What Is GPON

PON is a point to multi-point (P2MP) passive optical network. GPON, a type of PON technology, is defined by ITU-T Recommendation G.984.x. Figure 1 shows a GPON network.

Figure 1 GPON network 

• The optical line terminal (OLT) is an aggregation device located at the central office (CO) for terminating the PON protocol.
• Optical network units (ONUs) are located on the user side, providing various ports for connecting to user terminals. The OLT and ONUs are connected using an optical distribution network (ODN) for communication.
• The ODN is composed of passive optical components (POS), such as optical fibers, and one or more passive optical splitters. The ODN provides optical channels between the OLT and ONUs. It interconnects the OLT and ONUs and is highly reliable.
   NOTE:

  The ODN network is passive, indicating that no optical amplifier or regenerator is deployed on the ODN network, thereby reducing maintenance costs of outdoor devices.

Why Is GPON Required

As the wide use of broadband services and fiber-in and copper-out development, carriers require a longer transmission reach, higher bandwidth, reliability, and lower operating expense (OPEX) on services. GPON supports the following functions to meet these requirements:

• Longer transmission distance: The transmission media of optical fibers covers up to 60 km coverage radius on the access layer, resolving transmission distance and bandwidth issues in twisted pair transmission.
• Higher bandwidth: Each GPON port can support a maximum transmission rate of 2.5 Gbit/s in the downstream direction and 1.25 Gbit/s in the upstream direction, meeting the usage requirements of high-bandwidth services, such as high definition television (HDTV) and outside broadcast (OB).
• Better user experience on full services: Flexible QoS measures support traffic control based on users and user services, implementing differentiated service provisioning for different users.
• Higher resource usage with lower costs: GPON supports a split ratio up to 1:128. A feeder fiber from the CO equipment room can be split to up to 128 drop fibers. This economizes on fiber resources and O&M costs.

Basic Concepts

GEM Frame

In the gigabit-capable passive optical network (GPON) system, a GPON encapsulation mode (GEM) frame is the smallest service-carrying unit and the basic encapsulation structure. All service streams are encapsulated into the GEM frame and transmitted over GPON lines. The service streams are identified by GEM ports and each GEM port is identified by a unique port ID. The port ID is globally allocated by the OLT. Therefore, the ONUs connected to the same OLT cannot use GEM ports that have the same port ID. A GEM port is used to identify the virtual service channel that carries the service stream between the OLT and the ONU. It is similar to the virtual path identifier (VPI)/virtual channel identifier (VCI) of the asynchronous transfer mode (ATM) virtual connection.

Figure 1 shows the GEM frame structure.

Figure 1 GEM frame structure 

A GEM header consists of PLI, Port ID, PTI, and header error check (HEC) and is used for differentiating data of different GEM ports.

• PLI: indicates the length of data payload.
• Port ID: uniquely identifies a GEM port.
• PTI: indicates the payload type. It is used for identifying the status and type of data that is being transmitted, for example, whether the operation, administration and maintenance (OAM) message is being transmitted and whether data transmission is complete.
• HEC: ensures the forward error correction (FEC) function and transmission quality.
• Fragment payload: indicates the frame fragment.

The following section describes the GEM frame structure based on the mapping of the Ethernet service in GPON mode, as shown in Figure 2.

Figure 2 GEM frame structure 

• The GPON system parses Ethernet frames and maps data into GEM payloads for transmission.
• Header information is automatically encapsulated into GEM frames.
• The mapping format is clear and has good compatibility.

