FOA CFOT cheat sheet

Total internal reflection occurs when light traveling in the core strikes the core-cladding boundary above the critical angle; the critical angle is set by the refractive index difference between core and cladding.

Multimode fiber core diameters are typically 50 or 62.5 µm with 125 µm cladding; single-mode fiber core is about 8–10 µm with 125 µm cladding and a much smaller numerical aperture.

Attenuation is wavelength dependent: 850 nm is highest, 1310 nm is low and has zero material dispersion, and 1550 nm has the lowest loss (~0.2 dB/km for single-mode).

Dispersion has two parts: modal dispersion (multimode only, reduced by graded-index profiles) and chromatic dispersion (all fibers, minimized near 1310 nm).

Two common connector families: 2.5 mm ferrule (SC, FC, ST) and 1.25 mm ferrule (LC, MU); LC is the standard choice for high-density datacom and FTTH equipment ports.

Splices are fusion (lowest loss, ~0.05–0.1 dB) or mechanical (~0.1–0.5 dB); connectors typically add 0.3–0.75 dB per mated pair depending on polish and grade.

WDM multiplexes separate wavelengths on one fiber: CWDM uses 20 nm spacing across 1270–1610 nm, DWDM uses much tighter spacing for high-capacity transport.

Applications map to fiber type: 850 nm multimode for short-reach data center links, 1310/1550 nm single-mode for metro/long haul, and single-mode for FTTH PON networks.

Structured cabling hierarchy: entrance facility, equipment room, backbone cabling, telecommunications room, horizontal cabling, work area; horizontal distance limit is 90 m permanent link plus 10 m of patch cords for 100 m channel.

Fiber is a recognized horizontal medium (ISO/IEC and TIA allow it), and it is preferred where distance, EMI, or bandwidth exceed copper capability.

Backbone fiber counts must include spare capacity; typical practice is at least 25–50% spare strands and separate counts for voice/data and riser vs. campus runs.

Indoor cable ratings: OFNR/riser for vertical runs between floors, OFNP/plenum for air-handling spaces; outdoor cable must transition to indoor-rated cable within 15 m (50 ft) of entry.

Bend radius rules: at least 10× cable OD during installation and 10× (or manufacturer spec, often 15×) after installation for tight-buffered cable; never violate minimum bend radius when dressing in trays.

Connector color codes: UPC (blue) and APC (green, angled 8° with lower reflectance) must never be mated to each other; multimode is typically beige (OM1/OM2), aqua (OM3/OM4), lime (OM5).

Horizontal pathway separation from power: keep fiber away from unshielded power lines unless in a grounded metallic raceway; fiber is immune to EMI but cable jackets and electronics are not.

Label both ends of every strand and record polarity and port assignments; document link loss budgets and test results as part of the as-built record.

OSP cable types: loose-tube (gel-filled or dry) for aerial/duct/direct-buried, armored for rodent or direct-burial protection, and ADSS (all-dielectric self-supporting) for aerial spans without a messenger.

Minimum burial depth is governed by local code and soil/road class; typical values are 0.6–1.2 m (24–48 in), greater under roadways, with warning tape and tracer wire above the duct.

Aerial spans require strand or ADSS with sag calculated from span length, ice/wind loading, and NESC clearances; fiber must not carry mechanical load through the connectors.

Duct and conduit design uses fill ratio limits (commonly ≤40–50% for multi-cable pulls) and innerduct to protect cable and allow future pulls.

Fusion splicing in OSP is done in splice closures (inline, butt, or dome) with gel or heat-shrink sealing; closures must be re-enterable and rated for the environment.

OTDR testing from both directions is required to characterize splices and locate faults; bidirectional averaging removes the effect of backscatter coefficient mismatch at splices.

Locate and mark before digging; fiber is not detectable by metal locators unless a tracer wire or metallic member is present, so records and potholing are essential.

