In short: a BiDi (bidirectional) transceiver sends and receives over a single fiber instead of two, by using one wavelength for each direction. BiDi modules always work in pairs – an "upstream" (U) and a "downstream" (D) version – and they halve the number of fibers a link needs.
That makes BiDi a simple way to get more out of existing fiber, or to save on rented fiber between sites. Here is how it works and what to watch out for.
How BiDi works
A standard optical module needs two fibers: one to send and one to receive. A BiDi module has a tiny filter inside that lets it send on one wavelength and receive on another, both on the same fiber. The module at the other end does the opposite. A typical 1G pair, for example, sends at 1310 nm in one direction and at 1550 nm in the other.
That is why BiDi modules come in two versions:
- U (upstream), for example SFP-1G-BX-U: sends at 1310 nm, receives at 1550 nm.
- D (downstream), for example SFP-1G-BX-D: sends at 1550 nm, receives at 1310 nm.
One end of the link gets a U module, the other end a D module. Two U modules (or two D modules) will not work together.
Why use BiDi?
- Half the fiber: one fiber per link instead of two. If you rent dark fiber between sites, that can halve the cost.
- More links on existing cabling: a fiber pair that carried one link can carry two BiDi links.
- Simple to run: no multiplexer or extra equipment, just a matching pair of modules.
BiDi at every speed
| Speed | Typical reach | Where to find them |
|---|---|---|
| 1G SFP | 10, 40 and 80 km | 1G BiDi modules |
| 10G SFP+ | 10, 20 and 40 km | 10G BiDi modules |
| 10G XFP | 20, 40 and 80 km | XFP BiDi modules |
| 25G SFP28 | 10 and 40 km | 25G SFP28 modules |
| 40G / 100G | short multimode links | QSFP-40G-SR-BD, QSFP-100G-SR1.2 |
The 40G and 100G BiDi modules are a special case: they let data center operators run 40G or 100G over the duplex multimode cabling they already use for 10G.
What to watch out for
- Order a matching pair: one U and one D module with the same speed and reach.
- Single-mode fiber with a simplex connector: BiDi SFPs use a single LC connector and one fiber.
- Coding for each device: like any module, each end has to be coded for the equipment it goes into – see our SFP compatibility guide.
If you need many links over the same fiber rather than just one or two, CWDM or DWDM is the next step.
Frequently asked questions
Can I connect two BX-D modules to each other?
No. Both would send and receive on the same wavelengths. A BiDi link always needs one U and one D module.
Is BiDi slower than a normal two-fiber link?
No. A 10G BiDi link carries the full 10 Gbit/s in each direction, just like a standard 10G link.
Can I connect a BiDi module to a normal LR module?
No. A standard module sends and receives on separate fibers at one wavelength; a BiDi module uses one fiber and two wavelengths. Both ends of the link must be BiDi.
