What is GPON?

PON is a point-to-point multipoint (P2MP) active optical system. GPON refers to Gigabit Passive Optical Networks. GPON is defined by the ITU-T Recommendation G.984.x. GPON can be used to transport not only Ethernet however, but as well ATM as well as TDM (PSTN, ISDN, E1 and E3) traffic. GPON network is composed of two active transmission devices that are known as Optical Line Termination (OLT) and Optical Network Unit (ONU) or Optical Network Termination (ONT). GPON allows triple-play services as well as high-bandwidth, long range (up at 20km) as well as other features. Figure 1 illustrates an GPON network.

1. GPON network


IFgpon: GPON Interface

SNI: Service Node Interface

UNI: User to Network Interface

CPE: Customer Premises Equipment

An optical line terminal (OLT) is an aggregation device that is located in the office central (CO) to terminate PON protocol. PON protocol.

Optic Network Units (ONUs)/Optical network terminals ( ONTs) are situated on the user side and provide various ports to connect to terminals used by users. They are OLT and ONUs connect via an optical distribution network (ODN) for communication.

The ODN is made up from passive optical elements (POS) like optical fibers and some or all passive optical splitters. The ODN is a source of optical lines between OLT as well as the ONUs. It connects with the OLT with the ONUs and is extremely reliable. ODN is a passive network. ODN network is non-active, which means that there is no optical amplifier or regenerator is used in this ODN network, which reduces the maintenance cost of outdoor devices.

GPON History

In 1995, the first work on fiber-to-the-home architectures was carried out through the Full Service Access Network (FSAN) working group that was created by the major telecommunications service providers and system suppliers. In 1996, the International Telecommunications Union (ITU) carried out further research and standardization on two versions of PON. The previous ITU-T G.983 standard was based on Asynchronous Transfer Mode (ATM) and was called APON (ATM PON). The further improvements made to the original APON standard, as well as the gradual fading away from ATM as a protocol resulted in the final, complete Version of ITU-T G.983 being referred to frequently as broadband PON or BPON. A typical APON/BPON will provide six22 megabits of data per second (Mbit/s) (OC-12) for downstream traffic, and 150 Mbit/s (OC-3) of upstream bandwidth, even though the standard allows for higher speeds.

Its ITU-T G.984 gigabit-capable Passive Optical Networks (GPON) standard showed an increase over BPON in both bandwidth efficiency and total bandwidth due to the use of larger packets of variable length. In addition, the standards allow different bit rates however, the industry has settled on 2.488 gigabits/second (Gbit/s) for downstream bandwidth and 1.244 Gbit/s for upstream bandwidth. The GPON Encapsulation Method (GEM) provides extremely efficient packing of user traffic by using frame segmentation.

At the time of mid-2008 Verizon had installed more than 800,000 lines. British Telecom, BSNL, Saudi Telecom Company, Etisalat and AT&T were undergoing advanced tests with Britain, India, Saudi Arabia as well as the UAE and the USA in the United States, respectively. GPON networks are currently installed in a variety of networks around the world, and the trend patterns indicate a higher growth rate for GPON over other PON technologies such as EPON. According to Router-switch.com estimates, the worldwide OLT ports shipping is 5.8 million, while the 2014 worldwide ONT unit shipment stands at 48.5 million.

Why is GPON Required?

With the increasing usage of broadband services, as well as the growth of fiber-in and copper-out, ISP (Internet Service Provider) demands a wider transmission range, greater capacity, reliability, and lower operating costs (OPEX) on its services. GPON provides the following functions in order to meet these needs:

More distance for transmission The transmission medium of optical fibers can cover the entire 60-km distance at the top layer. solving bandwidth and transmission distance issues with the transmission of twisted pairs.

Greater bandwidth The GPON port is able to support the maximum speed of transmission in the range of 2.5 Gbit/s (Gbit/s) in the downstream direction , and 1.25 Gbit/s when it comes to the direction of upstream in order to meet the needs of high-bandwidth applications like high definition television (HDTV) and outside broadcast (OB).

A better user experience when using all services Flexible QoS supports traffic control in line with users and their services, as well as implementing differing service provisioning for various users.

Utilization of more resources at less expense: GPON supports a split ratio of 1:128. A feeder fiber that comes from CO equipment room CO equipment room is divided to up 128 fibers drop. This helps reduce fiber resources as well as O&M costs.

