Fibre laid without ducting: does ITU-T L.163 guarantee reliability?
7 min read
A specification for a link outside town cites “direct surface application to ITU-T L.163”, and the cost line shortens: no duct, minimal digging, fast timelines. Whoever cites it has usually stopped at the title. The Recommendation is real, it is from the ITU-T, and it is serious — but the first thing it says about itself, in a line no specification ever repeats, is which reliability it offers. Not in a footnote. On the cover, among the keywords.
The figure on the cover.
ITU-T L.163, Criteria for optical fibre cable installation with minimal existing infrastructure, approved on 29 November 2018 by Study Group 15, lists its own keywords right at the start, before the summary: “Best-effort reliability, closing the digital divide, developing countries, direct surface application, optical fibre cable installation, rural areas.” The first is not a technical detail buried at the back of the document: it is the first word the ITU-T itself attaches to it.
Clause 3.2.1 defines its subject in one line: “DSA cable: Optical fibre cable for direct surface application as defined in [ITU-T L.110].” DSA, direct surface application, is a cable designed for direct contact with the environment — laid on the surface or in a shallow groove, with no duct separating it from the ground along the route. No duct means none of the protection a duct provides by default: that is not a construction detail, it is the premise for everything that follows.
Where the difference shows.
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“At least one impact.” Clause 8.2.2 a), among the criteria for selecting the cable, carries a note on the expected level of resistance: “NOTE 1 – DSA cables should be crush resistant to withstand at least one impact by a road construction machine.” One impact, not a series. On a rural route crossed rarely, that makes sense; on an urban or industrial route, where a work site returns to the same spot more than once a year, the second hit is covered nowhere — the note does not rule it out, it simply does not address it.
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Depth is a variable, not a threshold. Clause 8.1.2 a) is blunt: “the depth of the trench or groove can be less than 0.5 m. The depth should be determined upon terrain, cost, construction speed, security, trust ability and longevity.” Cost and speed sit alongside security and longevity, on the same footing. In an even more extreme case — virgin terrain, clause 8.2.1 — the Recommendation allows burial less than 5 cm deep, the minimum needed to stop the cable sliding down a slope. A note next to clause 8.1.2 points to another document — 0.8 m, 0.6 m and 0.5 m depending on the traffic the route carries — but that document is marked [b-IEC TR 62691]: the “b-” prefix in L.163’s bibliography flags an informative reference, not a requirement of the Recommendation itself. Whoever writes “installed to L.163” has not written down a depth: they have left it to whoever digs. It is the same gap Italy’s minitrenching rules try to close with explicit thresholds — here it stays open.
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The cable you cannot see is the one that gets cut. Clause 8.1.1 e) treats visibility as a trade-off, not a requirement: “Shiny-colour polyethylene (PE) sheath could have a shorter life than black. Another option is to use black PE sheath with a distinctive stripe. The cable visibility issue may need to be decided at each community.” A shiny jacket, more visible and more easily stolen; a black jacket with a stripe, less attractive and harder for whoever digs later to spot. The Recommendation leaves the choice to the local community — it does not say which of the two, nor does it require the choice to be recorded anywhere.
The counterweight: the coordinates almost no specification demands.
There is one clause, 10.2, worth more than all the others combined, because it is the only defence left once depth and ducting are gone: “As-build [sic, error in the original] records along with video recording with optical fibre of the cable route co-ordinates (latitudes and longitudes) should be recorded, preferably in a geographic information system (GIS) will help from a centralised network operation centre (CNOC) team to guide the local maintenance team to reach the spot of outage with ease.” Route coordinates, in a GIS, to guide whoever repairs the fault straight to the spot. That is exactly the as-built documentation too many Italian specifications treat as an optional attachment — here the Recommendation treats it as a condition for the network’s survival, not a courtesy to the acceptance test.
The other side: not a Recommendation to avoid.
L.163 does not hide its own architectural limit: clause 1 states it bluntly, “This Recommendation considers only point-to-point network architectures.” And it is equally open about who it is for: “equally applicable to the developing countries where creation of telecommunications infrastructure is underway to bridge the digital divide”, and useful “in quick restoration of telecom services” after an outage. It even asks, in clause 8.3, for a preliminary installation trial on a short, representative stretch before committing to the full route: a method, not an improvisation.
The problem is not the Recommendation, explicit about what it is and what it does not promise. It is whoever cites it in a specification for an industrial route, a link between two sites, a repair that turns from temporary into permanent, without saying which reliability regime they are buying into.
What to write into the specification.
- Depth declared as an absolute minimum value, in centimetres, by terrain type — not “can be less than”, left to whoever installs it.
- Crush-resistance class with the number of impacts required, not “at least one”: two or more, depending on whether the route is rural, urban or industrial, with the test to back it up.
- Visibility and marking regime decided and written down — shiny jacket or black with a stripe — with the decision on record, not left “to each community”.
- As-built with route coordinates, delivered in a GIS, as a condition of acceptance — not a file attached once the work is finished, but an acceptance requirement.
- An explicit statement of the reliability regime accepted: if the specification cites L.163, it must state that a best-effort regime is being accepted — or spell out which additional clauses rule that out.
How it is checked on acceptance.
Acceptance testing measures the actual depth along the whole route, not the figure declared at quotation stage, and records the crush test outcome with the number of impacts withstood. It checks by eye that the visibility regime chosen matches what was written down — jacket and colour, not just the data sheet. It receives the GIS file with route coordinates and spot-checks it against the ground. And it closes with a signature on a point no conventional acceptance test asks for: written confirmation of which reliability regime was accepted for that route.
The point.
The figure on L.163’s cover is not a flaw to correct: it is information the specification has to carry through, clause by clause, into measurable requirements — depth as a threshold, crush class with the number of impacts, marking decided and written down, georeferenced as-built as a condition of acceptance, with the record to produce rather than an opinion. It is the work we do in every specifications and compliance engagement, for direct surface application as much as for conventional ducted installation: write the figure down, do not point at the standard. And the route coordinates, laying conditions and optical measurements do not stay a GIS file delivered once: they feed into the same site and network map, where an AI flags which section was laid under a best-effort regime and needs different monitoring, and the team responds — alongside CSIDIA, the group’s other company. Where an AI is needed on network data, it runs within the client’s perimeter: on-premise on autonomous machines needing no deep integration into the existing network, or on dedicated cloud with a data centre in Italy, with shared management between the two teams. We are already on the ground, with our own crews, with no subcontracting.
Are you about to write a specification that cites direct surface application, or do you have to accept a route someone else has already laid this way? Talk to an engineer: the site visit costs nothing, and it is the moment to write down which reliability regime you are working to.