BIDI 100G SRBD vs. 100G SR1.2 vs. 400G SR4.2
100G and 400G short-reach BIDI transceivers (SRBD, SR1.2, and SR4.2) share the same fiber type, connector, reach, and wavelengths, which makes them easy to confuse. Despite these similarities, they are not interoperable with one another. This article will explain what each designation means, why they cannot be mixed, and how the 400G SR4.2 module relates to the 100G types in breakout applications.
The cause?
While SRBD, SR1.2 and SR4.2 share the same optical characteristics, short-reach, dual wavelength transmission over duplex multimode, they are not interoperable with each other. SRDB ↔ SR1.2 will not link despite seeming identical on an optical level. Where as 400G SR4.2 can break out only to SR1.2 and never to the SRBD version.
Why? To put it bluntly the cause is a difference in FEC implementation and lane handling between the SRBD and the SR1.2 / SR4.2. These modules use PAM-4 optical lanes, which require Forward Error Correction (FEC). The SR1.2 / SR4.2 generation uses the standardized KP4 FEC, with a defined lane structure, applied consistently end-to-end. This is what allows an SR4.2 to break out cleanly into four SR1.2 links.
The SRBD generation predates the standardization of KP-FEC for these BIDI optics. As a result, SRBD modules do not share a guaranteed compatible FEC-and-lane implementation with SR1.2.
Side-by-side comparison.
| Parameter | 100G SRBD | 100G SR1.2 | 400G SR4.2 |
|---|---|---|---|
| EdgeOptic P/N | BIDI-100G-QSFP28-150 | BIDI-100G-QSFP28-151 | BIDI-400G-QSFP-DD-150 |
| Form factor | QSFP28 | QSFP28 | QSFP-DD |
| Aggregate rate | 100G (often dual-rate 40G/100G) | 100G | 400G |
| Connector | Duplex LC (2 fibers) | Duplex LC (2 fibers) | MPO-12 (4 fiber pairs) |
| Wavelengths | ~850 / 910 nm | ~850 / 910 nm | ~850 / 908 nm |
| Optical arrangement | 2 × 50G PAM-4 | 2 × 50G PAM-4 | 8 × 50G PAM-4 |
| Reach (OM3/4/5) | 70 / 100 / 150 m | 70 / 100 / 150 m | up to 150 m (OM5) |
| Interoperates with | SRBD only | SR1.2, or 4× from SR4.2 | SR4.2, or 4×100G to SR1.2 |
| Breakout role | 100G end-to-end | 100G, or breakout leg of SR4.2 | 400G, or 4×100G to SR1.2 |
| FEC | pre-KP-FEC | KP4-FEC | KP4-FEC |
"BIDI" here means that two wavelengths share each fibre, not that the whole link runs on one strand. SRBD, SR1.2, and SR4.2 use duplex fibre (a 2-fibre pair per 100G): each fibre carries two colours in one direction. This differs from true single fibre BIDI modules, which pair as Side A / Side B on a single strand. SRBD and SR1.2 modules are identical on both ends and don't need to be A/B paired.