A Passive Optical Network (PON) is a fiber-to-the-premises access technology that uses unpowered optical splitters to share a single fiber from a central office among many subscribers, reducing outside-plant cost and complexity.

Key components:

• Optical Line Terminal (OLT): provider’s endpoint in the central office.
• Optical Distribution Network (ODN): fibers, passive splitters (e.g., 1:16, 1:32, 1:64), connectors.
• Optical Network Unit/Terminal (ONU/ONT): customer endpoint.

How it works:

• Downstream: The OLT broadcasts data to all ONTs over a downstream wavelength (commonly 1490 nm for data; 1550 nm for RF video in legacy systems). Each ONT filters and decrypts only its own traffic (e.g., AES encryption).
• Upstream: ONTs transmit on a separate wavelength (commonly 1310 nm) using time-division multiple access (TDMA). The OLT assigns non-overlapping time slots via ranging and dynamic bandwidth allocation (DBA) to prevent collisions and adapt to traffic demand.
• Wavelength-division multiplexing (WDM) combines/separates the upstream and downstream wavelengths on the same fiber.
• The OLT manages framing and control (e.g., GEM in GPON, MPCP in EPON), timing, and error correction (FEC).

Standards and capacities:

• EPON (IEEE 802.3ah/av): 1G/10G symmetric.
• GPON (ITU-T G.984): ~2.5G downstream/1.25G upstream.
• XG-PON (G.987): 10G/2.5G; XGS-PON (G.9807.1): 10G/10G.
• 10G-EPON (802.3av): 10G rates.

Typical reach is 10–20 km with total optical loss determined by fiber length, split ratio, and components (the “power budget”).

Benefits: low outside-plant power/maintenance, high capacity, easy upgrades via wavelength coexistence. Trade-offs: shared bandwidth, splitter-induced loss, careful budgeting and security required.

• Standards/Framing:

  • GPON: ITU-T G.984, TDM with GEM encapsulation (Ethernet/IP mapped).
  • EPON: IEEE 802.3ah, native Ethernet framing, MPCP for control.
  • XG-PON: ITU-T G.987 (10G-PON), GPON evolution.
  • XGS-PON: ITU-T G.9807.1, 10G symmetric GPON evolution.

• Line rates (nominal):

  • GPON: 2.488 Gbit/s downstream, 1.244 Gbit/s upstream (asymmetric).
  • EPON: 1.25/1.25 Gbit/s line rate (≈1/1 Gbit/s payload, symmetric).
  • XG-PON: 10 Gbit/s downstream, 2.5 Gbit/s upstream (asymmetric).
  • XGS-PON: 10/10 Gbit/s (symmetric).

• Wavelengths/coexistence:

  • GPON: 1490 nm DS, 1310 nm US, optional 1550 nm RF video.
  • EPON (1G): 1490 nm DS, 1310 nm US.
  • XG-/XGS-PON: 1577 nm DS, 1270 nm US; designed to coexist with GPON on same ODN.

• QoS/DBA:

  • GPON/XG/XGS: T-CONTs, strict DBA and service tiers, strong multi-service QoS.
  • EPON: DBA via MPCP; QoS using Ethernet VLAN/802.1p; simpler L2 integration.

• Split ratio/reach:

  • GPON/EPON: typically up to 1:64 (1:128 feasible), ~20 km reach.
  • XG-/XGS-PON: higher optical budgets, up to 1:128–1:256, ~20–40 km variants.

• Efficiency/overhead:

  • GPON: 125 μs frames, GEM overhead; efficient for mixed TDM/IP.
  • EPON: minimal L2 overhead for Ethernet; easier, cheaper aggregation.
  • XG/XGS: improved coding/FEC vs GPON; higher throughput.

• Security:

  • GPON/XG/XGS: AES-128 per-ONT.
  • EPON: privacy/encryption options (AES in 10G-EPON; 1G commonly churning/AES implementations vary).

• Interoperability/ecosystem:

  • EPON: strong multi-vendor interop (IEEE), popular with cable/MSOs, Asia.
  • GPON/XG/XGS: dominant in telcos; interop improving but often vendor-profile specific.

• Use cases:

  • GPON/EPON: legacy 1G-era broadband.
  • XG-PON: higher DS for mass broadband.
  • XGS-PON: symmetric 10G for business, 5G fronthaul, premium residential.

