrast, 400G SR4.2 is technically more complex so the overall cost of related fibers or modules is higher, but it can support a longer reach.

In addition, 400G SR8 offers both flexibility and higher density. It supports fiber shuffling with 50G/100G/200G configurations and fanout at different I/O speeds for various applications. A 400G-SR8 QSFP-DD transceiver can be used as 400G SR8, 2x200G SR4, 4x100G SR2, or 8x50G SR.

400G SR4.2 & 400G SR8: Boosting Higher Speed Ethernet

As multimode fiber continues to evolve to serve growing demands for speed and capacity, both 400G SR4.2 and 400G SR8 help boost 400G Ethernet and scale up multimode fiber links too ensure the viability of optical solutions for various demanding applications.

The two IEEE 802.3cm standards provide a smooth evolution path for Ethernet, boosting cloud-based services and applications. Future advances point toward the ability to support even higher data rates as they are upgraded to the next level. The data center industry will take advantage of the latest multimode fiber technology such as OM5 fiber, and use multiple wavelengths to transmit 100 Gb/s and 400 Gb/s over fibers over short reaches of more than 150 meters.

Conclusion

The 400G era has fully arrived. Building on this foundation, the IEEE have progressively developed the 800G and 1.6T Ethernet standards, effectively meeting the growing traffic demands of hyperscale data centers and paving the way for future ultra-high-speed transmission.

At present, FS offers 400G RoCE network solutions, delivering superior performance with reliable 400G transceivers that have undergone rigorous testing on target devices to ensure perfect compatibility and high quality, meeting all your 400G network demands.