Currently, the mainstream technology for fixed network access is PON. In the early years, it was mainly used for operator home/business user access, but now it is gradually expanding to large enterprise parks (POL) and even industry network expansion (F5G), and the development momentum is unstoppable.

        So what kind of technology is PON technology? What is the network architecture? What is the data transmission method? Let's take a look below.

        PON technology is the general term for a class of technologies, including the early narrowband PON, APON, BPON, the current popular EPON, GPON, 10G EPON, XG-PON, XGS-PON, and the later 50G-PON...

        The foundation of all these PON technologies is the same, that is, the network architecture and data transmission method of PON.

        Regarding the network architecture of PON, we have to move out this classic networking diagram.

[Summary of Technical Points]:

        PON is a passive optical network with point-to-multi-point (P2MP) structure;

        The PON consists of an optical line terminal OLT, an optical network unit ONU and an optical distribution network ODN;

        The reason why PON is called a "passive optical network" specifically refers to the partial passivity of ODN (POS, passive optical splitter, is a physical optical device that does not require an external power supply);

        One-level light splitting or two-level light splitting can be used inside the PON (in actual projects, two-level light splitting is the majority);

        The access radius of a PON (the distance between OLT and ONU) is generally planned to be within 20 kilometers (20 kilometers is the physical distance). Among them, urban access is generally 5-10 kilometers, and remote mountain villages or sparse areas may have more than 10 kilometers or even 15 kilometers.

        After understanding the network architecture of PON, let's analyze how data is transmitted based on this architecture. Data                 transmission in general networks is based on two-way: from the originator to the sink, and from the sink to the originator. We also call it downlink and uplink.

        From the previous PON network architecture, we learned that PON is point-to-point.

Then there is actually only one optical fiber between the OLT and the POS. How to realize the transmission of upstream and downstream data in one optical fiber?

[Answer]: Divide different lanes, that is, plan different wavelengths.

[Analysis]: In a PON network, the wavelengths used for uplink and downlink communication are different. For example, EPON/GPON both belong to 1G PON technologies. Their downlink wavelength is planned to be 1490nm, and their uplink wavelength is planned to be 1310nm. They do not affect each other., it is possible to fully realize two-way full-duplex communication. In addition, there is also the CATV service of Radio and Television, with a separate wavelength planned at 1550nm.

Here, some friends have questions again:

Why is the downstream wavelength 1490nm and the upstream wavelength 1310nm, and not the other way around?

[Answer]: Cost considerations.

[Analysis]: When the standard was formulated, 1310nm optical devices were relatively mature and had low cost, while 1490nm had a higher cost. PON is a point-to-multi-point architecture, so we naturally have to place lower-cost devices on the multi-point side, so that the initial deployment cost of the PON network can be greatly reduced.

Let's continue to return to the introduction of data transmission in the PON network. PON uses broadcast to transmit data downstream, and TDMA to transmit data upstream. A series of questions came again:

Why does PON use broadcast to transmit data downstream?

[Answer]: Because it is simple and practical.

[Analysis]: The downstream direction of the PON refers to the direction from OLT to ONU. The comparative relationship between the number of senders and receivers in this direction is a "1 to many" relationship, so we will naturally choose to use broadcast to send data, because it is the most convenient way. At the same time, because the downlink is limited by the physical characteristics of the passive splitter, the data sent by the PON port is evenly distributed to each splitter after passing through the passive splitter. Its passive physical characteristics cannot control the connection or failure of a certain splitter. It can only realize the simple splitter function, so the downlink realizes the phenomenon of passive broadcasting based on the physical splitter characteristics.

The PON downlink uses broadcast, and each ONU can receive data from other ONUs. How to ensure data security?

[Answer]: The ONU actively filters and encrypts the data.

[Analysis]: On the one hand, the ONU will actively filter its own data according to the corresponding filtering conditions, such as         iltering and receiving its own data through the ONU ID (GEM-PORT ID); on the other hand, the data sent by the OLT to each ONU will be encrypted (such as GPON's AES-128), and the encrypted key is generated by each ONU and sent to the OLT. The ONU will not know the keys of other ONUs, so it is difficult to decrypt the data belonging to other ONUs.

Why does PON uplink use TDMA to transmit data?

[Answer]: Achieve simultaneous data transmission by multiple ONUs for fair competition.

[Analysis]: The upstream direction and downstream direction of the PON are opposite, that is, the direction from the ONU to the OLT.         The comparative relationship between the number of senders and receivers in this direction is a "many-to-one" relationship. Since it is a "many-to-one" relationship, we must not let each ONU send data for granted. Otherwise, there will be superposition of optical signals of the same wavelength (such as 1310nm of GPON), and the OLT will not be able to read the data after receiving it. Error frames or unknown frames will appear and discard them.

        Therefore, in this case, there must be an arbitration mechanism to protect our uplink data transmission from conflicts. This arbitration mechanism is TDMA, which divides the uplink into different time slots, and then allocates these time slots to different ONUs as needed. The ONUs send data in their own time slots.

        Therefore, we also see here that the light-emitting time of the ONU is strictly specified by the OLT, and it will not actively emit light, nor will it emit light for a long time. Once an ONU actively lights up or lights up for a long time, the ONU is a "rogue ONU" and will affect the entire service under the PON port.