In the ever-changing field of optical networking, revolutionary technologies continually reshape the landscape. A significant leap forward comes with the introduction of 400G SR4 transceivers, not only expanding short-reach options but also paving the way for the upcoming transition to 800G. This post explores the nuances of these transceivers, examining their impact on network infrastructure and their role in defining the future of data transmission.
The first generation of 400G transceivers was based on 8x56G PAM4 from the original QSFP-DD and OSFP MSA. Short-reach transceivers like the 400G SR8 and the innovative 400G SR4.2 "bidi" emerged from these MSAs, featuring eight lanes on both the host and optical sides. However, the 50G VCSELs used in the optical side lacked scalability for an 800G roadmap. The evolution in 100G VCSEL technology, driven by the demands of AI and ML clusters, led to the development of the 400G SR4.
The 400G SR4 shares a common optical interface across all form factors but differs significantly in how they interface with the host and the network.
Introducing the OSFP-RHS SR4, known as the 'Flat top,' designed for network adapters in AI and ML clusters, seamlessly integrating with GPU structures. Differentiating from legacy OSFP form factors, the OSFP-RHS eliminates the heat sink from the transceiver shell and adopts a 4x112G configuration on the host, diverging from the original 8x50G. It interfaces directly with the leaf level of the networking cluster, aligning with the demands of advanced computing environments.
The OSFP112 form factor, often called the 'Finned' transceiver, follows the type 2 physical specification of the OSFP MSA. Its defining feature is the heat sink, open to expose fins or closed with a smooth top. Departing from legacy OSFP transceivers, it adopts a 4x112G configuration on the host, deployed in the switching levels of AI and ML clusters for efficiency and adaptability, supporting specialized 2x200G or 2x100G ‘breakouts.’
Moving to the QSFP112, the QSFP version of a 112G SerDes on the host transceiver. Initially used in 400G network adapters and later in network switches, QSFP112 ports maintain backward compatibility with QSFP56 and QSFP28 transceivers.
Focusing on QSFP-DD, specifically the QSFP56-DD "QSFP-DD" SR4, it stands out by offering 400G SR4 compatibility with 56G SerDes QSFP-DD switches. Similar to the legacy 400G DR4 interface, this transceiver incorporates a DSP and gearbox for seamless retiming between four optical lanes and eight lanes at the host, ensuring optimal performance in diverse networking environments.
A crucial consideration for users of all 400G SR4 transceivers is the shift towards a lower back reflectance optical connector, the MPO-12/APC.
As we analyze these technological advancements, it's clear that the future of data transmission is being intricately shaped by these innovative transceiver technologies, promising heightened efficiency and adaptability in the ever-evolving optical networking landscape.