Optical fiber access network refers to the application form that uses optical fiber as the main transmission medium in the access network to realize user information transmission. It is not a traditional optical fiber transmission system, but a special optical fiber transmission designed for the access network environment.
History
The new revolution represented by the Internet is profoundly changing the traditional telecommunications concepts and architecture. With the gradual opening of access network markets in various countries, the relaxation of telecommunications regulatory policies, the intensification and expansion of competition, and the rapid emergence of new business needs, and the development of stable technologies (including optical fiber technology) and wireless technologies, access networks have begun to become the focus of attention. Under the huge market-driven potential, various access network technologies have been produced. With huge optical fiber communication capacity, optical fiber access plays an important role in trunk communications. In access networks, optical fiber access will also become the focus of development and long solution.
Basic composition
Optical access network (OAN) refers to using optical fiber as the main transmission medium to realize the information transmission function. It is connected to service nodes through optical line terminals (OLT) and connected to users through optical network units (ONU). The optical fiber access network includes remote equipment(optical network units) and central office equipment( optical line terminals), which are connected through transmission equipment. The main components of the system are OLT and remote ONU. They complete the conversion of signaling protocols from the Service Node Interface (SNI) to the User Network Interface (UNI) in the entire access network. The access device itself also has networking capabilities and can form various forms of network topology. At the same time, the access equipment also has local maintenance and remote centralized monitoring functions. It forms a maintenance and management network through transparent optical transmission and is integrated into the network management center for unified management through the corresponding network management protocol.
The role of the OLT is to provide an interface between the access network and the local switch, and to communicate with the optical network unit at the user end through optical transmission. It completely separates the switching function of the switch from user access. The optical line terminal provides maintenance and monitoring of itself and the user end. It can be placed directly at the exchange end together with the local switch, or it can be set up at the remote end.
The function of the ONU is to provide user-side interfaces for the access network. It can be connected to a variety of user terminals and has photoelectric conversion functions and corresponding maintenance and monitoring functions. The main function of the ONU is to terminate the optical fiber from the OLT, process optical signals and provide business interfaces for multiple small businesses, institutional users and residential users. The network end of the ONU is an optical interface, and its user end is an electrical interface. Therefore, ONU has optical/electrical and electrical / optical conversion functions. It also has digital/analog and analog/digital conversion capabilities for voice. The ONU is usually placed close to the user, and its location has great flexibility.
Optical access network (OAN) is divided into active optical network (AON) and passive optical network (PON) in terms of system distribution.
Fiber Access Network Types
Active optical network (AON)
Active optical networks can be divided into SDH-based AON and PDH-based AON. The central office equipment (CE) and remote equipment (RE) of the active optical network are connected through active optical transmission equipment. The transmission technology is SDH and PDH technology that have been widely used in the backbone network, but SDH technology is the main one. Below mainly discuss SDH (Synchronous Optical Network) system.
SDH based
The concept of SDH was first proposed by the Bell Communications Research Institute in the United States in 1985 and is called Synchronous Optical NETwork (SONET). It is composed of a set of hierarchical standard transmission structures and is suitable for various adapted and processed payloads (that is, the part of the network node interface bit stream that can be used for telecommunications services) on physical media such as optical fiber, microwave, satellite, etc. transfer on. This standard became the new standard for the American number system in 1986. The Consultative Committee on International Telegraph and Telephone (CCITT), the predecessor of the Standards Department of the International Telecommunication Union (ITU-T), accepted the SONET concept in 1988 and reached an agreement with the American Standards Institute (ANSI) to modify SONET and rename it to the Synchronous Digital Series ( Synchronous Digital Hierarchy (SDH), making it a universal technical system suitable for optical fiber, microwave, and satellite transmission at the same time.
