400G – Which form factor? (QSFP-DD, OSFP, CFP8)

QSFP-DD, OSFP, CFP8Guest Blog
QSFP-DD, OSFP, CFP8
QSFP-DD, OSFP, CFP8

We are all aware about the SFP+ for 10G and QSFP28 for 100G transmission. A small form factor pluggable, a low-power consumption and an interoperability between all systems vendors; these are the 3 successful criteria for a complete form factor.

However, we all remember the situation on 100G, characterised by 4 different form factors:

400G form factor
400G form factor

At a Telco, the transmission department needs a pluggable transceiver able to cover long reach also using some dedicated technologies such as Coherent detection. The datacentre team needs a small form factor, with the lowest power consumption and the lowest cost per unit, but their application is for short reach only (max 2km).

During the first instances of the 100G pluggable transceivers, the CFP form factor was preferred because it was impossible to make a transceiver less than 12W, even for intermediate reach. Once the technology and components availability were better, it was then feasible to do a CFP2, then a CFP4. Still today, the Coherent technology for 100G and 200G is only available on CFP and CFP2 form factors.

In parallel, the GAFA with their phenomenal need for additional DC capacity, have pushed the QSFP28 form factor for various short reach applications (DAC, PSM4, CWDM4 and SR4).
Today, with the technology maturity and QSFP28 wide-adoption, most of the 100G applications are available on QSFP28 form factor, with some exceptions for reach more than 40km, including Coherent detection.

For the 400G bitrate, some essentials interrogations must be raised before going deeper into the subject:

  • Who need the 400G pluggable transceivers?
  • For which application?
  • How about technology maturity?
  • Any interoperability with former form factors?

Following the market situation, 400G is a priority for the large DC (intra-DC application) and, at a lower scale, for the Telco transmission department. Because the 400G bitrate requires PAM4 modulation, the reach is becoming more and more challenging and is limited to few kilometres only. Longer reach will require Coherent detection technology and/or amplification, dispersion compensation, etc.

Again, we will observe a similar scenario for 400G, a dedicated form factor for the intra-DC application and another one for the transmission. However, thankfully it seems that “intermediate” form factors won’t happen for 400G.

Access / Intra-DC

400G form factor
400G form factor

400G is coming with 2 form factors for the Access/Intra-DC:

  • QSFP56-DD (also called QSFP-DD for QSFP Double Density)
  • OSFP (for Octal SFP)

Both form factors are running 8x lanes of 50G PAM4 on the electrical side while the optical side can be either 8x lasers of 50G PAM4 or 4x lasers of 100G PAM4. In the 4 lasers design, we add a “gearbox” converting 8x 50G PAM4 electrical to 4x 100G PAM4.

The QSFP56-DD is defined by the QSFP-DD MSA alliance (www.qsfp-dd.com) while the OSFP is defined by the OSFP MSA group (www.osfpmsa.org).

The 2 form factors are similar but have 3 key differences:

  • OSFP allows more power (15W) than the QSFP-DD (12W). The OSFP allows an early adoption because it’s easier to release a technology designed for 15W than 12W.
  • The QSFP-DD port is backward compatible with QSFP+ (40G), QSFP28 (100G) and QSFP56 (200G). The OSFP port requires a QSFP to OSFP converter module.
  • The OSFP integrates thermal management directly into the form factor, the QSFP-DD not.

Both QSFP-DD and OSFP are designed for intra-DC application including DAC, AOC and optical connection up to 2km. Additional variants will come for other applications (DCI) with longer reach and other technology like DWDM super channel.

Transmission

CFP8
CFP8

The CFP8 form factor, defined by the CFP MSA (www.cfp-msa.org), is radically different than QSFP-DD and OSFP as:

  • It allows up 24W power consumption
  • It has 16x channels 25G NRZ on the electrical side (instead of 8x 50G PAM4 for QSFP-DD and OSFP)
  • It has a MDIO management interface (instead of I2C for QSFP-DD and OSFP)

With its large space and max 24W power consumption, the CFP8 is intended for transmission application. Available in an initial version of 10km (CFP8 400GBASE-LR8), it has 16x electrical lanes of 25G NRZ which are converted to 8x lanes of 50G PAM4.

However, other variants are coming for longer reach, including Coherent detection technology. A version called CFP8 ZR (80km) will come at a later stage but it also opens the door for a CFP8 800G! By using the 16x electrical lanes and apply a 50Gbps PAM4 signal, it is feasible to reach 800G; then adding a DSP, Coherent detection and multiplexing lasers will enable the optical transmission. Clearly, this is not for today yet.

For 400G application, others form factors than the one listed above are also available, but for dedicated applications. We can list the COBO (Consortium for On-Board Optics) and the CDFP for cable application enabling 16x lanes of 25Gbps.

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