400G optical modules use 10 transmission standards: VR4, SR4, SR4.2, SR8, DR4, FR4, LR4, LR8, ER4, ZR4. Three parameters differentiate them: transmission distance, fiber type, and connector type. This guide lists specifications for each standard and provides a selection method by use case.
1. Naming and Transmission Distance
Code | Meaning | Distance | Fiber Type |
|---|---|---|---|
VR | Very Short Range | 0-50m | Multimode |
SR | Short Range | Up to 100m | Multimode |
DR | Distance Range | Up to 500m | Single-mode |
FR | Long Range (short) | Up to 2km | Single-mode |
LR | Long Range | Up to 10km | Single-mode |
ER | Extended Range | Up to 40km | Single-mode |
ZR | Zero-Dispersion Range | 80-120km | Single-mode |
2. Modulation
400G optical modules use PAM4 (4-level pulse amplitude modulation). Each symbol carries 2 bits, doubling per-channel rate relative to NRZ modulation.
3. Specifications by Standard
400GBASE-VR4
Distance: 50m on OM4, 30m on OM3
Fiber type: Multimode (MMF)
Wavelength: 850nm
Connector: MPO/MTO
Channels: 4 channels, 100G each, 400G total
Transmission: PAM4, 4 parallel fiber pairs
Application: intra-data center short-distance links
400G SR4
Distance: 100m on OM4, 70m on OM3
Fiber type: Multimode (MMF)
Wavelength: 850nm
Connector: MPO-12/MTP-12
Channels: 4 channels, 100G each, 400G total
Transmission: PAM4, parallel
Application: data center, cloud provider, and enterprise network interconnects
400G SR4.2
Distance: 150m on OM4, 100m on OM3
Fiber type: Multimode (MMF)
Wavelength: 850nm and 910nm (WDM, 2 wavelengths per fiber pair)
Connector: MPO-12/MTP-12
Channels: 8 channels, 50G each, 400G total
Transmission: PAM4, parallel, WDM increases per-fiber capacity
Application: large-scale cloud computing, HPC, medium-distance data center interconnects
400GBASE-SR8
Distance: 100m on OM4, 70m on OM3
Fiber type: Multimode (OM3/OM4/OM5)
Wavelength: 850nm
Connector: MTP/MPO-16
Channels: 8 channels, 50G each, 400G total
Transmission: 8 parallel fiber pairs
Application: intra-data center short-distance, rack-to-rack high-density links
400GBASE-DR4
Distance: 500m
Fiber type: Single-mode (SMF)
Wavelength: 1310nm
Connector: MPO-12/MTP-12
Channels: 4 channels, 100G each, 400G total
Transmission: PAM4, 4 parallel fiber pairs
Application: inter-data center links, building-to-building fiber connections
400GBASE-FR4
Distance: 2km
Fiber type: Single-mode (SMF)
Wavelength: 1270nm, 1290nm, 1310nm, 1330nm (CWDM)
Connector: Duplex LC
Channels: 4 wavelength channels, 100G each, 400G total
Transmission: CWDM, 4×100G over a single fiber
Application: short-range MAN links, inter-data center connections
400GBASE-LR4
Distance: 10km
Fiber type: Single-mode (SMF)
Wavelength: 1270nm, 1290nm, 1310nm, 1330nm (CWDM)
Connector: Duplex LC
Channels: 4 wavelength channels, 100G each, 400G total
Transmission: CWDM, same technique as FR4 with longer reach
Application: medium-to-long-range MAN and data center connections
400GBASE-LR8
Distance: 10km
Fiber type: Single-mode (SMF)
Wavelength: 1273nm, 1277nm, 1282nm, 1286nm, 1295nm, 1300nm, 1304nm, 1309nm (8-wavelength WDM)
Connector: Duplex LC
Channels: 8 wavelength channels, 50G each, 400G total
Transmission: 8-channel WDM
Application: same distance as LR4, different channel count, for high-speed links up to 10km
400GBASE-ER4
Distance: 40km
Fiber type: Single-mode (SMF)
Wavelength: 1295.56nm, 1300.05nm, 1304.58nm, 1309.14nm
Connector: Duplex LC
Channels: 4 wavelength channels, 100G each, 400G total
Transmission: LAN-WDM
Application: MAN and WAN backbone interconnects
400GBASE-ZR4
Distance: 80-120km
Fiber type: Single-mode (SMF)
Wavelength: 1528.77~1567.13nm
Connector: Duplex LC
Channels: 4 wavelength channels, 100G each, 400G total
Transmission: DWDM with 16-QAM modulation
Application: data center interconnect (DCI), carrier backbone networks, cross-city/region links
4. Parameter Comparison
Standard | Max Distance | Fiber Type | Wavelength | Connector | Channel Config |
|---|---|---|---|---|---|
VR4 | 50m | Multimode | 850nm | MPO/MTO | 4×100G PAM4 |
SR4 | 100m | Multimode | 850nm | MPO-12/MTP-12 | 4×100G PAM4 |
SR4.2 | 150m | Multimode | 850nm/910nm | MPO-12/MTP-12 | 8×50G PAM4 |
SR8 | 100m | Multimode | 850nm | MTP/MPO-16 | 8×50G PAM4 |
DR4 | 500m | Single-mode | 1310nm | MPO-12/MTP-12 | 4×100G PAM4 |
FR4 | 2km | Single-mode | 1270/1290/1310/1330nm | Duplex LC | 4×100G CWDM |
