Something You Should Know About Passive Optical Network
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Something You Should Know About Passive Optical Network

Posted on by FS.COM

From the previous article, we have learnt that a classical passive optical network (PON) consists of a central office node (CON), called an optical line terminal (OLT), one or more user nodes, called optical network units (ONUs) or optical network terminals (ONTs), and the optical distribution network (ODN), namely the fibers and PON splitters between them. However, you may still be confused with the interconnection between these passive optical equipment.

According to the PON architecture, as shown in the following diagram, we can divide a PON into three parts – the CO (i.e. OLT), ONU/ONT and ODN (includes fibers and optical splitters) to make it easier to understand.


PON includes EPON and GPON. They have the most popular versions of PON application at present. They have a similar or even said the same topology, but the protocols and chipset are quite different. EPON is a native Ethernet solution that leverages the features, compatibility and performance of the Ethernet protocol, while GPON leverages the techniques of SONET/SDH and Generic Framing Protocol (GFP) to transport Ethernet. Thus, EPON and GPON equipment, both OLT and ONUs, are not the same. In general, GPON equipment is more expensive than EPON. However, with the development of technology, most GPON OLT platform in the market currently can be also compatible with EPON which are more cost-effective.

How about the connection between these PON equipment? PON designs call for single-mode optical fiber, because multi-mode optical fiber cannot support the bandwidth, reach and multiple signal wavelengths enabled by PON applications in the way that single-mode can. PONs therefore do not require separate transmit and receive optical fibers, as would be the case for multi-mode optical fiber. Moreover, with the newest PON technology, called WDM-PON, only one fiber cable will be used to each ONU, because PON uses WDM technology to separate transmit and receiver directions on different wavelengths. This technology greatly reduce the quantity and cost of fiber cables. However, due to the limitation of fiber optic transmission, the maximum distance from OLT to ONU has been 20 km so far.

Come back to the topology diagram again, there is one OLT, and different numbers of ONU or ONT. In fact, the number of connected OUNs depends on the PON ports of your OLT and split ratio. For instance, here is a 4-port OLT and the split ratio is 1:32. So, it can support 128pcs ONUs. Similary, if the split ratio turns as 1:64, so the number of ONUs is 256. In general, this is limited by the transmission distance, splitter structure level, cable and connector loss, etc. We recommend 1:32 split ratio in your PON network within 20 km (note: 1:32, the maximum transmitting distance is generally 10 km.) in the premise of guaranteeing the subscriber bandwidth and network stability. Of course, if your PON network distance is short, the splitter structure level and transmission loss are small, 1:64 is also available. For GPON, the split ratio can reach up to 1:128.

In addition, we find that in the user side is ONUs while sometimes is ONTs. As we said in the previous article, they are almost the same things but for different definitions. Furthermore, ONUs are usually in the remote node, such as the building corridor or curb, near the customer premises but not located in the customer premises as ONTs. This is why we usually use “house” as ONT in the PON diagram.

With the mature technology of optical network, people believe that all-optical PONs can make a so-called “triple play” over one network, delivering voice, data and video. So, now comes the questions. What are the interface at OLT for a voice traffic? It is based on VoIP (Voice over IP). Each ONU has integrated SIP based gateway, and it is transparently passed to third party VoIP gateways through OLT. How about the TV connection? Assuming that we talk about the CATV (Cable TV), it is not to be connected to OLT at all. There is additional device, called EDFA is needed. CATV signal is injected at different wavelength, typically 1550 nm, and sent to the ONU’s, which has integrated CATV module working on this wavelength.

PON offers particularly attractive performance, efficiency and TCO advantages. Knowledge about PON is far more than these we have mentioned. For more information about PON knowledge, just keep your attention on our blog. Of course, if you are planning to purchase the PON equipment, such as GPON OLT, EPON ONU, or GPON ONT etc., welcome to visit or contact us by sending e-mail to

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