As the demand for higher bandwidth and faster data transmission continues to escalate, the networking industry is constantly innovating to meet these needs. In the realm of 400G Ethernet, two technologies, 400G SR4 and 400G SR4.2, have emerged as prominent solutions. While they may appear similar, each has unique features and benefits that cater to different networking scenarios. This article delves into the differences between 400G SR4 and 400G SR4.2, helping you understand which one might best fit your networking needs.

Understanding 400G SR4

400G SR4 is part of the IEEE 802.3bs standard. It leverages four parallel fibers for transmitting data and four parallel fibers for receiving data, supporting a total of eight fibers. Each fiber operates at a rate of 100 Gbps, thus aggregating to 400 Gbps. The "SR" in SR4 stands for "Short Reach," and the "4" indicates four lanes of transmission. 400G SR4 is a high-speed Ethernet solution designed to meet the increasing demand for data transmission in data centers and other high-performance networking environments.

The 400G SR4 interface uses an MPO/MTP (Multi-Fiber Push-On/Pull-Off) connector, which contains 12 or 16 fibers. In the SR4 configuration, 8 of these fibers are active (4 for transmitting and 4 for receiving), while the remaining fibers are either unused or reserved for future use.

400G SR4

Key Features of 400G SR4

High Data Rate: Provides a robust 400Gbps data rate, suitable for high-demand applications.
Single Wavelength: It uses 850nm wavelength VCSELs (Vertical-Cavity Surface-Emitting Lasers).
Short Reach: It supports a reach of up to 100 meters over OM4 multimode fiber and up to 70 meters over OM3 multimode fiber.
Applications: Optimized for short-distance communication, ideal for intra-data center connections.

FS 400G SR4 Transceivers

The FS 400G SR4 transceivers feature compact OSFP, QSFP112, and QSFP-DD form factors, making them ideal for high-speed data centers and high-performance computing networks. Key selling points include support for 400Gbps transmission rates, low power consumption, high-density, and multimode fiber connectivity. These transceivers offer an efficient and cost-effective solution while being compatible with existing 400G/100G infrastructure, ensuring seamless upgrades and exceptional performance. The following picture shows the 400G-to-400G link to ConnectX-7 of QSFP-DD 400G SR4.

FS 400G SR4.2 Transceivers

	OSFP-SR4-400G-FL	QSFP112-SR4-400G	QDD-SR4-400G
Center Wavelength	850nm	850nm	850nm
Connector	MTP/MPO-12	MTP/MPO-12	MTP/MPO-12
Cable Distance (Max.)	30m/50m@MMF	50m@MMF	60m/100m@MMF
Modulation	4x 106.25G PAM4	4x 100G PAM4	8x 50G PAM4
DSP	7nm DSP	Broadcom 7nm DSP	Maxlinear
Power Consumption	≤8.5W	≤9W	≤8W
Transmitter Type	VCSEL	VCSEL	VCSEL
Packaging Technology	COB	COB	COB
Application	InfiniBand 800G to 2x400G Breakout	Data Center	Data Center 400G to 2x200G Breakout 400G to 4x100G Breakout

Understanding 400G SR4.2

400G SR4.2, on the other hand, is a newer standard defined by the IEEE 802.3cm. 400G SR4.2 utilizes wavelength multiplexing to enhance the performance of each fiber pair, making it a more efficient solution. The primary difference between 400G SR4 and 400G SR4.2 lies in its wavelength division multiplexing (WDM) capabilities.

The optical lane arrangement is shown as follows. The leftmost four positions are labeled TR because they transmit wavelength λ1 and receive wavelength λ2. Conversely, the rightmost four positions are labeled RT because they receive wavelength λ1 and transmit wavelength λ2. Wavelength λ1 is traditionally set at 850 nm. The second wavelength, λ2, is nominally set at 910 nm, which supports longer reaches over OM5 multimode fiber, optimized for short wavelength multiplexing. As a result, the reach over OM5 is 50% longer than over OM4 and more than double that over OM3.

