LLD is where fiber networks go from architecture to actionable construction documents. Strand assignments, splice point placement, conduit fill, reel cut sheets, OTDR test plans — every detail resolved before a crew touches the first pole.
Low level design (LLD) is the detailed engineering phase that follows HLD design. Where high-level design establishes network architecture — route corridors, splitter placement, serving area boundaries, and optical budget — LLD converts that architecture into the construction documents that crews actually work from. If HLD answers "what are we building and where," LLD answers "exactly how do we build it, pole by pole."
LLD defines the complete physical construction package for a fiber network. That means exact splice point locations — not just general corridor routing, but the specific pole or vault where each splice closure goes and why. It means strand assignments for every fiber in the cable, so technicians know which buffer tube connects to which destination without making field decisions that should have been made at a desk. It means conduit fill calculations that tell a pull crew whether a given conduit segment has capacity before the reel shows up on the truck. It means span measurements that account for pole-to-pole distances as-measured, not as-estimated from GIS. It means reel assignments — specific cable lengths cut for specific segments — so there's no guessing about whether a reel will reach. It means slack storage locations, attachment detail drawings, and a complete OTDR test plan with acceptance thresholds.
LLD is what distinguishes a buildable design from a planning exercise. FTTH design that stops at HLD leaves the hardest engineering questions for field crews to answer under production pressure — which is the most expensive way to answer them.
Every Draftech LLD package is a complete construction document set. We don't deliver partial packages that require field engineering to fill in the gaps. The following deliverables are included in a standard LLD engagement:
Every fiber mapped from origin to destination — buffer tube, fiber color, splice sequence, and termination point. No field guessing on fiber assignments.
Closure-by-closure splice documentation showing incoming and outgoing buffer tube assignments, fiber counts, and slack management for each splice point.
Per-segment conduit fill analysis based on actual cable outer diameters and conduit inner diameter — flagging any segments at or above NEMA fill limits before a pull is attempted.
Attachment heights, strand and lashing wire specifications, down-guy locations, and NESC clearance compliance for every aerial pole on the route.
Field-measured span lengths, sag calculations for aerial cable, and tension data — all based on actual pole data, not GIS estimates.
Specific cable lengths assigned to each pull section, accounting for slack loops, splice tails, and reserve — so procurement orders the right lengths and crews pull the right reel.
Designated slack storage points with coil length specifications — planned upstream to prevent the field-improvised slack loops that create maintenance problems later.
Wavelength, pulse width, range, and acceptance threshold specifications for every fiber and span — so post-construction testing is standardized and results are comparable.
Documentation framework for construction crews to record field deviations, enabling accurate as-builts without a full redesign when conditions differ from plan.
The consequences of poor LLD show up in the field as specific, measurable cost events. This isn't abstract — every item below represents a category of rework our clients have experienced before working with us, and it's why LLD quality control is worth taking seriously.
Wrong splice point location: A splice case placed at the wrong pole or at the wrong point in a conduit run means a technician discovers the error mid-build. Re-routing and re-splicing to the corrected location requires pulling already-placed cable, re-cutting lengths, and re-scheduling the splice crew. Each rework event costs $800–$4,500 in field labor depending on location accessibility, crew size, and the extent of the re-route — and that's before accounting for schedule delay on a build where every day costs money.
Wrong conduit fill calculations: If the design overstates available fill capacity, a pull crew arrives at a pull point with the correct reel but the wrong assumptions about whether it can be pulled through. A field-discovered conduit conflict — discovering an undocumented existing cable, or discovering fill is already at limit — is a $2,000+ job site delay per incident when you account for crew mobilization, equipment staging, and the time to determine an alternate path. It can cascade into permit delays if the alternate path requires a new right-of-way.
Incorrect reel cut lengths: A cable cut 200 feet short of the required segment length means either a waste splice mid-span (adding optical loss, labor, and a new closure location to the asset database) or a reel that doesn't reach and requires emergency procurement of a new cut. Wrong reel lengths are a direct translation of LLD inaccuracy into construction cost.
Fiber count errors: An LLD that doesn't correctly account for the HLD fiber allocation will produce strand assignment sheets that run out of fibers before all destinations are served. Discovering a fiber count error in LLD is a redesign event — everything downstream of the error gets redone. Discovering it in the field after cable is pulled is significantly worse.
Draftech's LLD QC checklist catches 12 common design errors before packages ship to the field. The checklist includes fiber count validation against the HLD, splice point placement logic review, conduit fill margin verification, reel assignment continuity checks, and optical budget compliance at each splice location. We catch our own mistakes.
Not all LLD is the same. The design complexity, schedule, and risk profile of a fiber LLD package varies significantly by project type. Our team has production experience across all of the following:
Clean-slate network construction with no legacy plant conflicts. Greenfield LLD is the most straightforward type, but speed is usually the pressure — greenfield builds often have aggressive investor-driven schedules where HLD and LLD need to happen in parallel across multiple zones.
Brownfield overbuilds require LLD that accounts for existing attachments on shared poles and conduit infrastructure that may already be at or near capacity. Strand assignment schemes must navigate existing cable plant on the same routes, and make-ready analysis feeds directly into LLD sequencing.
Multi-dwelling and multi-tenant unit design introduces riser routing, IDF/MDF placement, in-unit wiring strategies, and horizontal cabling runs that require a different LLD discipline from outside plant work. Our MDU LLD packages include full riser diagrams, port assignment matrices, and equipment room layout drawings.
