Hello everyone!
We have learned about PON technology. Today, we will continue to learn about the XG(S) PON technology.
GPON Technology Evolution
XG(S)PON refers to two PON technologies: XG-PON and XGS-PON.
XG-PON, 10-Gigabit-capable passive optical network, provides asymmetric 10G transmission (Maximum downstream line rate: 9.953 Gbit/s, Maximum upstream line rate: 2.488 Gbit/s).
XGS-PON, 10-Gigabit-capable symmetric passive optical network, provides symmetric 10G transmission (Maximum downstream line rate: 9.953 Gbit/s, Maximum upstream line rate: 9.953 Gbit/s).
What Is XG(S)-PON?
XG(S)-PON is evolved from the existing GPON technology, the factors promoting the technology evolution to XG(S)-PON 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.
Spectrum Allocation of PON Technologies
The spectrum allocation of GPON and XG(S)-PON is as follows:
The GPON and 10G GPON use different wavelengths but can share the same optical fiber.
Specification Differences Between GPON, XG-PON, and XGS-PON
Network Structure
An XG(S)-PON network is of the point-to-multipoint (P2MP) type, which is the same as that of a GPON network.
The XG(S)-PON network contains an optical line terminal (OLT), optical network units (ONUs), 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 optical distribution network (ODN) is composed of passive optical components (POS) such as optical fibers, and one or more passive optical splitters. It interconnects the OLT and ONUs.
Working Principles of XG(S)-PON
System Transmit Principles
XG(S)-PON uses wavelength division multiplexing (WDM) to transmit data in different wavelengths on an ODN network.
In order to separate signals from multiple users on the same optical fiber, data is transmitted in different ways in the upstream and downstream directions:
Data is broadcast in the downstream direction.
Data is transmitted in the TDMA mode (based on timeslots) in the upstream direction.
Working Principles of Downstream
Working Principles for Downstream Transmission
Broadcast mode: The downstream frame length of XG(S)-PON is fixed at 125 us. In this mode, all ONUs can receive the same data. However data of different ONUSs is differentiated by XGEM port ID, and ONUs receive their own data by filtering.
XG(S)-PON Service Mapping
In the downstream direction:
The OLT sends data in broadcast mode. All services are encapsulated in the GEM port of the service processing unit and broadcast to all ONUs connected to the XG(S)-PON port. Then, the ONUs filter data based on the GEM port ID, only the GEM port that belongs to the ONU is reserved and decapsulated, and the Service is sent to the user device through the service interface of the ONU
Working Principle of Upstream
Working Principles for Upstream Transmission
TDMA mode: In the upstream direction, the XG(S)-PON transmits data in TDMA mode. The upstream link is divided into different timeslots. The upstream timeslots are allocated to each ONU according to the upstream bandwidth map field of the downstream frame.
Data flow forwarding in the upstream direction
Upstream direction: The TDMA mode is used in the upstream direction. Services are mapped to different GEM ports on the ONU, and then the GEM ports are mapped to T-CONTs and uploaded to the OLT. On the OLT side, the T-CONT first demodulates the GEM port unit, and then sends the demodulated GEM port payload to the XG(S)-PON MAC chip for demodulation. Finally, the XG(S)-PON MAC chip sends the demodulated GEM port payload to the related service processing unit for processing.
A GEM port identifies the service virtual channel between the OLT and the ONU in the XG(S)-PON system, that is, the channel that carries the service stream. The GEM port is similar to the VPI/VCI in the ATM virtual connection.
A T-CONT is the basic control unit of upstream service flows in an XG(S)-PON system and carries services in the upstream direction. All GEM ports must be mapped to a T-CONT, and the OLT transmits data upstream in DBA scheduling mode. A T-CONT can carry multiple or one GEM port according to the user configuration.
Each T-CONT is uniquely identified by an Alloc-ID.
The Alloc-ID is allocated by the OLT based on the XG(S)-PON port.
Each ONU supports multiple T-CONTs and can be configured with different service types.
Key Technologies
Ranging
The logic that 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).
