The construction of man in China is still very rapid, and the access network technology also plays a very important role in it, whether from the selection of technical methods or from the selection of materials, only by adapting to local conditions can we maximize the role of the access network technology.
1. Access Network Technology in Metro Transmission Networks
The core layer consists of core nodes, including exchange bureaus, long-distance bureaus, data centers, and gateway bureaus. It is responsible for large-capacity relay circuits between core nodes and interconnection with provincial/local long-distance networks, interconnection with other networks. The network structure is relatively stable, and the Business reliability and security requirements are high. The number of network nodes is small, the service capacity is large, and the circuit scheduling is frequent. Core layer networking technologies include Metro wavelength division, MSTP, and OXC.
If the traffic volume is not very large, the new Metro Transmission Network core layer can be networked using MSTP access network technology. The business of the metro core layer has a high degree of convergence, and the number of core device nodes is relatively small. Large-particle business transmission can be achieved through 10G devices or 40G devices. Due to the long development and application process of SDH Equipment, the cost of SDH-based MSTP System is relatively low, and mature network protection and large network bandwidth can be provided, it carries high-speed IP addresses, POS ports, and traditional SDH ports, and provides SDH link services at the same time to achieve interconnection between the Exchange Board, the gateway board, and the Exchange Board. The initial network construction uses MSTP technology to provide a low-cost Comprehensive Business Solution for the Metro Transmission Network core layer.
The Metro core layer does not require the transmission network to have L2 switching and processing functions. As long as it provides point-to-point high-speed connections to POS or GE/10GE interfaces), the MSTP of the core layer only needs to provide the data passthrough function. In terms of the relationship between the Metro Transmission Network and the IP network, the efficiency, flexibility, and cost of the current metro transmission network carrying IP data are still not resolved, and the traffic volume is not a particularly large Metro core layer, the IP network and the metro transmission network can be set up separately. The IP network node is independent of the transmission network node. Separate networking is conducive to taking full advantage of their respective technical advantages to achieve powerful business processing capabilities at the core layers of the two networks.
The core layer should adopt the metro Wavelength Division Technology for areas with extremely large business volumes, especially areas with high business traffic growth in the future. The Metro wavelength division technology can be used to unify the IP Broadband and the core layer of the Metro Transmission Network to the Metro wavelength division physical platform, the wavelength resources provided by the platform are responsible for SDH, MSTP, and IP Broadband Services respectively. This not only facilitates the unified management of networks, but also enables flexible allocation of wavelength resources to quickly meet the rapidly growing bandwidth requirements of the IP network and solve the problem of fast consumption of fiber resources in the fiber-optic direct connection mode, improves the utilization of network resources. In addition, the metro Wavelength Division provides a wavelength channel with protection, which can be used to transmit data services with better QoS Assurance than fiber-optic direct connections to enhance the survivability and robustness of the IP network. More importantly, the application of Metro wavelength division technology will provide a smoothly evolved physical platform for the future development of smart optical networks, which can avoid network convergence difficulties and difficulties in expansion caused by separation of networking, it lays the foundation for introducing intelligent OXC and adapting to future intelligence to provide diversified services and flexible bandwidth allocation.
In the future, the goal of the core layer network topology is to develop towards a mesh or grid network. It adopts a distributed control mechanism, applies OXC networking technology, and is based on new standards and technologies such as ASON and GMPLS. OXC-based Intelligent Optical Network is an important direction for the future development of the transport network, but the current technology is not yet mature, and business needs need to be explored.
The aggregation layer is composed of aggregation nodes and is responsible for business aggregation and guidance within a certain region. It requires strong business scheduling capabilities. The existence of the aggregation layer prevents access points from directly entering the core layer, resulting in large access network spans and serious trunk fiber consumption. The Convergence layer mainly uses MSTP, RPR, and Metro wavelength division technologies. MSTP is used in the aggregation layer to ensure support for traditional TDM services, optimize data service transmission, and improve bandwidth utilization. Using the L2 switching and aggregation functions of MSTP, you can save the service ports of the aggregation Layer Nodes and reduce network costs. At present and in the future, the TDM business will remain the most important source of revenue for telecom operators, and there is still room for growth. When the business needs are dominated by TDM business, create a convergence layer for the Metro Transmission Network to use MSTP.
