Optical transport and troubleshooting
BiDi optics: identify the pair and wavelengths
A single-strand BiDi link can help when few fibers are available. Before selecting it, document both modules and the full route. This guide covers pairs using complementary transmit and receive wavelengths; the BiDi name also appears in other architectures.
Identify the pair by part number, not color
Record the manufacturer, full part number and TX/RX wavelengths at each end. Compare A’s TX with B’s RX and B’s TX with A’s RX in the data sheets. U/D or A/B labels and latch colors do not replace those specifications.
Also confirm protocol, speed and each module’s support in its host and port. Complementary wavelength labels alone do not settle optical range or equipment compatibility.
Documented example: Cisco GLC-BX-U and GLC-BX-D
Cisco describes the 1000BASE-BX10 GLC-BX-U with 1310 nm TX and 1490 nm RX; the GLC-BX-D reverses these roles, using 1490 nm TX and 1310 nm RX. This example applies to that pair on a single strand of single-mode fiber.
Before using this pair in an existing system, check the host, port, software, connectors, route and power limits. The wavelengths shown apply only to the Cisco pair in this example.
Compare fiber savings with spare management
Compare a supported BiDi pair with a two-strand duplex link or another available route. Count genuinely available fibers and adaptation points, and consider which spare modules each end needs.
If the route contains filters, multiplexers or other wavelength-selective elements, verify that they pass both wavelengths in the intended directions. Before a substitution, record the working pair and coordinate any interruption.
Information for a link review
| Item | What to record | Purpose |
|---|---|---|
| GLC-BX-U | TX 1310 nm / RX 1490 nm | Compare with complementary GLC-BX-D endpoint |
| GLC-BX-D | TX 1490 nm / RX 1310 nm | Compare with complementary GLC-BX-U endpoint |
| Actual assembly | Host, speed, fiber, losses and RX limits | Confirm suitability before selecting the pair |
Before approaching an optical connection
Do not look into optical ports or fiber ends, even when no light is visible. Inspection, cleaning and measurement require the manufacturer’s tools and procedures. If the assembly is hybrid and also carries power, identify the complete system and arrange work with qualified personnel.
Technical questions
Are two identical BiDi modules always wrong?
It depends on the architecture. For the complementary single-strand pairs covered here, check the exact pairing specified by the manufacturer; the BX10 example uses U and D.
Can one spare strand count as a backup?
A free strand is not an operational backup until its route, terminations and compatible modules have been checked. Record the required changes and their interruption.
Can an adapter turn any link into BiDi?
No. A physical adapter does not define protocol or module optics. Select the complete system using its documentation.
Further documentation
- Cisco — SFP Modules for Gigabit Ethernet Applications Data Sheet — GLC-BX-U / GLC-BX-D pair: complementary wavelengths.
- Fluke Networks — 5 Vital Safety Rules for Fiber Optic Cables — Optical safety and suitable tools.
Third-party field experience: useful questions, without specification or safety authority.
- Networking — SFP+ Question — Question about a BiDi pair without a link.