DWDM technology principle and its application in metropolitan area network
DWDM technology principle and its application in metropolitan area network
Dense wavelength division multiplexing (DWDM) technology can make full use of the huge bandwidth resources of optical fiber, greatly increase the system transmission capacity and reduce transmission costs. Therefore, this technology has been widely used in long-distance and backbone network ultra-large capacity transmission. If DWDM technology is introduced into the metropolitan area network and the access network, the entire network will become a seamlessly connected whole, providing support and connection for all different services. Therefore, DWDM in the metropolitan area network has great advantages and development potential. It will become the inevitable evolution of the entire communication network to the all-optical network.
Since DWDM technology has not been applied in the metropolitan area network of our office, combined with the principles and performance advantages of DWDM technology, we have conducted some preliminary discussions on the application of DWDM technology in the metropolitan area network. The application in provides some references.
2 DWDM technical principles and performance advantages
2.1 Principle of Dense Wavelength Division Multiplexing
DWDM technology refers to placing more channels densely in the current 1.55μm band, and using an optical multiplexer (combiner) at the sending end to combine signal optical carriers of different specified wavelengths into a single fiber for propagation. At the receiving end, an optical demultiplexer (demultiplexer) separates these optical carriers carrying different signals at different wavelengths, so that multiplexed transmission of multiple optical signals can be achieved in a single fiber (see Figure 1) . In the ITU-T recommended standard, the channel spacing is specified as an integer multiple of 100 GHz. Now, people have been experimenting with 50GHz and 33.3GHz channel spacing, even narrower, and strive to make full use of the available bandwidth of the fiber.
2.2 DWDM system composition
The DWDM system is mainly composed of a light source, an optical amplifier, an optical multiplexer and an optical demultiplexer, which are briefly described as follows.
1. Light source
The length of the non-electric regeneration relay of the DWDM system has been increased from 50 ~ 60km to 500 ~ 600km. While the dispersion limited distance of the transmission system is required to be greatly extended, in order to overcome the nonlinear effect of the optical fiber, the use of light source technology is more advanced and the performance is more advanced It is an excellent laser. In short, the two outstanding characteristics of the DWDM system light source are: (1) has a certain chromatic dispersion tolerance; (2) standard and stable wavelength.
2. Optical Wavelength Converter (OTU)
DWDM can be divided into two types of open and integrated system structures. The so-called "open" means that in the same WDM system, SDH systems of different manufacturers can be connected. It uses wavelength conversion technology to convert the optical signal of the multiplexed terminal to a specified wavelength, so there are no special requirements for the multiplexed terminal interface, as long as these interfaces comply with the optical interface standard recommended by ITU-T G.957. The integrated WDM system does not use wavelength conversion technology, and the wavelength of the optical signal of the multiplexing terminal is required to meet the system specifications.
3. Optical amplifier (OA)
An optical amplifier is an active device that directly amplifies optical signals without undergoing optical / electrical / optical conversion. It can compensate for the loss of optical power in fiber transmission and extend the transmission distance of the communication system.
Working principle of erbium-doped fiber amplifier (EDFA): EDFA is mainly composed of erbium-doped fiber (EDF), pump light source, coupler, isolator and other components. It utilizes the nonlinear effect of erbium-doped fiber. The pump light (the pump wavelength of erbium-doped fiber amplifier is 980nm) is input into the erbium-doped fiber. When signal light is input, the phase and wavelength of the radiated light will spontaneously interact with the signal The light keeps the same, so that the optical signal with stronger power can be obtained at the output end, and the optical signal is amplified.
4. Optical multiplexer and optical demultiplexer
In a WDM system, a device that combines signals from different light source wavelengths is called a multiplexer. Conversely, a device that decomposes multiple wavelength signals sent through the same fiber into individual wavelengths and outputs them separately is called a demultiplexer. The same device can be used as a demultiplexer and a combiner. Generally, the insertion loss of the splitter and the combiner is small, the isolation is large, the in-band loss is flat, and the polarization dependence is low. The optical demultiplexers currently used in WDM systems mainly include arrayed waveguide gratings (AWG), phased array demultiplexers, tunable filters, interference film filters, and grating couplers.