If the existing SDH network still has a large amount of remaining capacity, it can meet the needs of future TDM business development, and when the new business mainly focuses on IP data business, you can consider using the RPR Technology for networking. RPR provides optimized data service transfer capabilities. It provides multiple levels of business types to meet diversified business needs of users.
When the traffic volume in a full range or partial area is large and the optical fiber supply is short, the metro wavelength division technology can be used in the partial area of the Convergence layer. The CWDM technology should be adopted based on economic considerations. Due to the small size of aggregation business, low-speed business can be aggregated to a wavelength through the T-MUX interface, in order to improve the wavelength utilization. In the current situation, the traffic volume at the aggregation layer is relatively small. Generally, the bandwidth needs can be met without the color Metro wavelength division technology. For the communication between the Metro Transmission Network and the IP network, the convergence layer IP Metropolitan Area Network and the Metro Transmission Network are preferred. The IP network node is independent of the transmission network node. In the future, the convergence layer will develop to the unified transmission platform. The access layer is at the end of the network to implement business access technology. The access layer is the most technically rich and cost-sensitive area. Currently, the access layer is mainly available in MSTP, RPR, and EPON. The access layer uses MSTP to replace some data network devices and reduce network costs. RPR networking can be used for IP traffic-dominated areas to optimize the technical capability of data service access networks. Because the main services in the access layer include 10 M/M Ethernet, 2 M, 34 M/45 M and other small particle services, the metro wavelength division technology is not applicable to this layer.
For the networking of Metro Transmission Networks and IP networks, technical maturity and network economics should be comprehensively considered. Different technical solutions can be used to achieve economic and flexible business access according to actual needs. In the access layer, the Metro Transmission Network should be able to provide a wide range of business interfaces to meet the needs of IP Service Access Network Technology and bearer, which is conducive to saving network investment and improving resource utilization. In some regions, if there is a shortage of transmission resources or the demand for user IP services is high, fiber-optic direct connection can still be used. Specific technologies should be determined through technical and economic analysis based on business needs and proportions of different business volumes.
2. Optical Fiber selection considerations
The next generation G.655 optical fiber with low dispersion slope is used. On the access layer of the Metropolitan Area Network, the channels are very intensive. For Systems Based on 2.5 Gbit/s and below, G.652 Optical Fiber bearer systems have good technical advantages, therefore, G.652 optical fiber is an option. G.652 and G.655 optical fiber are supported for Systems Based on 10 Gbit/s and higher rates; for the backbone layer of a man, a new type of optical fiber in G.655 optical fiber can be used, such as anhydrous peak Optical Fiber G652C, large effective area optical fiber, and low dispersion slope optical fiber, the new generation of Water-free peak Optical Fiber expands the available spectrum and shows its unique advantages, which will inevitably be widely used.
When selecting optical fiber in an existing network, there are many factors to consider. The two key factors are attenuation and dispersion. These two factors determine the choice of optical fiber, and ultimately affect the cost of network construction. The mainstream optical fiber of man is standard single-mode optical fiber (SMF), which has the smallest dispersion in the 1310nm area and the smallest attenuation in the 1550nm area. SMF is available in the O, S, C, and L bands, but the attenuation peak in the 1383nm zone, that is, the water peak, makes it not ideal in the E band. In order to open the E band of optical transmission, an enhanced single-mode optical fiber (E-SMF) appears, the water peak attenuation of 1383nm area is significantly reduced without affecting the dispersion characteristics of the optical fiber. Therefore, E-SMF in the 1260nm to 1625nm zone, all bands have availability. The wider wavelength zone makes the E-SMF more suitable for DWDM applications.
