Connectivity in the Era of Big Data: Adapting with QSFP28, SFP-DD, and DSFP

In today's global market, servers typically connect at speeds of 10 Gigabit Ethernet or 25 Gigabit Ethernet, which are standard for access rates. However, as data center networks advance from 100 Gigabit Ethernet to 400 Gigabit Ethernet architectures, we foresee a rise in the commercial adoption of servers with 100GE connections, especially in GPU cluster configurations. During this transition, server access rates are expected to bypass the 50GE mark and jump directly to 100GE. This trend reflects the ongoing need for greater data throughput and increased bandwidth to keep up with the escalation of data traffic.

QSFP28 Package Introduction

The QSFP28 format is designed to cater to the needs of 100-Gigabit applications. It enables four channels of data to be transmitted simultaneously, with each channel operating at a rate of 25 Gbps. Consequently, by leveraging all four channels, it accomplishes a combined data rate of 100 Gbps, making it the prevalent packaging form for 100G modules. This form factor is apt for catering to both 100-Gigabit Ethernet (with a quad 25-Gbps setup) and EDR InfiniBand applications. Nonetheless, limitations exist with the present QSFP28 interface.

The most common network interface cards (NICs) for servers in use today are rated for 10GE or 25GE, though some servers are equipped with 100GE QSFP28 NICs. These 100G NICs utilize four 25G physical lanes, and compatibility is restricted to switches that also have 100GE QSFP28 ports. The industry-leading 100GE QSFP28 Top-of-Rack (TOR) access switches boast up to 32 such ports. However, with a total switching capacity not exceeding 3.2Tbps, they can accommodate a maximum of only 32 servers. This limitation makes them inadequate for data centers requiring higher throughput and greater bandwidth and hinders the deployment of 100GE servers within such environments.

To address the growing needs for higher data throughput and larger bandwidth in data centers, both server NICs and access switches are progressing toward advanced 100GE interfaces. This next-generation 100GE interface reduces the number of physical lanes to two, each operating at 50Gbps. Not only does this halve the channel count from the older QSFP28 interface, but it also enhances port density, aligning with the future development trajectory of data center infrastructure.

Introduction to SFP-DD Package

Introduced at the Optical Fiber Communication (OFC) conference in 2017 by Alibaba, the Small Form-factor Pluggable-Double Density (SFP-DD) standard was developed to address the evolving needs of 400G data center networks. Alibaba's initiative led to the establishment of the SFP-DD Multi Source Agreement (MSA) organization on July 12th, 2017, with the mission of defining the optical interface, mechanical, and thermal standards for server and switch interconnects in next-gen 400G infrastructure.

The SFP-DD form factor is an evolution of the 4-lane data center modules, streamlining connectivity through just 2 lanes capable of supporting 25 Gbps Non-Return to Zero (NRZ) or 56 Gbps Pulse Amplitude Modulation 4-level (PAM4). It facilitates higher data rates such as 50 Gbps or 112 Gbps with PAM4, and lays the groundwork for future scalability to 200 Gbps (2x100G). This new package is designed to conserve precious real estate within data centers while also adhering to the emerging prerequisites for cost-efficiency, compact sizing, high port density, and reduced energy consumption.

Moreover, the innovative SFP-DD design guarantees backward compatibility with pre-existing SFP and SFP+ high-speed interfaces. This ensures a seamless transition from 25G SFP28 Active Optical Cables (AOC) to 100G, as well as setting a foundational tech platform for elevating single optical port capabilities from 50G to 100G. In tandem with QSFP-DD, SFP-DD can craft a holistic solution for 100G data center network access and facilitate interconnections amongst 400G switches.

DSFP Package Introduction

The Dual Channel Small Form-factor Pluggable (DSFP) is an optical module standard defined by the MSA specifically for 10G point-to-point applications. As a bidirectional optical module, DSFP houses two electrical channels and remains in line with the well-established SFP+ form factor. This compatibility ensures that DSFP offers highly dense, cost-effective optical module solutions that are particularly advantageous for the emerging 5G networks. Notably, DSFP modules can pack twice the installation density of traditional SFP modules, without sacrificing compatibility with the existing SFP infrastructure.

In terms of performance, DSFP modules are capable of handling up to 28 Gbps per electrical lane using Non-Return to Zero (NRZ) signaling, and they double their data rate to 56 Gbps per lane when employing the more complex Pulse Amplitude Modulation 4-level (PAM4) signaling. As a result, DSFP can support either 56 Gbps using NRZ or 112 Gbps using PAM4 for data transmission. Looking ahead, DSFP is expected to advance further, enabling 224 Gbps transmission using PAM4 technology.

The DSFP standard is emerging as a low-cost and efficient solution that boasts excellent signal integrity at high speeds, contributes to reduced printed circuit board (PCB) sizes, and simplifies the design and production process. Its introduction is set to accelerate 5G network deployment and safeguard customer investments. With DSFP adoption, communication devices can benefit from doubled module density, which in turn effectively doubles the port density and throughput of the equipment. This improvement in the cost-effectiveness of devices enhances the competitive edge of customer products.

In the marketplace, the DSFP form factor is gaining traction, with H3C's S9855 switch series already featuring DSFP ports. Additionally, companies like NVIDIA have rolled out network cards and switches equipped with DSFP ports, which are fully compatible with DSFP Active Optical Cables (AOCs).

How to choose the package

When selecting between QSFP28, SFP-DD, and DSFP modules, you should consider the following factors:

Bandwidth Requirements

Determine the total bandwidth needed for your network or connections. QSFP28 is typically utilized for 100Gbps connections, while SFP-DD and DSFP are designed to support higher rates (up to 100Gbps for SFP-DD and up to 200Gbps for DSFP).

Hardware Compatibility

Check which type of modules are supported by your existing network hardware such as switches and routers. Different hardware models may have specific slot designs and may not support all types of modules.

Port Density

If rack space is limited, you might need to consider port density. SFP-DD and DSFP can support higher rates and channel counts in a smaller footprint, aiding in increased port density.

Distance and Signal Integrity

Choose the appropriate module type and transmission medium (such as single-mode or multi-mode fiber) based on link length. Different modules have specific specifications and limitations regarding distance.

Ecosystem and Supply Chain

Consider the stability of the supply chain and the maturity of the module ecosystem. More established technologies are generally easier to get support for and have broader interoperability.

Overall, QSFP28 modules are currently quite common in the market and are suitable for existing 100Gbps network needs; SFP-DD modules provide a higher-density 100Gbps solution without increasing space utilization for data centers; and DSFP represents a new technology that may not be as widely applied as the former two but offers higher bandwidth potential. The choice of which solution to opt for will depend on your specific application requirements, budget considerations, and predictions on future technological trends.

Conclusion

In summary, choosing between the QSFP28, SFP-DD, and DSFP modules requires balancing current network demands with future growth, considering factors like bandwidth needs, space constraints, and budget. While QSFP28 is widely-used for 100Gbps connections, SFP-DD offers higher density for future-proofing your data center without taking up more space, and DSFP provides a high bandwidth potential, especially for emerging applications like 5G. Making an informed choice involves looking at what your network needs now and how it will evolve, ensuring you invest in technology that delivers both performance and value over time.