To prevent collisions, ranging is enabled when an ONU registers for the first time. The OLT 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 that reaches from ONUs to an OLT is the same, preventing collisions during upstream transmission.
In the ranging process, the OLT must open a window and pause upstream transmission channels of other ONUs.
Burst Optical/Electrical Technology
In the 10G PON 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.
The burst transmit function is supported by ONU-side optical modules.
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 burst receive function is supported by OLT-side optical modules.
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.
Note:
In the XG(S)-PON system, all data is broadcast downstream to ONUs. The transmission requires OLT-side optical modules to transmit optical signals continuously and ONU-side optical modules to receive optical signals continuously. Therefore, these optical modules are not required to support the burst receive and transmit function.
DBA
Working Principle
DBA: Dynamic Bandwidth Assignment
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.
The XG(S)-PON system uses the SBA+DBA mode to effectively use the bandwidth.
The SBA allocates bandwidth to TDM services to ensure high QoS.
Other services can use DBA to dynamically allocate bandwidth.
The implementation of DBA is closely related to the QoS guarantee mechanism.
The XG(S)-PON system supports dynamic bandwidth allocation through status reporting and OLT service monitoring (non-status reporting).
There are three mechanisms for reporting the XG(S)-PON status to the DBA:
PLOu-State Ind DBA;
DBRu-Piggy-back DBA;
Payload DBA;
The implementation mechanism of the DBA function is as follows:
The OLT or ONU performs congestion detection.
Report the congestion status to the OLT.
Update the bandwidth allocated by the OLT according to the specified parameters.
The OLT sends the grant according to the newly allocated bandwidth and T-CONT type.
Manages DBA operations.
Basis for DBA Implementation - T-CONT
Transmission containers (T-CONTs): dynamically receive the grants issued by the OLT, manage the upstream bandwidth allocation at the transmission convergence layer of the PON system, and improve the upstream bandwidth of the PON system.
BandwidthType
FB, NA, NAB, and BE
T-CONT Type
Type1
Type2
Type3
Type4
Type5
Relationship Between T-CONT Type and Bandwidth Type
T-CONTs are classified into five types. During upstream service scheduling, different types of T-CONTs are selected based on service types. Each T-CONT bandwidth type has specific QoS features, which are mainly reflected in the bandwidth guarantee. The QoS features are classified into fixed bandwidth, assured bandwidth, assured/maximum bandwidth, maximum bandwidth, and hybrid mode (corresponding to Type 1 to Type 5 of five T-CONT types).
Based on the service priority, the System sets the SLA for each ONU to limit the service bandwidth.
The maximum bandwidth and minimum bandwidth limit the bandwidth of each ONU. The assured bandwidth varies with the priority of the service. Generally, the voice service has the highest priority, the video service has the highest priority, the video service has the second-highest priority, and the data service has the lowest priority.
The OLT permits the bandwidth based on the service, SLA, and actual ONU conditions. Service with a higher priority can have higher bandwidth.
Line Encryption
Line encryption technologies are required to eliminate the data theft risk.
Upstream and downstream broadcast data can be encrypted using an encryption algorithm (AES-CTR+ cryptographic algorithm) by XG(S)-PON.
Advanced Encryption System (AES) is an international encryption algorithm.
Encrypts onty the payload in the GEM frame.
The XG(S)-PON System periodically exchanges and updates the AES key, which improves the reliability of the line data.
Line Encryption Process
To prevent the risks caused by the cracking of the encryption key, the key replacement technology is used to continuously update the encryption key to improve Security.
Features and Applications
The line encryption algorithm does not increase the load or occupy the bandwidth.
The line encryption algorithm does not cause transmission delay.
The line encryption function is recommended for scenarios that have high requirements for online security.
FEC
Context
Forward error correction (FEC) is mainly used for improving the 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, an error correction mechanism is introduced.
The mechanism can check and correct errors after data are transmitted to the peer end, for example, using FEC.
The mechanism can check errors after data is transmitted to the peer end but not correct errors.
The FEC of an XG(S)-PON port is described as follows:
Supports FEC in the upstream and downstream directions.
FEC is enabled in the upstream and downstream directions by default.
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.)
Thanks for reading!
