OTN technology solves the problem of metropolitan area network
In recent years, the rapid development of data services such as broadband, IPTV, and video services has put forward new requirements for operators' transmission networks. On the one hand, the transmission network must be able to provide bandwidth suitable for business development, and on the other hand, the transmission network can be carried out. Fast and flexible business scheduling and perfect and convenient network maintenance management (OAM function) to meet business needs.
OTN technology includes the complete system structure of the optical layer and the electrical layer, each layer of the network has a corresponding management and monitoring mechanism, and has a network survivability mechanism of the optical layer and the electrical layer, which can well meet the needs of the networking; and OTN technology It can provide powerful OAM function, provide perfect performance and fault monitoring function. The ODUk-based cross function of OTN equipment has increased the circuit switching granularity from SDH's 155M to 2.5G / 10G / 40G, thereby enabling flexible scheduling and protection of large-grain services. In addition, OTN equipment can also introduce an intelligent control plane based on ASON, which improves the flexibility and survivability of network configuration.
ITU-T has formulated a number of standards related to OTN technology around 2000, and established a relatively complete OTN standard system. However, due to the development of the transmission business from SDH signals to IP / Ethernet services, the relevant OTN standards are also being revised. In October this year, ITU-T passed the revised version of the OTN standard (G.709), enhancing OTN's ability to support GE / 10GE / 100GE and other Ethernet services. The new standard introduces ODU0, ODU4 and ODUflex that can flexibly adjust the bandwidth, and improves the multiplexing structure and mapping method of OTN, so that it can better apply to IP / Ethernet services.
OTN meets new demands of metropolitan area networks
With the rapid development of broadband services and the gradual rise of new services such as soft switching and 3G, IP services have gradually become the largest service type in metropolitan area networks. After investigating the current situation of some metropolitan area networks and combining with the future development trend, it can be summarized as the following characteristics:
First, there are many converged IP services from the core routers in the county to the city, and there is a trend of increasing year by year. Most of them are based on GE services, and 10GE interfaces will be used in the future.
The second is that the metropolitan IP core network mostly uses optical fiber direct connection networking, and the IP business scheduling requirements are less, only exist in a small number of large cities, and the business particles are mostly 2.5Gbit / or 10Gbit / s. In the future, it will mainly be 10GE / 40GE / 100GE particles and will be carried by WDM system.
The third is that the current dedicated line services are still mainly 2M and 155M, which are generally implemented through traditional SDH or ASON equipment; large-grain IP services are carried by the metropolitan area wavelength division system, which provides optical layer protection while saving fiber resources. In the future, large-grain private line services of GE and above will gradually increase, thus placing higher requirements on the scheduling capabilities of large-grain circuits.
Fourth, the current base station bearer mainly adopts MSTP, and it will evolve to PTN / IP bearer network in the future. The line interface type will be mainly GE and 10GE.
According to the analysis of the above characteristics, with the Ethernet and high speed of the service network interface, and the gradual increase in the demand for large-grain services, the demand for metropolitan WDM and OTN networking will become more and more urgent.
Table 1 Comparison of OTN crossover and multidimensional ROADM
Both OTN and multi-dimensional ROADM are transport network technologies suitable for large-grain business scheduling. A comparison of the two technologies is shown in Table 1.
As can be seen from Table 1, OTN crossover is more flexible than ROADM. In view of the relatively diversified service particles (GE, 2.5G and 10G) that need to be scheduled in the metropolitan area network in the future, it is recommended to use OTN crossover technology to complete the scheduling and protection of large particle services.
ROADM equipment widely used in foreign metropolitan area networks has not been recognized in the domestic market so far. There are two main reasons for this: relatively high equipment costs, but the rapid configuration and maintenance-free features that are not urgently needed by Chinese operators; due to the limitation of wavelength continuity, ROADM has great business scheduling The restrictions cannot meet the requirements of operators. In this respect, it is obviously not as good as the OTN electrical layer cross-connect equipment based on ODUk cross.
