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The electrical and optical interfaces of the QSFP28 100G optical fiber transceiver are defined based on its internal technical architecture and physical connection method, and there are essential differences between the two:
- Electrical interface: refers to the part where a module is connected to host devices such as switches and servers, responsible for the transmission of electrical signals.
- Optical interface: refers to the part where the module is connected to external fiber optic jumpers, responsible for the transmission of optical signals.
Electrical Interface
The electrical interface of the 100G optical fiber transceiver follows the QSFP28 MSA (Multi Source Protocol) standard, and its core specifications are as follows:
| Item | Specification |
| number of channels | 4 independent channels |
| Single channel rate | 25.78 Gbps (NRZ modulation) |
| Total bandwidth | 4 × 25.78 Gbps=103.125 Gbps (effective data rate 100G) |
| modulation technique | NRZ (Non Return to Zero Modulation) |
| Interface standards | CAUI-4（100G Attachment Unit Interface） |
| pin definition | QSFP28 standard 38 pin interface |
| hot-swapping | Support, can be plugged in and out without power failure |
| power supply voltage | 3.3V |
Regarding the CAUI-4 interface
CAUI-4 is the standard electrical interface for the QSFP28 100G optical fiber transceiver, which splits the 100G data stream into four 25G NRZ electrical signal channels and processes the signals internally through a Gearbox or DSP chip.
The optical interface of the QSFP28 100G optical fiber transceiver varies depending on the fiber transceiver types, mainly divided into two categories: MPO/MTP multi-core interface and duplex LC interface.
| Fiber transceiver | Optical interface type | Fiber type | Transmission distance | Wavelength | Channel architecture |
| SR4 | MPO-12 | MMF OM3/OM4 | 70-100m | 850nm | 4×25G parallel |
| SR2 | MPO-12 | MMF OM3/OM4 | 70-100m | 850nm | 2×50G PAM4 |
| PSM4 | MPO-12 | Single mode fiber | 500m | 1310nm | 4×25G parallel |
| CWDM4 | Duplex LC | Single mode fiber | 2km | 1271-1331nm | 4×25G + WDM |
| LR4 | Duplex LC | Single mode fiber | 10km | LAN-WDM | 4×25G + WDM |
| ER4 | Duplex LC | Single mode fiber | 40km | LAN-WDM | 4×25G + WDM |
| LR1 | Duplex LC | Single mode fiber | 10-20km | 1310nm | 1×100G PAM4 |
1. MPO/MTP Interface（muliti fiber parallel）
MPO (Multi fiber Push On) interface is used for parallel transmission of multi-core optical fibers, commonly found in short-range modules such as SR4, SR2, PSM4, etc.
| Item | Value |
| Connector Type | MPO-12 (12 core fibers) |
| Fiber optic usage | SR4:8 cores (4 transmit + 4 receive), 4 cores unused |
| End face type | UPC (flat polishing) |
| Typical transceiver type | SR4, SR2, PSM4 |
2. Duplex LC Interface (Dual fiber wavelength division multiplexing)
The duplex LC interface uses 2-core optical fibers and achieves 100G transmission through wavelength division multiplexing (WDM) technology, commonly found in medium to long distance modules such as LR4, ER4, CWDM4, LR1, etc.
| Item | Value |
| Connector Type | Duplex LC (one transmitter and one receiver) |
| Number of optical fibers | 2 cores |
| End face type | UPC (flat polishing) or APC(80 ) |
| Typical module | LR4, ER4, CWDM4, LR1 |
Interface differences between different architectures
| Fiber transceiver types | Electrical interface (host side) | Optical interface (fiber optic side) | Signal conversion method |
| SR4 | 4×25G NRZ | MPO-12（4×25G optical） | 4-to-4 direct pass-through |
| SR2 | 4×25G NRZ | MPO-12（2×50G PAM4） | 4:2 Gearbox |
| LR4 | 4×25G NRZ | Duplex LC（4×25G + WDM） | 4-channel WDM |
| LR1 | 4×25G NRZ | Duplex LC（1×100G PAM4） | 4:1 Gearbox + PAM4 |
Conclusion
The electrical interface of the QSFP28 100G optical fiber transceiver is uniformly 4×25G NRZ (CAUI-4), which connects to the host through the standard 38 pins of QSFP28. The optical interface varies depending on the module type – short-range modules (SR4/SR2) use the MPO-12 interface, and medium-to-long-range modules (LR4/LR1) use the full-duplex LC interface, depending on the transmission distance and technical architecture.
| Interface type | Connection object | Signal type | Channel configuration | Typical module |
| Electrical interface | Switch/Server | 4 x 25G NRZ electrical signal | 4-channel parallel | All QSFP28 modules |
| Optical interface (MPO) | Multi core fiber optic patch cords | 4 x 25G or 2 x 50G optical signal | Parallel transmission | SR4、SR2、PSM4 |
| Optical Interface (LC) | Duplex LC patch cords | 1 x 100G PAM4 or 4 x 25G + WDM | Wavelength Division Multiplexing | LR4, ER4, CWDM4, LR1 |
There are also new single wavelength single lambda PAM4 technologies and 100Gbe BIDI transceivers that use simplex LC interfaces, such as 100G DR1, FR1, BIDI-10, BIDI-80, etc