2. 3 DWDM network management system:
Since the DWDM system can carry SDH, PDH and other unrestricted digital or analog signals, its network management system should be separated from the network management of the transmitted service layer and sent to the upper network management layer through the Q3 interface. This can increase the diversity of DWDM bearer services.
At the sending end, the optical monitoring (OSC) signal with a wavelength of? S (1510nm) generated by this node is inserted to complete the transfer of frame synchronization bytes, official bytes and overhead bytes used by the network management. The network management system transmits overhead bytes to other nodes through the physical layer of the optical monitoring channel or receives overhead bytes from other nodes to manage the DWDM system, realize functions such as configuration management, fault management, performance management, and security management, and manage with the upper layer. The system is connected. In order to prevent the optical monitoring channel in a section of optical fiber from being disconnected in both directions, and the network element management system cannot obtain the monitoring information of the network element, the DWDM system must have the protection function of the monitoring channel.
2.4 Optical fiber system suitable for DWDM
In a DWDM system, since multiple optical signals are transmitted simultaneously in one fiber, the density of optical power in the fiber is increased, and nonlinear phenomena such as four-wave mixing (FWM) are easily induced. In addition to conventional G.652 fiber, there are dispersion-shifted G.653 fiber and non-zero dispersion G.655 fiber.
G.652 fiber has very low loss at 1.3m and 1.5m, especially at 1.55m, the loss is less than 0.2dB / km, which is very beneficial for long-distance transmission, but the dispersion is relatively large, about 17ps /nm.km. Because G.652 optical fiber has been laid in large quantities in China, it is the current first choice to use the original optical fiber to achieve ultra-high speed transmission using DWDM technology.
G.653 fiber is also called dispersion-shifted fiber. It has the smallest dispersion and the lowest loss in the 1550nm window. It is the best choice for single-wavelength systems. However, due to the crosstalk phenomenon of high-speed transmission, it is difficult to open multiple WDM systems. The use of G.653 fiber is not recommended.
G.655 optical fiber is to move the zero dispersion point of G.653 optical fiber to 1570nm or 1510 ~ 1520nm, successfully overcome the shortcomings of G.652 dispersion limitation and G.653 can not open WDM, the upgrade is very flexible, both Applicable TDM system can also be applied to WDM technology. However, the price of the optical cable using the optical fiber is 1.5 times that of the conventional optical cable.
2.5 Performance advantages of WDM system
1. Large transmission capacity and high transmission rate
In the PDH phase, the transmission rates of optical fiber lines are mostly 34Mbit / s and 140Mbit / s. In the SDH phase, the transmission rates are mostly 155Mbit / s, 622Mbit / s, 2.5Gbit / s and 10Gbit / s. Since SDG adopting TDM mode transmits 10Gbit / s or 40Gbit / s, it also needs related modulation technology and more advanced laser, which will make the cost extremely high and difficult for users to accept. With WDM, each wavelength can not only transmit 2.5Gbit / s SDH signals, but also transmit optical carrier signals of 10Gbit / s and 40Gbit / s or more, making the transmission capacity on one fiber several times larger than single-mode fiber To dozens of times,
2. Optical fiber system has long transmission distance and simple transmission equipment
The WDM system uses a 1550nm window with the lowest loss of quartz fiber, its transmission loss is smaller and the transmission distance is longer, and the EDFA technology, external modulation, electrical absorption and other methods make the allowable loss and dispersion of the WDM system relay section larger and the transmission distance The extension from tens of kilometers to hundreds of kilometers or more. The WDM system uses an optical amplifier to replace the original electric regenerator, which greatly reduces the number of SDH repeaters, saves costs, and simplifies equipment.
