According to the development trend of telecommunication networks in recent years, the concept of “access network” has been proposed by the ITU-T. The access network refers to all devices between the backbone network and user terminals. Its length is generally a few hundred meters to a few kilometers, so it is vividly called the "last mile." Since the backbone network generally adopts the structure of optical fibers and the transmission speed is fast, the access network becomes the bottleneck of the entire network system. The access methods of the access network include copper wire (ordinary telephone line) access, optical fiber access, fiber optic coaxial cable (cable cable) hybrid access, wireless access, and Ethernet access.
Optical Access Network Overview
Optical fiber access network (OAN) refers to using optical fiber as the main transmission medium to implement the information transmission function of the access network. It is connected to the service node through the optical line terminal (OLT) and to the user through the optical network unit (ONU). The optical fiber access network includes remote devices—optical network units and central office devices—optical line terminals that are connected by transmission devices. The main components of the system are the OLT and the remote ONU. They complete the conversion of signaling protocols from the service node interface (SNI) to the user network interface (UNI) throughout the access network. The access device itself also has networking capabilities and can form various forms of network topology. At the same time, the access device also has local maintenance and remote centralized monitoring functions, forms a maintenance management network through transparent optical transmission, and is integrated into the network management center through corresponding network management protocols.
The role of the OLT is to provide an interface between the access network and the local exchange, and to communicate with the optical network unit of the subscriber through optical transmission. It completely separates the switching capabilities of the switch from user access. The optical line terminal provides maintenance and monitoring for itself and the user side. It can be placed directly on the exchange office together with the local exchange, or on the remote side.
The role of the ONU is to provide the user side interface for the access network. It can access a variety of user terminals, while having the photoelectric conversion function and the corresponding maintenance and monitoring capabilities. The main function of the ONU is to terminate the optical fiber from the OLT, process optical signals and provide service interfaces for multiple small businesses, business users, and residential users. The network end of the ONU is an optical interface and the user end of the ONU is an electrical interface. The ONU therefore has optical/electrical and/or optical conversion functions. It also has voice/digital and analog/digital conversion capabilities. The ONU is usually placed closer to the user and its location has great flexibility.
The optical access network (OAN) is divided into active optical networks and passive optical networks from the system allocation. In this paper we only discuss passive optical networks. No light source network is a kind of point-to-multipoint optical fiber transmission and access technology, refers to the optical distribution network (ODN) between the OLT and ONU, without any active electronic equipment, it includes passive light based on ATM Network APON and passive optical network based on IP E/GPON. It consists of an office-side optical line terminal (OLT), a user-side optical network unit (ONU), and an optical distribution network (ODN). It is called passive because it does not have any electronic devices between the Optical Line Terminal (OLT) and the Optical Network Unit (ONU). The devices used include optical fibers and optical splitters. Source device. Passive optical networks have advantages such as simple equipment, low investment, easy installation, flexible networking, and avoidance of electromagnetic interference and lightning. Optical fiber access network adopts optical fiber as the transmission medium, has long transmission distance, large transmission capacity, strong anti-interference, low attenuation, light weight, etc. It is an ideal solution for wired broadband access technology and represents the broadband access network. Direction of development.
1. (I) GPON System Overview
GPON (Gigabit-Capable Passive Optical Network, Gigabit Passive Optical Network) is the latest generation of broadband passive optical integrated access standard based on the ITU-T G.984.x standard. It has high bandwidth, high efficiency, large coverage, and rich user interfaces. Many advantages are considered by most operators as an ideal technology for realizing broadband network access and integrated transformation.
2. Reference structure of GPON
The reference structure of the GPON access network is shown in Figure 2-1. GPON is mainly composed of an optical line terminal (OLT), an optical distribution network (ODN), and an optical network unit (ONU). The OLT is located At the access network office. Its location can be at the interface of the local exchange within the office, or it can be interfaced with the remote concentrator or multiplex at the remote module in the field. It provides the network side and core for the access The interface of the network and communicates with the user-side ONU through one or more ODNs. The OLT and the ONU are master-slave relationships. They control the ODN, perform real-time monitoring, and manage and maintain the same passive Optical network system.
Figure 2-1 GPON system model diagram
(where SNI is the service node interface, UNI is the user network interface, ODN is the optical distribution network, and TE is the terminal device).
The function of the optical network unit (ONU) is to provide a direct or remote user-side interface for the optical access network, which is located at the user side of the ODN, processes optical signals and provides service interfaces for several users. ONU/ONT of the GPON system (ONT is an ONU that is used for FTTH and has a user port function) can be placed in different locations such as transfer boxes, building/splice boxes, companies/offices, and homes to form an FTTCab (optical fiber to transfer box). FTTB/C (Fiber to Building/Distribution Box), FTTO (Fiber to the Office), and FTTH (Fiber to Home).
GPON adopts TDM broadcast mode in the downlink direction (OLT to optical network unit) and TDMA (Time Division Multiple Access) mode in the upstream direction (ONU to OLT). It can flexibly form topology structures such as tree, star and bus. The GPON system requires that the optical transmission system between the OLT and the ONU use a single-mode optical fiber conforming to the ITU-T G.652 standard, and uplink and downlink generally adopt a wavelength division multiplexing Technology to implement single-fiber two-way uplink and downlink transmission, and uplink uses a wavelength of 1260 nm to 1360 nm. The wavelength of 1480 nm to 1500 nm is used downward. In addition, the GPON system can also use the third wavelength Mode (1540 nm to 1560 nm wavelength) to implement CATV service bearer.
The concept of Passive Optical Network (PON) has a long history. It has The characteristics of high bandwidth and transparency of network protocols. At the same time, the point-to-multipoint structure saves fiber resources and reduces costs. After 20 years of development, Ethernet technology has been completely dominated the local area network with its simple, It has in fact proved to be the best carrier for carrying IP data packets. With the increasing proportion of IP services in metro and trunk transmission, Ethernet is gradually penetrating access, metro and even backbone networks through improvements in Transmission rate and manageability. The combination of Ethernet and PON produces EPON. It also has the advantages of PON and Ethernet, and has become a hot technology in the field of FTTP/FTTH.
Figure 1 shows the EPON system schematic. It can be seen that the EPON access system has the following features:
It is only optical and passive components such as optical fibers and optical splitters between the OLT and the ONU. It is not necessary to rent the equipment room, no power supply, and no active equipment maintenance personnel. Effective saved.
EPON adopts the Ethernet transmission format and is also the mainstream technology of the user LAN/premises network. Both have natural integration and eliminate the cost factors brought about by the conversion of the transmission protocol.
The single-fiber wavelength division multiplexing technology (downlink 1490nm, uplink 1310nm) requires only one trunk fiber and one OLT, and the transmission distance can reach 20 kilometers. The ONU side distributes up to 32 users through the optical splitter, so the cost pressure Of the OLT and the backbone optical fiber can be greatly reduced;
Both the uplink and the downlink are at a gigabit rate. The downlink uses the bandwidth for different users to encrypt broadcast transmissions, and the uplink uses Time Division Multiplexing (TDMA) to share the bandwidth. High-speed broadband fully meets the bandwidth requirements of The access network customers, and can be flexible and flexible to dynamically allocate bandwidth according to changes in user requirements;
With a point-to-multipoint structure, the number of ONUs and a small amount of user-side optical fibers can be used to easily expand and upgrade the system, fully protecting the operator's investment.