Gigabit Passive Optical Network

Gigabit Passive Optical Network

A longer transmission reach, higher bandwidth, reliability, and lower operating expense (OPEX) on services.

Longer transmission distance

Higher bandwidth

Better user experience on full services

Higher resource usage with lower costs

What is GPON?

PON is a point to multi-point (P2MP) passive optical network, GPON stands for Gigabit Passive Optical Networks. GPON is defined by ITU-T Recommendation G.984.x. GPON can transport not only Ethernet, but also ATM and TDM (PSTN, ISDN, E1 and E3) traffic. GPON network consists of mainly two active transmission equipments, namely- Optical Line Termination (OLT) and Optical Network Unit (ONU) or Optical Network Termination (ONT). GPON supports triple-play services, high-bandwidth, long reach (up to 20km), etc.

Figure 1 shows a GPON network.

IFgpon: GPON InterfaceSNI: Service Node InterfaceUNI: User to Network InterfaceCPE: Customer Premises EquipmentThe optical line terminal (OLT) is an aggregation device located at the central office (CO) for terminating the PON protocol. Optical network units (ONUs)/Optical network terminal (ONTs) 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. 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.

GPON History

Starting in 1995, work on fiber to the home architectures was done by the Full Service Access Network (FSAN) working group, formed by major telecommunications service providers and system vendors. The International Telecommunications Union (ITU) did further work, and standardized on two generations of PON. The older ITU-T G.983 standard was based on Asynchronous Transfer Mode (ATM), and has therefore been referred to as APON (ATM PON). Further improvements to the original APON standardas well as the gradual falling out of favor of ATM as a protocolled to the full, final version of ITU-T G.983 being referred to more often as broadband PON, or BPON. A typical APON/BPON provides 622 megabits per second (Mbit/s) (OC-12) of downstream bandwidth and 155 Mbit/s (OC-3) of upstream traffic, although the standard accommodates higher rates.

The ITU-T G.984 Gigabit-capable Passive Optical Networks (GPON) standard represented an increase, compared to BPON, in both the total bandwidth and bandwidth efficiency through the use of larger, variable-length packets. Again, the standards permit several choices of bit rate, but the industry has converged on 2.488 gigabits per second (Gbit/s) of downstream bandwidth, and 1.244 Gbit/s of upstream bandwidth. GPON Encapsulation Method (GEM) allows very efficient packaging of user traffic with frame segmentation.

By mid-2008, Verizon had installed over 800,000 lines. British Telecom, BSNL, Saudi Telecom Company, Etisalat, and AT&T were in advanced trials in Britain, India, Saudi Arabia, the UAE, and the USA, respectively. GPON networks have now been deployed in numerous networks across the globe, and the trends indicate higher growth in GPON than other PON technologies like EPON. According to Huanetwork estimation, 2014 global OLT ports shipment is 5.8 million, 2014 global ONT unit shipment is 48.5 million.

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Why is GPON Required?

As the wide use of broadband services and fiber-in and copper-out development, ISP (Internet Service Provider) requires 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.

GPON System Overview

Introduction to the GPON SystemMainstream 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 4 shows the 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.

Figure 5 Downstream communication principle of GPON

GPON Downstream TransmissionAll data is broadcast to all ONUs from the OLT. The ONUs then select and receive their respective data and discard the other data. Figure 5 shows the details.

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.

Figure 6 Upstream communication principle of GPON

GPON Upstream TransmissionIn 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 6 shows the details.

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 Basic Concepts

GEM FrameIn 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 2 shows the GEM frame structure.

Figure 2 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 3.

Figure 3 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.

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).

Table 1 T-CONT types

Bandwidth Type

T-CONT Type

Type 1

Type 12

Type 3

Type 4

Type 5

Fixed Bandwidth

X

No

No

No

X

Assured Bandwidth

No

Y

Y

No

Y

Maximum Bandwidth

Z = X

Z = X

Z = X

Z

Z X + Y

Description

The fixed bandwidth is reserved for specific ONUs or specific services on ONUs. It cannot be used by other ONUs even if no upstream service streams are carried on the specific ONUs.