In the future, with the application of wavelength transparent optical networks in the Metropolitan Area Network, the system will work beyond the range of signal regeneration relay distance. Due to the high dispersion coefficient between SMF and E-SMF, the dispersion distance of the 10 Gbit/s system is limited to about 70km. For a long ring network, the dispersion compensation module (DCM) is required ), this dispersion compensation module is actually composed of optical fibers with a negative dispersion coefficient, which is used to reduce the accumulation of positive dispersion values of optical fibers. When this module is used for ultra-long distance, they will lead to a rise in the system price and a great attenuation. The price of a DCM module is almost the same as the price of the compensated optical fiber, and the resulting attenuation will require additional amplifiers to be added in the ring. This dispersion limit makes SMF suitable for less than 70km.
Non-zero dispersion displacement optical fiber (NZ-DSF) is a good choice for the application of more than 70km, the zero color scatter position of the NZ-DSF relative to SMF in a longer wavelength point. The attenuation and dispersion of NZ-DSF in 1550nm zone is suitable for high performance transmission. NZ-DSF was initially designed for long distance optimization, and the next generation of NZ-DSF will have ideal performance in man.
The Metro NZ-DSF provides DWDM availability from 1440nm to 1625nm, including C, S and L bands, because the dispersion coefficient of the metro NZ-DSF is less than half of the SMF, therefore, it may provide twice the dispersion restriction distance of SMF. In the future system, the working distance of NZ-DSF optical fiber will reach 200km without extra dispersion compensation, and of course no Dispersion Compensation Optical Fiber (DCF) and optical amplifier.
Although the NZ-DSF with positive and negative dispersion coefficient can make the 10 Gbit/s system work distance in the C band is more than 200 km, but the fiber with positive dispersion coefficient is recommended, for many reasons. First, the optical fiber with normal color dispersion coefficient can provide a longer working distance and is compatible with 40 Gbit/s systems, and is compatible with existing systems and access applications. In addition, the 10 Gbit/s and 40 Gbit/s systems must be compensated by standard dispersion modules. The current standard DCM is an optical fiber with a negative dispersion coefficient, they cannot compensate the NZ-DSF of the negative dispersion coefficient.
Although SMF with a higher positive dispersion coefficient can be used to compensate the NZ-DSF of the negative dispersion coefficient, the SMF of 1km can only compensate the NZ-DSF of the negative dispersion coefficient of 2km, so a large amount of SMF is required, this will definitely increase the network attenuation so that the compensation is unrealistic. At the same time, due to the inconsistent dispersion slope, this kind of compensation will lead to a great difference in dispersion accumulation between different wavelengths of the system. In the future, 40 Gbit/s systems require stricter color dispersion restrictions, and all optical fiber dispersion accumulation must be compensated. Considering that the 40 Gbit/s system has higher dispersion compensation requirements, to be compatible with other systems, we recommend that you use optical fiber with normal color dispersion in the man environment. The zero color scatter of the negative dispersion coefficient NZ-DSF is above 1620nm. It has a low dispersion coefficient in the L band, and a high dispersion coefficient in the 1310nm, the low dispersion of the L band will increase the nonlinear crosstalk between channels, this feature limits the use of the DWDM System in this region. The high dispersion coefficient of 1310nm limits its availability.
Because of the normal color dispersion coefficient of the city NZ-DSF zero color scatter is roughly at 1400nm. It has a relatively low dispersion coefficient in 1310nm, and its dispersion coefficient is only equivalent to 1/4 of the Negative Dispersion NZ-DSF, the typical value is-6 ps/nm. km. In comparison, E-SMF or smf in the 1310nm area with zero color scatter, will have the maximum dispersion restriction distance of a single channel.