GPON System Overview

Introduction to the GPON System

Mainstream PON technologies include broadband passive optical network (BPON), Ethernet passive optical network (EPON) and gigabit-passive optical network (GPON). Incorporating the ATM Encapsulation mode, BPON is mainly used to carry ATM services. As the time passes for the demise of ATM technique, BPON will also be eliminated. EPON can be described as an Ethernet passive optical network technology. GPON is a gigabit , passive optical network technology that is currently the most commonly used standard technological advancement in optical accessibility.

Figure 2. Working basis of GPON network

The GPON network it is the OLT connects to an optical splitter via one optical fiber which is then connected to the optical splitter. then connected to the the ONU. different wavelengths in both the downstream and upstream directions for the transmission of information. In particular, wavelengths vary between 1260 nm and 1360 nm in the upward direction and 1480 nm up to 1500 nm for the direction downstream.

The GPON utilizes WDM for transmitting data at different wavelengths for upstream and downstream on the identical ODN. Data is broadcast in downstream direction and is transmitted using the TDMA mode (based on timeslots) in the upstream direction.

GPON Downstream Transmission

All data is sent to all ONUs via the OLT. The ONUs will then choose and receive their own data and then discard the rest of the data. Figure 3 shows the particulars.

Figure 3 Downstream communications the principle behind GPON

Main features:

Supports point-tomultipoint (P2MP) multicast transmitting.

Broadcasts the same data to all of the ONUs and distinguishes ONU data by GEM Port ID.

This allows an ONU to obtain the requested data via ONU ID.

GPON Upstream Transmission

In the direction of upstream, each ONU is able to send information to OLT only within the time slot that is allowed and assigned from the OLT. This guarantees that each ONU transmits data in a specific order, thus avoiding downstream data conflicts. Figure 4 illustrates the particulars.

Image 4 Upstream Communication principles of GPON

Main features:

It supports time division multiple access (TDMA).

Data transits on a time slot that is exclusive.

Couples optical signals to the optical splitter.

It detects and stops collisions via the ranging.

GPON Basic Concepts

GEM Frame

In the gigabit-capable optical network (GPON) system the GPON Encapsulation Mode (GEM) frame can be described as the most compact unit for carrying service and is the basic encapsulation structure. Each service stream is encapsulated within this GEM frame and then transmitted via GPON lines. They are identified via GEM ports. Each GEM port can be identified with an unique port ID. This port ID is allocated globally through the OLT. This means that ONUs linked with the OLT are not able to use GEM ports with the identical port ID. The GEM port is used for identifying the virtual channel that transmits an information stream from the OLT as well as the ONU. It is similar in structure to the digital path identification (VPI)/virtual channel identifyr (VCI) that are part of an Asynchronous Transfer Mode (ATM) online connection. Figure 5 illustrates what is the GEM frames structure.

Figure 5 GEM frame structure

A GEM header is composed comprising PLI, port ID PTI and header error check (HEC) as well as being used to differentiate information from various GEM ports.

PLI: stands for the length of the data payload.

Port ID: Uniquely identifies a GEM port.

PTI: refers to the type of payload. It can be used to determine the type and status of data being transmitted, for instance whether the operation administration , and maintenance (OAM) message is being transmitted , and whether data transmission has been completed.

HEC is a means of ensuring Forward error correction (FEC) function as well as the transmission quality.

Fragment payload is the name given to how much frame was used.

The next section will describe what is the GEM frame structure that is based in the mapping function of Ethernet service to Ethernet service GPON the mode depicted in Figure 6.

Figure 6 GEM frame structure

The GPON system reads Ethernet frames and converts the information into GEM payloads that can be sent Smartnet.

Information about the header is automatically encapsulated in GEM frames.

The mapping format is easy to understand and compatible.


Transmission Container (T-CONT) is an upstream service provider upstream direction of the GPON system. Every GEM ports are mapped to TCONTs. Service streams are then transmitted upstream using The OLT’s Dynamic Bandwidth Allocation (DBA) scheduler. T-CONT is the main control unit for the upstream service stream within the GPON system. Each T-CONT can be identified using an Alloc-ID. The Alloc-ID is assigned through OLT via the GPON port in the OLT and the T-CONTs that are used by ONUs linked with the GPON port of the OLT are not able to share the same Alloc-IDs.

There are five kinds of T-CONT. The T-CONT type is selected based on the scheduling of different kinds of service stream upstream. Each bandwidth type comes with its specific high-quality of service (QoS) characteristic. QoS is usually expressed by the bandwidth guarantee that can be divided into assured, fixed assuring, non-assured, best effort, as well as hybrid types (corresponding to type 1 and the type 5 described on Table 1.).

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