PON is the main technology used in optical fiber broadband access. It has been 15 years since China Telecom started large-scale commercial use in 2006, but the application of PON technology has not declined at all. Not only is it still the main technology for optical fiber broadband access, but it has gradually penetrated into private networks, local area networks and Intra-family networking. For example, the recently popular FTTR uses PON technology
2.The meaning of passive optical network
PON is composed of optical line terminal OLT, optical distribution network ODN and optical network unit ONU.
The OLT is a central office device. User equipment includes ONT and ONU. ONT is an optical network terminal, usually used by a single user, also known as an optical modem. FTTO and FTTH user terminals belong to ONT. ONU is usually shared by multiple users and is mainly used in FTTB. User equipment can also be collectively referred to as ONU
PON is the abbreviation of Passive Optical Network, which means passive optical network. PON technology simultaneously accesses multiple users through one optical fiber. Before the emergence of PON technology, other optical access technologies needed to achieve the same function, and there were active convergence devices in ODN. The PON technology replaces the active convergence device with a passive optical splitter. There is no active device in the ODN, so this technology is called a passive optical network. That is to say, the passive optical network means that the ODN part is passive.
3. The key component of PON: optical splitter
The optical splitter is a key device in PON. It is a passive device based on optical power splitting, which can divide one optical signal into multiple paths and complete the opposite process.
The picture above is a 1×2 splitter. When one optical signal enters, it is divided into two identical signals, and the optical power of the split signal is only half of the original signal. Cascading 1×2 splitters once becomes 1×4, and cascading again becomes 1×8……, the splitting ratio of our commonly used optical splitters is 1×4, 1× 8. 1×16……. In FTTR, a 1×5 optical splitter with unequal optical ratio is also used.
According to different connection methods, splitters are mainly divided into two types: box type and insert type. Cassette splitter is also called pigtail type, mainly used in fiber optic cable transfer box; plug-in splitter is also called connector type, mainly used in fiber optic cable splitter box.
When the split ratio of the splitter increases by 1 level, the insertion loss increases by about 3dB. For splitters with the same splitting ratio, the insertion loss of the plug-in type is about 0.2dB larger than that of the cassette type.
4. The technical principle of PON
The downlink of PON (from the OLT to the user) uses the broadcast method. The optical splitter broadcasts the downlink optical signal to each ONU connected to the same splitter. Each ONU receives its own signal and discards the signal that does not belong to it. Lose.
The uplink of the PON adopts the time division multiplexing (TDM) method, and each ONU communicates with the OLT in different time slots.
The uplink and downlink of the PON use different wavelengths, and the OLT and ONU adopt the optical transceiver and optical module, so that different wavelengths can be transmitted in one optical fiber.
5. The difference between different PON technologies
Currently used PON technologies mainly include EPON and GPON, as well as 10G-EPON and XG-PON. Although there are great differences between different PON technologies, from the user's point of view, the main differences lie in line rate, available bandwidth and working wavelength.