3. Blocking of Metro optical cable lines
Due to the development of municipal construction, it is very frequent for the communication line engineering and maintenance department to cooperate with municipal construction in the transformation and cutover of the Urban optical cable line, in addition, the increase in road trimming, expansion and expansion, and other road excavation projects, and all kinds of planned, unplanned, pre-planned, or non-scheduled, unexpected construction work are being carried out, regardless of the day or day, every moment is threatening the security of optical fiber lines in communication channels and their pipelines. To this end, communication line engineering maintenance personnel are required to avoid or minimize interruption of communication during construction and troubleshooting, ensures communication security, stability, and reduces economic losses and adverse social impact caused by communication blocking.
However, at present, the self-protection capability of the optical fiber transmission system is limited. In the case of a full-resistance fault on the optical fiber transmission line, for example, only the protection system of the optical fiber transmission equipment is used, it is difficult to ensure the safety and smoothness of the line. For example, in SDH Transmission Systems with loop self-healing function, if the optical fiber transmission ring is not a real physical cable ring, the optical fiber line is blocked at a certain point, the entire SDH Transmission ring may be interrupted. Another example is the automatic monitoring system for optical cable lines that have emerged and used in recent years. Although it can complete real-time and automatic monitoring of optical cable lines, however, it cannot prevent and predict sudden interruption of optical fiber cables caused by external force, or protect the optical fiber transmission system in the case of a fault in the optical fiber cable line. That is to say, no matter which optical fiber cable is completely blocked or part of the core is blocked, it will cause a certain duration of communication interruption to the optical fiber communication system that is not protected by another physical cable transmission route.
At present, most of the metro relay optical fiber cables and user trunk cables are at least 24 cores, most of which are in use, and when the optical fiber cables are blocked, from block to complete repair, 6 ~ 10 h. Even if there is a planned cutover, communication may be interrupted for 1 hour under the current technical conditions ~ 6 h. This is a serious communication loss caused by high-speed, broadband, and large-capacity optical fiber transmission, especially for major optical fiber cable blocking obstacles with many transmission systems and Long interruptions, it will not only cause serious economic losses to the telecom operation department, but also cause serious adverse social impact.
To provide users with high-quality, efficient, secure, and smooth communication lines, more effective protection measures must be taken. For example, dual-route mutual protection is a very effective protection measure. Through this mutual protection, neither Sudden Line Blocking, link blocking, or optical cable line cutover will cause obvious communication interruptions or user-perceived communication interruptions.
Although the automatic monitoring system for optical fiber lines that has emerged in recent years can achieve real-time automatic monitoring of optical fiber cables, it cannot prevent sudden blocking of optical fiber cables caused by prediction of external force, the optical transmission system cannot be protected when the optical cable line fails. If one optical cable is completely blocked or some of its fiber cores are blocked, optical systems that do not use another physical optical cable to transmit routes will cause long-time service interruption.
In addition, the Municipal Construction of the metro optical cable line is also frequently relocated and cutover, to provide users with high-quality, efficient, secure, and smooth communication services, it is required that the optical fiber line cutover should not interrupt the communication circuit as much as possible. Even if it is not interrupted, the interruption time should be compressed to a minimum to ensure the security and stability of the communication network, reduce the economic losses and adverse social effects caused by communication blocking. Currently, for optical cable lines in operation, the cutover force is between 0 and ~ . For general users, there is no impact if users are prepared by prior notice. For some important and special large users, such as foreign and foreign-funded enterprises, due to their own time difference or daily difference ~ The cutover at may also affect the communication. If the optical transmission system can have reliable dual-route protection for physical optical cables, whether it is abrupt cable line blocking, fiber link blocking, or optical cable cutover, can ensure that the communication is not obviously interrupted, the user does not feel that there is a communication interruption) or can ensure that it is an instant interruption. At the very least, it can also ensure a short interruption without causing serious adverse effects. The following describes several physical dual-route protection methods for optical fiber transmission in the construction and operation and maintenance of Metro optical fiber lines.