With the emergence of more large-granularity business cross-scheduling requirements in the metropolitan area network, especially the increase in GE business scheduling and protection requirements, the use of OTN cross-devices with OADM / ROADM to achieve more flexible scheduling and protection recovery functions is An optional networking mode. Among them, OADM / ROADM is mainly used for wavelength up and down and WDM ring network protection in WDM systems; and OTN mainly completes end-to-end performance monitoring, circuit scheduling and protection, and flexible service adaptation and aggregation.
Application of OTN in Metropolitan Area Network
According to the above analysis, the application of OTN in the metropolitan area network will mainly appear in the way of combining metropolitan WDM. OTN is positioned to provide the bearer of large particle services at GE and above. Therefore, it is very important to discuss the application of OTN in the metropolitan backbone / aggregation and access layer, and to discuss the application of ODUflex.
At the metropolitan backbone / aggregation layer, when the demand for large-scale GE and above services between different areas in the metropolitan area network or access to long-distance networks reaches a certain scale, and there are scheduling, aggregation, and protection recovery requirements, it can be used in the metropolitan area network OTN / WDM network is deployed in the core / aggregation layer. The application of OTN network mainly exists in the following two scenarios: TDM and Ethernet dedicated line services carrying GE particles and above. Customer equipment can be directly connected to the OTN network, and can also be connected to the OTN network through the access / aggregation layer SDH / PTN network; as a bearer network for upper-layer networks such as IP, SDH, and PTN, when SDH / PTN and other networks have more than GE When the sub-wavelength-level relay circuit is required, it can be connected to the OTN network, which can implement scheduling and protection, and achieve the purpose of saving fiber or channel resources.
The newly introduced ODUflex technology of OTN is similar to the VC cascade technology in SDH, which can provide flexible service access capabilities within the same ODUk (k = 2, 3, 4), realize flexible adaptation to service bandwidth, and increase bandwidth Utilization rate to meet different bandwidth requirements of users. Especially for some new services such as FC, CPRI, etc. have better adaptability.
At the metro access layer, the access layer is near the end of the network, so cost is an important factor in the choice of technical solutions. OTN devices applied at the access layer mainly appear in the form of box-type devices, and are applied in combination with CWDM. In addition to providing the above-mentioned backbone / aggregation layer services, the access layer OTN currently considers two main applications as CPRIoverOTN and PONoverOTN.
In the construction of 3G networks, the quality of network coverage is crucial. Traditionally, the macro base station equipment is used as the main network construction method. The main problem is that operators have to spend a lot of time and expense in renting computer rooms and lines. The core idea of ​​the new network coverage concept is to separate the baseband processing (BBU) and radio frequency section (RRU) of the traditional macro base station into two devices, baseband processing and radio frequency extension, and use fiber connection between the two. A BBU can connect multiple RRUs, thereby further improving the sharing efficiency of the baseband pool. The distributed base station can realize the centralized placement of larger capacity BBUs, which saves site resources to a greater extent. The public radio interface specification CPRI is a standard interface for connecting BBU and RRU initiated by companies such as Ericsson, Huawei, NEC, Nortel Networks and Siemens. The CPRI interface can be used for multiple 3G standards and future LTE.
Currently, fiber optic direct drive and WDM / OTN technologies can meet CPRI transmission requirements. The use of OTN to carry CPRI interface signals can improve the bandwidth utilization of optical fibers, support longer-distance transmission, provide complete protection capabilities and rich optical layer management, support any topology networking, simplify operation and maintenance management, simple expansion, and improve wireless New business launch speed.
The benefits of using OTN to carry PON are similar to the above CPRIoverOTN, mainly to extend the transmission distance of PON and provide protection. The latest G.709 standard passed by ITU-T has already regulated the mapping method of OTN transmitting CPRI and GPON signals.
Currently, mainstream transmission equipment manufacturers are developing OTN related products, of which Huawei's OTN commercial equipment has been widely put into market application. In order to meet the growing demand for IP services and large-grain private line services, and to adapt to the development trend of transmission network technology, China's communications industry should increase investment in R & D of OTN technology to accelerate the development, standardization, and promotion of OTN equipment.
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