It applies to services that are sensitive to service quality. The services can be TDM or VoIP services.

The assured bandwidth is available at any time required by an ONU. When the bandwidth required by the service streams on the ONU is smaller than the assured bandwidth, the system can use the DBA mechanism to allocate the remaining bandwidth to services on other ONUs.

Because DBA is required, this type provides a less real-time performance compared with the fixed bandwidth.

This type is the combination of the assured bandwidth and maximum bandwidth. The system assures some bandwidth for subscribers and allows subscribers to preempt bandwidth. However, the total used bandwidth cannot exceed the maximum configured bandwidth.

It applies to VoIP services.

This type is the maximum bandwidth that can be used by an ONU, fully providing the bandwidth required by the ONU.

It applies to IPTV and other high-speed Internet services.

This type is the combination of the fixed, assured, and maximum bandwidth. It supports the following functions:

Reserves bandwidth for subscribers and the bandwidth cannot be preempted by other subscribers.

Provides the bandwidth to an ONU at any time when required

Allow subscribers to preempt some bandwidth. (The total used bandwidth cannot exceed the maximum configured bandwidth.)

NOTE:In Table 1, X indicates the fixed bandwidth value, Y indicates the assured bandwidth value, Z indicates the maximum bandwidth value, and No indicates not involved.

GPON Networking Application

GPON is a passive optical transmission technology that applies in FTTx solutions, including fiber to the building (FTTB), fiber to the curb (FTTC), fiber to the door (FTTD), fiber to the home (FTTH), fiber to the mobile base station (FTTM), fiber to the office (FTTO), and fiber to the WLAN (FTTW), for voice, data, video, private line access, and base station access services. Figure 7 shows FTTx networking applications.

Figure 7 FTTx networking applications

The FTTx network applications in GPON access have the following in common: The data, voice, and video signals of terminal users are sent to ONUs, where the signals are converted into Ethernet packets and then transmitted over optical fibers to the OLT using the GPON uplink ports on the ONUs. Then, the Ethernet packets are forwarded to the upper-layer IP network using the uplink port on the OLT.

FTTB/FTTC: The OLT is connected to ONUs in corridors (FTTB) or by the curb (FTTC) using an optical distribution network (ODN). The ONUs are then connected to user terminals using xDSL. FTTB/FTTC is applicable to densely-populated residential communities or office buildings. In this scenario, FTTB/FTTC provides services of certain bandwidth for common users.

FTTH: The OLT connects to ONTs at user homes using an ODN network. FTTH is applicable to new apartments or villas in loose distribution. In this scenario, FTTH provides services of higher bandwidth for high-end users.

FTTO: The OLT is connected to enterprise ONUs using an ODN network. The ONUs are connected to user terminals using FE, POTS, 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.

FTTD: uses existing access media at user homes to resolve drop fiber issues in FTTH scenarios.

FTTM: The OLT is connected to ONUs using an ODN network. The ONUs are then connected to wireless base stations using E1. The OLT connects wireless base stations to the core IP bearer network using optical access technologies. This implementation mode is not only simpler than traditional SDH/ATM private line technologies, but also drives down the costs of base station backhaul. FTTM is applicable to reconstruction and capacity expansion of mobile bearer networks. In this scenario, FTTM converges the fixed network and the mobile network on the bearer plane.

FTTW: The OLT connects to ONUs using an ODN network, the ONUs connect to access points (APs) using GE for WLAN traffic backhaul. FTTW is the trend in Wi-Fi construction.

GPON Solution Provider Huanetwork

Huanetwork is a independent GPON solution provider, working with over 500 ISPs and carriers in the world. Huanetwork provides Huawei OLT at very competitive price, Huawei is the world leader in GPON market with 36% marketshare. Full GPON solution (OLT, ODN, ONU) include technical support service are all available at Huanetwork, many ISPs starts their GPON journey by telnet accessing Huanetwork GPON LAB remotely, which is a free service provided by Huanetwork aiming at bridging the digital divide.

Interested in GPON solution price, contact sales@huanetwork.comInterested in Huanetwork LAB test, contact support@huanetwork.com

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