Passive optical networks (PON) technology was available in the middle of the 90s. Since the huge development of the network, various standards have been established and matured. PON developed from the first ATM PON (APON) and then evolved in Broadband PON (BPON) which is compatible with APON. Later, arisen Ethernet PON (EPON) and Gigabit PON (GPON) bring great improvement in data transmission distance and bandwidth. This tutorial will introduce GPON technology.
GPON Components
GPON is defined by ITU-T recommendation series G.984. GPON represents an increase in bandwidth compared with APON and BPON. GPON can be applied in many areas. In fiber to the desktop (FTTD) application, GPON is distributed via single-mode, simplex optical fiber connectors, and passive optical splitter typically using angled polish connectors (APC) to provide precision terminations. There are four main components in this GPON system: the optical line terminal (OLT), the transmitting media (cabling and components), the fiber optical splitter, and the optical network terminal (ONT).
OLT
OLT is a device that serves as the service provider endpoint of a passive optical network. It is an active optical aggregation device that is usually located in a data center or the main equipment room. An OLT converts the optical signals transmitting over fiber to the electrical signals and presents them to a core switch. OLT distributing signal is connected with backbone cabling or horizontal cabling through optical splitters, which are connected to the optical network terminal at each work area outlet.
Fiber Optic Splitter
Fiber optic splitter, also known as a beam splitter, is an integrated waveguide optical power distribution device. With this fiber optic splitter, multiple devices can be served from a single fiber. It’s one of the most important passive devices in the optical fiber network. It’s especially useful in GPON, EPON FTTx, etc. PON typically connects a single fiber from an OLT to multiple ONUs. The connectivity between OLT and ONUs is achieved by using fiber optical splitters. The number of the outputs in the splitter determines the number of splits. The split ratios often contain 1:4, 1:8, 1:16, 1:32 and 1:64. The insertion loss of a typical 1×32 optical splitter ranges from 17 dB to 18 dB. Fiber optic splitter includes fused bi-conical taper (FBT) splitter and planar lightwave circuit (PLC) splitters.
The two-layer splitter can be used from OLT to the user end. Generally, a device where the first layer of the splitter is used is also called FDC (Fiber Distribution Center), and the second layer of the splitter is called ONT or FAB (Fiber Access Box).
Transmitting Media
GPON transmits signals through the passive, physical cabling infrastructure. The transmitting media include copper, fiber optic patch cords, enclosures, adapter panels, connectors, splitters, and other materials. All these transmitting media components should be factored in the channel loss budget to get a better system performance.
Optical Fiber:
- Standard ITU-T L.110 Cable Type: Corning Glass; Metallic Armored Cable; continuous welded stainless tube overlaid by wired armor Installation Environment: Direct buried, under water and moisture proof Temperature: -10 deg C to +40 deg C Typical Outside Diameter: 11.0 mm Allowable Tensile Strength: >3,000 N Crush Resistance: 5000 N/100 mm (testing must show no optical loss upto 1500 kg/100 mm) Allowable Bending Radius: 110 mm (during installation); 220 mm (after installation)
- Type I Cable: For aggregation link between backbone network to aggregation node and access link between aggregation node and access node.
- This type of cable could be 12/24 core type
- Cable laying can be ADSS (All Dielectric Self Support) type, Underground (Directly Buried) or Aerial.
- Type II Cable: For link between access node to client (End Device). This could be Ethernet cable or optical cable (6 Core).
ONT/FAB
ONT also called the modem, connects to the termination point (TP) with an optical fiber cable, and connects to your router via a LAN / Ethernet cable. It converts the optical signals to electrical signals to deliver to the end device. ONT always has multiple Ethernet ports for connection to IP services such as CPUs, phones, wireless access points, and other video components.
GPON Loss Budget
PON is typically composed of OLT and ONUs and other optical transmission media such as fiber cables and connectors which have been pointed out before. Link loss can be caused by these components (cable, connectors, patch cords, splices, couplers, and splitters). Link loss is very important in designing optical access network. The link budget is shown in the following table. This budget covers all optical components between OLT and ONU.
The table below shows the Loss budget for the GPON system
| Bandwidth (nm) | Items | Path Loss (dB) | The number of Splitters that can be used |
| 1310 | Minimum optical loss | 13 | 1 |
| Maximum optical loss | 28 | 2 | |
| 1490 | Minimum optical loss | 13 | 1 |
| Maximum optical loss | 28 | 2 |