GPON is a flexible gigabit per second fiber connection proposed by ITU-T. It carries multiple services with ATM cells and GEM frames to support broadband full-service access for commercial and residential services. GEM(GPON Encapsula TIonMetod) is similar to the universal frame procedure GFP and is a unique packaging method of GPON. It not only has the ability to transmit Gigabit per second reflected in the name, but also is a new PON technology that is essentially different from the existing PON system.
With the continuous rapid development of the Internet and the popularity of personal computers, various value-added multimedia and IP services have proliferated. There is already an xDSL access technology that can not meet its huge bandwidth needs, and access to the network has become a "bottleneck" for the development of the entire network. Taking this reality into account, the Full Service Access Network(FSAN: Full Service Access Network) began standard research on PON systems above lGb/s in January 2001.
FSAN tries to seek a full-service access scheme that is flexible, programmable, high-speed, efficient, low-cost, and has rich business and bandwidth management capabilities that can effectively support existing and future businesses, and gradually forms the GPON series of standard specifications.
2 GPON Reference Configuration
The network structure of GPON consists of GPON's local device OLT Line Terminus, remote access device ONT(OpticalNetwork Unit) and ODN(Optical Division Network), in which ODN does not contain any electronic source devices, All ODN optical branch devices consist of passive devices such as passive optical branches(Plitter) and do not require valuable active devices. In the GPON network structure, an OLT can have multiple GPON modules, each of which leads to an independent GPON access network and is connected by passive optical branches and optical fibers to multiple different types of ONUs, as shown in Figure 1[l].
On the Structure and Key Techniques of GPON System
Figure L GPON system reference configuration
GPON's local device OLT is mainly responsible for high-speed connections to the wide area network or backbone metropolitan area network, and transmits high-speed connected data information through ODN to the remote access device ONU. As a high-performance optical network platform, packet data and TDM applications in transmission formats such as IP/SONET/SDH / ATM / CWDM are supported on the GPON architecture, and the above application services can be transparently transmitted. To meet the strict requirements of TDM for real-time and chattering, it has a complete service guarantee mechanism and troubleshooting capability, with a downlink rate of up to 1.244 Gb/s or 2.488 Gb/S.
The ONU remote access device is mainly responsible for the connection with the client device, which acts as a hub for the client's multi-service data stream. It can be transparently transmitted in the original format of the packet data and TDM service stream, ensuring the integrity and unity of the service performance. It also provides users with rich network interface types and standard rates to meet users 'needs for multiple services.
ODN is responsible for data signal transmission of passive optical fiber access lines. Its transmission distance is shorter than that of active optical fiber access systems. The coverage range is within 40km. The network structure is flexible and there is no need for a separate computer room. The maintenance is relatively simple. Therefore, this access network structure can economically provide all kinds of users with full-service access services.