Optical fiber has many advantages such as wide bandwidth, strong ability of long-distance transmission, good confidentiality and strong anti-interference ability etc., which is the main technology to realize the future access network. The FTTH mode refers to that optical fiber runs through users' home, generally only needs one or two subscriber lines. It is not economical in the short term, but it is the long-term development direction and the ultimate solution for access networks. Pure optical fiber access network refers to the direct connection of optical fiber to users without other transmission media application scenarios, which can be divided into Fiber To The Office (FTTO) and Fiber To The Home (FTTH). The FTTO mode refers to enterprise networks that need dozens to hundreds of user lines. In this scenario, as long as the number of user lines exceeds a certain amount, it is cost-effective to use optical fiber.
The PON transmission scheme transmits narrowband service signals at 1310nm wavelength region and broadband service signals at 1550nm wavelength region. The benefit of doing so is: the price of 1310/1550nm wavelength division multiplexing devices is cheap, which can economically transmit narrowband services. Meanwhile, the fiber optic loss at 1550nm wavelength region is small, suitable for transmitting broadband signals. Because PON is a structure mainly based on optical fiber using added splitting technology, it is unsuitable to adopt bus-type structure, mostly adopting passive dual star (DSP) or tree-type structures. APON based on ATM is the development trend of PON. APON is a passive optical network supporting broadband services. In this structure, the distance from ONU to the central office can be up to 10 kilometers. The downstream rate is 155Mb/s or 622Mb/s, and the upstream rate is 155Mb/s. Both downstream and upstream directions adopt cell-based transmission schemes. It can directly be compatible with ATM's OD network.
Currently, SDH has firmly established itself in the core network. It is necessary to bring the great technical advantages of SDH into the access network area, making the functions and interfaces of SDH as close to the user as possible. The inherent flexibility of SDH enables network operators to more quickly and effectively provide long-term and short-term business demands required by users and networking needs. For the rapidly developing cellular communication system, adopting SDH system is particularly suitable. It can quickly and flexibly provide the required 2Mbps transparent channel, fiber access price. For large enterprises requiring high reliability and high quality services, SDH can provide ideal network performance and service reliability. At this point, SDH system can be directly used to connect with users in point-to-point or ring topology form. This can increase transmission bandwidth, improve network management capabilities, simplify maintenance work, and reduce operation and maintenance costs.
PON is an inexpensive choice. APON adopts time-division multiplexing (TDM) technology in the downstream direction, and performs TDMA protocol in the upstream direction. Only one user is allowed to transmit at a given moment in the upstream direction. There are several main transmission technologies for PON, including FDM, TDM and DWDM, and they will continue to develop. In some PON systems, CAT is transmitted as a digital signal on the same optical cable but at different wavelengths. Since PON uses wavelength division multiplexing and light source power separation technology, the equipment on the network is shared, and the service is transparent, thus supporting a good network management system, reducing operating costs, and realizing comprehensive services.
From SDH in the core network to the access network, overall development trends show that the application of optical fiber access networks first replaces feeder cables with feeder optical fibers, then continues to advance towards users, but the cost becomes increasingly higher. Therefore, currently, optical fiber usually only reaches the distribution box by the roadside, i.e., the Service Access Point (SAP). A Optical Network Unit (ONU) also needs to be set up here to achieve photoelectric conversion and branching functions. The ultimate goal of pure optical fiber access network is to push optical fiber to residential users, at which point ONU will be set up at the user's residence. Currently, fiber to the home is not practical because the price of optical fiber is still too expensive. With the significant drop in optical fiber prices, optical fiber will eventually replace copper wires to achieve FTTH. The HFC system developed based on CATV is currently the most suitable for application and the best broadband access network for transitioning to FTTH. It uses optical fiber for broadband transmission and coaxial cable for connecting user terminals, with a bandwidth approaching 1000MHz, providing multiple services and having broad development prospects.
