Technical Structure and Practical Application of Optical Fiber Access Network

Before giving you a detailed introduction to the technology of the optical fiber access network, let's first understand the structure of the optical fiber access network, and then fully introduce the practical application of the technology. The topological structure of the optical fiber access network refers to the geometric arrangement of transmission lines and nodes, which indicates the layout of the nodes in the network and their interconnection. The network topology has an important impact on network functions, cost and reliability. The three basic topology structures are bus, ring, and star, which can generate bus-star, double star, double ring, and bus-bus applications, they have their own characteristics and complement each other.

1. Bus Structure

The bus structure is a network structure consisting of optical fiber as a public bus (bus), and various user terminals are directly connected to the bus through a certain coupler. This structure is a series structure with the characteristics of sharing trunk optical fiber, saving line investment, easy addition and deletion of nodes, and less interference with each other. However, the disadvantage is the accumulation of losses, the user receiver has a high requirement on the dynamic range and is highly dependent on the trunk optical fiber.

2. annular structure

A ring structure refers to a network structure in which all nodes share an optical fiber link and the optical fiber link is connected to a closed loop at the beginning and end. The outstanding advantage of this structure is that it can achieve self-healing of the network, that is, without external intervention, the network can restore the transmitted business from failure in a short period of time.

3. Star Structure

A star structure is used by each user terminal to exchange information through a star coupler with control and exchange functions at a central node (in the terminal board). This structure is a parallel structure. It is easy to upgrade and scale up because it does not involve the accumulation of losses. Each user is relatively independent and has a strong business adaptability. However, the disadvantage is that the cost of optical fiber is high and the reliability of the central node is extremely high. The star structure is divided into single star structure, active Double Star structure and passive Double Star structure.

1) single star structure: This structure uses optical fiber cables to directly connect the olts located in the China Telecom exchange office with users, basically point-to-point connections, similar to the existing copper access network structure. Each household has a separate line and is directly connected to the Telecommunications Board. Therefore, a single star can be compatible with the existing copper network. Users are independent of each other and have good confidentiality. Upgrading and resizing are easy, as long as the devices at both ends are changed, new services can be activated and the adaptability is strong. The disadvantage is that the cost is too high, and each household needs a single pair of optical fiber or one optical fiber Bidirectional Wavelength Division Multiplexing). To access thousands of households, thousands of cores of optical fiber cables are required, which is difficult to handle, in addition, each household needs a dedicated light source detector, which is quite complex.

2) active binary structure: It adds an active contact between the center and the user. The Central Bureau shares optical fiber with the active contact, and transmits large-capacity information to the active contact and then small-capacity information to thousands of households by using TDM or OFDM. The advantage is that the flexibility is strong. The source contacts of the central bureau share optical fiber cables, and the number of Optical Fiber cores is small, reducing the cost. The disadvantage is that the active connection is complex, costly, and inconvenient to maintain. In addition, if you want to introduce new broadband services and upgrade the system, you need to replace all the optical devices, or it is difficult to use the wavelength division multiplexing overlay scheme.

3) passive Double Star structure: This structure maintains the advantage of Optical Fiber sharing in the active Double Star structure, and replaces the active contact with the passive shard for easy maintenance, high reliability, and low cost. Because of a series of measures, the confidentiality is also good, it is a good access network structure.

Optical fiber Access Network

According to the position of the optical network unit ONU, optical fiber access methods can be divided into the following types: FTTB optical fiber to the building); FTTC optical fiber to the roadside); FTTZ optical fiber to the residential area); FTTH optical fiber to the user ); FTTO optical fiber to office); FTTF optical fiber to floor); FTTP optical fiber to pole); FTTN optical fiber to neighbors); FTTD optical fiber to door); FTTR optical fiber to remote unit ).

Among them, FTTB optical fiber to the building), FTTC optical fiber to the roadside), FTTH optical fiber to the user) three forms. FTTC is mainly used to provide services for residential users. The ONU optical network unit is set on the roadside, that is, the electrical signals from ONU are sent to users, generally, videos are transmitted over coaxial cables and telephones are transmitted over twisted pair wires. The ONU of FTTB is located in the distribution box of the building. It is mainly used in integrated buildings, remote medical care, distance education, and large entertainment venues to serve large and medium-sized enterprises and business users, provides high-speed data, e-commerce, visual text and other broadband services. FTTH is to place ONU in the user's home to provide various comprehensive broadband services for home users. FTTH is the ultimate goal of the optical fiber access network, but each user needs a pair of optical fiber and a dedicated ONU, as a result, the cost is expensive and implementation is very difficult.

Advantages and disadvantages of optical access networks

Compared with other access technologies, the optical fiber access network has the following advantages:

1) The optical fiber access network can meet users' needs for various services. In addition to making phone calls and watching TV, people also want high-speed computer communication, Home Shopping, home banking, remote education, and VOD) and high definition TV (HDTV. It is difficult to implement these services using copper wires or twisted pair wires.

2) optical fiber can overcome some restrictions that copper cables cannot overcome. Low Optical fiber loss and low frequency bandwidth remove the limitation of small copper wire diameter. In addition, the optical fiber is not subject to electromagnetic interference, ensuring the signal transmission quality. Replacing copper with optical fiber cables can solve the congestion problem of underground communication pipelines in cities.

3) The performance of the optical fiber access network is constantly improved, and the price of copper cables is continuously increasing.

4) The optical fiber access network provides data services with a sound monitoring and management system that can adapt to the needs of the Digital Network for broadband integrated services in the future, breaking the "bottleneck" and making the information highway unobstructed.

Of course, compared with other access network technologies, fiber-optic access networks also have some disadvantages. The biggest problem is the high cost. In particular, the closer the Optical Node is to the user, the higher the access device cost shared by each user. In addition, compared with wireless access networks, fiber-optic access networks also require pipeline resources. This is why many emerging carriers are optimistic about fiber-optic access technology, but have to choose wireless access technology. Currently, the main reason affecting the development of optical fiber access networks is not technology, but cost. So far, the cost of optical fiber access networks is still too high. However, the use of fiber-optic access networks is an inevitable trend in the development of fiber-optic communication. Although the steps for developing fiber-optic access networks vary from country to country, fiber-to-user access networks are recognized as the development goals of access networks.