Low-Level Design (LLD) for Fiber Broadband

Low-Level Design (LLD) is the phase of a fiber broadband project where approved concepts become final, buildable engineering. It builds on a validated High-Level Design (HLD) and field survey data. LLD produces the design that becomes the basis for contract documents, permit and inspection approvals, and construction. The finished design also serves as an asset management record for the network owner.

This white paper is Chapter Three of the Fiber Broadband Association’s (FBA) Building Better Broadband series. It was written by the FBA Deployment Specialists Committee and its Engineering Best Practices working group. It follows Chapter One (Planning and High-Level Design) and Chapter Two (Fielding and Base Mapping), and it leads into Chapter Four (Permitting and Approvals). The guidance comes from FBA members’ experience engineering and deploying fiber networks. It is written for ISPs, engineering firms, and OSP designers who want fewer permit resubmissions, fewer field changes, and more predictable cost and schedule.

What the Chapter Covers

Part 1: Network Engineering Development
LLD turns a conceptual route into a buildable alignment. Two tests guide that work: jurisdictional compliance and constructability.

Jurisdictional compliance: where infrastructure may legally go, including ROW ownership, Public Utility Easements (PUEs), clearance rules, and permit formats.
Constructability: whether it can be built safely and economically, based on soil and geology, restoration, equipment access, and traffic control.

The chapter also covers:

Fiber-count strategy for FTTx feeder and distribution networks and for middle-mile routes.
Conduit configuration, with a recommended minimum of n+1 conduits and at least three conduits for rural FTTx: feeder/distribution, drops or MST tails, and a spare.
Horizontal Directional Drilling (HDD) bore profiles, using a 1:10 depth-to-distance ratio as the efficient guideline and 1:5 as the practical minimum.
Vault and handhole spacing: spans up to about 2,500 feet for large cables, and often under 1,000 feet for cables under 24-count.
Aerial engineering: make-ready, one-touch make-ready policies, pole-by-pole and span engineering to National Electrical Safety Code (NESC) requirements, and hardware and anchor planning.

Part 2: Optical & Performance Engineering
The chapter compares centralized split architectures with cascaded distributed split architectures:

Centralized: splitters sit at a Central Office (CO) or Fiber Distribution Hub (FDH).
Cascaded distributed: splitters are placed closer to subscribers.

It explains how subscriber density, optical loss budget, and maintenance approach should drive the choice. It also covers enclosure placement and slack guidelines:

About 10 feet (3 meters) for splice closures
About 50 feet at underground splice locations
50 feet plus the roadway width at road crossings

Part 3: Systems, Data & Documentation
LLD software and data interoperability decide how fast and how well networks get engineered. The chapter lays out a “right tool for the right job” model:

GIS is the enterprise system of record for planning and HLD.
CAD produces the precise, permit- and construction-ready plan sets.
Both are used for as-builts.

Integration tools from Esri, Autodesk, and Safe Software (FME) help bridge the two platforms. Standard formats such as Shape, DWG, and GeoJSON keep designs intact as data moves between systems. A “design freeze” at key stages prevents rework.

Part 4: Deliverables & Transition
The chapter defines what makes an engineering package permit-ready:

Plan and profile drawings
ASCE 38-22 utility quality levels (QL-D through QL-A)
Conflict confidence ratings
Two-factor verification of critical inputs such as ROW limits
Risk resolution logs
Staged internal reviews, including Professional Engineer (PE) review
QC review gates and constructability reviews
A formal turnover to construction

Why It Matters

Missing details, poor assumptions, or oversights in construction packages and permit applications lead to delays, cost overruns, and late launches. When LLD is done thoroughly and has validation and quality control built in, the handoff to construction goes smoothly, with predictable cost and schedule and few surprises.

