For metropolitan area networks and long-distance networks, if the optical transport layer has the capability of remote reconfiguration, it can greatly reduce operating costs. Operators have also realized this potential, and have added requirements for reconfigurable optical add-drop multiplexers (ROADM) and multi-dimensional optical switches in the recent bidding for business networks.
Concern about the new technology of ROADM began when the telecommunications industry fell, at that time, equipment manufacturers have reduced investment in new product development. There are few companies that can seriously study the opportunities that ROADM brings, while some companies are scrambling to complement their product lines at the fastest speed and at the lowest cost. But thankfully: we can also meet the new market demand of ROADM in other ways. What is ROADM?
"ROADM" refers to a network element (or node) that can be dynamically added and dropped to service wavelengths through remote reconfiguration, and the power of service wavelengths can also be managed. Figure 1 is a general model of ROADM nodes, as shown in the figure,
The components of ROADM vary with different applications. However, most ROADMs have the modules listed here.
The main functional modules of ROADM nodes include:
â— Front and rear optical amplifiers.
â— Generation and termination of optical service channel (OSC).
â— Wavelength drop.
â— Wavelength on the road.
â— The power monitoring of the aggregate channel or single channel within the node can be performed in multiple places within the node.
â— Available / unavailable and optional wavelength monitoring, signal-to-noise ratio monitoring of optical signals in the entire node.
â— Power / attenuation control of up and down and through wavelengths.
â— Dispersion compensation.
â— Gain equalization of pre and post amplifiers.
ROADM products provided by system integrators vary with different applications. From relatively inexpensive basic systems to highly flexible and scalable platforms, system providers need to choose different system structures and different underlying device technologies to meet the different cost and performance requirements put forward by the market.
Generally, system integrators adopt a modular structure to improve the deployment efficiency of ROADM systems. They hope to provide the required functions at the lowest price and provide further expansion space to meet the new demands brought by network expansion.
However, as the configuration complexity increases, the ROADM control environment also becomes more and more complicated due to factors such as the increase in the number of interoperable elements, the increase in the variety of device technologies, and the physical distribution of system functions. The main challenges faced by ROADM designers of key optical modules are: too many types of optional technologies, and these technologies are evolving too fast; in addition, the market requires that ROADM systems must be cost-effective modular systems that support multiple applications.
The two main device technologies for manufacturing ROADM are the wavelength selective switch (WSS) technology based on bulk optics and the integrated multiplexer / demultiplexer / optical switch / variable optical attenuator (VOA) based on planar optical path (PLC) ) And detector technology. In equipment based on WSS technology, the optical performance monitor is usually a discrete device, usually placed in a separate package circuit, and an N-port optical switch is placed in front of it, so that several points to be measured share a monitor. In this type of device, the function of the control circuit can only be achieved if the nodes can communicate with each other. PLC-based devices usually integrate a power monitor in each channel, which eliminates the need for external power monitoring equipment. However, it only provides single-channel power monitoring at one point, which has a great limitation on the control structure.
Because these technologies are in the process of rapid evolution, and which technology has an absolute advantage is unpredictable, system integrators must carry out modular design so that when they are replaced with new devices in the future, the amount of work required for redesign is minimal. Finally, in order to meet the requirements of lower performance and capacity requirements and higher cost requirements, the control system must still be able to operate normally after removing certain devices. The above not only puts forward a certain degree of requirements for the consistency of the interface, but also requires abstracting the underlying functions as much as possible in order to provide a unified interface to the remaining nodes.
Figure 2 shows various embedded control intelligence at the ROADM node. ROADM here is based on WSS technology.
Embedded control includes several different control mechanisms and levels. Designers can integrate these functions themselves, but the complexity and device cost are very high.
The so-called "embedded control intelligence", as the name implies, its control circuit is local to circuit components and optical devices. Depending on the level of intelligence provided by the optical module, the control circuit can work on the processor of the circuit assembly or the processor inside the optical module. There are three local control circuits. The first control circuit, because all the required excitation signals come from the device, such as the current of the laser and the temperature of the substrate, so it is local. The second case is: the bandwidth required for control is very wide, which eliminates the possibility of communication between circuit components, so the control function can only be completed locally. The third situation is that control takes place at the device level, but high-level cooperation is required to achieve the control function. For example, the setting of the control target changes the control behavior according to the state of the system (such as the setting of VOA and the gain of EDFA).
The above local control is performed within a single optical device or module, and the task of the coordination control layer is to bundle multiple different modules and control functions into a working ROADM system. The specific functions of this layer include:
â— Import remote feedback information into local control circuit.
â— Monitor whether the operation status of the local control circuit meets the regulations.
â— Coordinate the work between various local circuits during startup, shutdown and failure.
â— According to the configuration and status of the node, set the operation tasks for the lower-level subsystems (such as the coordination of on-off services and the power control of on-off services)
â— When applying multi-point control circuit, coordinate the work of this node and other nodes.
