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A 5G radio site is not one optical link. The same tower connects a remote radio unit to a distributed unit, that distributed unit to a centralised unit, and the centralised unit to the core. Each segment has its own distance, its own latency allowance and its own fiber situation, but they are often ordered as one line item called "5G optics". The result shows up later as a fronthaul port that will not synchronise, a midhaul span that needs an amplifier, or a backhaul ring that runs out of capacity two years early.
This guide separates the three segments, sets out the module that belongs in each, and lists the checks that matter when a mobile operator or tower company asks for 5G backhaul optics.
Fronthaul, Midhaul and Backhaul Are Not the Same Link
The 3GPP split puts the radio unit at the top of the tower, the distributed unit at the cabinet or the edge site, and the centralised unit in an aggregation building. In a centralised RAN, one building may aggregate dozens of sites over the same fiber ring, which is why the wavelength plan matters as much as the rate.
| Segment | Typical distance | Rate per site today | Common module | Fiber and notes |
|---|---|---|---|---|
| Fronthaul (RU to DU) | 300m to 10km | 10G or 25G | SFP28 25G 10km, SFP28 25G BiDi | Single fiber BiDi where dark fiber is scarce, eCPRI and O-RAN 7.2 timing is tight |
| Midhaul (DU to CU) | 10km to 40km | 25G to 50G | SFP28 25G 40km, QSFP28 50G | CWDM or DWDM where the ring is shared, latency and sync budget still constrained |
| Backhaul (CU to core) | 40km to 120km | 100G to 200G | QSFP28 100G LR4, ER4, ZR4 | Duplex single-mode, DWDM on congested rings, coherent above 120km |
| Aggregation ring | 40km to 200km | 100G to 400G | DWDM transponder and muxponder cards | Mux and demux, amplifiers and channel monitoring as the ring grows |
The distance in the table is the optical route, not the map distance. Rural sites with long feeder routes and many splices behave like a much longer span, and the reach code has to be chosen from the measured loss.
What Changes When the Site Is Not a Data Center
Mobile sites add constraints that a data hall does not have:
- Temperature. Outdoor cabinets and unheated shelters swing from below freezing to well above ambient in the same year. Modules with industrial temperature range remove one failure mode from the maintenance list.
- Fiber scarcity. Many midhaul and fronthaul routes have one or two fibers available, so single-fiber BiDi or a CWDM and DWDM overlay is often the only way to add capacity without new cable.
- Timing. Fronthaul and midhaul carry synchronisation as well as traffic. A module that links successfully but drops PTP or SyncE packets is not a working link.
- Remote sites. Diagnostics matter more when the site needs a truck roll. DDM readings for optical power and temperature let a fault be identified before anyone drives out.
- Power and space. Amplifier sites need power, cooling and a cabinet. Choosing a reach code that avoids one is a real cost decision, not only an optical one.
Reading a Backhaul Requirement
When a mobile operator or integrator sends a requirement, five items decide the module:
- Measured route length and splice count for each hop.
- Rate per site today, and the capacity the ring is expected to carry in three years.
- Number of fibers available between each pair of sites.
- Wavelength plan already in use on the ring, so new channels do not collide with an existing CWDM or DWDM grid.
- Sync requirement: whether the link must carry PTP and SyncE with the accuracy the RAN split demands.
What APEX Supplies for 5G Transport
APEX covers the three segments from one source. For fronthaul, the 25G SFP28 range includes 25G 10km duplex modules and single-fiber BiDi versions for routes where only one fiber is available, with the timing behaviour the O-RAN split needs. For midhaul and long feeder runs, the same family extends to 40km and 80km in duplex, DWDM and CWDM versions, so a ring can be overlaid without new cable. For backhaul, the 100G QSFP28 ER4 and ZR4 parts cover 40km to 80km spans, and the passive CWDM and DWDM modules handle channel add and drop where the ring is already lit.
Where a ring outgrows pluggable optics, APEX supplies OTN and DWDM transport platforms with muxponder and transponder cards, so the same partner can quote the passive layer, the line cards and the optics together. Modules are coded for the host platform, and any mix of rates and reaches can be quoted as one order with matching spares per site.
Frequently Asked Questions
Can one module type serve fronthaul and backhaul?
No. Fronthaul is short and timing critical, backhaul is longer and capacity critical. A 25G SFP28 module is the natural fronthaul part, while backhaul at 40km and beyond belongs to 100G QSFP28 LR4, ER4 or a DWDM channel on the ring.
When should I use BiDi instead of duplex optics?
When only one fiber is free between the sites. BiDi modules transmit and receive on different wavelengths over a single strand, which doubles the capacity of an existing cable without new duct work or splicing.
Do I need coherent optics for 5G backhaul?
Below 120km, coded and amplified pluggables usually cover the span. Beyond that, or where several 100G services share one fiber pair, coherent modules on a DWDM grid and the line system around them become the practical route.
What affects synchronisation in a fronthaul link?
The module has to pass timing traffic transparently and keep latency stable. Ask for the module class that matches the host port, then verify PTP and SyncE after commissioning rather than assuming a working link means working timing.
Next Step
Send the site list with the measured distance per hop, the rate per site, the number of fibers between each pair of sites and the wavelength plan already in use. We will confirm the module for each segment, quote the mix together, and supply samples for one ring before the volume rollout starts.
