GPON Type B protection technology

Overview of Type B Protection

Type-B protection is one of the four PON protection types defined in the PON standard. It is also the most commonly used type of PON protection. The general model is defined in the ITU-T standard document.

Its main mechanism is active/standby switchover. When the backbone line of the GPON OLT cannot transmit information to the GPON ONU, the protection line is used to transmit information. Therefore, services will not be interrupted due to faults between devices.

Type B protection is classified into Type B single-homing and Type B dual-homing, the following sections describe the differences between the two networks.

Type B single-homing protection

Type B single-homing protection network

Type B single-homing protection mainly protects the backbone fiber and different PON ports of the same OLT. It has two types of PON ports and fibers: active/standby. Multiple PON ports and optical fibers cannot forward packets at the same time. 

The following figure shows the network topology.

-Typical network topology of Type B single-homing protection-

Working principle of type B single-homing protection

When the optical fiber or PON port of the active line is faulty, the ONU immediately switchover services to the standby line.

The conditions for triggering the automatic protection switchover are as follows:

• The backbone fiber is broken.
• The active PON port of the OLT is faulty.
• Line quality deterioration

Active/standby switchover process

Generally, the work port (the PON port connected to the working link) is in the active state, and the protect port (the PON port connected to the protection link) is in the standby state.

Troubleshooting when the active optical fiber is faulty

• When the work port detects the LOS alarm (caused by the fault of the active optical fiber), it immediately disables the transmit function of the optical module.
   
• The protect port detects the LOS alarm on the work port, enables the transmission function of the optical module, and performs ONU ranging.
   
• If the protect port and the protect-side link are normal and the ONU ranging is successful, the port recovery alarm is reported.
   
• The status of the work port and the protect port is switched. In this case, the work port is in the standby state, and the protect port is in the active state. The switchover is complete.