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Services · ODN/PON design and optical budget
Testing is won before the cable goes in the ground.
Attenuation, connectors, splices, splitters and a declared margin: the optical budget is calculated item by item, and the optical class is verified while changing it still costs one line of specification.
A failed test rarely comes from bad installation.
It comes from a calculation never made. The optical budget — the maximum loss a link can absorb while staying within the equipment specification — takes ten minutes with pen and paper, and it is almost always done afterwards, once the cable is already buried. The items are known: 0.35 dB/km at 1310 nm and about 0.25 dB/km at 1550 on G.652.D fibre, 0.02-0.05 dB per fusion splice, 0.2-0.3 dB per clean connector pair, and a splitter that weighs more than all the rest combined.
On the ODN the delicate point is precisely the splitter. Clause 6.6 of ITU-T G.671 sets ranges, not single values: 8.2-10.6 dB for a 1:8, 13.3-17.5 dB for a 1:32. And a cascade is not equivalent to a single stage of the same overall ratio: 1:4 plus 1:8 gives 13.6-18.0 dB against the 13.3-17.5 of a 1:32. These are tenths that, added to a 2-3 dB design margin, decide whether the network stays in class B+ (28 dB, ITU-T G.984.2) or has to be redone at the first extra splitter.
Services
How an optical network is sized.
01
Start from the services and the real distances.
How many units served, at which split ratio, along which routes. Distances come from the survey and from the datasheet of the cable actually installed, not from a textbook figure.
02
Every item with its own number.
Cable attenuation, splices, connector pairs, splitters from the thresholds in clause 6.6 of ITU-T G.671, and a declared design margin — normally 2-3 dB for ageing, temperature and future repairs.
03
The optical class is verified first.
The total is compared with the classes of the planned equipment: B+ and C+ for GPON (ITU-T G.984.2), N1, N2, E1 and E2 for XGS-PON (ITU-T G.9807.1). For multi-site interconnection we assess whether a CWDM or DWDM grid fits better (ITU-T G.694.2 and G.694.1).
04
The design becomes the specification.
Split ratios, normal or extended reach, allowed losses as numbers, test methods and thresholds. Alternatives come with their residual margin beside them: the choice stays with the client.
An example, step by step.
Illustrative case, not a real client. A local operator has to cover a district of 320 dwellings: ten 12 km primary links leave the exchange, each with a 1:4 in a zone cabinet plus a 1:8 close to the buildings. The supplier’s quotation uses the “typical” datasheet losses — 6.4 and 9.6 dB — and the sums appear to work with margin to spare.
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Redo the sums with the standard thresholds
In place of the “typical” figures we use the maxima from clause 6.6 of ITU-T G.671: 7.4 dB for the 1:4 and 10.6 dB for the 1:8, so 18.0 dB for the cascade instead of the 16.0 dB in the quotation.
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The other items, one by one
Cable: 12 km × 0.35 dB/km = 4.2 dB. Ten fusion splices at 0.05 dB = 0.5 dB. Six connector pairs at 0.3 dB = 1.8 dB. Total loss: 24.5 dB.
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The margin shows where it fails
With a 3 dB design margin the total reaches 27.5 dB, half a decibel under the 28 dB of class B+ (ITU-T G.984.2). One future repair with an extra splice takes it out of budget: the design is compliant on paper and fragile in real life.
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Three alternatives, with the numbers beside them
A single-stage 1:32 (17.5 dB) recovers half a decibel but removes flexibility; shortening the primary link saves 1.4 dB every 4 km; class C+ equipment (32 dB) or XGS-PON E1 (33 dB, ITU-T G.9807.1) takes the residual margin from half a decibel to 4.5 and 5.5 dB. The cost of each is on the table.
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The client decides, the specification records
The choice stays with the operator. The resulting specification states the split ratio, normal or extended reach, the maximum value for each item, the margin and the test thresholds: at testing exactly the same items will be measured.
In this example the correction happens before installation: the estimate is that ten primary links do not have to be rebuilt and 3-4 weeks of rework are avoided, against a failed test discovered with the cable already buried. The calculation, the datasheets and the specification stay inside the client’s perimeter — on-premise or on a dedicated cloud in Italy — and are shared with nobody else.
An illustrative example on a typical case: the assumptions are recalibrated on your own data.
What you are left with.
- An optical budget calculated item by item, with the design margin declared rather than hidden inside the totals.
- Split ratios and allowed losses taken from clause 6.6 of ITU-T G.671, not from a “typical” value on a commercial datasheet.
- An optical class verified against the planned equipment, with the residual margin left for growth.
- An ODN specification with losses written as numbers and test thresholds consistent with the calculation.
- A comparison of the alternatives — cascade, single stage, higher class — with the margin of each next to its cost.
Who has this problem.
- Operators and network contractors designing or reviewing an ODN before a tender or a contract lot.
- Companies with several sites to connect that must choose between dark fibre, CWDM and DWDM.
- Anyone with a link already installed who needs to know whether it supports a move to XGS-PON without being rebuilt.
- Technical departments that must state the allowed losses of an ODN without falling back on generic wording.
Two ways of working.
On call 24/7 for faults and restorations
The damage located by OTDR before any digging, splicing in the field in agreed windows including nights, the link re-certified before service is restored.
In both cases measurements, native traces and as-built records stay yours: on-premise in your own environment or on a dedicated cloud in Italy, in directly staffed data centres.
Frequently asked questions
The questions we are asked most often.
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How many dB does a 1:32 splitter lose, and how do I put it into the ODN optical budget?
Clause 6.6 of ITU-T G.671 sets ranges, not single values: 13.3-17.5 dB for a 1:32 and 8.2-10.6 dB for a 1:8. What enters the optical budget is the standard maximum, not the “typical” figure on a commercial datasheet. And a 1:4 plus 1:8 cascade gives 13.6-18.0 dB: it is not equivalent to a single-stage 1:32.
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What design margin is reasonable to leave on a new backbone?
Common practice reserves 2-3 dB for ageing, temperature swings and future repairs, which add at least one splice not foreseen in the original design. A link handed over with zero residual margin risks falling out of budget at the first maintenance visit. The margin belongs in the calculation, declared rather than hidden inside the totals.
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Will the link I have already installed support a move to XGS-PON, or do I need a different optical class?
The sums are redone item by item — cable attenuation, splices, connector pairs, splitters — and the total is compared with the equipment classes: B+ 28 dB and C+ 32 dB for GPON (ITU-T G.984.2), N1, N2, E1 and E2 for XGS-PON (ITU-T G.9807.1). That comparison says how much margin is really left. Where it is not enough, the alternatives come with their residual margin next to the cost, and the choice stays with the client.
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How do I state the allowed ODN losses in a specification without resorting to generic wording?
Every item with its own number: fibre attenuation from the datasheet of the cable actually installed — for a G.652.D roughly 0.35 dB/km at 1310 nm and 0.25 dB/km at 1550 — 0.02-0.05 dB per fusion splice, 0.2-0.3 dB per clean connector pair, splitters from the thresholds in clause 6.6 of ITU-T G.671. Then split ratio, normal or extended reach, declared margin and test thresholds consistent with the calculation.
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To connect two company sites, is CWDM or DWDM the better option, and which ITU-T grid do I cite?
The grids are defined by ITU-T G.694.2 for CWDM and G.694.1 for DWDM: the choice depends on the number of channels, the distance and the margin available on the real route. The specification should also fix the multiplexer loss threshold, which is an optical budget item like any other. The comparison is made on the numbers of that route, not on a general rule.
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