3. The network is more intelligent
One of the goals of the future development of the optical fiber network is to achieve unified transmission network monitoring and successfully incorporate TMN. The current PDH network management frame structure has fewer management bits and poor network management capabilities; although SDH adds rich management and maintenance overhead bits to the frame structure, due to the different information models of various manufacturers, the network management systems of different manufacturers are at the interface Cannot communicate with each other; the WDM system sets up important network management monitoring channels to transmit the network management information of the WDM system, and its network management is closer to the TMN mode.
4. Suitable for transmitting multimedia comprehensive business information
Because the optical carrier signals transmitted in the same optical fiber are independent of each other, they can transmit different signals with different transmission characteristics, and their channels are completely transparent to the data format, regardless of the signal rate and modulation method, so that multiple formats of business signals, such as Multimedia information such as voice, data, and video can be transmitted with high quality in the WDM system, which improves service quality.
3 Application of DWDM technology in metropolitan area network
The SDH technology currently used in metropolitan area networks is mainly aimed at voice services based on circuit switching. However, from the perspective of network expansion, adding equipment to the traditional SDH network meets the needs of data services. From the perspective of network efficiency, there are disadvantages such as low efficiency, high cost, and low utilization. DWDM brings unique competitive advantages to the metropolitan area network and data communications to bring highly competitive solutions.
3.1 The difference between DWDM in the application of metropolitan area network and wide area network DWDM has achieved great success in the application of wide area network, but when constructing the broadband metropolitan area network, the two are very different.
1. Device requirements
The transmission distance of the wide area network is thousands of kilometers or even across the ocean. Many amplifiers and other equipment are required in the middle. The laser, multiplexing devices and components are very demanding, and the system cost is expensive. The transmission distance of the metropolitan area network is generally within 100km, and it is not very sensitive to the transmission attenuation value of the optical fiber, which eliminates the need to use external modems and optical amplifiers and the corresponding channel balancing requirements. Operators are therefore relatively free to choose optical components with lower specifications, which greatly reduces the overall system cost.
2. Supported business types, flexibility and network cost requirements
Compared with wide area networks, metropolitan area networks have lower requirements in terms of transmission capacity and distance, but have higher requirements in terms of the types of services supported, flexibility, and network costs. From the perspective of service channels, long-distance networks mainly provide 622Mbit / s, 2.5Gbit / s and 10Gbit / s channels, while metropolitan area networks require optical interfaces to support SDH, ATM, Gigabit Ethernet, etc. from 100Mbit / s to 2.5Gbit / All signals within the range of s can carry different types of services. DWDM is a pure physical layer technology. Its operation is completely independent of the type of information it carries. It can provide transparent services based on wavelengths and flexibly transmit signals in any format.
3. The difference between network topology and protection
From the perspective of network / service topology and protection, the fiber / service topology used in long-distance networks is mostly linear, and service protection is performed by OXC, DXC, or higher-level protocols. In contrast, the topology of a metropolitan area network is a ring or mesh structure, and the service topology is a radial structure. The transmission system itself must provide protection or support a customer-level protection mechanism. This requires the metropolitan area DWDM system to have a richer networking capability and can form an optical channel self-healing ring to improve the security and reliability of the network.
4. The metropolitan area network has higher requirements on the system's modularity level and add-drop multiplexing capability
The expansion of long-distance networks is mainly carried out by increasing the capacity and effectively using idle wavelengths in a way that does not affect business traffic. However, due to the suddenness and uncertainty of IP services in the metropolitan area network, the metropolitan DWDM system is required to have the ability to add new nodes to the loop and increase capacity between unpredictable endpoints. This puts forward higher requirements on the system's modularity level and add-drop multiplexing capability. In addition, the metropolitan area network DWDM system must be able to provide any number of wavelength add-drop multiplexing capabilities at any network node to meet the needs of dynamic scheduling services.
3.2 DWDM metropolitan area network structure
A basic structure of the DWDM metropolitan area network is the DWDM self-healing ring structure (see Figure 3).
