1. The basic structure of fiber access network

Optical access network (OAN) refers to the use of optical fiber as the main transmission medium to achieve access network’s information transmission function. The optical line terminal (OLT) is connected to the service node and the optical network unit (ONU) is connected with the user. The fiber access network includes remote devices - optical network units and local terminal equipment - optical line terminals, which are connected by transmission equipment. The main components of the system are the OLT and the remote ONU. They complete the conversion of the signaling protocol from the service node interface (SNI) to the user network interface (UNI) throughout the access network. Access device itself also has networking capabilities can form a variety of forms of network topology. At the same time, the access device also has function of local maintenance and remote centralized monitoring and forms a maintenance management network through a transparent optical transmission, and through the corresponding network management protocol incorporates the network management center for unified management.

The role of the OLT is to provide an interface with the local switch for the access network and to communicate with the optical network unit of the client through optical transmission. It separates the exchange function of the switch from the user access completely. The optical line terminal provides maintenance and monitoring of itself and the client terminal, which can be placed directly with the local switch at the switch office or at the remote end.

The role of the ONU is to provide the user side interface for the access network. It can access a variety of user terminals, with photoelectric conversion function and the corresponding maintenance and monitoring functions. The main function of the ONU is to terminate the fiber from the OLT, to process optical signals and to provide business interfaces for multiple small businesses, business users, and residential users. The network side of the ONU is an optical interface, and its client terminal is an electrical interface. The ONU, therefore, has functions of optical/electrical and electrical/optical conversion. ONU is usually placed which is closer to the user with great flexibility.

Fiber access network (OAN) is divided into two types: active optical network (AON) and passive optical network (PON).

2. Active fiber access network

Active optical networks can be divided into SDH-based AON and PDH-based AON. Central office equipment (CE) and remote equipment (RE) of the active optical network are connected through active optical transmission equipment. Transmission technology is SDH and PDH technology, which is widely used in the backbone network, but SDH technology is the main.

SDH is also known as Synchronous Optical Network (SONET). 

With the introduction of SDH technology, the transmission system not only has the function of providing the physical process of signal propagation but also provides the function of signal processing, monitoring and so on. SDH can support a variety of circuit layer services, such as variable rates of asynchronous digital series, DQDB, FDDI, ATM, and a variety of new businesses in the future. SDH technology achieves cross connection and add-drop multiplexing connections provides flexible up / down circuitry and dynamic network topology to enhance network flexibility and security for business development. It achieves the protection and height of the circuit and optimal utilization of communication capabilities in a greater geometry range so as to lay the foundation for enhanced networking capabilities with just a few seconds to re-network. In particular, SDH self-healing ring can recover rapidly within tens of milliseconds after the circuit failure. These advantages of SDH make it a basic transport network for the broadband service digital network.