• GPON (G.984): 2.488 Gbit/s downstream, 1.244 Gbit/s upstream; common split 1:32–1:64 (up to 1:128 with high budget); typical reach 10–20 km (max spec 60 km with constraints).
• EPON (802.3ah/1G-EPON): 1.25/1.25 Gbit/s; splits 1:16–1:32 (up to 1:64); reach ~10–20 km (max 20 km).
• 10G-EPON (802.3av): 10/1 Gbit/s (asymmetric) or 10/10 Gbit/s (symmetric); splits 1:32–1:64 (up to 1:128 with optics); reach 10–20 km (up to 30–40 km with high-budget optics).
• XG-PON (XG-PON1, G.987): 10 Gbit/s downstream, 2.5 Gbit/s upstream; splits 1:32–1:64 (1:128 feasible); reach 10–20 km (max spec 40 km).
• XGS-PON (G.9807.1): 10/10 Gbit/s; splits 1:32–1:64 (1:128 in high-power classes); reach 10–20 km (up to 40 km).
• NG-PON2 (TWDM-PON, G.989): 4–8 wavelengths at 10/10 each; per-wavelength 10/10 Gbit/s (aggregate 40–80 Gbit/s); splits 1:32–1:64; reach 10–20 km (up to 40 km); more costly optics.
• 25G PON (25GS-PON MSA/G.9804-series): 25/10 or 25/25 Gbit/s; splits 1:32–1:64 (1:128 possible with N2 optics); reach 10–20 km (up to 30–40 km).
• 50G PON (G.9804): 50/12.5 or 50/50 Gbit/s profiles; splits 1:32–1:64 (1:128 with higher budgets); reach 10–20 km (targets up to ~40 km).

Typical design trade-offs:

• Higher split ratios reduce reach and margin; extended reach typically requires lower splits or higher optical budgets (e.g., B+/C+/N1/N2 classes).
• Practical deployments often target ~28–31 dB budgets, ≤20 km fiber length, differential reach ≤20 km, and 1:32 or 1:64 split to balance cost and performance.

• OLT (Optical Line Terminal)

  • Located at the provider’s central office.
  • Aggregates subscriber traffic to the core network; manages PON trees.
  • Performs ranging, registration, time-division multiplexing, and Dynamic Bandwidth Allocation (DBA).
  • Enforces QoS, encryption (e.g., AES on GPON GEM frames), and service policies.
  • Hosts optics for downstream/upstream wavelengths (e.g., GPON 1490/1310 nm; XG(S)-PON 1577/1270 nm).
  • Interfaces to aggregation/edge routers via Ethernet/MPLS.

• ONT/ONU (Optical Network Terminal/Unit)

  • Located at customer premises (ONT) or as a remote unit feeding multiple users (ONU).
  • Terminates the PON, converts optical to user interfaces (Ethernet, POTS, Wi‑Fi).
  • Authenticates and registers with OLT; supports QoS queues/T‑CONT types and encryption.
  • May integrate routing, NAT, VoIP, and management (TR‑069/OMCI).

• Passive Optical Splitters

  • Unpowered devices that divide one optical signal into many (1:2 to 1:64/1:128).
  • Types: PLC (planar lightwave circuit) and FBT; PLC preferred for high split ratios/flat spectral response.
  • Introduce insertion loss that drives power budget and reach planning.
  • Support tree or cascaded split topologies.

• Fiber and ODN (Optical Distribution Network)

  • Feeder fiber from OLT to splitter(s); distribution fiber from splitters to drops; drop fiber to ONT.
  • Includes cables, splices, connectors, enclosures, attenuators.
  • Determines reach and reliability; designed to meet optical power budgets and wavelength plans.
  • Carries downstream broadcast and upstream TDMA traffic on separate wavelengths; optional 1550 nm overlay for RF video (GPON).

Performance:

• Topology: PON is shared (TDMA), point-to-point/active Ethernet is dedicated per user. PON contends for bandwidth; AE doesn’t.
• Throughput: GPON ~2.5/1.25 Gbit/s shared; XGS-PON 10/10; 25G-PON up to 25 G. Active Ethernet delivers 1/10/25/100 G per subscriber line rate.
• Latency/Jitter: PON adds DBA scheduling and ranging (typically sub‑millisecond to a few ms under load); AE is consistently low (tens–hundreds of microseconds).
• Symmetry: PON often asymmetric except XGS/25G-PON; AE trivially symmetric.
• Oversubscription/QoS: PON relies on DBA and profiles; bursty traffic can affect neighbors. AE isolates users, simpler QoS with predictable performance.
• Reach: PON ~20 km typical (long-reach variants exist); AE depends on optics (10–80 km with LR/ZR).
• Reliability: PON has splitter/feeder single‑point risks; protection (Type B/C) adds cost. AE failures are per-link; more fibers/switch ports to manage.
• OAM/SLA: AE better for strict SLAs and low latency apps; PON fine for broadband/SMB, acceptable for many enterprise cases with XGS-PON.

Cost:

• Outside plant: PON uses one feeder and passive splitters (low CapEx, no field power), minimizing fiber counts. AE requires one fiber per subscriber and active aggregation ports, raising fiber and port costs.
• Central office/electronics: PON OLT ports serve 32–128 ONTs, lowering per‑subscriber port cost. AE needs a switch port per user; optics and power per port increase CapEx/OpEx.
• Power/space: PON significantly lower (no remote power); AE higher due to active gear.
• Upgrades: PON can coexist via WDM, incremental OLT/ONT swaps; AE upgrades are per-link optics/switch refresh.

