What's the four Key Technologies of 10G GPON?

Ranging

The logic reaches from optical network units (ONUs) to an optical line terminal (OLT) vary. The round trip delays (RTDs) between an OLT and ONUs also vary depending on time and environment. Therefore, collisions may occur when ONU sends data in TDMA mode (in this mode, only one of the ONUs connecting to a PON port sends data at a moment), as shown in Figure 1.

To prevent the collisions, ranging is enabled when an ONU (HG8245) registers for the first time. The OLT (MA5683T) measures the RTD of each ONU in the ranging process and calculates the equalization delay
(EqD) of each ONU to ensure that the values of Teqd, which is equal to RTD plus EqD, of all
ONUs connected to the same PON port are the same. Therefore, the logic reaches from ONUs
to an OLT are the same, preventing collisions during upstream transmission.

Burst Optical/Electrical Technology

In 10G GPON upstream direction, Time Division Multiple Access (TDMA) is used. An optical
network unit (ONU) transmits data only within the allocated timeslots. In the timeslots that are not allocated to it, the ONU disables the transmission of its optical transceiver to prevent other
ONUs from being affected. The optical line terminal (OLT) then receives the upstream data
from each ONU in a burst manner based on timeslots. Therefore, to ensure normal running of
the 10G GPON system.

Ranging can be implemented to prevent cells transmitted by different ONUs from conflicting
with each other on the OLT. However, the ranging accuracy is ± 1 bit and the cells transmitted
by different ONUs have a protection time of several bits (not a multiple of 1 bit). If the ONU-
side optical modules do not support the burst transmit function, the transmitted signals overlap
and distortion occurs.

• The distance from each ONU to the OLT varies and therefore the optical signal attenuation varies for each ONU. As a result, the power and level of packets received by an OLT at different timeslots various.
• If the OLT-side optical modules do not support the burst receive function, the OLT may restore incorrect signals because only the level greater than the threshold is considered valid and the signals with the level lower than the threshold cannot be restored.

DBA

The OLT uses DBA to dynamically adjust the upstream bandwidth allocated to different ONUs
to address the burst traffic on the ONUs, meeting the ONU upstream bandwidth requirements
and improving the utilization of the PON upstream bandwidth.

In the preceding figure,

• The DBA module in the OLT consistently collects DBA reports and uses the DBA algorithm to calculate the upstream bandwidth allocated to each ONU.
• The OLT sends the calculated result to each ONU using a bandwidth (BW) map.
• Each ONU transmits burst upstream data using permitted timeslots defined in the BW map.

Highlights and Applications

• Based on ONUs' burst upstream service traffic, the OLT dynamically allocates an upstream bandwidth to each ONU in real time, improving upstream bandwidth utilization on PON ports.
• More users are supported on a PON port.
• Higher service bandwidths with burst requirements are supported than those before DBA is applied.

FEC

Forward error correction (FEC) is mainly used for improving transmission quality of a line.
No ideal digital channel is available in practice. As a result, bit errors and jitter occur when digital signals are being transmitted over any transmission medium, deteriorating transmission quality on lines.

To resolve the problem, error correction mechanism is introduced.

• The mechanism can check and correct errors after data is transmitted to the peer end. such as FEC.
• The mechanism can check errors after data is transmitted to the peer end but not correct errors.

Highlight and Application

• Does not require retransmission and provides a high real-time performance
• Requires an additional bandwidth (Users must balance the transmission quality and bandwidth.)
• Checks and corrects errors after data is transmitted to the peer end, but does not apply to services for which retransmission is enabled
• Applies to data transmission on the network that has a poor quality
• Applies to services that have a low requirement on delay (The delay is large if retransmission is configured for services.)

Configuration Guide

The FEC function of 10G GPON as follows:

• Supported only in the downstream direction.
• FEC is enabled by default.
• The FEC function cannot be configured manually.

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What‘s 10 GPON?

10G GPON is evolved from the existing GPON technology, the factors promoting the technology
evolution to 10G GPON are as follows:

• Developing services require higher bandwidths and the GPON technology cannot meet bandwidth requirements.
• Innovative access technologies on the user side require higher bandwidths and the GPON technology will face bandwidth bottlenecks.
• A greater split ratio and a longer transmission distance increase network construction investments.

Specification Differences Between 10G GPON and GPON

The following table lists specification differences between the two technologies.

Specifications	10G GPON (G.987 and G.988)	GPON (G.984)
Wavelength	Downstream: 1575-1580 nm Upstream: 1260-1280 nm	Downstream: 1480-1500 nm Upstream: 1290-1360 nm
Center wavelength	Downstream: 1577 nm Upstream: 1270 nm	Downstream: 1490 nm Upstream: 1310 nm
Transmission rate	Downstream: 9.95328 Gbit/s Upstream: 2.48832 Gbit/s	Downstream: 2.48832 Gbit/s Upstream: 1.24416 Gbit/s
Frame structure	x GPON encapsulation mode (xGEM)	GEM

XGEM Frame

An XGEM frame is the minimum service carrying unit in an 10G GPON system and is also the basic encapsulation structure. All the services must be encapsulated in XGEM frames for transmission. An XGEM frame is composed of a header and payload, as shown in Figure 1.

Figure 1 XGEM frame structure

Service Multiplexing

GEM ports and T-CONTs divide a PON network into virtual connections for service multiplexing, as shown in Figure 2.

Figure 2 Working principles of service multiplexing in an 10G GPON system

GEM Port

A GPON encapsulation mode (GEM) port is a virtual service channel that carries a service flow between the OLT (MA5683T, MA5680T, MA5608T) and an ONU (HG8245, HG8240) in an 10G GPON system. The GEM port is similar to the virtual connection (identified by VPI/VCI) in asynchronous transfer mode (ATM). VPI is the acronym for virtual path identifier and VCI is the acronym for virtual channel identifier.

• Each GEM port is identified by a unique XGEM port ID.
• The XGEM port ID is globally allocated according to the 10G GPON port by the OLT.
• A GEM port can carry one or more types of services.

T-CONT

A transmission container (T-CONT) is the basic control unit of upstream service flows in an 10G GPON system, and is also the unit for carrying service flows in the upstream direction. All the GEM ports are mapped to T-CONTs, and the OLT uses dynamic bandwidth allocation (DBA) to schedule upstream transmission.

• A T-CONT can carry one or more GEM ports according to user configurations.
• A T-CONT is identified uniquely by Alloc-ID.
• The Alloc-ID is allocated according to the 10G GPON port by the OLT.
• An ONU supports multiple T-CONTs configured for various service types.

Networking Diagram

A 10G GPON network is of the point-to-multipoint (P2MP) type, which is the same as that of a GPON network. Figure 1 shows a 10G GPON networking diagram.

Figure 1 Networking Diagram 

The 10G GPON network contains an optical line terminal, optical network units, and an optical distribution network (ODN).

• 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 types of ports for connecting to user terminals. The OLT and ONUs are connected through a passive ODN for communication.
• The Optical distribution network (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.

Transmit Principles

10G GPON uses wavelength division multiplexing (WDM) to transmit data in different wavelengths on an ODN network. Figure 2 shows the working principles.

Figure 2 Transmit Principles 

• In the downstream direction, data is broadcast and the center wavelength is 1577 nm (the wavelength ranging from 1575 nm to 1580 nm).
• In the upstream direction, data is transmitted in time division multiple access (TDMA) mode and the center wavelength is 1270 nm (the wavelength ranging from 1260 nm to 1280 nm).

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