The SDH network is a revolution of the original PDH (Plesiochronous Digital Hierarchy) network. PDH is asynchronous multiplexing. When a low-speed branch signal is accessed and received at any network node, processes such as multiplexing, code conversion, code rate adjustment, timing, scrambling, and descrambling must be carried out on the node. PDH only The electrical interface is stipulated, but the line system and optical interface are not uniformly stipulated, making it impossible to establish a global information network. 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 other processes. Through the definition of multiple containers C, virtual containers VC, and cascaded multiframe structures, SDH can support multiple circuit layer services, such as asynchronous digital series at various rates, DQDB, FDDI, ATM, etc., as well as possible in the future. Various new businesses emerge. A large number of spare channels in the segment overhead enhance the scalability of the SDH network. Through software control, the original method of manually changing wiring in PDH realizes cross-connection and add-drop multiplexing connections, provides flexible circuit add/drop capabilities, and makes the network topology dynamically variable, enhancing the ability of the network to adapt to business development. Flexibility and security enable optimal utilization of circuit protection, height and communication capabilities within a larger geometric range, thereby laying the foundation for enhanced networking capabilities and re-networking in just a few seconds. In particular, the SDH self-healing loop can quickly recover within tens of milliseconds after a circuit failure. These advantages of SDH make it the basic transmission network for broadband business digital networks.
The main advantages of applying SDH (synchronous optical network) in access networks are: SDH can provide ideal network performance and service reliability; the inherent flexibility of SDH makes the SDH system particularly suitable for cellular communication systems that are developing extremely rapidly. Of course, considering the high sensitivity of access networks to costs and the harsh operating environment, SDH equipment suitable for access networks must be highly compact, low-power consumption and low-cost new systems with promising market application prospects.
The latest development trend of SDH for access networks is to support IP access. Now it is necessary to support at least the mapping of Ethernet interfaces. Therefore, in addition to carrying voice traffic, part of the SDH payload 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, using the VC12 cascade method to support IP transmission is also a more efficient method. In short, as a mature and reliable transmission technology that provides major business revenue, it will continue to improve in the foreseeable future to support the smooth transition from circuit-switched networks to packet networks.
Based PDH
Plesiosynchronous digital series (PDH) has been widely used in access networks due to its cheap features and flexible networking functions. In particular, the SPDH equipment introduced in recent years has introduced the SDH concept into the PDH system, further improving the reliability and flexibility of the system. This improved PDH system will still be widely used for a long time.
Passive optical network (PON)
Passive optical network (PON) refers to the optical distribution network (ODN) between OLT and ONU without any active electronic equipment. It includes ATM-based passive optical network APON and IP-based PON. APON’s business development is implemented in stages, with the initial focus being VP dedicated line services. Compared with ordinary dedicated line services, the VP dedicated line service equipment provided by APON has low cost, small size, power saving, reliable and stable system, and certain advantages in performance-price ratio. The second step is to implement primary group and secondary group circuit simulation services, and provide enterprise intranet connections and enterprise telephone and data services. The third step is to implement the Ethernet interface to provide Internet access services and VLAN services. In the future, it will be gradually expanded to other services and become a veritable full-service access network system.
APON uses a cell-based transmission system to allow multiple users in the access network to share the entire bandwidth. This statistical multiplexing method can make more effective use of network resources. An important factor in whether APON can be widely used is the price issue. Today’s first-generation actual APON products have limited business supply capabilities and high costs. Their market prospects are uncertain due to the setback of ATM worldwide, but their technical advantages are obvious. Especially considering the operation and maintenance costs, in newly built areas, highly competitive areas or areas that need to replace old copper cable systems, laying PON systems at this time, whether it is FTTC or FTTB, is a far-sighted choice. Whether the performance-price ratio can be improved to a level acceptable to the market in the next few years is the key to the survival and development of APON technology.
The upper layer of IPPON is IP. This method can make fuller use of network resources, easily achieve dynamic allocation of system bandwidth, and simplify complex equipment in the middle layer. PON-based OANs do not require the installation of expensive active electronics in external stations, thus enabling service providers to cost-effectively deliver the required bandwidth to enterprise users.
Passive optical network (PON) is a pure media network that avoids electromagnetic interference and lightning effects from external equipment, reduces the failure rate of lines and external equipment, improves system reliability, and saves maintenance costs. It is an ideal choice for telecommunications maintenance. Technology that the department has long awaited.
The advantages of passive optical networks:
(1) The passive optical network is small in size, has simple equipment, low installation and maintenance costs, and relatively small investment.
(2) Passive optical equipment networking is flexible, and the topology can support tree, star, bus, hybrid, redundant and other network topologies.
