How to Choose a 400G Transceiver: SR8, DR4, FR4, LR4 and ZR
To choose a 400G transceiver, match three things: the port form factor on your switch (QSFP-DD, OSFP or QSFP112), the link distance, and the fibre and connector you have. In practice that means SR8 for up to 100 m on multimode, DR4 for 500 m, FR4 for 2 km and LR4 for 10 km on single-mode, and 400ZR or ZR+ for data centre interconnect.
The details matter, because modules that look interchangeable on a datasheet can use different connectors, break out differently and draw very different power. This guide walks through the form factors, each common 400G type, connectors, breakout to 4 x 100G, power and a final checklist.
QSFP-DD vs OSFP vs QSFP112
The form factor is fixed by the switch or NIC, so start there. All three carry 400G, but the electrical interface differs.
| Form Factor | 400G Electrical Lanes | Backward Compatibility | Notes |
|---|---|---|---|
| QSFP-DD | 8 x 50G PAM4 | Accepts QSFP28, QSFP56 and QSFP+ modules | Most common 400G Ethernet switch port |
| OSFP | 8 x 50G PAM4 | Does not accept QSFP modules without an adapter | Larger body, more thermal headroom, common in AI and HPC platforms |
| QSFP112 | 4 x 100G PAM4 | Accepts earlier QSFP modules on most hosts | Newer 400G NICs and switches with 100G SerDes |
The optical side of a given type, such as 400G-DR4, is the same whichever form factor carries it, so a QSFP-DD DR4 can link to an OSFP DR4. What you cannot do is plug a QSFP-DD module into an OSFP cage, or assume a QSFP112 module will work in a QSFP-DD port. Our QSFP transceivers guide covers the family history in more depth.
400G Transceiver Types Compared
| Type | Fibre | Reach | Optical Lanes | Connector | Breakout |
|---|---|---|---|---|---|
| 400GBASE-SR8 | Multimode | 70 m OM3, 100 m OM4 | 8 x 50G, 850 nm | MPO-16 | 8 x 50G or 2 x 200G |
| 400GBASE-SR4.2 (BiDi) | Multimode | 70 m OM3, 100 m OM4, 150 m OM5 | 4 fibre pairs, 2 wavelengths | MPO-12 | No |
| 400GBASE-DR4 | Single-mode | 500 m | 4 x 100G, 1310 nm | MPO-12 APC | 4 x 100G-DR |
| 400G-XDR4 / DR4+ | Single-mode | 2 km | 4 x 100G, 1310 nm | MPO-12 APC | 4 x 100G-FR1 |
| 400GBASE-FR4 | Single-mode | 2 km | 4 x 100G CWDM | LC duplex | No |
| 400G-LR4 | Single-mode | 10 km | 4 x 100G CWDM | LC duplex | No |
| 400ZR / ZR+ | Single-mode, DWDM | Around 120 km (ZR); longer with ZR+ | 1 coherent wavelength | LC duplex | No (ZR+ can run lower rates) |
400G-SR8
SR8 runs eight 50G PAM4 lanes over eight fibre pairs at 850 nm, reaching 70 m on OM3 and 100 m on OM4. It uses a 16-fibre MPO-16 connector, which is not mateable with MPO-12, so it needs its own trunks and patch cords. SR8 is popular for short links in AI clusters and for breaking out to 50G or 200G servers.
400G-SR4.2 and SR4/VR4
SR4.2 is a BiDi design: two wavelengths share each fibre, so 400G runs over four fibre pairs on a standard MPO-12. That lets you reuse the 12-fibre multimode trunks already installed for 40G-SR4 and 100G-SR4. Newer 400GBASE-SR4 (100 m on OM4) and VR4 (50 m on OM4) use 100G per lane over MPO-12 and mostly appear in QSFP112 modules.
400G-DR4
DR4 is the most common single-mode 400G type inside data centres. It sends four 100G PAM4 lanes over four fibre pairs at 1310 nm, reaching 500 m. Its MPO-12 APC connector uses eight of the twelve fibres. The big advantage is breakout: each lane can feed a separate 100G-DR module.
400G-XDR4 / DR4+
XDR4 (also sold as DR4+ or DR4-2) is DR4 with a stronger optical budget, reaching 2 km. It is not an IEEE type, but it is widely used where links run between halls or buildings on a campus and you still want 4 x 100G breakout, in this case to 100G-FR1 modules.
400G-FR4
FR4 multiplexes four CWDM wavelengths (1271, 1291, 1311 and 1331 nm) onto a single fibre pair, reaching 2 km over LC duplex. It is the easy upgrade path for existing duplex single-mode cabling, because nothing on the patch panel changes. It cannot break out, since all four lanes share one fibre.
400G-LR4
LR4 uses the same four CWDM wavelengths as FR4 with a stronger optical budget for 10 km on LC duplex. The common 10 km version follows the 100G Lambda MSA; the IEEE equivalent, 400GBASE-LR4-6, is specified for 6 km. The older 400GBASE-LR8, with eight 50G wavelengths, also reaches 10 km but is less common in new builds. For 40 km, IEEE defines 400GBASE-ER8.
