51.2T Network Upgrades: Mastering SerDes Tech in Optical Transceivers

NADDOD Jason Data Center Architect May 17, 2024

In today's rapidly evolving data-centric landscape, the role of optical transceivers as critical components in network communications cannot be overstated. These devices are central to managing the vast data flows in modern networks, making technological advancements in this area crucial for future-ready infrastructure. This article delves into the intricate world of optical transceiver packages, including SFP, SFP+, SFP28, QSFP+, QSFP28, QSFP56, QSFP112, QSFP-DD, DSFP, and OSFP. We will examine their intricate relationship with SerDes (Serializer/Deserializer) technology—focusing on channel count dynamics and their pivotal role in defining network capacity and efficiency. Our exploration will provide a detailed overview of the technological distinctions and enhanced functionalities that these optical transceivers bring to the fore in 51.2T networking environments.


Optical Transceiver and SerDes Evolution


1. SFP: The Entry Point for Small Form-Factor Pluggable

The Small Form-factor Pluggable (SFP) is a lightweight version of the Gigabit Interface Converter (GBIC) module, designed to support gigabit rates. Typically, SFP transceivers are equipped with a single gigabit SerDes channel, establishing the foundational technology for small form-factor pluggable transceivers.


2. SFP+: Advancing to 10Gbps

Building upon the SFP, the SFP+ supports 10Gbps Ethernet and other high-speed standards. Though similarly named, SFP+ transceivers enhance performance through a more advanced single-channel SerDes setup, facilitating faster data transmission speeds.




3. SFP28: The 25Gbps Frontier

Created for 25Gbps Ethernet, SFP28 transceivers incorporate a single 28G SerDes channel. This setup manages to deliver approximately 25Gbps after accounting for protocol overhead, providing a robust solution for the increasing bandwidth demands.




4. QSFP+: Elevating to 40Gbps with Quadruple Channels

The Quad Small Form-factor Pluggable Plus (QSFP+) features four independent 10Gbps SerDes channels. This multi-channel configuration quadruples the throughput to 40Gbps, substantially increasing bandwidth within a compact package.




5. QSFP28: The 100Gbps Contender in the 25G SerDes Age

The QSFP28 maintains the quad-channel layout of the QSFP+ but increases each channel's capacity to 25Gbps. This adjustment enables it to support a combined output of 100Gbps, thus addressing contemporary high-speed networking demands efficiently.


qsfp28NADDOD - QSFP28-100G-SR4


6. QSFP56 & QSFP112: Transitioning to the 200G/400G Era

QSFP56 and QSFP112 mark advancements by providing 50Gbps and 100Gbps per channel, respectively, while still using a four-channel SerDes technology. Both transceivers achieve 200Gbps and 400Gbps total data rates, respectively, after accounting for protocol overhead—indicating a significant evolution in data handling and transmission capabilities.


qsfp112NADDOD - QSFP112-400G-SR4


7. QSFP-DD: The 400G Octal-Channel Double Density Innovation

The QSFP-DD, where “DD” stands for Double Density, expands QSFP’s four channels to eight. By adding more pins and enhancing the design, QSFP-DD achieves 400Gbps or higher transmission rates (such as QSFP-DD 8x56=400G), utilizing more efficient SerDes technology and increased channel numbers. NADDOD is a leading global optical transceivers manufacturer, offering cutting-edge solutions to meet the growing demands of high-speed networking.


qsfp-ddNADDOD - QSFP-400G-FR4


8. DSFP: The Dual-Channel 100G Solution

The Dual Small Form-factor Pluggable (DSFP) offers two independent channels, each capable of 28G or 56G SerDes, allowing a single transceiver to provide 50G/100G connectivity in a more compact footprint.


9. OSFP: The Future-Oriented Eight-Channel Design

The Octal Small Form-factor Pluggable (OSFP) brings forth an innovative design with eight independent SerDes channels (56G/112G/224G per channel), facilitating transmission rates of 400G/800G/1600G. The OSFP's slightly larger size is beneficial for better heat dissipation and maintaining signal integrity at these higher rates.


NADDOD, a leading global optical transceivers manufacturer, introduces the 800G NDR transceivers embedded with Broadcom VCSEL and Broadcom DSP. These transceivers deliver high bandwidth, low latency, high-speed connectivity, and excellent thermal dissipation capabilities. They enable superior performance for AI and hyperscale datacenters with higher reliability and ROI, pushing the boundaries of high-speed data transmission and setting new industry standards.


osfp 800g



The Impact of SerDes Rate and Channel Count on Optical Transceivers

The relationship between the SerDes rate and the channel count is crucial to a transceiver's performance. Enhancements in the channel count or single-channel rate are the primary strategies for advancing the total transmission capabilities of optical transceivers.


