The ABC of the PON Network: OLTs, ONUs, ONTs and ODNs

In recent years, telecommunications companies have increasingly focused on the concept of the “Fiber to the Home” (FTTH), which means that the technologies are evolving rapidly and continuously. There are two important types of systems that enable FTTH broadband connections: active optical networks (AON) and passive optical networks (PON). In this article we present the ABC of the PON network. This includes the basic components and related technology, including OLT, ONT, ONU and ODN.

 

A passive optical network (PON) is a system in which fiber optic cables and signals are available to the end user throughout the way or most of the path. Depending on where the PON ends, the system can be described as “Fiber to the Curb” (FTTC), “Fiber to the Building” (FTTB) or "Fiber to the Home" (FTTH). The difference between WDM-PON vs GPON vs XG-PON.

 

A PON consists of an Optical Line Terminal (OLT) in a local switch and several Optical Network Units (ONUs) near the end users. There are currently two major PON standards: Gigabit Passive Optical Network (GPON) and Ethernet Passive Optical Network (EPON). But no matter what kind of PON network it is, the same basic topology structure exists. A Gigabit Ethernet Passive Optical Network (GEPON) system generally consists of an Optical Line Terminal (OLT) in the service provider's switch and several Optical Network Units (ONUs) or Optical Network Terminals (ONTs) near end users. In addition, an Optical Distribution Network (ODN) is used for transmission between OLT and ONU/ONT.

 

The OLT is an apparatus that integrates the L2/L3 switch function in the GEPON system. OLT devices generally include a rack, the control and switch module (CSM), the EPON Link module and the PON-Card (ELM), redundancy protection, 48V DC power supply modules or a 110/220V AC power module, and a fan. PON-Card and power supply support hot-swappables, while other modules are permanently installed. The main function of OLTs is to control the flow of information via the ODN from a central switching facility in both directions. The maximum distance supported for transmission via the ODN is 20 km. OLT has two floating directions: upstream (retrieving and distributing different types of data and voice traffic from users) and downstream (retrieval of data, voice and video traffic from the Metro network or from a long-haul network and sending to all ONT modules within the ODN).

 

ONUs convert optical signals transmitted via optical optical signals into electrical signals. These electrical signals are then sent to individual subscribers. In general, there is a certain distance or a separate access network between ONU and the end user's premises. In addition, ONUs can send, aggregate and maintain various types of data coming from the customer (“grooming”) and send them to the OLT. Grooming is a process in which the data stream is optimized and reorganized in order to be made available more efficiently. OLT supports bandwidth allocation, which allows a smooth transfer of data to the OLT, which normally arrives in the form of packages from the customer. ONU can be connected using various methods and cable types such as twisted pair copper wire, coaxial cable, fiber optic or Wi-Fi.

 

Essentially, ONTs are the same as ONUs. ONT is a term used by ITU-T (International Telecommunication Union), while ONU is a term "Institute of Electrical and Electronics Engineers" (IEEE). They both refer to the user page in the GEPON system. In practice, however, there are certain differences between ONTs and ONUs, depending on their location. ONTs are usually directly on site at the customer's place.

 

The ODN provides the optical transmission medium for the physical connection of the ONUs to the OLTs. The range is 20 km or more. Within the ODN, fiber optic cables, fiber optic connectors, passive optical PON splitters and auxiber-mart components work closely together. The ODN includes in particular five segments: the feeder fiber, the optical distribution point, the distribution fiber (distribution fiber), the optical access point (perpic access point) and fiber-fibre end cable (drop fiber). The feed fiber runs from the optical distribution frame (ODF) in the control center (Central Office, CO) to the distribution points for remote coverage. The distribution fibre runs from the optical distribution point to the optical access point and distributes optical fiber connections across adjacent areas. Fiber end cables connect the optical access point with user connections (ONTs) and thus include the fiber optic connection to the user household. In addition, the ODN is the actual path for PON data transmission and its quality directly affects the performance, reliability and scalability of the PON system.