Rural BEAD builds at 3–7 addresses per route mile involve long feeder spans, infrequent splice locations, and terrain challenges that require careful reel cut planning. Span distances can exceed standard reel lengths in low-density territory, requiring spliced mid-span sections that must be engineered, not improvised.
Hybrid routes require LLD that transitions between aerial and underground plant, including riser protection at transition points, handhole placement at cable transition locations, and conduit-to-aerial transitions with appropriate bend radius protection. These transitions are common points of field errors when the LLD doesn't address them explicitly.
Encased crossings under railroads and limited-access highways require LLD sections that specify bore specifications, casing pipe requirements, end seal details, and permit drawing sets conforming to the applicable railroad or DOT standards. We've designed crossing packages for Class I railroad submissions and state DOT permit applications across multiple states.
Project type drives LLD scope: A BEAD rural route and an MDU fiber project require completely different deliverables. When you engage Draftech for LLD services, we scope the deliverable set to match the project type — not a one-size-fits-all template. Tell us what you're building and we'll tell you exactly what the LLD package needs to contain.
A fiber LLD package that's technically complete but ignores the practical realities of construction management creates friction for project managers coordinating field crews. Draftech LLD packages are designed to be as useful to a PM scheduling subcontractors as to a splice technician in the field.
Our LLD packages include construction sequencing notes — not just a design drawing, but guidance on the logical order of construction activities: which cable sections need to be pulled before splice access is practical, where parallel aerial and underground segments require coordination, and which sections have permit dependencies that should be addressed before mobilizing crews.
Permit reference numbers are embedded in the LLD plan sheets at the relevant locations. When a crew is working a section that crosses a county road or requires utility notification, the permit number and any special conditions are visible in the construction document — they don't need to search a separate permit file.
Flagged make-ready sections are clearly identified in the LLD — spans where make-ready work must be completed before fiber construction can proceed. This allows project managers to phase construction around make-ready timelines and avoid mobilizing aerial fiber crews to sections where pole attachment clearances haven't been obtained yet.
For large-scale projects, we also provide a construction zone index — a master sheet cross-referencing plan sheet numbers to geographic zones and construction phases, so a PM can pull the relevant sheets for a specific crew's work zone without paging through the full package. These project management integrations add minimal time to the LLD process but significantly reduce field coordination overhead.
When LLD is paired with our field survey work, the construction package is also tied to real GPS-verified pole locations, not GIS estimates. Crews working from GPS-anchored LLD have dramatically fewer field discrepancies — and fewer calls back to the engineering team asking which pole is which.
If you're looking for more detail on how detailed design packages affect project execution, our splice point placement guide covers a significant source of field rework in LLD-to-construction handoffs. For the full picture of common design errors, the LLD quality control checklist is the right starting point.
A complete fiber LLD package includes strand assignment sheets that map every fiber buffer tube to its destination, splice diagrams for each closure, conduit fill calculations for every segment, pole attachment detail drawings with heights and span lengths, span charts, reel cut sheets with specific cable lengths assigned to each pull section, slack coil and storage locations, an OTDR test plan with acceptance thresholds, and an as-built preparation package. The LLD is the document construction crews work from — it needs to answer every field question before a crew picks up a tool.
High-level design (HLD) establishes the network architecture — serving area boundaries, splitter placement, fiber allocation, route corridors, and optical budget. Low-level design (LLD) converts that architecture into construction documents. HLD tells you what to build and where; LLD tells the crew exactly how to build it — pole by pole, conduit by conduit. HLD is a planning document. LLD is an engineering package. You cannot build a fiber network from an HLD alone.
Every Draftech LLD package goes through an internal QC process before it ships. A second engineer who didn't touch the original design reviews the package against the HLD and checks it against our 12-point LLD checklist. The checklist covers fiber count consistency, splice point placement logic, conduit fill margins, reel assignment continuity, optical budget compliance at every splice point, clearance compliance on aerial spans, permit drawing accuracy, and BOM completeness. We catch our own mistakes before they become field problems.
Technically yes, but practically it's a significant risk. LLD without HLD means the engineer doing the pole-level design is also making architecture decisions — fiber counts, splitter placement, serving area boundaries — that should have been resolved upstream. The most common result is discovering mid-LLD that the fiber count assumptions are wrong and the entire strand assignment scheme needs to be redone. We'll do LLD-only engagements for clients who have a solid existing HLD, but we'll always review the HLD first and flag any issues before we start LLD work.
A 100-mile LLD typically runs 6–10 weeks depending on route complexity, aerial vs. underground mix, number of splice points, and how clean the HLD input data is. Aerial rural routes in flat terrain are faster — a well-organized aerial project at 100 miles could be done in 5–6 weeks. Underground urban routes, railroad crossings, or complex MDU sections add time. We scope every project individually and will give you a milestone schedule at kickoff.
ARE YOU AN LLD DESIGN FIRM?
This page describes the service we deliver to clients. If you provide fiber LLD production and are looking for a consistent subcontract pipeline, we have ongoing capacity needs in this discipline.
Whether you have a completed HLD that needs LLD production, or you need both HLD and LLD from scratch, our engineering team is available to scope the project. We work with ISPs, municipalities, cooperatives, and BEAD subgrantees across all 48 continental states.
Contact Our Engineering TeamOr email us directly at info@draftech.com — we reply within one business day.