FOA CFOS/W cheat sheet

FTTH topologies: point-to-point (home run, one fiber per subscriber, dedicated port) vs point-to-multipoint PON (feeder fiber shared by a splitter, 1:2 to 1:128).

PON power budget: link loss (dB) = sum of fiber attenuation (dB/km × km) + connector losses + splice losses + splitter insertion loss; must stay under the class budget (B+ 28 dB, C+ 32 dB per ITU-T G.984/G.987).

Splitter loss rule of thumb: 1:2 = 3.5 dB, 1:4 = 7.2 dB, 1:8 = 10.5 dB, 1:16 = 13.7 dB, 1:32 = 16.6 dB, 1:64 = 20.5 dB, 1:128 = 23.5 dB (excess loss included).

GPON downstream 2.488 Gbit/s, upstream 1.244 Gbit/s, 1490 nm down / 1310 nm up; XGS-PON is symmetric 9.953 Gbit/s with 1577 nm down / 1270 nm up.

Wavelength plan: 1310 nm upstream, 1490 nm downstream, 1550 nm RF video overlay; WDM coexistence filters separate legacy and next-generation PONs on the same feeder.

Fiber-to-the-premises variants: FTTH (fiber to living unit), FTTB (basement/building, then copper or LAN inside), FTTC/FTTN (cabinet/node, VDSL2 or G.fast over copper tail).

ODN is passive between OLT and ONT: feeder cable (OLT to splitter), distribution cable (splitter to access point), drop cable (access point to subscriber).

Downstream PON is broadcast/TDM with AES encryption per ONT; upstream is TDMA with burst scheduling by the OLT using ranging and equalization delay.

OLT: terminates PON ports, aggregates uplinks (10/100 Gbit/s), performs DBA, ranging, encryption and management (OMCI for ONTs).

ONT/ONU: subscriber-side CPE terminating the PON; SFU (single family unit) vs MDU/MTU multi-dwelling variants; provides Ethernet, VoIP POTS, RF video, Wi-Fi.

Optical splitter types: fused biconical taper (FBT, 1:2 to 1:4, wavelength-sensitive) vs planar lightwave circuit (PLC, 1:2 to 1:64, broadband, uniform).

ODF and fiber distribution frame: cross-connect point for patch cords, splice trays and pigtails; OLT ports are patched here to feeder cables.

Fiber access terminals (FAT), street cabinets, and aerial/underground closures house splices and splitter modules; drop terminals serve 4-16 subscribers.

Connector types: SC/APC (green, 8° angled, return loss ≥60 dB) is standard in FTTH; SC/UPC (blue, ≥50 dB) and LC for high-density; APC and UPC must never be mated.

Drop cable: 2.0×3.0 mm flat or 5 mm round, 1-2 fibers, often with steel strength members; indoor riser uses LSZH jacket; outdoor aerial uses self-supporting figure-8.

Power and space: OLT rack needs -48 VDC or AC, cooling, and grounding; cabinets need environmental sealing (IP54+) and lightning/ground protection.

Bend radius rule: ≥20× cable outer diameter during installation, ≥10× in service; for 250 µm bare fiber ≥30 mm, for 900 µm tight buffer ≥15 mm.

Cable pulling tension: typically ≤ 600 N for outdoor drop and ≤ 270 N for indoor drop; never exceed manufacturer rating and never pull by the fiber.

Duct capacity and fill: no more than 40-50% conduit fill; innerduct subducts allow later cable addition without re-excavation.

Aerial construction: minimum sag per span tables, clearance above road/rail/power per local code, lashing or ADSS self-support, and separate messenger strand.

Splicing: fusion splice typical loss ≤0.1 dB; mechanical splice ≤0.3-0.5 dB; splice tray stores 12-24 splices with bend radius control and fiber routing.

Connectorization: factory pigtail splicing vs field-installable connectors; field polish/cleave quality drives return loss and insertion loss (≤0.3 dB typical, ≤0.5 dB max).

Safety: laser class 1M/3R at 1310/1490/1550 nm - never look into fiber or connector; use IR viewer/card, cap unused ports, and treat fiber shards as hazardous waste.

Documentation and labeling: every cable, port, splitter leg and closure gets a unique ID; as-built records and OTDR traces are handed over at acceptance.

Insertion loss test: light source + power meter (OLTS) at 1310/1490/1550 nm, one reference per method (1-jumper, 2-jumper, 3-jumper); record both directions.

OTDR: measures distance, splice/connector loss, reflectance and total link loss; set IOR, pulse width, range and averaging correctly; use launch and receive reels for end connectors.

Acceptance thresholds (typical): connector ≤0.5 dB, fusion splice ≤0.1 dB (≤0.15 dB field), link loss within PON class budget with ≥3 dB margin for aging/repairs.

Reflectance limits: APC ≤ -60 dB, UPC ≤ -50 dB; high reflectance causes upstream burst errors and ranging instability.

Continuity and polarity: verify fiber mapping with VFL (visible red) for shorts/bends and with OLTS for correct A-B/B-A pairing; document polarity per port.

Common faults: dirty or scratched endface (inspect and clean every connector, IEC 61300-3-35 pass/fail), macrobend (loss at 1625/1650 nm), bad splice, splitter port swap, wrong patch.

Power meter readings: dBm is absolute, dB is relative; loss (dB) = P_in(dBm) - P_out(dBm); splitter ratio check via expected loss table.

Certification package: OLTS results per wavelength, OTDR bidirectional traces, endface inspection records, labeling and as-built drawings signed off at PON acceptance.

Troubleshooting order: physical layer first (power, LED, fiber continuity, endface), then link layer (ONT registration, PLOAM, ranging), then service layer (VLAN, IP, PPPoE/DHCP).

ONT status LEDs: PON link (registration with OLT), LOS (loss of signal), Auth/Fail (OMCI provisioning), LAN, and VoIP; LOS blinking with PON off usually means fiber break or dirty connector.

Ranging and equalization: ONT reports Ranging Time; OLT assigns Equalization Delay so all upstream bursts align - a wrong value causes upstream collisions.

Optical power window at ONT: typically -8 to -27 dBm for GPON class B+; below sensitivity causes LOS, above overload (> -8 dBm) saturates the receiver.

Distance/attenuation check: OTDR event distance × IOR (1.468 for SMF) yields physical length; compare with as-built to locate faults.

Escalation matrix: subscriber CPE → drop/terminal → distribution/splitter → feeder/OLT; isolate by swapping patch cords and testing at the FAT.

Common service faults: wrong VLAN or service profile, ONT not provisioned, duplicate serial number, splitter leg swapped, and RF video overlay issues (1550 nm).

Post-repair verification: repeat OLTS/OTDR, confirm ONT online and throughput test (iPerf/speed test), update records, and close the ticket with evidence.