Let’s explore the key technologies of GPON.

The key technologies of GPON include Ranging, Upstream dynamic bandwidth allocation, and Downstream AES encryption.

Ranging

Why do we need to measure the distance?

The distances between ONUS and OLTS are different. The transmission time of optical signals on the optical fiber is different, and the time when the optical signal arrives at each ONU is also different.

The OLT allocates different timeslots to each ONU to transmit upstream data. How to ensure that each ONU can accurately locate timeslots?

How to avoid conflicts between upstream data from multiple ONUs and implement frame synchronization?

Ranging Principle

The ranging process is as follows:

The OLT obtains the round trip delay (RTD) of the ONU through the ranging process and calculates the physical distance of each ONU.

Then, the OLT specifies a proper equalization delay (EqD)

In the ranging process, a window needs to be opened, that is, a quiet zone to suspend the upstream transmission channels of other ONUS. The OLT opens the window by setting BWmap to null without authorizing any timeslot.

In the PON upstream transmission, the TDMA mode is used for access. One OLT can be connected to multiple ONUs. The shortest distance between the ONU and the OLT may be tens of meters and the longest actual physical distance may be up to 40 kilometers. Optical signals are transmitted over optical fibers, and the transmission delay is 5 ps per kilometer. Due to the change of the ambient temperature and the aging of the components, the transmission delay changes constantly. To implement the TDMA access, it is ensured that the upstream data of each ONU is inserted into the specified timeslot after the public optical fiber is confluent, no collision occurs between ONUs, and the gap is not too large. The OLT needs to accurately measure the distance between each ONU and the OLT, so as to control the time of sending the upstream data by each ONU.
The OLT obtains the round trip delay (optical splitter) of the ONU through the ranging process and then specifies a proper equalization delay (EqD) to ensure that no conflict occurs on the optical splitter when each ONU sends data.
During the ranging process, a window, namely, the Quiet Zone, needs to be opened to suspend the upstream transmit channels of other ONUs. The OLT opens a window by setting the BWmap to null and does not authorize any timeslot.

Ranging Result

Through the RTD and the EqD, the data frames sent by the ONUs are synchronized, so as to ensure that no conflict occurs on the optical splitter when the ONUs send data.

All ONUs are at the same logical distance and send data in the corresponding timeslots.

Upstream Dynamic Bandwidth Allocation

Why is DBA Required?

What is DBA?

Dynamic bandwidth assignment (DBA)
DBA can dynamically allocate upstream bandwidth in microseconds or milliseconds.

Why DBA is required?

Improve the upstream line bandwidth utilization of the PON ports.
Connect more Users to the PON ports.
Provide users with better services at higher bandwidths, especially for services that have bandwidth bursts.

DBA Working Mechanism

SR-DBA: status report-DBA

SR-DBA Implementation

The DBA module of the OLT continuously collects DBA report information, performs calculations, and sends the calculation result to each ONU in the form of a BW Map.

Each ONU sends upstream burst data in their respective timeslots according to the BW Map information to occupy the upstream bandwidth.

The GPON system uses the SBA+DBA mode to achieve effective bandwidth utilization.

TDM services are configured with a bandwidth through SBA to ensure high QoS.
Bandwidths for other services are dynamically allocated through DBA.
The implementation of DBA is closely related to the QoS guarantee mechanism.
The GPON system supports dynamic bandwidth allocation through status reporting and OLT service monitoring (non-status reporting).

There are three mechanisms for the GPON to report DBA status:

PLOu-State Ind DBA
DBRu-Piggy-back DBA
Payload DBA

The implementation mechanism of the DBA function consists of the following parts:

The OLT or ONU performs congestion detection.
Report the congestion status to the OLT.
Update the bandwidth to be allocated by the OLT according to the specified parameters.
The OLT sends authorization according to the newly allocated bandwidth and T- CONT type.
Manage DBA operations.

Basis for DBA Implementation – T-CONT

Transmission containers (T-CONTS): Dynamically receives the authorization from the OLT, manages the upstream bandwidth allocation at the transmission convergence layer of the PON system.

Bandwidth type:

FB, AB, NAB, and BE

T-CONT type:

Type1
Type2
Type3
Type4
Type5

Relationship Between T-CONT Types and Bandwidth Types

According to service priorities, the System sets the SLA for each ONU and limits the bandwidth of the services.

The maximum and minimum bandwidths limit the bandwidth of each ONU at extreme conditions to ensure that the bandwidth varies according to the service priorities. Generally, the voice service has the highest priority, followed by video and data services.

The OLT allocates the bandwidth according to the service, SLA, and actual ONU conditions. The higher the priority is, the higher the bandwidth is.

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).

Downstream AES Encryption

Why Encryption ls Required?

The broadcast technology is used in the GPON downstream direction. On the same PON port, data is the same on the backbone optical fiber and after optical splitting. The data received by each ONU is the same. How to ensure that the data intended for one ONU is not parsed by other ONUs?

GPON supports AES128 encryption for downstream broadcast data

Advanced Encryption System (AES) is an international encryption algorithm.

Only the payload in a GEM frame is encrypted.

The GPON system periodically exchanges and updates the AES key to improve the reliability of the line data.

AES encryption in the GPON system

The OLT initiates a key replacement request. The ONU responds to the request, generates a new key, and sends the new key to the OLT three times.

After receiving the new key, the OLT starts key Switching and notifies the ONU (also three times) of the frame number of the new key by using related commands. The ONU switches the check key in the corresponding data frame.