Multimode and singlemode fibres: Comparing 400G access types

Guest Blog

400G is now under release over multiple form-factors: QSDFP-DD, OSFP and CFP8. While the short-term deployment is on the access side, and mainly for intra-DC application, other applications such as DCI and transmission are also going on the 400G direction.

As we are now getting familiar with the form-factors and the PAM4 modulation, it is time to investigate and understand the different optics types and their applications.

Intra-DC / Intra-Building

Multimode fibre

400GBASE-SR8 – Point-to-point over multimode fibre, 100m
• Connector: MPO-24
• Max reach: 100m, over multimode OM4 fibre
• Fibres: 16 fibres (ribbon patchord)
• Form factors: QSFP56-DD, OSFP
• Upgrade from: n/a
• Alliance: IEEE 802.3cm (breakout: 802.3cd)

The 400GBASE-SR8 is technically simple but requires a ribbon patch cord with 16 fibres. While 40GBASE-SR4 and 100GBASE-SR4 use 8 fibres (MPO-12), the 400GBASE-SR8 is not the product to use for a 40G or 100G link upgrade.

The product is built with 8 VCSEL lasers emitting at 850nm with a 50Gbps PAM4 modulation and doesn’t include any gearbox, so the overall cost of the module remains low.

400GBASE-SR4 – Point-to-point over multimode fibre, 100m
• Connector: MPO-16
• Max reach: 100m, over multimode OM4 fibre
• Fibres: 8 fibres (ribbon patchord)
• Form factors: QSFP56-DD, OSFP
• Upgrade from: 40GBASE-SR4, 100GBASE-SR4
• Alliance: IEEE 802.3cm (IEEE name is 400GBASE-SR4.2)

The 400GBASE-SR4 is technically more complex than the 400GBASE-SR8 but it has the great advantage of being interoperable with an upgrade from 40GBASE-SR4 and 100GBASE-SR4.
It has 4 VCSEL 850nm lasers cadenced at 100Gbps PAM4 thanks to the internal gearbox converting 8x 50Gbps PAM4 to 4x 100Gbps PAM4. The cost of that module is higher than the SR8 but will drop over time due to the volume and gearbox price erosion.

400G-BiDi

The 400-BiDi is under development and will be different compared to the famous 40G-SRBD and 100G-SRBD/SWDM4. It allows for a 40G or 100G transmission over a pair of multimode fibres.
The 400G BiDi will have an MPO connector allowing a breakout application from 1x QSFP-DD BiDi to 4x QSFP28 BiDi; which can be used for TOR switch to servers.

Singlemode fibre

400GBASE-FR8 / 400GBASE-FR4 – Point-to-point over singlemode fibres, 2km

• Connector: Dual LC
• Max reach: 2km, over singlemode fibre
• Fibres: 2 fibres
• Form factors: QSFP56-DD, OSFP
• Upgrade from: 10GBASE-LR, 40GBASE-IR4, 100GBASE-CWDM4
• Alliance: 100G Lambda MSA (400GBASE-FR4) | IEEE 802.3bs (400GBASE-FR8)

The 400GBASE-FR4/FR8 is the 100GBASE-CWDM4 equivalent and the most common solution for short reach (max 2km) over a pair of single-mode fibres. It is a cost-effective solution built on 4x CWDM lasers (FR4) or 8x CWDM lasers (FR8) with an embedded optical multiplexer/demultiplexer. The FR4 requires a gearbox (8x 50Gbps PAM4 to 4x 100Gbps PAM4) while the FR8 doesn’t.

400GBASE-LR8 / 400GBASE-LR4

• Connector: Dual LC
• Max reach: 10km, over singlemode fibre
• Fibres: 2 fibres
• Form factors: QSFP56-DD, OSFP, CFP8
• Upgrade from: 10GBASE-LR, 40GBASE-LR4, 100GBASE-LR4
• Alliance: 100G Lambda MSA (400GBASE-LR4) | IEEE 802.3bs (400GBASE-LR8)

Up to 10km over a pair of singlemode fibres, the LR8 and LR4 are the 400G standard as these will be interoperable with most of the 400G form factors. The 400GBASE-LR8 is built with 8x EML lasers running at 50Gbps PAM4 while the 400GBASE-LR4 is built with 4x EML lasers operating at 100Gbps PAM4. Both are multiplexed following the LWDM scheme. In term of signal integrity, the 400GBASE-LR4 remains challenging but has lower power consumption than the LR8.

400GBASE-DR4

• Connector: MPO-12
• Max reach: 500m over singlemode fibre
• Fibres: 8 fibres
• Form factors: QSFP56-DD, OSFP
• Upgrade from: 40GBASE-PSM4, 100GBASE-PSM4
• Alliance: IEEE 802.3bs / 802.3cd

In DC infrastructure, the breakout architecture is commonly used to connect four servers to a single port in the switch (TOR – Top of the rack switch): 4x 10G (SFP+) to 1x 40G (QSFP+) or 4x 25G (SFP28) to 1x 100G (QSFP28).

The 400G-DR4 is exactly the equivalent of the PSM4, except that it requires a very specific 100G QSFP28 at the other end: the QSFP28 100GBASE-FR or 100GBASE-DR, operating with a single laser at 100Gbps PAM4.

The 400GBASE-DR4 is built with 4x lasers operating at the same wavelength. Another version called XDR4 is also available and is very similar to the DR4 except that it has a longer reach (2km instead of 500m).