Acceptance testing after construction includes continuity, insertion loss, OTDR traces, and often PMD/CD or polarization tests for long-haul routes.

Power budget = transmitter output power − receiver sensitivity; subtract all connector, splice, and fiber losses plus a margin (commonly 3 dB for repairs and aging) to confirm the link closes.

Loss budget formula: total loss = (N connectors × connector loss) + (M splices × splice loss) + (fiber length × attenuation at the operating wavelength).

Choose fiber type by reach and speed: OM4 supports 10GBASE-SR to ~400 m and 40/100G to ~150 m; single-mode with 1310/1550 nm optics covers kilometers to tens of kilometers.

PON architecture: OLT at the central office, passive splitters (1:32 or 1:64 typical), ONT at the subscriber; the split ratio directly reduces the optical budget available per subscriber.

Redundancy options: ring topologies with protection switching, dual-homed paths, and diverse routing so a single cut does not isolate a site.

Connector and splice counts should be minimized; each mated connector pair costs ~0.3–0.75 dB, so long links use splices instead of connectors where possible.

Wavelength planning: separate transmit and receive wavelengths (e.g., 1310 nm upstream / 1490 nm downstream in GPON, with 1550 nm for RF video) must be allocated to avoid interference.

Document the design with a link-loss spreadsheet per span and verify against the standard's maximum channel loss (e.g., 10GBASE-LR allows 6.2 dB over 10 km).

Reference test method matters: one-jumper (1-cord) reference gives lower measured loss than two-jumper or three-jumper methods; always state the method used and keep the reference cords clean and matched.

OLTS (light source + power meter) measures end-to-end insertion loss at the link wavelength; test at 850/1300 nm for multimode and 1310/1550 nm for single-mode as specified.

OTDR measures distance vs. loss, locates events (splices, connectors, bends, breaks), and reports reflectance; use a launch cable (and receive cable) longer than the dead zone to see the first/last events.

OTDR dead zones: event dead zone (shortest resolvable spacing between reflective events) and attenuation dead zone (recovery after a reflective event); shorter pulse widths improve resolution but reduce range.

Set OTDR IOR (index of refraction) and backscatter/helix factors from the cable manufacturer; wrong IOR shifts all reported distances.

Visual fault locator (VFL) is a red laser used to find breaks, tight bends, and bad connectors near the tester; it does not measure loss quantitatively.

Inspection and cleaning: use a fiber inspection scope; a single dirty endface can add several dB and damage mating ferrules. Always inspect, clean, and re-inspect before mating.

Pass/fail criteria come from the standard (e.g., TIA-568 loss limits) and the design budget; record both measured values and the calculated limit for each link.

FTTH topologies: point-to-point (home run) and PON (point-to-multipoint with passive splitters); PON is the dominant architecture for residential broadband.

GPON: 2.488 Gbps downstream / 1.244 Gbps upstream, 1490 nm down / 1310 nm up, with 1550 nm reserved for video overlay; split ratios commonly 1:32 or 1:64.

XGS-PON: symmetric 10 Gbps, 1270 nm upstream / 1577 nm downstream; can coexist with GPON on the same fiber using WDM.

Optical distribution network (ODN) loss budget must account for splitters (a 1:32 splitter adds ~16–17 dB), connectors, splices, and fiber; the class of the PON optics (e.g., Class B+) sets the allowed loss.

ONT placement options: fiber to the home (ONT indoors), fiber to the building (ONT in a telecom room), fiber to the curb (ONT in a pedestal); the choice affects powering and subscriber electronics.

Drop cable is typically 1–2 fiber, often flat or figure-8, installed from the splitter/pedestal to the ONT; protect it from bend radius violations and UV exposure.

Service activation requires measuring optical power at the ONT (e.g., −8 to −27 dBm receive window for GPON) and verifying link status before provisioning.

Regulatory and construction considerations: right-of-way permits, pole attachment agreements, and make-ready work drive FTTH deployment schedules and cost.