400ZR and ZR+
400ZR is a coherent module defined by the OIF for data centre interconnect, carrying 400G on a single tunable DWDM wavelength over around 80 to 120 km of amplified point-to-point fibre. ZR+ modules, such as those following the OpenZR+ MSA, add stronger FEC and multiple line rates for longer metro and regional links. Both plug straight into a router or switch port, removing the need for a separate transponder in many designs. They are a strong fit for telecom operators and for linking data centres across a city, but the host port must support their higher power class.
Connectors: MPO-16, MPO-12 APC and LC
- MPO-16: used by SR8. A different key position means it will not mate with MPO-12 cables. Check whether the module calls for a flat (UPC) or angled (APC) polish before ordering cables.
- MPO-12 APC: used by DR4 and XDR4. The angled polish is required on single-mode parallel optics; mating APC to a flat UPC connector causes high loss and can damage end faces.
- MPO-12 UPC: used by multimode SR4.2 and SR4, matching most existing 40G/100G multimode trunks.
- LC duplex: used by FR4, LR4 and ZR. Works with standard duplex single-mode patching.
Parallel optics also need correct polarity end to end. Our MTP/MPO polarity guide explains Type A, B and C, and you can source matching MTP/MPO trunks and jumpers or LC patch cords for each type.
Breakout to 4 x 100G
Breakout lets one 400G switch port serve four 100G devices, which is a common way to connect 100G servers or older 100G switches to a new 400G leaf. The rule is that the 100G end must use the same optical lane: one 100G PAM4 wavelength.
- 400G-DR4 to 4 x 100G-DR: up to 500 m, using an MPO-12 APC to four LC duplex breakout cable.
- 400G-XDR4 to 4 x 100G-FR1: up to 2 km, same cabling approach.
- 400G-SR8 to 8 x 50G or 2 x 200G-SR4: for multimode server breakout.
A 400G-DR4 will not link to 100G-LR4 or 100G-CWDM4 modules, because those use four 25G wavelengths. The switch port also has to be configured for breakout mode. For short in-rack fan-out, a breakout DAC or AEC is often simpler.
Power and Thermal Considerations
400G optics draw far more power than 100G. Short-reach types such as SR8 and DR4 typically sit around 8 to 12 W, FR4 and LR4 at the upper end of that range or above, and coherent ZR modules higher again, often 15 W or more. Multiply that by 32 or 36 ports and the optics alone can add several hundred watts per switch.
Check the switch’s supported power class per port before ordering ZR or long-reach modules; some platforms only allow high-power optics in certain ports. Keep airflow direction consistent across the rack, and remember that equipment rooms in hot climates such as the UAE leave less thermal margin. OSFP’s built-in heat sink gives more headroom, which is one reason it is common on AI platforms. Our AI infrastructure team regularly balances these limits for dense GPU fabrics.
400G Transceiver Choosing Checklist
- Port type: QSFP-DD, OSFP or QSFP112, and the exact switch or NIC model at each end.
- Distance: up to 100 m (SR8/SR4.2), 500 m (DR4), 2 km (FR4 or XDR4), 10 km (LR4), longer (ZR/ZR+).
- Fibre plant: multimode grade (OM3, OM4, OM5) or single-mode, and whether it is parallel MPO or duplex LC.
- Breakout: if you need 4 x 100G, choose DR4 or XDR4 and matching 100G-DR or FR1 modules.
- Connector and polish: MPO-16, MPO-12 APC, MPO-12 UPC or LC, with the right polarity.
- Power class: confirm the port supports the module’s power, especially for ZR.
- Coding: order modules coded and tested for your platform.
Browse the full 400G transceiver range, or the 100G QSFP28 modules you need for breakout.
Frequently Asked Questions
What is the difference between 400G DR4 and FR4?
400G-DR4 uses four parallel fibre pairs over an MPO-12 APC connector and reaches 500 m, and it can break out to four 100G-DR links. 400G-FR4 multiplexes four wavelengths onto one duplex LC fibre pair and reaches 2 km, but it cannot be broken out.
Which connector does 400G SR8 use?
400G-SR8 uses a 16-fibre MPO-16 connector over multimode fibre, reaching 100 m on OM4. MPO-16 is keyed differently from MPO-12, so it needs dedicated MPO-16 trunks and patch cords.
Can a 400G transceiver connect to 100G?
Yes, through breakout: a 400G-DR4 connects to four 100G-DR modules and a 400G-XDR4 to four 100G-FR1 modules. The 100G modules must use a single 100G wavelength, so 100G-LR4 and CWDM4 will not work.
Is QSFP-DD compatible with OSFP?
No, QSFP-DD and OSFP are different sizes and will not fit each other’s ports. However, a QSFP-DD module and an OSFP module of the same optical type, such as 400G-DR4, can link to each other over fibre.
How far can 400ZR reach?
400ZR is designed for data centre interconnect over around 80 to 120 km of amplified point-to-point DWDM fibre. ZR+ modules extend this to longer metro and regional distances with stronger error correction.
Talk to TIFNEX
Send us your switch and NIC models, link distances, fibre type and quantities, and our engineers will recommend the right 400G optics, breakout modules and MPO or LC cabling, coded and tested for your platform. We quote within 24 hours and deliver across the UAE, Egypt and the GCC. Contact TIFNEX or message us on WhatsApp at +971 54 300 8970.
