FOA CFOS/DC cheat sheet

Link loss budget: total budget (dB) = transmitter output (dBm) − receiver sensitivity (dBm); usable budget = total budget − safety margin (typically 3 dB for connectors/repairs) − connector losses (0.3–0.75 dB mated pair) − splice losses (0.1–0.3 dB each) − fiber attenuation (dB/km × length).

Attenuation reference values at 1310/1550 nm: OS2 single-mode ≈ 0.35 dB/km at 1310 nm and 0.20–0.25 dB/km at 1550 nm; OM3/OM4 multimode ≈ 3.0–3.5 dB/km at 850 nm and ≈ 1.0–1.5 dB/km at 1300 nm.

Modal bandwidth rule: OM3 ≈ 2000 MHz·km, OM4 ≈ 4700 MHz·km at 850 nm; OM5 adds wideband short-wavelength division multiplexing (SWDM) capability. Reach is set by the lesser of loss-limited and bandwidth-limited distance.

Fiber/transceiver pairing rule: 10GBASE-SR uses OM3/OM4 at 850 nm; 10GBASE-LR uses OS2 at 1310 nm; 100GBASE-SR4 uses 8-fiber MPO with OM4; 100GBASE-LR4 uses OS2 with 4-wavelength LAN-WDM around 1310 nm.

Polarity decision rule: duplex LC links use A-to-B (straight) patch cords; MPO trunk method A is straight (key-up to key-down), method B is flipped (key-up to key-up), method C is paired-flipped. Polarity type must be consistent across the entire channel.

Topology selection: centralized (star from one cross-connect) minimizes active equipment and eases administration; zone/distributed cabling reduces cabling volume and cost in high-density areas; a mix is chosen by port density, distance, and moves-adds-changes (MAC) rate.

Distance planning inputs: horizontal permanent link ≤ 90 m plus ≤ 10 m of patch cords (100 m channel); backbone distances are limited by the chosen application and by loss/bandwidth budgets, not by a single fixed rule.

Redundancy and route diversity: protected rings or dual-homed paths need physically separate routes (different conduit/duct), and failure analysis must assume a single cut; span loss and OSNR are checked for the worst-case path, not the normal path.

Work breakdown structure (WBS) decomposes scope into deliverable-oriented packages; every WBS element must map to an owner, a duration estimate, and an acceptance criterion before scheduling.

Critical path method: the critical path is the longest-duration path through the network; total float = late start − early start. Any activity with zero float is critical and drives the completion date.

Cost and schedule baselines are approved versions of the plan; changes after baseline approval go through formal change control (request, impact assessment, approval, rebaselined documents).

Earned value formulas: CV = EV − AC; SV = EV − PV; CPI = EV/AC; SPI = EV/PV. CPI or SPI below 1.0 signals cost or schedule overrun against the baseline.

Risk register content: risk description, probability, impact, risk score (probability × impact), owner, response strategy (avoid, mitigate, transfer, accept), and trigger.

Procurement lead time rule: long-lead items (cable reels, MPO trunks, cabinets, transceivers) are ordered from the approved design and released schedule; delivery dates are tied to installation milestones in the schedule.

Acceptance testing gates: visual inspection, continuity/polarity verification, insertion loss (OLTS) and OTDR testing, then documentation handover; each gate has pass/fail criteria defined before work starts.

Stakeholder and communications planning: identify roles (owner, designer, installer, tester, IT operations), define reporting cadence and escalation path, and record decisions in a decision log to prevent scope creep.