Read the White Paper

Whitepaper FAQs

What is Low-Level Design (LLD) in fiber broadband deployment?
LLD is the engineering phase that turns a validated High-Level Design and field survey data into final, buildable, permittable deliverables. It covers aerial and underground infrastructure, optical performance, material definition, digital design data, and permitting requirements. Its outputs form the basis for contract documents, permit approvals, and construction.
What is the difference between High-Level Design (HLD) and Low-Level Design (LLD)?
HLD sets the design path. It’s used to assess feasibility, cost, homes passed, permit needs, and schedule for a go/no-go capital decision. LLD takes that path down to procurement-level detail: exact routes, fiber counts, conduit, structures, splice architecture, and plan sets ready for permitting and construction.
Where does LLD fit in the engineering pathway?
The FBA engineering pathway runs from Planning and HLD, to Fielding and Base Mapping, to LLD, to Permitting Approvals, to Construction Readiness. LLD is the bridge between validated design intent and physical construction.
How many conduits should a fiber route include?
The white paper recommends at least n+1 conduits, where n is the number needed for the initial cable plant. Rural and other low-density FTTx builds usually benefit from at least three: one for feeder and distribution, one for drops or MST tails, and one spare. High-growth corridors, middle-mile routes, and data center interconnections may call for more.
How do you design a horizontal directional drilling (HDD) bore profile?
Bore profiles set the depth, entry and exit angles, and utility clearances for an HDD installation, within the limits of the drilling equipment. A 1:10 ratio (10 feet of horizontal travel for every foot of depth) is the efficient guideline, and 1:5 is the practical minimum. For example, to reach 13 feet of depth under a county drain, bore pits would ideally sit about 130 feet from each side of the crossing, with about 65 feet as the minimum.
How far apart should vaults and handholes be placed?
It depends on the cable. Large-diameter cables can sometimes be pulled through spans approaching 2,500 feet. Smaller and lower-count cables, typically under 24-count, often need access points less than 1,000 feet apart. Structures also make natural branching points for future expansion when placed at major intersections, near commercial districts, and next to growing residential areas.
What is make-ready, and why does it matter for aerial fiber?
Make-ready is the work needed on existing poles before a new attachment can go up safely, such as moving existing attachments or replacing poles. It often drives the feasibility, cost, and schedule of aerial builds. Power make-ready is usually done only by the electric utility or pole owner. Where one-touch make-ready is allowed, a qualified contractor can move existing communications attachments, which can shorten schedules. When make-ready and permit delays pile up, an underground build may be worth evaluating.
Should a fiber network use a centralized or cascaded split architecture?
Both can produce reliable, scalable networks. The right choice depends on what the project needs to achieve. Centralized: concentrates splitters in a hub, which simplifies maintenance but needs larger feeder cables and more distribution fiber. Cascaded distributed: places splitters closer to subscribers, which cuts distribution fiber and suits larger or lower-density areas, but adds splice points and more complex loss calculations.
How much slack should be planned in a fiber design?
Common guidelines from the white paper: About 10 feet (3 meters) for most splice closures, About 50 feet at underground splice locations, About 50 feet plus the roadway width at road crossings, to support future repairs, Aerial slack should be enough to lower the closure to a ground-level work area, since splicing multiple times from a bucket is discouraged.
Should LLD be done in GIS or CAD?
Usually both, each for a different job. GIS is the enterprise system of record and is best for planning and HLD. CAD provides the precision needed for LLD, permit applications, and construction packages. Integration tools from Esri, Autodesk, and Safe Software’s FME move data between the two. Standard exchange formats such as Shape, DWG, and GeoJSON help keep designs consistent across contractors.
What makes an engineering package permit-ready?
A permit-ready package includes:

- Plan and profile drawings built on vetted base maps
- Utility data documented to ASCE 38-22 quality levels
- Supporting calculations such as load calculations, bore profiles, traffic control plans, and restoration details
- Two-factor verification of critical inputs
- An active risk resolution log
- Staged internal reviews and QC gates, including PE review where required