Advantages and disadvantages of Optical Fiber Access Network

At present, fiber-optic access networks are still one of the most common access networks. their special features include large communication capacity, high quality, and high flexibility. We believe that with the rapid development of the communication industry, the optical fiber access network technology will be more powerful. From the perspective of the entire telecommunication network, the whole network can be divided into two major parts: public network and user resident network CPN. CPN belongs to the user. Therefore, the general meaning of the telecommunication network refers to the public telecommunication network. Public telecommunication networks can be divided into long-distance networks, relay networks, and access networks. The combination of long distance network and relay network is called the core network. Compared with the core network, the access network is between a local switch and a user. It is mainly used to connect the user to the core network. The access network consists of a series of transmission devices between the business node interface SNI) and the user network interface UNI.

In recent years, the new technological revolution represented by the Internet is profoundly changing the traditional telecom concept and architecture. With the gradual opening of access network markets in various countries, the telecom control policy has relaxed, with the increasing competition and the rapid emergence of new business demands, wired technology, including optical fiber technology, and wireless technology, access networks have become the focus of attention. Driven by huge market potential, various access network technologies have emerged. Optical Fiber Communication has the advantages of large communication capacity, high quality, stable performance, anti-electromagnetic interference, and strong confidentiality. In trunk communication, optical fiber plays an important role. In the access network, optical fiber access will also become the focus of development. Fiber-optic access network is a long-term solution for broadband access development.

I. Basic Structure of the optical fiber Access Network
The optical fiber access network (OAN) uses optical fiber as the main transmission medium to implement the information transmission function of the access network. The optical line terminal (OLT) is connected to the business node, and the optical network unit (ONU) is connected to the user. The optical fiber access network includes remote devices-optical network units and local devices-optical line terminals, which are connected through transmission devices. The main components of the system are OLT and remote ONU. They complete the transformation from the service node interface (SNI) to the user network interface (UNI) related signaling protocol in the entire access network. The access device also has networking capabilities and can form various network topologies. The access device also provides local maintenance and remote centralized monitoring functions. A Maintenance and Management Network is formed through transparent optical transmission, and integrated into the management center through the corresponding Network Management Protocol.

The role of the OLT is to provide interfaces between the access network and local switches, and communicate with the optical network units of the user end through optical transmission. It completely isolates the switch switching function from user access. The optical line terminal provides maintenance and monitoring for itself and the user end. It can be directly placed together with the local switch on the switch end or on the remote end. ONU is used to provide user-side interfaces for access networks. It can be connected to multiple user terminals and has the photoelectric conversion function and corresponding maintenance and monitoring functions. The main function of ONU is to end the optical fiber from the OLT, process optical signals and provide business interfaces for multiple small enterprises, business users and residential users. The network side of ONU is the optical interface, while the user side is the electrical interface. Therefore, ONU has the optical/electrical/optical conversion function. It also provides the number/module and mode/number conversion functions for conversational sounds. ONU is usually placed near the user, and its location is flexible.

Optical fiber access network (OAN) is divided into two types: Active Optical Network (AON) and Passive Optical Network (PON and PassiveOpticaOpticalNetwork.

Ii. Active Optical Fiber Access Network
Active optical networks can be divided into SDH-based AON and PDH-based AON. CE (a local device of an active Optical Network) and RE (a remote device) are connected through an active optical transmission device. The transmission technology is a widely used SDH and PDH Technology in the backbone network, this article mainly discusses SDH Synchronous Optical Network (SDH) systems.

1. SDH-based Active Optical Network
The concept of SDH was first proposed by the bell Institute of Communications in 1985, called Synchronous Optical Network SynchronousOpticalNETwork, SONET ). It is composed of a complete set of standard transfer structures, applicable to a variety of adaptive processing of the net load, that is, the network node interface bit stream can be used for telecommunications services) it is transmitted over physical media such as optical fiber, microwave, and satellite. This standard became a new standard of the US Digital System in 1986. CCITT, the predecessor of the International Telecommunications Union standards department ITU-T), accepted the SONET concept in 1988 and reached an agreement with American Standards Association ANSI, the modified SONET is renamed SynchronousDigitalHierarchy and SDH in the Synchronous Digital series, making it a general technical system suitable for both optical fiber, microwave, and satellite transmission.

SDH is a revolution for the original PDHPlesiochronousDigitalHierarchy quasi-synchronous digital series. PDH is asynchronous, when a low-speed branch signal is connected to a network node, the node must perform the following operations: Re-connection, code conversion, code speed adjustment, timing, scrambling, and decoding, in addition, PDH only specifies the electrical interface, and there is no uniform regulation on the Line System and optical interface, so it is impossible to establish the global information network. With the introduction of SDH technology, the transmission system not only provides the function of the physical process of signal propagation, but also provides the function of signal processing, monitoring and other processes. SDH supports the business of multiple circuit layers by defining multiple container C and virtual container VC and cascade frame structures, such as asynchronous digital series, DQDB, FDDI, and ATM at various rates, as well as various new services that may emerge in the future. A large number of standby channels in the segment overhead enhance the scalability of SDH network. The software control enables the original PDH to manually change the wiring method to achieve cross-connection and plug-and-play multiplexing connections, provide flexible upper/lower circuit capabilities, and make the network topology dynamically changeable, this enhances the flexibility and security of the network to adapt to business development, and enables optimal utilization of circuit protection, height, and communication capabilities within a larger geometric range, thus laying the foundation for enhanced networking capabilities, you can rebuild the network in just a few seconds. In particular, SDH Self-healing ring can be quickly restored within dozens of milliseconds after the circuit fails. These advantages of SDH make it the basic transmission network for broadband business digital networks.