T-CONT

Transmission container (T-CONT) is a service carrier in the upstream direction in the GPON system. All GEM ports are mapped to T-CONTs. Then service streams are transmitted upstream by means of OLT's dynamic bandwidth allocation (DBA) scheduling. T-CONT is the basic control unit of the upstream service stream in the GPON system. Each T-CONT is identified by Alloc-ID. The Alloc-ID is allocated by the GPON port of the OLT, and the T-CONTs used by ONUs connected to the same GPON port of OLT cannot have the same Alloc-IDs.kangyu

There are five types of T-CONT. T-CONT selection varies during the scheduling of different types of upstream service streams. Each T-CONT bandwidth type has its own quality of service (QoS) feature. QoS is mainly represented by the bandwidth guarantee, which can be classified into fixed, assured, non-assured, best-effort, and hybrid modes (corresponding to type 1 to type 5 listed in Table 1).

 

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). Adopting the ATM encapsulation mode, BPON is mainly used for carrying ATM services. With the obsolescence of the ATM technology, BPON also drops out. EPON is an Ethernet passive optical network technology. GPON is a gigabit passive optical network technology and is to date the most widely used mainstream optical access technology.

Figure 1 shows the working principle of the GPON network.

Figure 1 Working principle of the GPON network 

• In the GPON network, the OLT is connected to the optical splitter through a single optical fiber, and the optical splitter is then connected to ONUs. Different wavelengths are adopted in the upstream and downstream directions for transmitting data. Specifically, wavelengths range from 1260 nm to 1360 nm in the upstream direction and from 1480 nm to 1500 nm in the downstream direction.
• The GPON adopts WDM to transmit data of different upstream/downstream wavelengths over the same ODN. Data is broadcast in the downstream direction and transmitted in the TDMA mode (based on timeslots) in the upstream direction.

GPON Downstream Transmission

All data is broadcast to all ONUs from the OLT. The ONUs then select and receive their respective data and discard the other data. Figure 2shows the details.

Figure 2 Downstream communication principle of GPON 

Main features:

• Supports point-to-multipoint (P2MP) multicast transmission.
• Broadcasts the same data to all ONUs and differentiates ONU data by GEM port ID.
• Allows an ONU to receive the desired data by ONU ID.

GPON Upstream Transmission

In the upstream direction, each ONU can send data to the OLT only in the timeslot permitted and allocated by the OLT. This ensures that each ONU sends data in a given sequence, avoiding upstream data conflicts. Figure 3 shows the details.

Figure 3 Upstream communication principle of GPON 

Main features:

• Supports time division multiple access (TDMA).
• Transits data on an exclusive timeslot.
• Couples optical signals on an optical splitter.
• Detects and prevents collisions through ranging.

GPON Networking Applications

GPON is a passive optical transmission technology that applies in FTTx solutions, including fiber to the office (FTTO), for voice, data, video, private line access. Figure 1 shows FTTO networking applications.

Figure 1 FTTO networking applications 

 

GPON Frame Structure

Figure 1 shows the GPON frame structure.

Figure 1 GPON frame structure 

Upstream GPON Frame

An upstream GPON frame has a fixed length of 125 µs. Each upstream frame contains the content carried by one or more T-CONTs. All ONUs connected to a GPON port share the upstream bandwidth

• All ONUs connected to a GPON port send their data upstream at their own timeslots according to bandwidth map (BWmap) requirements.
• Each ONU reports the status of data to be sent to the OLT using upstream frames. Then, the OLT uses DBA to allocate upstream timeslots to ONUs and sends updates in each frame.

In Figure 1, an upstream GPON frame consists of the physical layer overhead upstream (PLOu), PLOAM upstream (PLOAMu), power level sequence upstream (PLSu), dynamic bandwidth report upstream (DBRu), and payload fields, as described in Table 1.

The OLT is connected to enterprise ONUs using an ODN network. The ONUs are connected to user terminals using GE, POTS, E1, or Wi-Fi. QinQ VLAN encapsulation is implemented on the ONUs and the OLT. In this way, transparent and secure data channels can be set up between the enterprise private networks located at different places, and therefore the service data and BPDUs between the enterprise private networks can be transparently transmitted over the public network. FTTO is applicable to enterprise networks. In this scenario, FTTO implements TDM PBX, IP PBX, and private line service in the enterprise intranets.