FOA CFOS/D cheat sheet

Link power budget: required margin = minimum transmit power − receiver sensitivity; the sum of fiber attenuation (dB/km × km), splice loss (typ. 0.1 dB each), connector loss (typ. 0.3–0.75 dB per mated pair), and passive splitter loss must not exceed that margin.

Splitter loss in dB: 1×N ideal loss = 10·log10(N); add ~0.3–0.5 dB excess loss per stage. 1×32 ≈ 15 dB ideal, ~17 dB practical; cascaded stages add their dB values.

Distance limits by Ethernet rate over OM3/OM4: 10GBASE-SR ≈ 300/400 m; 40GBASE-SR4 ≈ 100/150 m; 100GBASE-SR4 ≈ 70/100 m. OS2 with 100GBASE-LR4 reaches 10 km; 100GBASE-ER4 reaches 40 km.

Wavelength window choice: 850 nm multimode (VCSEL, short reach), 1310 nm single-mode (zero-dispersion, 10 km class), 1550 nm single-mode (lowest attenuation ~0.2 dB/km, long haul, but EDFA band).

Bend-insensitive multimode (OM3/OM4/OM5) minimum bend radius typically 7.5 mm vs 10× cable OD for standard; OM5 adds wideband 850–953 nm for SWDM shortwave wavelength division multiplexing.

Polarization mode dispersion (PMD) and chromatic dispersion budget must be checked at ≥10 Gbps over long single-mode spans; PMD coefficient in ps/√km, dispersion in ps/(nm·km).

Fusion splice loss target ≤0.1 dB, mechanical splice ≤0.3 dB, field connector ≤0.3 dB (0.75 dB max per TIA-568). OTDR bidirectional averaging is required for accurate splice loss reporting.

Topology selection rule: star/home-run for campus and FTTx (centralized split), ring for metro/backbone protection, bus discouraged for high-count access. PON split ratio 1:32 typical, 1:64 or 1:128 with budget and class B+/C+ optics.

Reflectance budget: APC (8° angled) connectors ≤ −60 dB reflectance, UPC ≤ −50 dB, PC ≤ −40 dB. High reflectance degrades analog/RFoG and 10G+ links; never mate APC to UPC.

Scope baseline = scope statement + WBS + WBS dictionary; change to any of these requires formal integrated change control (CCB approval) before work proceeds.

Critical path method: float (slack) = late start − early start = late finish − early finish; the critical path has zero float and determines minimum project duration.

Earned value formulas: CV = EV − AC; SV = EV − PV; CPI = EV/AC; SPI = EV/PV; EAC = BAC/CPI (typical variance) or AC + (BAC − EV) (atypical); VAC = BAC − EAC.

PERT three-point estimate: expected = (O + 4M + P)/6; standard deviation = (P − O)/6; use for schedule and cost uncertainty on splice/termination labor.

Risk register fields: ID, description, cause, probability, impact, score (P×I), response strategy (avoid, mitigate, transfer, accept), owner, trigger, contingency reserve.

Communication and RACI: each WBS work package has exactly one accountable owner; responsible/consulted/informed may be multiple. Stakeholder register updated at each phase gate.

Quality gates for OSP: pre-construction site survey, cable reel test (OTDR on delivery), installation inspection, end-to-end link certification (OLTS per TIA-606/ISO 14763), as-built documentation handover.

Procurement lead time dominates FTTx schedules: fiber cable 6–12 weeks, OLT/ONT 4–12 weeks, permits and pole attachments often 60–120 days. Order long-lead items before design freeze.

Change control and contingency: management reserve covers unknown-unknowns (outside baseline); contingency reserve covers identified risks (inside baseline). Use earned schedule for schedule performance on long OSP builds.

TIA-568 maximum channel insertion loss for 10GBASE-SR over OM4 (400 m) ≈ 2.6 dB; verify per-link using manufacturer link budget tables, not generic rules.

Attenuation coefficients: OS2 ≤0.35 dB/km at 1310 nm and ≤0.25 dB/km at 1550 nm; OM4 ≤3.5 dB/km at 850 nm. Use these for budget calculations.

PON power budget classes (ITU-T G.984/G.987): Class B+ 28 dB, Class C+ 32 dB, Class D 35 dB. Split ratio and reach must fit the class budget.

Connector loss allocation in a link budget: count every mated pair as one connector event; a patch panel with two adapters equals two connector events.

OTDR dead zone: event dead zone ~1 m, attenuation dead zone ~4 m for standard modules; use short-pulse settings for closely spaced splices.

Chromatic dispersion limit for 10G NRZ single-mode ≈ 800–1000 ps/nm; at 1550 nm OS2 (~17 ps/nm·km) this allows ~50–60 km before compensation.

WBS rule: 8–80 hour work packages at the lowest level (work package), decomposed to activities for scheduling; 100% rule - children sum to parent scope.

Schedule compression: crashing adds cost (e.g., second splice crew), fast-tracking adds risk (parallel design and construction). Apply only to critical path activities.

Cost baseline = sum of activity cost estimates + contingency reserve; management reserve sits above baseline and is not in EV calculations.