LR8 | 10km | Single-mode | 8 wavelengths (1273-1309nm) | Duplex LC | 8×50G WDM |
LR4 | 10km | Single-mode | 1270/1290/1310/1330nm | Duplex LC | 4×100G CWDM |
ER4 | 40km | Single-mode | 1310/1550nm | Duplex LC | 4×100G WDM |
ZR4 | 120km | Single-mode | 1528.77-1567.13nm | Duplex LC | 4×100G DWDM 16QAM |
5. Selection Method
By Distance
Under 100m: VR4, SR4, SR8 (intra-data center)
500m to 2km: DR4, FR4 (inter-data center, building-to-building)
Up to 10km: LR4, LR8 (MAN, medium-to-long inter-data center)
40-120km: ER4, ZR4 (long-distance links, backbone)
By Fiber Type
Multimode: VR4, SR4, SR4.2, SR8 — lower cost than single-mode, short-distance use
Single-mode: DR4, FR4, LR4, LR8, ER4, ZR4 — used above 500m
By Application
Intra-data center interconnect: VR4, SR4, SR4.2, SR8
Inter-data center / building-to-building: DR4, FR4
MAN, medium-to-long distance (within 10km): LR4, LR8
WAN, long-distance (above 40km): ER4, ZR4
6. Connector / Form Factor Types
The tables above list the fiber-side connector for each standard (Duplex LC, MPO-12, MPO-16). Separately, modules use one of 3 host-side form factors: QSFP-DD, OSFP, QSFP112. Form factor determines electrical interface, power budget, and host port compatibility — independent of the transmission standard (SR4, DR4, etc.), which is available in multiple form factors.
QSFP-DD (Quad Small Form Factor Pluggable Double Density)
Channels: 8×50G, 400G total
Power: 12-15W
Size: comparable to QSFP28
Backward compatibility: supports QSFP+, QSFP28 (100G/200G modules fit QSFP-DD ports)
Application: data center switches, routers, servers; high port density deployments
OSFP (Octal Small Form Factor Pluggable)
Channels: 8×50G, 400G total
Power: 15-20W
Size: larger than QSFP-DD
Backward compatibility: none direct; requires a converter module for QSFP28 and earlier
Thermal design: higher heat dissipation capacity than QSFP-DD, supports future 800G modules
Application: long-distance links, MAN/WAN, high-power modules
OSFP-RHS (Riding Heat Sink)
A heat sink variant of the OSFP form factor, with the heat sink mounted on top of the module rather than inside the cage (the alternative variant is OSFP-FUSE, with a passive heat sink built into the cage)
Used for higher-power modules such as coherent ZR/ZR+ transceivers, where additional thermal dissipation is required
Channel configuration and backward compatibility are the same as standard OSFP
QSFP112
Channels: 4×112G PAM4, 400G total (higher per-channel rate than QSFP-DD's 50G)
Power: 10-15W, approximately 3.5W (29%) lower than comparable 400G modules
Size: compatible with QSFP-DD cage dimensions
Backward compatibility: supports QSFP28, QSFP56
Application: upgrading existing 200G networks to 400G, HPC, telecom networks
Form Factor Comparison
Form Factor | Channel Config | Power | Backward Compatibility | Heat Dissipation | Hot-Pluggable | Application |
|---|---|---|---|---|---|---|
QSFP-DD | 8×50G | 12-15W | QSFP28 (100G/200G) | Good | Yes | Data center, high density |
OSFP | 8×50G | 15-20W | None (converter required) | Excellent | Yes | Long distance, future 800G |
OSFP-RHS | 8×50G | 15-20W+ | None (converter required) | Excellent (external heat sink) | Yes | High-power coherent modules (ZR/ZR+) |
QSFP112 | 4×112G | 10-15W | QSFP28, QSFP56 | Standard | Yes | 200G-to-400G network upgrade |
7. Compatibility Notes
OSFP and QSFP-DD modules interoperate across the two ends of the same link when both ends use the same transmission standard — for example, both 400G-SR4 or both 400G-DR4. Form factor difference between the two ends does not affect link operation.
A 400G OSFP module cannot be inserted into a QSFP-DD port, and a QSFP-DD module cannot be inserted into an OSFP port: the two form factors have incompatible physical dimensions and connector protocols. A 100G QSFP28 module cannot be inserted directly into an OSFP port either; a converter module is required, and link speed after conversion remains at 100G.
Across the 10 transmission standards, fiber-side connectors fall into 3 types. Duplex LC is used by FR4, LR4, LR8, ER4, and ZR4 — WDM standards running over 1-2 fiber pairs. MPO-12 (APC) is used by VR4, SR4, SR4.2, and DR4 — 4-lane parallel-fiber standards. MPO-16 (APC) is used by SR8 — an 8-lane parallel-fiber standard, with 8 fibers for transmit and 8 for receive.
