2016年12月27日星期二

Science and Technology of GPON



Passive optical networks (PON) technology was available in the middle of 90s. Since the huge development of 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 about 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 which serves as the service provider endpoint of a passive optical network. It is an active Ethernet 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 Ethernet switch. The OLT replaces multiple layer 2 switches at distribution points. 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.



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.

Fiber Optic Splitter

Fiber optic splitter, also known as 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 and 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 the splits. The split ratios often contain 1:4, 1:8, 1:16, 1:32 and 1:64. The insertion loss of a typical 1x32 optical splitter ranges from17 dB to 18 dB. Fiber optic splitter includes fused biconical taper (FBT) splitter and planar lightwave circuit (PLC) splitters.

ONT

ONT, also called the modem, connects to the termination point (TP) with an optical fiber cable, and connects to your router via an 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 as the following table. This budget covers all optical components between OLT and ONU.

GPON Power Budget

The transmitter’s power and receiver’s sensitivity are two parameters that influence the reach of the access network. How to calculate the power budget? The formula is “P=FCA*L+SL+Penalties”. P represents power budget. FCA is fiber cable cable attenuation in dB/m. L is the distance and SL is a splitter loss. Penalties stands for additional loss such as the splice and connectors. The following table shows the required power budget for different GPON configurations.

Table2. The minimum power budget for different GPON configurations

Now let’s calculate the reach of a network system. Suppose that the power budget is about 23 dB. A single-mode fiber cable operating at the wavelength of 1550 nm is used. SL is 14 dB and there are two mechanical splices (0.5 dB/per splice) and two connectors (0.5 dB/per connector). So the maximum reach of the network can be calculated as (23-14-2*0.5-2*0.5)/0.3≈23km.

Conclusion
GPON is the most complex of all PONs. But it’s the best one of all PONs. GPON has the benefits of saving costs for moves and adds or other changes, low price per port on passive components, easy installation and low installation costs. So GPON gains the popularity in today’s diverse and ever-changing technology applications.

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