The QSFP28 100G FR single laser, a competitor to the CWDM4?

Guest Blog

Thanks to the 400G technology, a new QSFP28 100G module type is coming.  The QSFP28 100GBASE-FR, also called QSFP28 100G single lambda, will be a single laser and a single receiver of 100Gbps instead of being 4x25Gbps (4x lasers and 4x receivers).

Back to the regular QSFP28 4x25Gbps
The QSFP28 100Gbps is a parallel optic, 4x 25Gbps. It has 4 lasers and 4 receivers, valid for all optical versions (100GBASE-SR4, CWDM4, LR4, ER4L).

The electrical interface is 4x25Gbps NRZ (CAUI-4) and the optical side is also 4x 25Gbps with MPO connector (for 100GBASE-SR4 and PSM4) or LC connector (for 100GBASE-CWDM4, LR4, ER4L).

With a maximum power consumption of 3.5W (by MSA), the QSFP28 100GBASE-SR4, CWDM4 and LR4 are mature products and largely used in data centres and service providers networks on the access side. While the cost for the CWDM4 has dropped due to the increasing worldwide volume, it is now reaching the bottom limit. Dropping the cost down requires a significant updated QSFP28 design.

How is the 400G transceiver architecture helping the QSFP28 100G single laser?
400G QSFP56-DD and OSFP are coming with 3 key evolutions

  • A PAM4 modulation, doubling the bitrate to 50Gbps/lane
  • A baud rate of 50Gbauds, reaching a bandwidth of 100Gbps/lane (50Gbauds PAM4)
  • A gearbox and PAM4 modulator embedded in a DSP

Considering that the QSFP28 electrical host-interface is always 4x25Gbps NRZ, different alternatives are now available to “re-design” the QSFP28 with building blocks:

400G QSFP56-DD and OSFP

NRZ to PAM4 Electrical
The gearbox and PAM4 modulator convert 4 lanes of 25Gbps NRZ to 2 lanes of 50Gbps PAM4. It remains an electrical conversion.

50Gbps to 100Gbps
The gearbox converts 2 lanes of 50Gbps PAM4 to a single lane of 100Gbps PAM4. While it remains electrical, the key difference is the baud rate which is converted from 25Gbauds to 50Gbauds.

Laser multiplexing
With 2 lasers of 50Gbps PAM4 each, these can be multiplexed over a “super-channel” DWDM 100GHz, as an example.

A “single lambda” or a “super channel” QSFP28
By assembling the above building blocks, 2 architectures are feasible, with their advantages, applications and costs.

The single laser 100Gbps PAM4, also called single lambda or 100GBASE-FR
single lambda or 100GBASE-FR

While the QSFP28 electrical interface remains of 4 lanes of 25Gbps NRZ (CAUI-4), the signal is converted to 2 lanes of 50Gbps PAM4, then to a single lane of 100Gbps PAM4. The 2-step conversion is realised into a DSP.

With its single 100Gbps PAM4 lane, the module requires only one laser; drastically reducing the complexity of the optical part (with one laser instead of four and no filter required). However, the DSP brings another complexity of power consumption and cost. Because of the high chromatic dispersion, this module is only intended for short reach, up to 2km at a first step.

A super channel transceiver with 2 lasers of 50Gbps PAM4

with 2 lasers of 50Gbps PAM4

Another way to stay at a baud rate of 25Gbauds (and reduce the chromatic dispersion, etc.) is to mux 2x DWDM 50GHz lasers and make a super-channel 100GHz, compatible with regular 100GHz DWDM mux/demux:
100GHz DWDM mux/demux:

The module has a DSP converting 4 channels of 25Gbps NRZ to 2 channels of 50Gbps PAM4 on the electrical side. On the optical side, it has 2 lasers of 50Gbps PAM4 multiplexed together to make a “super-channel” laser, compatible with regular 100GHz DWDM mux and demux. It has the advantage of working at a baud rate of 25Gbauds but it remains a challenging module in term of cost (DSP + 2 lasers), power consumption and reach.

Probably too expensive for a short reach application, it remains an effective module for an upgrade from 10Gbps DWDM to 100G DWDM without changing the passive infrastructure. However, active equipment will be required such as EDFA, DCM, etc.

Which one to compete against the QSFP28 CWDM4?
The key application for a QSFP28 CWDM4 is the data centre, leaf-spine architecture. It consists of a dual-fibre module with duplex LC connector, maximum 2km. The main cost for this module is the 4 DML lasers and the filter assembly.

The “super-channel” transceiver (with 2 lasers) is not the right candidate, as it requires a DSP converting NRZ to PAM4, plus 2 EML lasers. The “super-channel” is intended for 100GHz DWDM application, for DCI (Data Centre Interconnect).

The single-laser QSFP28 (100GBASE-FR), requires a DSP and just one laser, removing the need for a filter; drastically relaxing the complexity of transceiver assembly. The cost for the assembly and component is inevitably lower except for the DSP which remains at a high cost. The DSP price erosion remains key for a potential swap from CWDM4 to 100GBASE-FR for new deployment. Today, the cost for the QSFP28 100GBASE-FR is nearly double the cost of the 100GBASE-CWDM4.

A breakout application together with 400GBASE-DR4
In classical leaf-spine architecture, it is common to have the servers connected to the switch with breakout topology: 4x10G or 4x25G. With QSFP28 100GBASE-FR and QSFP-DD 400GBASE-DR4, 4 servers can be connected at 100Gbps on a single 400Gbps port at the top-of-rack switch side.

The QSFP-DD 400GBASE-FR4 is the equivalent of QSFP+/QSFP28 PSM4, while the QSFP28 100GBASE-FR is the equivalent of SFP+ IR/LR, SFP28 IR/LR, a completely different technology, but the same application.

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