1. Background
First of all, as users have higher and higher requirements for broadband access, existing broadband access methods, such as ADSL and LAN access, have short transmission distance, limited access bandwidth, low security, and no QoS. Issues such as good guarantees have increasingly failed to meet the needs of users. Second, the rapid development of optical access technologies, from active optical access technologies (PDH, SDH, MSTP, point-to-point Ethernet systems) to PON passive optical access technologies (APON, BPON, GPON, EPON, GEPON) . Finally, due to the cost of optical fiber itself, the cost of equipment for optical transceiver modules, OLT and ONU, and the supporting cost of existing fiber-to-the-home, the current equipment cost and line cost of fiber-to-the-home are significantly higher than before. decline. Therefore, the fiber-to-the-home access method has gradually matured, and it has gradually become a hot spot in the communication industry at home and abroad. In the near future will become an important means of user access. However, which fiber access technology is more suitable for the large-scale development of FTTH among the current many fiber access technologies?
2. Active fiber access technology
2.1 PDH
PDH technology is widely used in the field of optical access due to its maturity. Its security, reliability and high QoS guarantee performance make it still an important optical fiber access technology method for telecom operators for a period of time. However, the traditional PDH technology inevitably has certain limitations in access applications, mainly reflected in:
(1) Lack of unified network management;
(2) Lack of networking capabilities;
(3) Poor protection for business;
(4) Lack of effective bearing means for the rapidly growing IP data services;
(5) Capacity expansion and upgrade lack flexibility;
(6) The interface is single, the equipment is stacked, there are many external cable connections, and the number of fault points increases, which brings difficulties to maintenance.
PDH optical access technology is mainly used for point-to-point small capacity private line enterprise users.
2.2 SDH
Compared with PDH, SDH, which is widely used in fiber access of enterprise and business customers, has the following obvious advantages:
(1) A unified bit rate and a unified interface standard facilitate the interconnection between devices;
(2) The network management capability is greatly strengthened;
(3) With self-healing protection function.
The main disadvantage of SDH is that it is designed to transmit TDM information. This technology lacks the functions required to process information other than traditional voice information based on TDM technology, and is not suitable for transmitting ATM and Ethernet services other than TDM.
SDH optical access technology is mainly used for point-to-point large-capacity private line enterprise users, inter-office or tandem point (POP) communication.
2.3 MSTP
Based on SDH, it also realizes the service access, processing and transmission of TDM, ATM, Ethernet and other services, and provides unified network management MSTP, which has the following advantages:
(1) Provide multiple physical interfaces to meet the rapid access to new services. While ensuring compatibility with traditional TDM services, it can provide flexible access to multiple services. Typical businesses are: IP, ATM, SDH, FR.
(2) Because it is based on the existing SDH transmission network, it can be well compatible with the existing technology and guarantee the existing investment.
(3) MSTP uses VC virtual cascade technology to effectively use bandwidth and achieve bandwidth management of smaller particles.
(4) MSTP uses LCAS technology to ensure that the number of virtual cascades can be adjusted dynamically without interrupting the data flow.
(5) MSTP technology supports meshing, tree, star, multi-ring switching and other networking methods, which can improve the scalability of the network and facilitate flexible and efficient configuration of the system environment.
(6) High reliability of transmission and automatic protection recovery function. MSTP inherits the protection features of SDH, and the automatic protection recovery is less than 50ms to ensure users' satisfaction with the service.
The disadvantages of MSTP are:
(1) The bandwidth utilization rate is low;
(2) The maximum bandwidth provided is limited;
(3) Mainly realize the second layer function and the relatively simple three layer function;
(4) Insufficient ability to provide flexible services.
(5) Occupation of optical fiber is more
The applications of MSTP are mainly located in the inter-office or inter-junction communication and the point-to-point communication of large enterprises and users.
2.4 Point-to-point Ethernet system
Point-to-point Ethernet system is the most direct Ethernet fiber access technology. Each user is directly connected to a user optical interface of the central office Ethernet switch via a pair of optical fibers. In the point-to-point Ethernet system, through the extended Ethernet OAM protocol, user-side equipment can be remotely managed through the central office switch, thereby providing carrier-grade operational and manageable Ethernet access.
2.4.1 Advantages
(1) High access bandwidth and convenient network upgrade;
(2) The network layer is simple, and the access network and user Ethernet are seamlessly connected;
(3) The Ethernet switch is placed in the building, community or central office room, and the central office and the user terminal are directly connected by optical fiber, and the entire access network structure is simple;
(4) The business opening rate is high and the investment recovery is fast;
(5) User-side equipment can be managed remotely through the central office switch, and line detection and fault location can be easily performed at the central office, reducing maintenance difficulty.
2.4.2 Disadvantages
(1) It is necessary to re-lay the optical fiber line;
(2) Each user occupies one / pair of optical fiber, the number of optical fibers is large, and the construction is difficult;
(3) Because the inherent mechanism of Ethernet technology does not provide end-to-end packet delay, packet loss rate and bandwidth control, it is difficult to guarantee the quality of service of real-time services, and it is difficult to provide TDM services;
(4) Maintenance cost is very high;
(5) Lack of security mechanism guarantee;
2.4.3 Application
When the users are very dense, the space requirements and costs of the equipment room increase rapidly, so it is not suitable for high-density user areas, and is more suitable for scattered users.
3. Passive optical fiber access technology
Passive optical network (PON) refers to the optical distribution network (ODN) between the OLT (optical line terminal) and the ONU (optical network unit) without any active electronic equipment. Its typical topology is a point-to-multipoint star structure (as shown in Figure 1). No node equipment is needed at the optical branch point, only a simple passive optical splitter needs to be installed, so it has the advantages of saving optical cable resources, sharing bandwidth resources, saving equipment room investment, high security, low comprehensive network construction costs, low maintenance costs High reliability and other advantages.
Figure 1 Topological diagram of passive optical network (PON)
Disadvantages of PON fiber access technology:
(1) The initial investment cost is too high;
(2) Its topology makes the user not have the protection function or the protection cost is too high.
The application of PON optical fiber access technology is mainly suitable for scattered small enterprises and resident users, especially those user areas are scattered, and the users in each area are relatively concentrated in a small area of ​​dense user areas.
At present, there are five types of PON-based fiber access technologies: APON, BPON, GPON, EPON, and GEPON. Because APON and BPON are based on ATM, and ATM is not the direction of development, and its rate is limited, the equipment is complex, and it cannot meet the users' high bandwidth and Low cost requirements, therefore, APON and BPON are not the development direction. This article mainly introduces EPON / GEPON and GPON fiber access technologies.
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