XG(S)-PON Principles (4): Key technologies

Hello, everyone!

Today, I want to continue to explain the XG(S)-PON technology. Now, I will start to explain the key technologies of XG(S)-PON. There are five key technologies: ranging, FEC, burst optical/electrical technology, line encryption and DBA. In this article, I will explain ranging.

Ranging

In the upstream direction, XG(S)-PON technology uses TDMA mode. This means that each ONU on the OLT PON port has a tape-defined time slot when it can broadcast data. Broadcasting data outside the assigned time slot causes a collision on the optical splitters. This phenomenon can also occur in TDMA mode, as we can see in Figure 1.

Figure 1. Collision on the optical splitter – without ranging

There is a logical reach between OLT and ONUs that varies, RTD (Round Trip Delay) also varies and depends on time and environment. This can lead to a collision when the ONU broadcasts in TDMA mode. Therefore, when the ONU first connects to the network (ODN) and when it registers, then ranging is enabled.

Figure 2. ONUs emit data in TDMA mode with ranging

The distance between OLT and ONU goes from ten meters to 60 or more km. The delay during broadcasting is about 5 ps/km. The OLT must measure the exact distance between it and the ONU device. During the ranking process, the OLT measures the RTD (Round Trip Delay)and calculates the E_qD (Equalization Delay). Based on these two data, OLT calculates T_eqdin this way:

T_eqd = RTD + E_qD.

During the ranking process, the OLT must open a window (called a quiet zone), and in this way, it pauses the broadcast of the ONU device in the upstream direction. OLT opens a window setting BW_map = 0 and does not authorize any time slot. And the end of this process, the logical distance between OLT and ONU devices are the same, and there is no collision in the upstream direction.

This is the end of this article. I will continue to write about this topic in the next article >> XG(S)-PON (5): Key technologies.

Thank you!