The main advantage of applying SDH to access networks is that SDH can provide ideal network performance and business reliability; the inherent flexibility of SDH makes it especially suitable for developing cellular communication systems using SDH systems. Of course, considering the high cost sensitivity of the access network and the bad performance of the operating environment, SDH devices suitable for access networks must be a new type of system with high compact power consumption and low cost, and their market application prospects are promising.

The Latest Development Trend of SDH for access networks is to support IP access. At present, at least Ethernet interface ing is required. In addition to carrying voice traffic, some SDH net loads can be used to transmit IP services, so that SDH can also support IP Access. There are many supported methods. In addition to the existing PPP method, it is also highly efficient to support IP transmission using the hierarchical scheme of VC12. In short, as a mature and reliable transmission technology that provides the main business revenue, it will continue to improve and support the smooth transition from a circuit switching network to a branch network in the foreseeable future.

2. PDH-based Active Optical Network
With its cheap features and flexible networking functions, the quasi-synchronous digital series PDH has been widely used in access networks. Especially in recent years, the SPDH equipment has introduced the SDH concept into the PDH system, which further improves the reliability and flexibility of the system. This improved PDH system will still be widely used for a long time.

Iii. Passive Optical Fiber Access Network
Passive Optical Network (PON) refers to the optical Distribution Network (ODN) between the OLT and The ONU. It does not have any active electronic devices. It includes an ATM-based Passive Optical Network (APON) and an IP-based PON. APON business development is implemented in stages, and the initial stage is the VP leased line business. Compared with common leased line services, APON provides low-cost and small-size VP leased line service equipment, featuring power saving, reliable and stable system, and advantageous in performance-to-price ratio. The second step is to implement a group and secondary group circuit simulation service to provide Intranet connections and enterprise telephone and data services. Step 3: Implement Ethernet interfaces to provide Internet and VLAN services. In the future, it will gradually expand to other services and become a real full-service access network system.

APON uses a cell-based transmission system that allows multiple users in the access network to share the entire bandwidth. This method of Statistics reuse can make more effective use of network resources. An important factor in whether APON can be widely used is the price issue. At present, the first generation of actual APON products have limited service supply capability and high costs. The market prospect of the first generation is uncertain due to the global failure of ATM, but its technological advantages are obvious. In particular, considering the operation and maintenance costs, the PON system is deployed in highly competitive regions in new regions or regions that need to replace the old copper system, or FTTB is a far-sighted choice. Whether or not the price-performance ratio can be improved to an acceptable level in the market in the next few years is the key to the survival and development of APON technology.

The upper layer of IPPON is the IP address. This method makes full use of network resources, so it is easy to dynamically allocate system bandwidth and simplify the complex devices in the middle layer. PON-based OAN does not require expensive active electronic devices to be installed on external sites, so service providers can provide enterprise users with the required bandwidth cost-effectively.

Passive Optical Network (PON) is a pure media network that avoids electromagnetic interference and lightning impact on external devices, reduces the failure rate of lines and external devices, and improves system reliability, at the same time, it saves maintenance costs and is a technology that the telecommunications maintenance department has long expected. The advantages of the passive optical access network are embodied in the following aspects:
1) The non-source optical network is small in size, the equipment is simple, the installation and maintenance costs are low, and the investment is relatively small.
2) flexible networking of passive optical devices. The topology can support network topologies such as tree, Star, bus, hybrid, and redundant.
3) easy to install. It has indoor and indoor shapes. The room exterior can be directly mounted on the wall or placed on the "H" pole without the need to rent or build an equipment room. However, the active system requires photoelectric and electro-optic conversion, and the equipment manufacturing costs are high. dedicated sites and data centers are required. The remote power supply problem cannot be solved, resulting in heavy daily maintenance workload.
4) Passive Optical Networks are suitable for point-to-multiple-point communication. Only passive optical splitters are used to distribute optical power.
5) The Passive Optical Network is a pure media network that completely avoids electromagnetic interference and lightning impact. It is ideal for use in areas with poor natural conditions.
6) from the perspective of technology development, the expansion of the passive optical network is relatively simple and does not involve equipment transformation. Only the software upgrade and one-time purchase of hardware equipment are required for long-term use, laying the foundation for fiber entry, ensures user investment.

4. optical access network topology
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.

V. Form of Optical Fiber Access Network
According to the position of the optical network unit ONU, the optical fiber access methods can be divided into the following types:

FTTB optical fiber to building); FTTC optical fiber to roadside); FTTZ optical fiber to residential area); FTTH optical fiber to user); FTTO optical fiber to office); FTTF optical fiber to floor ); FTTP optical fiber to pole); FTTN optical fiber to neighbor); 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.

Vi. 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.

1. Various broadband wireless access network technology competitions
2. Future Development Trend of Access Network Technology
3. FTTH leads the revolution in Broadband Access Technology
4. Discussion on the Development Trend of Wireless Network Access Technology
5. Quick Access Network Management