What are the electrical interfaces of an optical module?
Dec 22, 2025|
The correspondence between Arabic numerals and Greek numerals is as follows:
| Arabic Numeral | Greek Numeral (originating from Roman numerals) | Arabic Numeral | Greek Numeral (originating from Roman numerals) |
|---|---|---|---|
| 10 | X | 100 | C |
| 40 | XL | 400 | CD |
The Ethernet 802.3 standard defines CAUI as follows:
CAUI 100 Gb/s attachment unit interface.
The C here represents the Greek numeral 100. Following this logical continuity, the expressions CDAUI, etc., can be derived.
| Optical Module Type | Ethernet Interface |
|---|---|
| 100 Gp/s | XAUI |
| 100 Gp/s | CAUI |
| 40 Gp/s | XLAUI |
| 400 Gp/s | CDAUI |
CAUI-10 and CAUI-4 refer to a 10-channel 100G module (10×10G) and a 4-channel 100G module (4×25G), respectively.
The 8×50G Ethernet 400G optical module has a CDAUI-8 electrical interface.
SFI and XFI
Typically, the datasheet for an optical module will include a label for an electrical interface.
For example, something like this: "The optical module's electrical interface is an SFI interface."
Many engineers consider Ctrl+C and Ctrl+V the ultimate winning formula when writing specifications, leading to many interesting situations.
For example, if an engineer specifies that an XFP-packaged optical module supports the SFI interface, or an SFP+-packaged optical module supports the XFI interface, often if it works, some people will assume it's acceptable and correct.
Actually, SFI stands for SFP+ high speed serial electrical interface, which is a high-speed serial electrical interface used in SFP+ optical module packaging.
XFI stands for the high-speed serial electrical interface for XFP modules, with a nominal baud rate of 9.95-11.1 Gb/s. The SFP+ package is an enhanced version of SFP, increasing the speed from 2.5 Gb/s to 10 Gb/s.
The "X" in XFP stands for the Roman numeral 10. Both are 10G optical module packages, but the difference is that XFP is much larger. To make smaller packages, the CDR inside the optical module was removed, resulting in SFP+.
SFP+ lacks Cordless Diffraction (CDR), necessitating the addition of pre-emphasis and equalization processing for electrical signals on the system motherboard.
This is the most significant difference between XFI and SFI.
Because XFI and SFI electrical signals are designed for 10Gb/s speeds, with the advent of single-channel speeds of 25~28Gb/s, the electrical signals will be based on the 25GAUI C2M or CEI-28G-VSR electrical interface standards.