Bottom line: PON is usually far cheaper per subscriber and power-efficient but offers shared, higher-latency, less predictable performance; active Ethernet/point-to-point costs more but delivers dedicated, deterministic bandwidth and lower latency.

PONs (GPON/XG(S)-PON/EPON) are inherently secure and reliable, but with nuances.

Security

• Downstream is broadcast; confidentiality via AES-128 encryption per GEM port (GPON/XG(S)-PON). Keys are provisioned by OLT; ONUs decrypt only authorized traffic.
• Upstream uses TDMA per ONU timeslot; privacy is inherent (other ONUs can’t hear), though the OLT can see all.
• ONU authentication via serial number/LOID and optional 802.1X; access control via whitelist and binding to service profiles.
• Management: OMCI (GPON) and OAM (EPON) must be access-controlled; vendor features may add integrity, anti-replay, and secure provisioning. Harden OLT/ONU firmware and disable unused services.
• Threats: physical tapping of fiber, rogue/jamming ONUs (upstream laser on outside timeslot), misconfigurations. Mitigations: rogue-ONU detection/shutdown, transmit-power leveling, optical link monitoring, encryption, port isolation, storm control, DHCP/ARP protection at ONT.

Reliability

• Passive outside plant has high MTBF; no power in splitters. Forward Error Correction (Reed–Solomon) and robust power budgets aid BER.
• Protection options: Type B (dual feeder or dual OLT interface), Type C (dual OLT and dual distribution), rapid protection switching (<50 ms possible). Redundant OLTs, splitter redundancy, ringed feeder routes.
• ONT/ONU battery backup for power outages; optical link monitoring for predictive maintenance.

QoS mechanisms

• GPON: T-CONT types and DBA
  • Type 1: fixed bit rate (voice, TDM)
  • Type 2: assured
  • Type 3: assured + non-assured
  • Type 4: best-effort
  • Type 5: composite
  • DBA (status-reporting/non-reporting) allocates upstream grants. GEM ports map services to T-CONTs.
• EPON: MPCP with GATE/REPORT and LLIDs; OLT DBA assigns timeslots per service flow.
• Scheduling: strict priority + WRR/WFQ, hierarchical QoS; CIR/EIR, shaping/policing; VLAN PCP/DSCP mapping; per-subscriber and per-service queues; latency/jitter guarantees for real-time traffic; IGMP snooping/proxy for multicast control.

• Installation (CAPEX)

  • Network design: surveys, GIS, permits, make-ready (aerial) or locates (underground). Make-ready can be $10k–$50k per mile; pole attachment $7–$30/pole/year.
  • Construction: aerial $25k–$60k/mile; underground $60k–$200k/mile (urban higher). Cost per home passed: aerial $700–$1,500; underground $1,200–$3,000.
  • Central office: OLT chassis $20k–$100k; PON line cards $1k–$3k per port (GPON), higher for XGS-PON; aggregation/backhaul upgrades as needed.
  • ODN/optics: fiber, closures, cabinets, splitters ($20–$100), connectors; splicing/testing (OTDR, IL/RL).
  • Drop/CPE: drop fiber and install $150–$400/home; ONT/ONU $60–$150; gateway/Wi‑Fi $80–$200. Cost per home connected typically $300–$800 incremental.
  • Key design: split ratio (1:16–1:64), power budget, reach, coexistence filters if planning upgrades, aerial vs underground mix, density/take‑rate forecasts.

• Maintenance (OPEX)

  • Routine: cleaning connectors, OTDR monitoring, fiber asset records, inventory/spares, software/firmware.
  • Faults: fiber cuts, ONT failures, weather/rodents; truck roll $100–$300.
  • Ongoing costs: pole rent, locates, NOC monitoring, SLA handling. OPEX often $2–$6/sub/month (scale dependent).
  • Consider: redundancy (ring/backhaul), emergency restoration kits, training, safety, documented procedures.

• Upgrades

  • GPON to XGS‑PON: overlay via coexistence elements; reuse ODN and drops; replace/add OLT line cards/optics and ONTs for upgraded tiers.
  • Costs: XGS‑PON OLT ports/optics higher than GPON; ONT swap $80–$200 per subscriber; minimal field work if power budget/split ratios were planned.
  • Plan for: wavelength coexistence (video RFoG if present), split-ratio right‑sizing, power budget margin, central-office space/power, backhaul/core scaling, IPv6/QoS.
  • Vendor/interop: standard compliance (ITU‑T G.984/G.9807.1/G.989), avoid lock‑in, certify ONTs.