Optical fiber access systems can also be divided into active and passive systems. Active systems have PDH and SDH based distinctions, and their topologies can be ring, bus, star or hybrid types, also there are point-to-point applications. Passive systems are PON (Passive Optical Network), which can be divided into narrowband and broadband. Currently, the standardized broadband PON is ATM-based PON, namely APON. The PON itself is a point-to-multipoint system downstream and multipoint-to-point system upstream. Upstream requires solving multi-user contention issues, currently most upstream use TDMA (Time Division Multiple Access) technology. Optical fiber access network (OAN) is an access network using optical fiber transmission technology, i.e., a communication system where all or part of the transmission between the local exchange office and users adopts optical fiber transmission. Optical fiber access network can also be divided into Passive Optical Network (PON) and Active Optical Network (AON), among which the development of passive optical network is faster.
China allows optical cables to spread across urban and rural areas. In urban access network construction, wherever conditions permit, optical cables should be used to replace copper cables as much as possible. When the transmission distance is greater than 2 kilometers and the number of users exceeds 300 households, optical fiber transmission should be adopted; telephone transmission quality poor cable lines should be replaced with optical fiber access systems first; new residential areas, development zones, financial districts, business districts should all adopt optical fiber access methods entirely; for colleges, universities, government agencies, wherever conditions permit, fiber access should be prioritized; gradually achieve fiber entering rural areas. In rural access network construction, optical fiber access method should be adopted for centralized users who are far away, while relatively scattered users can adopt wired access and wireless access combined methods. For units that need computer networking, actively recommend optical fiber networking methods; for scattered users, batch implementation of fiber to the curb should be planned. With the development of optical fiber access technology, our country has also begun a grand optical fiber access project. In actual work, in order to ensure that trunk optical cables have sufficient redundant capacity and consider voice, data, CATV and other services, the main trunk optical cables in medium-sized cities will be no less than 48 cores, and the main trunk optical cables in small cities will be no less than 24 cores. Urban areas promote fiber entering buildings. In-city optical cables should go through underground pipelines as much as possible, suburban and rural areas optical cables should be buried directly as much as possible, and in special geographical environment areas, a small amount of overhead optical cables can be used.
For large enterprises that require bandwidth greater than 34Mbps, directly setting the SDH add/drop multiplexer (ADM) at the user's place to connect with STM-N service nodes via STM-1 channels has been proven to be an economically feasible way. This connection can be either point-to-point or achieved through a ring structure. For situations where the bandwidth requirement is far less than 34Mbps, adopting lower-speed multiplexers or sharing ADMs is more economical and effective. In order to fully utilize the advantages of SDH, SDH needs to be further expanded to small-bandwidth users, especially wireless users, providing 64Kbps level access and integrating existing and new service transmission platforms. Using STM-0 sub-rate connections (SubSTM-0) is an economical and effective plan for small-bandwidth users, while maintaining the overall SDH management capability and function. Currently, ITU-T Study Group 15 has developed a new recommendation G.708, which specifies two interfaces, SSTM-2n interface for transmitting TUG-2 and SSTM-1k interface for transmitting TU-12. For most ordinary enterprise users, terminal multiplexers located at the roadside (DP points) can be used to provide bandwidth for a large number of users with 2Mbps as the basic unit. Users requiring less than 2Mbps bandwidth can be solved by service multiplexers or subsequent PON.
Although full-digital, broadband optical fiber user loop (FITL) has the advantages of wide bandwidth, large capacity, and the ability to provide various services, its cost is relatively high, making it difficult to implement all at once. An ideal broadband access solution is the single-star type optical fiber to home (SS-FTTH) method of replacing copper twisted pairs with optical fiber access network. If such a broadband access network is built at once, it would be the most ideal for achieving B-ISDN. However, considering the current situation, the implementation cost of SS-FTTH is too high, and the market for broadband interactive services is still uncertain, so this ideal OAN solution is currently difficult to implement. A more feasible solution is to adopt Passive Optical Network (PON), which reduces the initial construction cost of OAN by using special point-to-multipoint multiple access protocols, allowing numerous optical network units to share optical road terminals and numerous users to share optical network units.
Related thematic articles: Market status of optical fiber access networks