SFP-DD – Next Generation SFP Module to Support 100G

Cables, Uncategorized

New transceiver MSA form-factor enables doubling the SFP bandwidth and supporting fast line rates while maintaining SFP backwards compatibility.

Recently, a group of industry suppliers gathered to form a new transceiver module form-factor or Multi-Source Agreement (MSA). The agreement aims to foster the development of the next generation of SFP form-factor used in DAC and AOC cabling as well as optical transceivers. Mellanox often leads these sorts of technology developments and is a founding member of the SFP-DD MSA, as well as both QSFP-DD and OSFP MSAs.

While all the specs are not final yet, it’s called the SFP-DD or Small Form-factor Pluggable – Double Density. The “double density” refers to offering two rows of electrical pins enabling two-channels instead of the traditional one-channel in an SFP architecture – the smallest industry standard form-factor available today for data center systems.

New designs offer improved EMI and thermal management and will enable 50G and 100G PAM4 signaling in each channel for 100G and 200G support and up to 3.5 Watts of thermal dissipation; the same as the current QSFP28 which is about 2.5 times larger than the SFP-DD.

Bottom line:

First products on the market will likely be based on 50G PAM4 signaling and will feature two channels, offering 100G in SFP-DD form-factor. These new switches and network adapter configurations will enable increased bandwidth switch plate density, essentially doubling today’s current density.

This advancement will enable 100G (2x50G PAM4) in a tiny SFP port and 50G and 100G links to servers and storage in the smallest MSA available, with the highest number of 100G front-panel pluggable ports in a Top-of-Rack switch.  Eventually, two channels of 100G PAM4 will enable 200G per SFP-DD device.


Maintaining Popular Breakout Cabling to Servers

With the advent of new 8-channel form factors such as QSFP-DD, OSFP and COBO, a new 2-channel form factor was needed to enable 4-to-1 breakouts for servers and storage.

These time-tested data center Top-of-Rack breakout or splitter cable configurations can be maintained going forward to 400G with the SFP-DD in both copper DAC and AOC cables enabling supporting 10G, 25G, 50G, 100G and eventually 200G to the server such as:

  • 40G QSFP+ -to-Quad 10G SFP+
  • 100G QSFP28-to-Quad 25G SFP28
  • 100G QSFP28-to-Dual 50G QSFP28
  • 400G QSFP-DD-to-Quad 100G SFP-DD
  • 400G QSFP-DD-to-Dual 200G SFP-DD

Servers today typically support one or two CPUs per server but are heading towards supporting four and eight CPUs per server in the future, with additional DRAM and FLASH on board and PCIe Gen4 at 16GT/s requiring more server uplink bandwidth.  Today, 10G and 25G uplinks are popular and some hyperscale companies also require 50G uplinks. At four and eight CPUs per server, 100G and 200G uplinks will be required.

Mellanox recently introduced two new 100G AOCs breakout cables that feature 100G-to-Quad 25G SFP28 and 100G-to-Dual 50G QSFP28. They are also available in copper DAC cabling. These breakout configurations can also be made using transceivers and splitter passive fiber cables if optical connectors are needed to detach fibers from the transceivers.

Similarly, new QSFP-DD and SFP-DD breakout cables will be available in the future to support new 50G PAM4-based switches and network adapters.

Mellanox 100G DAC and AOC Product Line based on QSFP28 and SFP28


The SFP-DD new form-factors tie with Mellanox’s recent 200GbE Spectrum-2 switch IC announcement which is based on 50G PAM4 signaling and points to future 200G and 400G switch, network adapter, cable and transceiver developments from Mellanox.


Poised to Support the Next 5-10 years

By doubling the number of lanes, and at the same time doubling the number of bits per clock sent with PAM4 modulation, the SFP-DD bandwidth can transfer 100G versus the SFP28 at 25G. This translates into or four times the bandwidth of SFP28. In the future, the SFP-DD MSA goal is to support 100G PAM4 modulation enabling 200G (2x100G) per SFP-DD package which translates to eight times the current SFP28 bandwidth in the same physical space.

PAM4 Modulation versus NRZ

100G in a SFP-DD form-factors is the so-called Ethernet Alliance “holy grail” in high-speed interconnects. 100G is likely to be the next “10G” which has been the main stay in data centers for the last 10+ years. SFP-DD enables 100G in the smallest form-factors available and is likely to be around for many years to come – starting out in hyperscale and later moving into the enterprise and storage.



The MSA members will develop operating parameters, signal transmission speed goals, and protocols for the SFP-DD interface, which expands on the popular SFP pluggable form factor.  Targets include:


  • DAC reach: 3-meter 28 AWG Direct Attach Copper (DAC) aka Twinax,


  • SFP Backwards Compatibility: with the SFP28 and SFP+ so that upgrades are easy and support for slower devices in new 50G PAM4 systems.


  • Break Out Support: Using the next generation 8-channel 400G QSFP-DD in a switch, the SFP-DD can be used in a quad breakout configuration of four 100G. Similarly, for dual breakouts of 200G to dual 100G or quad 50G.


  • Higher power dissipation: With advanced thermal designs, the SFP-DD goal is to support up to 3.5 Watts – equal to the current and much larger QSFP28 MSA.

Comparison of SFP-DD with QSFP28 and QSFP-DD


The SFP-DD MSA founding members include: Mellanox Technologies, Alibaba, Broadcom, Brocade, Cisco, Dell EMC, Finisar, HPE, Huawei, Intel, Juniper Networks, Lumentum, Molex, and TE Connectivity.

Mellanox offers complete end-to-end solutions of switches, network adapters, cables and transceivers supporting both the SFP+ for 10G line rates and the SFP28 for 25G line rates. Soon 50G PAM4 for 200G and 400G systems and interconnects for both Ethernet and InfiniBand.


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About Brad Smith

Brad is the Director of Marketing at Mellanox, based in Silicon Valley for the LinkX cables and transceivers business focusing on hyperscale, Web 2.0, enterprise, storage and telco markets. Recently, Brad was Product Line Manager for Intel’s Silicon Photonics group for CWDM4/CLR4 and QSFP28 product lines and ran the 100G CLR4 Alliance. Director of Marketing & BusDev at OpSIS MPW Silicon Photonics foundry. President/COO of LuxSonar Semiconductors ( Cirrus Logic) and co-founder & Director of Product Marketing of NexGen, a X86-compatible CPU company sold to AMD - now the X86 product line. Brad also has ~15 years in technology market research as Vice president of the Computer Systems group at Dataquest/Gartner; VP/Chief Analyst at RHK and Light Counting networking research firms. Brad started his career at Digital Equipment near Boston with the VAX 11/780 and has served as CEO, president/COO and on the board-of-directors of three start-up companies. Brad has a BSEE degree from the University of Massachusetts; MBA from University of Phoenix and holds 2 optical patents

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