Low-Level Design (LLD)
Detailed route, splice, and asset design for fibre networks — the buildable output a construction team can actually work from.
Low-Level Design (LLD) takes the architecture from HLD and turns it into something a construction team can actually build. Route plans down to the metre, splice points, fibre allocation, drop design, cabinet placement — the level of detail where every decision has a knock-on effect on cost, timeline, and operational quality.
We deliver LLDs that hold up under construction scrutiny: routes that match what’s actually there, splice schedules that don’t fall apart in the field, BOMs that match what builders need to order.
What LLD covers that HLD doesn’t
HLD answers “what shall we build.” LLD answers “exactly where, with exactly what.” The shift produces six new artefact classes:
1. Route plans at metre-level resolution. Backbone and distribution routes drawn against accurate base mapping, with chambers, pole locations, and service drop points pinned.
2. Splice schedule. Per closure, which fibres are spliced through, which are dropped to a service, which are reserved. Drives BOM at part-number level and constrains fault-finding for the network’s life.
3. Fibre allocation tables. Which premise is on which fibre. Determines OLT port assignment, fibre route, and provisioning workflow.
4. Asset placement. Cabinet, splitter, and splice closure locations confirmed against ROW, easement, and access constraints.
5. Drop design. Service drops from the distribution to the premise — aerial, underground, or hybrid per premise.
6. BOM at part-number level. What the construction team actually orders, with quantity and supplier-aligned part numbers.
What’s in an LLD deliverable
A complete LLD pack contains:
- Route drawings — geographically accurate, scaled, layered (fibre, ducts, chambers, closures, drops)
- Splice schedule per closure
- Fibre allocation table — every fibre, every premise
- Asset list — cabinets, splitters, closures, with locations and types
- Drop register — every premise, drop method, attachment point
- Final BOM — part-numbered, quantity-confirmed
- Construction notes — sequencing, civils detail call-outs, test points
- Operator submission package where applicable (RUS loan design, BT/Openreach DPA, local authority)
Tools and platforms
We work in the platform that fits the project:
- GIS-native (QGIS, ArcGIS, or PostGIS-backed) where the client manages assets in GIS
- CAD output (DWG via AutoCAD or Civil 3D) where construction or permit submission requires it
- Specialised platforms (VETRO, FOND, IQGeo Comsof, OSPInsight) where the client has standardised on one
- Hybrid where the data lives in GIS but submission is in CAD
The choice doesn’t change the design — it changes the production format.
Standards we work to
LLDs are built to the operator and funder standards governing the project:
- UK: Openreach DPA standards for PIA-fed networks, BT and Virgin Media construction standards, MBNL where applicable
- US: RUS loan design specifications, BEAD documentation, individual operator standards (Frontier, Lumen, regional ILECs)
- NESC compliance for any aerial design — clearances, structural loading, attachment height
- Local authority and DOT standards for ROW, permits, traffic management
Inputs we need
LLD builds on HLD output plus:
- Approved HLD — topology, hub locations, splitter architecture
- Survey or walkout data — pole condition, duct availability, chamber locations, premise verification
- Asset records — existing fibre routes, occupied conduits, joint-use arrangements
- Premise database — accurate addresses with build status (premises that exist, are under construction, won’t exist for 2 years)
- Operator standards documentation
Where survey data is missing or stale, we either commission survey or flag it as a constraint that needs walking before construction.
Common pitfalls in outsourced LLD work
Splice schedules that fail in the field. Bad splice schedules are the most common reason LLD output gets rejected by experienced splicers. We design splice schedules that account for slack handling, fibre identification, and future-add capacity — not just connectivity.
BOM that doesn’t match what the contractor can order. Generic BOMs at spec level (e.g. “144F single-mode cable”) don’t survive procurement. We build BOMs at part-number level aligned with the operator’s preferred suppliers.
Asset locations that ignore real-world ROW. Splitter cabinets placed in the abstract “right” location that turns out to be private property, contested ROW, or impossible-to-access. Our LLDs sanity-check asset locations against real access before locking them.
Lost continuity from HLD. When HLD and LLD are run by different teams or even different vendors, the architecture intent gets reinterpreted. We run continuity into LLD by carrying senior engineers from HLD into LLD review.
Typical timelines
- Small coverage area (under 5,000 premises) — 6-10 weeks
- Mid-sized rollout (5,000-25,000 premises) — 10-20 weeks, typically phased
- County-scale (25,000+ premises) — programme work, rolling delivery in 5,000-10,000 premise batches
How we deliver
LLD runs on our integrated UK-and-offshore model. Production drafting and detailed route work happens in our offshore team. Splice schedule design, asset placement decisions, and final BOM happen onshore with senior engineers who’ve delivered FTTH networks at scale. Every package goes through onshore QA before client release.
Talk to us about an LLD project
Tell us coverage, HLD status, operator, and timeline. We’ll scope and price within two business days for projects under 10,000 premises; larger programmes warrant a scoping call to discuss phasing.
Typical deliverables
- Detailed route plans with splice diagrams
- Final bill of materials with part numbers
- Splice schedules and fibre allocation tables
- Cabinet, splitter, and closure locations
- Drop and pathway design
- Construction-ready drawings (PDF, CAD, or both)
Who buys this
Project owners moving from HLD into build, where construction teams need detailed buildable design.
Talk to us about delivery options
Tell us what you need delivered, what your timeline is, and what format the downstream team needs the output in. We'll come back with scope, price range, and proposed approach.