The alarm and event processing layer generates events based on the trigger points set by the operation, management, maintenance, and service provisioning (OAM & P) systems, aggregates events generated by local control and coordinated control functions, and is responsible for unbinding, system verification, event shielding, and priority Settings.
The OAM & P layer provides all necessary functions of the human-machine interface. The OAM & P layer provides command translation for local script interfaces or remote management systems. It parses the commands into configuration information and query operations within the ROADM node. The OAM & P layer is also responsible for communication events and alarms caused by system configuration changes. It sends messages to the management system or local script interface through the northbound interface. In addition, this layer also assists in completing the online upgrade of system hardware and software.
Figure 2 describes the traditional development model. System integrators use discrete or low-level optical devices to integrate a ROADM system. System integrators are responsible for the development of control intelligence at all levels, including local control, coordinated control, alarm processing, communications, and OAM & P. The integrator has complete control over platform development, and customizes and develops the control program and interface of each optical device to implement the entire system most effectively.
This development model can reduce material costs, but because a system includes a variety of technologies, such a development model requires considerable investment in the development of control systems. Due to limited resources, the pressure to reduce operating expenses is also increasing, so the development of ROADM is very difficult. Now, equipment vendors who want to develop ROADM (even if there have been several with such capabilities) have few that can afford this high internal development cost. Transfer to the upper end of the supply chain. Although there are various restrictions, but in order not to let go of the opportunity of ROADM. Some equipment vendors started to mobilize their partners to help them fill this gap in the product line. Other equipment manufacturers turn to optical device providers to reduce development risks. In line with the trend of outsourcing in the optical network industry, equipment vendors have pushed device providers to the upper end of the value chain, allowing them to share the risks of more ROADM development.
Device providers have responded positively. They have begun to provide smart optical modules with control circuits and software to equipment vendors, and sometimes even complete line cards. Figure 3 shows a typical model for implementing ROADM control functions based on the current telecommunications supply chain.
The use of intelligent modules and integrated products can simplify the control of ROADM. However, many existing obstacles limit the level of intelligence that the module can provide.
Ideally, this upward movement of device providers in the value chain will bring system integrators highly integrated and intelligent off-the-shelf devices, allowing them to integrate ROADM products quickly and efficiently. To make this development model truly effective, equipment manufacturers require:
â— There are multiple supply sources for key optical devices.
â— The device has a standard control interface, abstracting various optical characteristics of the lower layer.
â— The control interface of all components must be consistent, so that the system can be developed quickly and coordinated.
However, the actual situation is that the optical modules required by ROADM are still far away from commercial use; the diversity of technology and architecture hinders the timely development of products; due to the different degrees of intelligence of the modules provided by intelligent module providers and the inconsistent logical interfaces, Therefore, the secondary development of key devices is particularly difficult.
As shown in Figure 3, such a situation will force the workload of the local control and coordination control layer to increase, and the material cost will increase. System integrators must carry out individual adaptation work according to the requirements of each control device and module for the coordinated control layer. Similarly, due to the special requirements of multiple system providers, the level of smart functions that device providers can package into their products is also greatly limited. Optimizing the outsourcing model The current ROADM supply chain model is not efficient because the lower level of the coordination control function is not standardized. If system integrators, device providers, and contract manufacturers agree to use standard communication interfaces, configuration modes, and alarm handling, then product modules and line cards can communicate with higher-level systems and be compatible with the requirements of multiple system manufacturers. In this way, a universal coordinated control framework can be formed, which can help system integrators to develop products in time, and allow system integrators to use scarce resources in differentiated development work to increase the added value of the system.
The final formation of control standards and coordination framework will of course lag behind the deployment of the first batch of ROADM. At present, system integrators who want to develop ROADM in time can choose to rely on a device or line card supplier, or cooperate with a third-party control solution provider to develop consistent value-added products for device providers that system integrators are optimistic about. Control plan.
No matter which method is adopted, system integrators can use common and consistent control interfaces and control behaviors to transfer more ROADM functions and development risks to the enhanced intelligent modules developed by suppliers. The functions that can be transferred to this enhanced intelligent module include:
â— Coordinating control function, feedback monitoring inside and between packaging circuits
â— Alarm and event handling
â— Online firmware and software upgrade
â— Unified structure of automatic rationing business and resource management
To realize the mixing and matching of devices and modular structure, the key is to design a unified control system. This requires system integrators to play a more active role in the development of a single device, or in the evolution of standards, or in the process of using third-party control solutions. The first solution (system integrators participating in the development of a single device) requires system integrators to invest rare R & D resources, and often leads to a single source of the system, resulting in increased material costs. The second scheme (system integrators participating in the evolution of standards) may reduce development costs, but usually standardization work takes a long time and is not suitable for a rapidly evolving market. It is a feasible idea to use a third-party control scheme for system integration, but the system integrator needs to specify the control interface of the device provider.
In short, by standardizing the framework of optical network node control and coordination, or by outsourcing the general control of optical devices to third parties, system integrators can greet the new opportunity of ROADM more quickly at a lower cost. By building a universal control platform, system integrators can add new functions to outsourced smart modules or line cards without spending a long time and high cost.
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