400G SR4.2

Key Features of 400G SR4.2

Wavelength Multiplexing: Each fiber pair carries two wavelengths of 850nm and 910nm, effectively doubling the capacity.
Extended Reach: Support up to 150 meters over OM5 multimode fiber and up to 100 meters over OM4 multimode fiber, providing greater flexibility in deployment and addressing some of the limitations of 400G SR4 in larger data centers.
Fiber Efficiency: 8 fibers (4 transmit and 4 receive) technically, but with each fiber carrying two wavelengths, reducing overall material and installation costs.
Applications: Suitable for data centers requiring longer reach connections within the same multimode infrastructure.

FS 400G SR4.2 Transceivers

FS QSFP-DD 400G SR4.2 transceivers enable seamless breakout to 4x QSFP28 100G SR1.2 optics, facilitating the creation of 400G connections to meet the increasing demand for higher data rates. These transceivers comply with the IEEE 802.3 protocol and the 400GAUI-8/PAM4 standard. They feature built-in digital diagnostics monitoring (DDM) for real-time access to operating parameters, making them ideal for 400G Ethernet and data center interconnect applications. Moreover, this breakthrough architecture is achieved while leveraging existing fiber cable infrastructure, thereby optimizing cost-effectiveness.

FS 400G SR4 Transceivers

	QDD-SR4.2-400G
Center Wavelength	850nm,910nm
Connector	MTP/MPO-12
Cable Distance (Max.)	70m@OM3/150m@OM5
Modulation	8x 53.125G PAM4
DSP	70m/150m@MMF
Power Consumption	≤12W
Transmitter Type	VCSEL 850nm
Packaging Technology	COB
Application	Data Center 00G to 4x 100G Breakout

400G SR4 vs. 400G SR4.2

In the rapidly evolving realm of data centers and high-performance computing networks, the demand for increased data throughput has made 400G optical transceivers a pivotal technological choice. Among these, 400G SR4 and 400G SR4.2 are prominent solutions, each offering distinct differences in transmission methods, distances, and application scenarios. Understanding these differences is crucial for network architects and engineers to select the most suitable solution for their specific deployment needs.

The following table shows a clear comparison between them.

	400G SR4	400G SR4.2
	400G SR4	400G SR4.2
Wavelength Multiplexing	1 wavelength (850nm)	2 wavelengths (850nm and 910nm)
Reach	100m over OM4 and 70m over OM3	150m over OM5 and 100m over OM4
Fibers	8 fibers	8 fibers
BiDi Technology	/	Support
Infrastructure Requirements	Relies on traditional OM3 and OM4 multimode fibers.	Benefits from using OM5 fiber, which is optimized for WDM and can take advantage of the dual-wavelength approach.
Infrastructure Requirements	Relies on traditional OM3 and OM4 multimode fibers.
Cost and Installation	Generally, the cost is lower due to the use of a single wavelength and existing OM3/OM4 multimode fiber infrastructure.	Potentially higher upfront costs due to the need for OM5 fiber and more complex transceiver technology, but offers greater scalability and future-proofing.
Applications and Use Cases	Ideal for smaller data centers or segments within data centers where distances are relatively short.	Better suited for larger data centers with more significant distances between switches and servers.

Summary

Both 400G SR4 and 400G SR4.2 play crucial roles in the evolution of high-speed networking within data centers. Choosing between them largely depends on your specific needs regarding reach, existing infrastructure, and future scalability. 400G SR4 is a solid choice for short-reach, cost-sensitive deployments, while 400G SR4.2 offers enhanced performance for more extensive and future-proof data center environments.

By understanding the differences and advantages of each technology, network architects and IT professionals can make informed decisions to optimize their network performance and accommodate the ever-growing demand for higher data rates and improved efficiency. Both 400G SR4 and 400G SR4.2 represent significant advancements in Ethernet technology, ensuring that modern data centers can keep pace with the rapid evolution of digital communication.

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