What an FTTH access link carries
A fibre to the home network is a passive optical tree. The optical line terminal sits in a central office, a passive splitter divides one port across many homes, and an optical network unit terminates the link at each subscriber. The optics sit at both ends of that tree: one module in the line terminal port, one module in each subscriber unit, and nothing active in between.
That arrangement is what makes access optics a budget question rather than a distance question. One port pays for the whole tree, so split ratio, connector count and total loss all land on the same part, and the rate is usually the lower of the two directions rather than the headline figure. The column behind this page is GPON and FTTH access products, and the neighbouring rate families are collected on optics selection guides.
What the access network constrains
Rate comes first, and access rates are asymmetric by nature. Downstream runs at 2.5G and upstream at 1.25G in GPON, and symmetric 10G in the newer access generation. The optics are built for that asymmetry: a diplexer at each end sends transmission and reception on two different wavelengths over one strand, so a single fibre serves the whole subscriber.
Loss is the second constraint. A splitter of 1:32 puts roughly fifteen decibels of loss into the path before any fibre is counted, splices and connectors add more, and the optical budget class of the port decides how far the tree can reach. The third is the environment: much of the access equipment sits in street cabinets, pole boxes or basement installations where the temperature range is far wider than in a data centre. The fourth is scale, because an access network is measured in thousands of subscriber units, and every unplanned variant in the field becomes a repeated spares problem.
Recommended optics for FTTH GPON access
| Module | Rate | Media / wavelength | Reach class | Where it usually lands |
|---|
| GPON OLT port module | 2.5G down / 1.25G up | SMF, diplexed 1490 / 1310 nm | 20 km class | Line terminal port feeding a passive tree |
| GPON subscriber module | 1.25G up / 2.5G down | SMF, diplexed 1310 / 1490 nm | 20 km class | Subscriber units in homes and small business sites |
| Combo access port module | 2.5G and 10G | SMF, four wavelength sets | 20 km class | Operators moving subscribers onto symmetric 10G |
| Symmetric 10G access modules | 10G / 10G | SMF, diplexed 1270 / 1577 nm | 20 km class | New build symmetric residential and business services |
| 10G SFP+ aggregation optics | 10G | SMF, 1310 / 1550 nm or CWDM | 10 to 40 km class | Uplinks from the access node to the metro layer |
| Passive splitter modules | Per port | SMF, 1:2 up to 1:64 | Passive element | Dividing one port across a subscriber group |
| Video overlay module | Per channel | SMF, 1550 nm | 20 km class | Carrying broadcast video on the same tree |
Reach classes are the standard ones for each family and the working limit is the measured loss on the tree, splitter included. The rate families are listed on the 10G and PON 25G and 50G pages, and the one strand case is compared on BiDi transceivers.
Optical budget, split ratio and the class of the port
The budget is spent from the port outwards: splitter loss, fibre loss, splice loss, connector loss and a margin for ageing and maintenance. A tree that is planned to the edge of the class works on the bench and fails in service on the coldest morning of the year, so the margin belongs in the plan from the start.
Split ratio and reach trade against each other directly: halving the split ratio buys several decibels, which either extends the span or pays for the extra connectors that a business route needs.
One strand, wavelength plans and business overlays
Access optics use a diplexer pair rather than two fibres, so one strand reaches the subscriber and the upstream and downstream wavelengths never collide. That same property is what lets a wavelength plan run alongside the access service: an overlay wavelength on the same tree can carry a business circuit or mobile backhaul without touching the residential subscribers.
Coding, temperature and subscriber unit variants
Coding is routine in access networks: the module is programmed for the line terminal or subscriber unit platform named on the request, tested on that model, and it is not manufactured by that brand. Coded parts for the common access platforms are collected on the Nokia, Huawei and ZTE pages, and the same rule applies to each of them.
Temperature grade belongs on the purchase line, not in a general note. A conditioned central office position and a pole mounted subscriber unit are different requirements, and the difference shows up as a link that resets in summer. Group positions that can share one part type, hold one coded part per group, and treat a subscriber unit variant as a separate line rather than a substitute.
How to choose, in order
- Fix the access rate per direction: 2.5G down and 1.25G up, or symmetric 10G.
- Add up the budget: splitter loss, fibre, splices, connectors and a maintenance margin.
- Set the split ratio and check that the reach still closes at the chosen class.
- Confirm the diplexer wavelengths and whether an overlay wavelength is needed.
- Write the temperature grade per position: office, street cabinet or subscriber unit.
- Set the coding per platform and hold one coded spare per group.
Frequently asked questions
Can a GPON module be used in a symmetric 10G port?
No. The rates and the wavelength pairs differ, and the port expects the matching part. Combo ports are the exception, because they carry both sets at once.
How much budget should be left for maintenance?
Leave a working margin rather than nothing. Access trees gain splices and patch points over their life, and a plan that closes with zero margin is a plan that will be revisited.
Is one strand enough for a business subscriber?
Yes for the access service itself. Where a business route needs its own path, the usual answer is a second wavelength on the same strand rather than a second fibre.
Do subscriber units need a different part from the office end?
Yes. The two ends transmit on different wavelengths, so the parts are not interchangeable even at the same rate.
What do you need to quote an access roll-out?
The line terminal model and release, the subscriber unit models, the splitter plan, the measured tree lengths and losses, the temperature grade per position, and the quantity per part with spares as separate lines.
Send the line terminal and subscriber unit models, the splitter plan and the measured tree losses, and we come back with a coded part per position, the passive elements to match and the spare lines set out separately. Related reading: PON at 25G and 50G, GPON and FTTH access products and all optics selection guides.
