Comprehensive analysis of the basic principles of the optical fiber Access Network Technology

There are many things worth learning about the optical fiber access network. Here we mainly introduce the basic principles of the optical fiber access network technology. 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.