TCPI (to-complete performance index) = (BAC − EV)/(BAC − AC); >1 means remaining work must be more efficient than plan.

Milestone vs. deliverable: milestones have zero duration and mark phase gates; deliverables are verifiable outputs in the WBS.

Lessons learned and closure: final acceptance certificate, as-built drawings, OTDR/OLTS test reports, warranty start date, and archive of project records.

OM5 wideband multimode supports SWDM over 850–953 nm; OM3/OM4 restricted to ~850 nm for shortwave WDM.

Single-mode OS2 is required for spans >550 m at 10G, >300 m at 40G, and all long-haul/FTTx feeder links.

MPO/MTP 12- and 24-fiber connectors: polarity types A (straight), B (reversed), C (crossed pairs); method B is the default for parallel optics (SR4).

APC vs UPC: APC for PON feeder and RFoG (low reflectance), UPC for data center patch; mixing causes >14 dB loss and damage.

Armored or direct-buried cable required where conduit is absent; rodent-resistant and LSZH jackets for indoor risers per local code.

Fiber count design rule: provision ≥25–50% spare strands per cable and ≥1 spare tube per closure for future growth.

Risk score matrix: probability (1–5) × impact (1–5); scores ≥15 require mitigation plan and owner; ≤4 accept and monitor.

Quality metrics for OSP: splice yield (≥98% acceptable), first-time-right certification rate, rework hours per km, and closure failure rate.

Contingency reserve is drawn only with project manager authority; management reserve requires sponsor approval.

Stakeholder engagement: permit authorities, pole owners, utility locates (call 811/one-call 2–3 business days ahead), and landowners are high-influence external stakeholders.

Safety and compliance: laser safety class 1/1M for telecom, confined-space entry permits for vaults, and arc-flash/grounding checks at huts.

Example: 10 km OS2 at 1550 nm = 10 × 0.25 = 2.5 dB fiber; 4 splices × 0.1 = 0.4 dB; 2 connector pairs × 0.5 = 1.0 dB; total 3.9 dB - compare to transceiver budget minus margin.

Design margin: reserve ≥3 dB for aging, repairs, and temperature; do not consume full transceiver budget.

Reflectance and return loss: return loss (dB) = −10·log10(reflected/incident); higher positive value = better (e.g., 55 dB return loss is better than 40 dB).

Fiber characterization: OTDR measures attenuation and event loss; OLTS measures end-to-end insertion loss and length; use both for acceptance.

Test wavelength must match operational wavelength: 850/1300 nm for multimode, 1310/1550 nm for single-mode.

Analogous estimating uses historical similar projects (fast, low accuracy); parametric uses unit rates (e.g., $/km, $/splice); bottom-up is most accurate and most costly.

Reserve analysis: contingency for known risks, management reserve for unknown; both excluded from EV until used.

Forecast EAC variants: EAC = AC + bottom-up ETC when original estimate is flawed; EAC = BAC/CPI when current variance persists.

Schedule baseline is approved version of the schedule; progress measured against it, changes via integrated change control.

Resource leveling changes start/finish dates (may extend critical path); resource smoothing uses float without changing critical path.

Communication plan defines who, what, when, how, and format; escalation path and reporting cadence per stakeholder group.

PON downstream uses 1490 nm, upstream 1310 nm, video overlay 1550 nm; keep 1310/1490/1550 budgets separate.

Splitter placement: centralized (one stage at CO) simplifies ops; distributed (cascaded) saves fiber but adds loss and reflection points.

Feeder vs distribution: feeder from OLT to splitter, distribution from splitter to ONT; design spare fibers per feeder and per distribution cable.

Maximum PON reach with Class B+ is ~20 km at 1:32; Class C+ extends to ~20 km at 1:64 or 60 km at lower split.

ONT receive sensitivity typically −27 to −8 dBm; verify optical power at ONT within vendor range at commissioning.

Project charter authorizes the project and names the sponsor; without it, no baselines or budget authority exist.

RACI vs. RASCI: RASCI adds Support role; each row has one Accountable.

Phase gate review criteria: deliverable acceptance, risk review, budget check, go/no-go decision recorded.

Regulatory: FCC/state right-of-way, NEPA/environmental review for new aerial/buried routes, and local franchise agreements for FTTx.

Contract types: fixed-price shifts cost risk to seller; T&M shifts to buyer; use fixed-price for well-defined splicing scopes.

Tier 1 test = OLTS insertion loss + length; Tier 2 = OLTS + OTDR trace; specify Tier 2 for backbone and high-count links.

Acceptance criteria per TIA-568: multimode 850 nm max 2.6 dB for 10GBASE-SR at 300 m OM3; verify against link budget table for actual length.

OTDR trace events: reflective (connectors, mechanical splices) vs. non-reflective (fusion splices, bends); ghost reflections appear beyond end of fiber.

Bidirectional OTDR averaging: splice loss = (L→R + R→L)/2; report gainers separately.

Documentation: test reports, fiber assignments, port maps, and as-built drawings handed over at acceptance.