(3) Easy to install, it has indoor and outdoor types. Its outdoor type can be hung directly on the wall or placed on the “H” pole, without the need to rent or build a computer room. Active systems require photoelectric and electro-optical conversion, and the equipment manufacturing costs are high. Special venues and computer rooms are required. Remote power supply problems are difficult to solve, and daily maintenance workload is heavy.
(4) Passive optical networks are suitable for point-to-multipoint communications and only use passive optical splitters to distribute optical power.
(5) The passive optical network is a pure media network, which completely avoids electromagnetic interference and lightning effects, and is extremely suitable for use in areas with harsh natural conditions.
(6) From the perspective of technological development, passive optical network expansion is relatively simple and does not involve equipment modification. It only requires equipment software upgrades. Hardware equipment is purchased once and used for a long time. This lays the foundation for optical fiber to the home and ensures user investment.
Features of optical fiber access network
1. The network coverage radius is generally small and does not require repeaters. However, due to the sharing of optical fibers by many users, optical power distribution or wavelength distribution may require the use of optical fiber amplifiers for power compensation;
2. It is required to meet the transmission requirements of various broadband services, with good transmission quality and high reliability;
3. Wide range of application;
4. The investment cost is high, network management is complex, and remote power supply is difficult.
Access Formation
According to the location of the optical network unit (ONU), fiber access methods can be divided into the following types:
- FTTB (fiber to the building);
- FTTC (fiber to the curb);
- FTTZ (fiber to the community);
- FTTH (fiber to the home);
- FTTO (fiber to the office);
- FTTF (fiber to the floor);
- FTTP (fiber to the pole) ;
- FTTN (Fiber to the Neighborhood);
- FTTD (Fiber to the Door);
- FTTR (Fiber to the Room).
The most important and widely used are FTTB (Fiber to the Building), FTTC (Fiber to the Curb), and FTTH (Fiber to the home).
FTTC mainly provides services for residential users. The optical network unit (ONU, or optical network terminal ONT) is set up on the roadside, that is, near the user’s residence. The electrical signals from the ONU ONT are then transmitted to each user. Video services are generally transmitted using coaxial cables and twisted pairs. Line transmission telephone service.
FTTB’s ONU ONT is installed at the wiring box in the building. It is mainly used in comprehensive buildings, telemedicine, distance education, and large entertainment venues. It serves large and medium-sized enterprises and institutions and commercial users, providing high-speed data, e-commerce, visual Broadband services such as graphics and text.
FTTH places ONUs ONTs in users’ residences to provide home users with various comprehensive broadband services. FTTH is the ultimate goal of optical fiber access networks, but each user requires a pair of optical fibers and a dedicated ONU, so it is expensive to implement. very difficult.
Advantages Disadvantages
Compared with other access technologies, optical fiber access networks have the following advantages:
(1) Optical fiber access network can meet users’ needs for various services. People’s demand for communication services is increasing. In addition to making phone calls and watching TV, people also hope for high-speed computer communications, home shopping, home banking, distance learning, video on demand (VOD), and high-definition television (HDTV). These services are difficult to implement using copper wires or twisted pairs.
(2) Optical fiber can overcome some limitations that copper cables cannot overcome. Optical fiber has low loss and wide frequency, which relieves the limitation of small copper wire diameter. In addition, optical fibers are not subject to electromagnetic interference, ensuring signal transmission quality. Using optical cables instead of copper cables can solve the problem of congestion in urban underground communication pipes.
(3) The performance of optical fiber access networks continues to improve and prices continue to decrease, while the prices of copper cables continue to rise.
(4) The optical fiber access network provides data services and has a complete monitoring and management system, which can adapt to the needs of future broadband integrated service digital networks, break the “bottleneck”, and make the information highway smooth.
Of course, compared with other access network technologies, optical fiber access networks also have certain disadvantages. The problem is the cost is higher. In particular, the closer the optical node is to the user, the higher the access equipment cost shared by each user. In addition, compared with wireless access networks, optical fiber access networks also require pipeline resources. This is also the reason why many emerging operators are optimistic about optical fiber access technology, but have to choose wireless access technology.
Today, the main factor affecting the development of optical fiber access networks is not technology, but cost. However, the use of fiber optic access networks is an inevitable trend in the development of fiber optic communications. Although the steps for developing fiber optic access networks vary across countries, fiber to the home is a recognized development goal of access networks. [3]