From Figure 3, we can clearly see the support of the DWDM system for different services. According to the signal transmission direction in the ring, the self-healing ring is divided into unidirectional ring and bidirectional ring; according to the ring protection mechanism, the DWDM self-healing ring is divided into 1 + 1 ring protection, 1: 1 protection and 1: N protection ring Wait. In addition, when transmitting SDH services, it is also necessary to coordinate the problems of duplication and conflict between the DWDM protection mechanism and the SDH protection mechanism.
In DWDM self-healing ring metropolitan area network structure, OADM (optical add-drop multiplexer) is the most critical component. Its role is to add separation wavelength flexibly in the DWDM loop.
3.3 Technical Features of Metropolitan DWDM
1. Business matching is direct and convenient
The traditional metropolitan area network adopts TDM technology, which is most suitable for providing 2Mbit / s circuits to satisfy 64kbit / s voice services, but this structure is not suitable for statistical multiplexing data services. After the DWDM technology is adopted in the metropolitan area network, routers or ATM switches can be used to directly match packets or cells to the wavelength without using the SONET or SDH method to adapt the DWDM transmission network. Thus, TDM is replaced by optical layer DWDM, which increases bandwidth utilization, promotes networking, and reduces costs.
2. Save fiber resources
Some large users require a large channel bandwidth, which exceeds STM-16 and adopts TDM such as STM-64 technology. It is currently difficult to meet their requirements; if a higher rate TDM system is used, all terminal equipment needs to be updated, and DWDM Then the existing terminal equipment can be retained, eliminating the need to use additional fiber. It is also easily compatible with existing SONET / SDH networks or old optical terminal equipment working based on asynchronous protocols.
3.4 Main Problems Existing in Metropolitan DWDM Technology
At present, the main problems of metro DWDM technology are in the following aspects:
1. Equipment cost
Although the transmission distance of the metropolitan area network is relatively short, due to the large number of optical add-drop multiplexers (OADM) used in the metropolitan area network, each add-drop multiplexer introduces a certain loss, so in the metropolitan area network Optical amplifiers must be used, which greatly increases the cost. If the price of upgrading the metropolitan area network with DWDM is higher than the price of using TDM and laying new fiber, it will limit the application of DWDM in the metropolitan area network. Now equipment manufacturers are working hard to improve the equipment so that the price of DWDM equipment used in metropolitan area networks is lower than the price of long-distance DWDM equipment.
2. technical problem
A large amount of OADM is required in the DWDM metropolitan area network. In order to control the network flexibly, the OADM should be programmable and can be controlled by software. However, the current DWDM add-drop multiplexing technology is not very mature, and there is not a lot of commercial use, especially for systems with many wavelengths. In addition, optical cross-connect equipment is immature.
3. Device compatibility issues
Since the current DWDM network standards have not been fully formulated, there are still problems with the interconnection between multiple manufacturers' equipment. In this case, a metropolitan area network can only use the equipment of one manufacturer, or add corresponding conversion equipment to achieve the interconnection between the equipment of different manufacturers, but with the formulation of WDM optical network standards, these problems can be solved. .
4. Network management technology is immature
Due to the lag in the formulation of relevant standards, the DWDM network management system currently has a relatively lagging technology problem. For example, although the DWDM systems of most manufacturers are equipped with an EMS network element management system, the upward interface of the network element management system is not uniform, and there are Q3 interfaces, CORBA interfaces, etc., which makes it difficult for the management systems to be compatible and interoperable; Most DWDM systems do not yet support important functions such as system error performance monitoring and connection integrity. The function of DWDM network management system needs to be further strengthened and perfected.
4 Conclusion
DWDM has been widely used in long-distance trunk networks. In metropolitan area networks, the cost of equipment is still very expensive, the technology is not mature, and the compatibility of equipment is poor. It is difficult to apply it for the time being, but due to the DWDM technology has The economics of capacity expansion, bit rate and protocol transparency, good scalability, etc. With the development of the market and the development and utilization of new low-cost metropolitan area network equipment, DWDM technology has broad application prospects in metropolitan area networks . It is believed that in the near future, DWDM technology will also show great brilliance in the application of the metropolitan area network of our office.
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