As technology has advanced, optical transceivers have evolved from single-channel to multi-channel configurations, with SerDes speeds increasing from 10Gbps to 112Gbps, or even higher, thereby enabling upgrades across a range of data rates from 1G to 800G.


The evolution of SerDes technology not only determines the data transmission speeds of networks but also affects the dimensions, power consumption, and cost of optical transceivers. Additionally, to ensure proper communication, the SerDes rates at both ends of the network connection facilitated by the optical transceivers must be compatible. If they are not, adjustments may be necessary using gearbox modules to modify the SerDes rates either in the switch or the transceiver itself to achieve interoperability.


SerDes Challenges in Optical Modules Amid the 51.2T Switch Chip and CX7 Network Card Upgrades

Currently, leading internet companies primarily adopt TD4: 8T + TH4: 25.6T network configurations. Broadcom’s TD4 and TH4 switch chips typically operate at a 56G SerDes rate, similar to the server CX6 network cards, which also function at 56G SerDes rates.


TOR (Top of Rack) switches utilize a single TD4 chip with interface configurations such as 48x100G + 8x400G or 40x200G. LEAF/SPINE switches deploy a 25.6T single-chip switch with interfaces like 64x400G or 128x200G.


For downstream connections in TOR switches, DSFP56 100G DAC/AOC cables or QSFP56 200G cables split into two 100G DAC/AOC cables are used to connect with CX6 network cards. Upstream connections employ QSFP56 200G optical transceivers or QSFP-DD56 400G optical transceivers to link with LEAF/SPINE switches.


With the introduction of Broadcom’s TH5 51.2T switch chip, the SerDes rate of switch lanes is progressively upgraded from 56G to 112G, resulting in increasingly diverse network architectures.


In the context of 51.2T, LEAF and SPINE switches utilize single-chip switches of either 128x400G or 64x800G. Optical transceiver selections are based on single-lane 112G SerDes, such as QSFP112 400G or OSFP112 800G, which can support splitting into two 400G connections.


Strategic Approaches for TOR Switch Enhancements

At this stage, there are two different schemes for TOR switches:


Scheme One: Continue using the Broadcom TD4 chip, featuring interface configurations such as 48x100G + 8x400G or 24x200G + 8x400G. Downstream connections with CX6 remain unchanged, utilizing single-channel 56G SerDes DAC/AOC cables. For upstream connections transitioning to 400G, gearbox-equipped QSFP-DD/OSFP optical transceivers are used. These have shifted from an SR8 to an SR4/DR4 configuration, connecting with the 400G QSFP112 SR4/DR4 at the LEAF layer.

The gearbox within the optical transceivers facilitates the conversion from an electrical side with 8 channels of 56G SerDes to an optical side with 112G SerDes. This scheme is suitable for customers with lower server bandwidth upgrade needs during a period when 112G SerDes technology is just maturing, and the price difference between optical transceivers with and without a gearbox is not significant.


Scheme Two: Implement the newly released Broadcom TD5 16T chip, which supports 112G SerDes, with interface forms like 24x400G + 8x800G or 40x400G. Downstream connections utilize QSFP112 DAC/AOC cables that connect directly to CX7 network cards. This configuration is suitable for customers requiring higher server bandwidth upgrades. Over time, the cost of non-gearbox optical transceivers is expected to decrease significantly, offering benefits like reduced power consumption and lower latency, and supporting LPO optical module interconnection. However, CX7 network cards are currently in high demand, primarily due to advancements in AI, making them scarce and more expensive.


Comparative Analysis:

Both schemes offer distinct advantages and challenges. The primary challenge lies in the transition from 56G SerDes to 112G SerDes, exacerbated by the increasing demand spurred by AI advancements. An optimal solution for many internet companies would be a TOR switch that supports dual SerDes rates: 56G SerDes for downstream and 112G SerDes for upstream connections. This arrangement allows for the continued use of mature, cost-effective CX6 network cards while facilitating a gearbox-free connection to 51.2T LEAF/SPINE switches using 112G SerDes. This strategy leads to lower networking costs, reduced power consumption, and improved latency, while also supporting future generations of LPO optical module interconnections.


As a leading and professional provider of innovative optical networking solutions to HPC, AI, and data center markets, NADDOD is at the forefront of the 51.2T era, offering advanced 800G/400G optical transceivers. These products are essential in addressing the increasing demands for high-speed, reliable network infrastructure, ensuring that modern data centers can efficiently manage the escalating data volumes necessary for next-generation computing applications.