BiDi and BX optics: more bandwidth on the fibre you already have
"BiDi" gets used for two quite different things. Both send traffic in both directions using more than one wavelength, and both exist to save you from pulling new fibre, but they solve different problems and they are not interchangeable.
BX: one fibre instead of two
A BX optic runs a full-duplex link over a single strand. It transmits on one wavelength and receives on another, so the two directions share the fibre without colliding. The connector is simplex LC — one ferrule, not two.
Because the wavelengths are asymmetric, BX modules only work in matched pairs. One end is upstream, the other downstream, and Cisco marks them -U and -D:
- 1G BX10 — 1310nm TX / 1490nm RX upstream, 1490 / 1310 downstream, to 10km.
- 1G BX40 and BX80 — the same idea at longer reach, with 80km parts pairing 1490 and 1550nm.
- 10G and 25G BX — 1270nm TX / 1330nm RX upstream, 1330 / 1270 downstream.
Order two of the same and you will have two modules transmitting on the same wavelength and listening on another neither is using. Always order a -U and a -D together.
BX is worth it when fibre count is the constraint: a leased strand, a congested riser, a run to a building where pulling a second fibre means digging.
SR-BiDi: 40G or 100G on duplex multimode
This is the other meaning, and the more commercially interesting one. An SR-BiDi optic uses two fibres — ordinary duplex LC, same as a 10G SR — but sends two wavelengths down each one, around 850 and 910nm. Two fibres carrying two lanes each gives you four lanes in total.
The point is what it replaces. A 40GBASE-SR4 or 100GBASE-SR4 needs eight fibres on an MPO-12 ribbon trunk. An SR-BiDi needs the two-fibre duplex LC patch cord already running to that switch. Upgrading a 10G link to 100G becomes a module swap at each end instead of a cabling project.
The trade is reach and price. BiDi parts cost more per module than the SR4 equivalent and typically reach around 100m on OM4 rather than 150m. Against the cost of pulling ribbon trunk through an occupied data hall, that is usually not a close decision.
Which one you want
If the problem is not enough strands, you want BX — one fibre per link, at 1G, 10G or 25G. If the problem is duplex fibre where the optic wants a ribbon trunk, you want SR-BiDi, at 40G, 100G or 400G.
Both ends must be the same technology. A BiDi will not talk to a conventional SR, and a BX-U will not talk to anything but a BX-D of the same reach class.
A note on naming
Vendors are inconsistent here. Cisco uses BX for the single-fibre parts and SR-BD or SR1.2 for the duplex BiDi ones; Juniper calls the latter BXSR; Arista uses SRBD. We file all of them under the same filter values — 1000BASE-BX, 10GBASE-BX and 25GBASE-BX for single-fibre, and 40GBASE-SR-BIDI, 100GBASE-SR-BIDI and 400GBASE-SR-BIDI for the duplex kind — so you can find the right one without knowing which vendor's word for it you started with. Search by the OEM part number you have and you will land on the match regardless.