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How 224G PAM-4 architectures will enable next generation data centers

24 August 2023
With 224G capable interfaces being implemented in data centers, connect and transceiver vendors are already releasing devices to support the next generation of cloud computing applications. Source: kubais/AdobeStock

Current data center architecture and processor options are implementing 224G PAM-4 as the most advanced signaling option available. 224G-capable interfaces are now being implemented to run between servers, switches and backplanes to provide the fastest possible data rates over copper to support advanced computing in the data center. End-to-end solutions and IP are still in development, but interconnect components and transmission media are being developed to support signaling at these high data rates.

In this article, an overview of the interconnect architecture options now being implemented in modern data center architecture, specifically targeting applications like artificial intelligence (AI) and other highly parallelized cloud workloads, will be presented. Connector and transceiver vendors are already releasing products for building out these architectures, and this will support the next generation of advanced cloud computing applications in data centers.

Future data center architecture

In 2018, the IEEE 802.3bs standard committee made the decision to implement PAM-4 signaling to implement a channel bandwidth doubling in order to reach 50 Gbps per lane data rates for 200 GbE (4x 50 Gbps Rx/Tx differential pairs) and 400 GbE (8x 50 Gbps Rx/Tx differential pairs). The move to PAM-4 solidified its implementation as the standard modulation solution for electrical and optical interfaces in data center environments, and today the implementation has spilled over into PCIe Gen 6.0.

PAM-4 has been the major enable up to 224G per lane channels, which require 56 GHz bandwidth per lane. But extension of signaling bandwidth up to 56 GHz greatly increases interconnect losses for wired cable runs, both in terms of dielectric loss and copper loss. In the case where cable run losses are too great, there will be a switch from copper to fiber.

Short runs: Passive copper

Passive copper cabling and printed interconnects are not going away and they will still continue to be implemented in systems targeting 56 GHz bandwidths. The shortest cable runs and printed circuits in packaging and boards are all passive copper.

Short passive copper cable runs operating at 224G cannot be implemented as flying lead connections due to extremely high copper and radiation losses. These cable options cannot span more than several inches before reaching their destination, so these options are only appropriate for flyover cables at 224G. In links between servers/switches, active copper is needed.

PCB-based architecture is nearing its maximum capabilities on commercially available laminates and copper foils, the enabler of the next bandwidth doubling to 448G is still an open question.PCB-based architecture is nearing its maximum capabilities on commercially available laminates and copper foils, the enabler of the next bandwidth doubling to 448G is still an open question.

Moderate runs: Active copper

Active copper cabling has similar loss (in dB per unit length) as passive copper cables. The difference between these is that active copper uses amplification in a transceiver to boost the signal strength so that it can be sent over longer copper cable runs. This allows older passive cable runs to now be used at higher bandwidths by effectively increasing the link loss budget.

Determining the use of active copper versus passive copper in interconnect design requires three pieces of information:

  • Insertion loss per unit length along the copper cable
  • Tx and Rx Gain provided by the active copper transceiver pairs
  • Rx threshold in the Rx transceiver
  • Total loss allowance on the channel

Transceiver options for active copper cabling still fit within a standard form factor connector, such as SFP. This can minimize mechanical redesigns as new product revisions are developed for data center infrastructure.

Long runs: Fiber optic cable

Finally, the longest length links will continue to run on top of fiber, just as has been the case in prior generations of data center architecture. Multimode and single-mode fiber is used in various situations, with single-mode fiber dominating longer links.

Currently, companies are demonstrating 224G VCSEL transceiver solutions operating at 850 nm, which would mean implementation in standard multimode fiber optic cable. As equipment upgrades to takes advantage of 224G per lane capabilities, existing fiber runs will still be useful for longer interconnects between servers and switches.

Summary

In summary, passive copper links, whether on the PCB or over cable, are increasingly becoming impractical for high-speed data transfer, including in data center architecture. At 56 GHz bandwidths implemented in 112G and 224G signaling environments over single pairs, the loss encountered will be 4x the value seen in a typical 100 GbE implementation over two Rx and two Tx pairs. The result is 25% link length allowance that becomes unsuitable for longer cable runs in packed racks.

As it appears the current PCB-based architecture is nearing its maximum capabilities on commercially available laminates and copper foils, the enabler of the next bandwidth doubling to 448G is still an open question. While every 448G interconnect could use PAM-4 with active copper, there may be other approaches that enable the next doubling:

  • Novel low-loss/low-Dk HDI PCB materials
  • Higher-order PAM or QAM/QPSK signaling implementation
  • Implementation of active high-bandwidth amplifiers on every PCB interconnect
  • Embedding of optical interconnects in PCB substrates

The first two outcomes are much more likely than complete transformation of PCB architecture to fiber or usage of active transceivers on every PCB interconnect.

To contact the author of this article, email GlobalSpecEditors@globalspec.com


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