Fronthaul Latency and Timing Optics: Optics Selection Guide

Two budgets on one fronthaul link

Fronthaul connects the baseband unit to the radio unit, and in an open radio access network it carries the split traffic between them, commonly 25G eCPRI over one or two fibres. The link has to move that traffic and pass timing, because the radio units and baseband units have to agree on time to a much finer resolution than the data path needs.

Both budgets run over the same fibre and the same optics, so fronthaul selection is not only a distance question: the module has to sit inside a latency and asymmetry budget the timing chain can absorb, and keep doing that at an outdoor temperature. The column behind this page is optics for 5G backhaul, the rate families are on 25G SFP28 and 10G, and the timing side is on telecom synchronisation products.

What the fronthaul link has to carry

Distance sets the first constraint: radios on a mast or pole sit within a few hundred metres of a hub in a dense area and up to ten kilometres away in a rural one, so the same network mixes short and long runs under the same management.

Timing sets the second. Frequency synchronisation and time of day are transported over the packet network, and the radio needs both to hold the air interface. Where the timing chain runs across the optical link, the link has to be transparent to it: no re-timing that discards the marker, no asymmetric delay that shifts the time estimate, no link flap that resets the chain and takes minutes to re-converge.

Recommended optics for fronthaul and timing links

Reach classes are the standard ones for each family and the working limit is the measured loss on the route, including the passive layer where several radio units share a strand. The WDM options are compared on CWDM, DWDM and SWDM4 and the single fibre case on BiDi against duplex.

Latency, asymmetry and the timing chain

An optical module adds a small delay, and the number itself is rarely the problem: two modules of the same part number can differ slightly, and a timing chain that corrects for a fixed asymmetry quietly absorbs a variable one.

The practical rules are to buy one part type for a timing path rather than mixing types, to keep both directions on the same part type where the link is part of a time chain, and to avoid mid-path re-timing that strips the timing marker. Where a link carries time, the module delay and the measured asymmetry belong in the link record, so a replacement can be checked against the first unit fitted.

Temperature, cabinets and outdoor radio units

Outdoor radio units see a much wider range than a data centre, so the temperature grade is a per position decision, not a network-wide one. A module rated for a conditioned room will work on a mild day and fail in a sealed cabinet in summer, and the failure usually shows up as a link that drops at the hottest hour and recovers by evening.

The same applies to dust and airflow direction: a shuttered outdoor cabinet with no airflow heats up, and the module diagnostics are how a maintenance team sees it coming. Ask for the grade in writing per position and check the reported temperature after the first hot week rather than at commissioning.

Sparing, coding and a multi-generation radio estate

A fronthaul estate is rarely one generation of radio: the same hub can serve 10G and 25G sectors, indoor and outdoor units and more than one radio vendor. Group positions that can share a part, hold one part type per group, and keep a coded spare per group rather than a percentage of the population.

Coding matters here as anywhere else: the part is programmed for the platform named on the request and tested on that model, and it is not manufactured by the brand of the host. A spare coded for the wrong hub sits in the store until it is re-coded, a delay the network cannot afford during a cell outage.

How to choose, in order

1. Split the site list into indoor and outdoor positions and record the temperature grade.
2. Measure each route including the passive layer and set the reach class from the measurement.
3. Decide where a single fibre strand is the constraint and where a wavelength will be added.
4. Identify which links carry timing and keep one part type along each timing path.
5. Group positions that can share a part and set the coded spare per group.
6. Write the link record with part number, channel and measured delay before turn-up.

Frequently asked questions

What rate does fronthaul need?
25G is the common working rate for one 5G sector set, with 10G still in service on older sectors and 50G appearing on the higher order ones. The right answer comes from the split and the number of sectors on the link.

Do I need industrial temperature parts?
For radio units in outdoor cabinets or on poles, yes; for a conditioned hub a standard grade is usually enough. It is a per position decision and belongs on the purchase line.

Can fronthaul and timing share one optical link?
Yes, and that is the usual arrangement. The requirement is transparency: the link must not re-time or drop the timing marker, and one module type along the path keeps the asymmetry figure repeatable.

Is a passive mux a problem for timing?
No. Passive mux modules add loss and delay but no re-timing, so they fit a timing path as long as the loss budget includes the mux.

What do you need to quote a fronthaul roll-out?
The site list with indoor or outdoor placement, the measured distances and loss, the strands available per site, the rates per sector, the timing requirement, and the quantity per part with spares as separate lines.

Send the site list with placement, distances, strand counts and the rates per sector, and we come back with a part per position, the reach class and the coded spare lines. Related reading: fronthaul optics for O-RAN 7.2, how SyncE works in 5G networks, and all optics selection guides.