What single-fibre working means
A BiDi, or single-fibre, link carries both directions of a connection over one strand. Instead of a separate transmit and receive strand, the two ends transmit on different wavelengths in the same fibre and separate them with a filter inside the module, so one strand does the work of a pair. It is the standard answer when a route is out of strands but not out of reach, and it is used from access networks up to 100G uplinks.
The consequence to plan for: the two ends are not interchangeable. A single-fibre link is ordered as a matched pair of ends, and a replacement has to match the end it replaces, which makes labelling and the spares list part of the plan rather than an afterthought.
Why operators choose single-fibre working
Three situations push a link onto one strand. A duct or a building entry where fibre is exhausted and pulling more is slow or expensive. A site where the pair count was planned for one service and a second has to be added without new cable. And an access or fronthaul build where each remote site has only one strand available and duplex optics would need a second run. In all three the fibre saved is worth more than the small premium on the module. Where strands are plentiful, duplex optics remain the simpler and cheaper choice, and the comparison is set out on BiDi versus duplex optical transceivers.
Recommended single-fibre optics by rate
| Module | Rate | Wavelength pair | Reach class | Typical single-fibre use |
|---|
| 1.25G SFP BiDi | 1.25G | 1310 / 1550 nm pair | 10 to 40 km | Access spur and legacy links reached on one strand |
| 10G SFP+ BiDi | 10G | 1270 / 1330 nm pair | 10 to 60 km | 10G access and mobile backhaul on one strand |
| 25G SFP28 BiDi | 25G | Single-strand wavelength pair | 10 to 40 km | 5G fronthaul and mid-haul where one strand is free |
| 100G QSFP28 BiDi | 100G | Single-strand wavelength pair | 10 to 80 km | 100G uplink on a route with no spare strand |
| Muxed single-fibre channels, CWDM or DWDM | 10G to 100G | Channel pair per direction | 10 to 80 km | Several links sharing one strand through a passive mux |
| Duplex LR or ER, for comparison | 10G to 100G | SMF, 1310 nm | 10 to 40 km | When two strands exist and spare count matters more |
The reach column lists the standard classes at each rate; the working limit is the measured span loss, and the wavelength pair in use sets the budget. The duplex equivalents sit on the 10G, 25G and 100G rate pages, and the single-fibre family on the BiDi transceivers page.
Pairing, labelling and spares: the usual failure
Two ends of the same type will not come up, and that is the most common commissioning fault on a single-fibre route. Each end carries a suffix or a colour code for its direction, and the fibre plan has to show which end goes where. Label both ends with the link name and the end type, record the pair in the spares list, and keep one spare of each end rather than one spare in total, because a spare of the wrong end is no spare at all. The same rule applies to a replacement in service: swapping a module without checking the end type turns a one hour outage into a two hour one.
Reach, monitoring and what it cannot do
Single-fibre working does not remove the link budget. The same reach classes apply, and the wavelength pair sets the loss budget, so a span planned for a duplex LR part has to be checked again for a single-fibre part on the same route. Monitoring changes shape as well: with one strand carrying both directions, a fault is per direction rather than per fibre, and a test run from one end sees its own transmit path and the far end receive path at the same time. That helps isolate a dead laser, but it means both ends have to be documented before the first fault, not during it. What single-fibre working cannot do is extend a strand that is already wavelength exhausted: where several links share one fibre, the answer is a muxed plan, described on the single-fibre multiplexing page and the WDM transceivers page, rather than another pair of standalone modules.
Where single-fibre optics are used
Access and mobile transport are the heaviest users: 1.25G and 10G parts on spurs where one strand reaches the site, and 25G SFP28 parts on fronthaul and mid-haul where a remote radio unit has a single strand available, as described in the optics for 5G backhaul notes. On the data side, 10G, 25G and 100G parts serve uplinks and inter-building links where the duct is full. Passive optical access is a different case again: a PON splitter already works on one strand per direction over a shared tree, which is why the GPON OLT and FTTH access platforms are planned separately from point to point single-fibre links.
How to choose, in order
- Count the strands actually free on the route, not the strands on the drawing.
- Fix the rate from the host port, then the reach class from the measured span loss.
- Choose the wavelength pair and write down which end goes where before ordering.
- Confirm the far end is the matching opposite end, and record both in the spares list.
- Decide whether one link or several will share the strand; if several, plan the mux and the channel pair.
- Label both ends of every link and keep one spare per end type.
Frequently asked questions
Can I put one single-fibre module at each end and expect the link to come up?
No. These modules are ordered as a matched pair with opposite wavelength assignments. Two ends of the same type transmit and receive on the same wavelengths and the link will not come up.
Is single-fibre working shorter or slower than the duplex part at the same rate?
No. The reach classes at each rate are the same, and the wavelength pair in use sets the loss budget. The span still has to be measured and checked against that budget.
Does it work on multimode fibre?
No. Single-fibre bidirectional transmission is a single-mode technique based on wavelength separation. Multimode links use duplex or parallel optics at short reach instead.
Can several single-fibre links share one strand?
Only with a muxed plan, where each link gets its own channel pair through a passive mux. Standalone modules do not share a strand; without the mux the two links interfere.
What should be in the request for a single-fibre route?
The rate and the host port, the measured span loss, how many strands are free, which end is which, and whether the strand will be shared with other links. The host model then fixes the coding.
Send the rate, the host port, the measured span and the strands available, and we confirm the wavelength pair, the reach class and the coding, with the two ends identified for each link. Related reading: BiDi versus duplex for the comparison, single-fibre multiplexing for shared strands, and all optics selection guides.
