Control Technologies Supporting APN step3

2.1 Challenges toward enabling more on-demand optical path usage

APN step3, which supports the spread of APN services, aims to promptly respond to diversified service needs by providing optical paths while efficiently using wavelength resources. To achieve this, further advances in on-demand optical path provisioning technologies [3, 4] are required. One of these advanced features is plug and play, which enables an optical path to be established anywhere, anytime within the APN area, simply by connecting an APN terminal to the APN, supporting more on-demand use case scenarios. For example, in a remote-production use case where footage from a sports or event venue is sent to a broadcasting station, an APN terminal can be brought to the venue according to the event schedule, and a temporary optical path can be established between the venue and editing site. This eliminates the need to permanently install an APN terminal at each venue, reducing the number of APN terminals used per user. APN node ports connected to APN terminals can also be shared alternately among users. This utilization is expected to improve cost efficiency per user and lead to the widespread use of APN services.

To automatically establish optical paths, the APN controller needs to recognize the connection relationship between the APN terminal and the access port of the Ph-GW to which the terminal connects. This ensures that optical signals from and to the APN terminal are transferred according to the designed optical path route by appropriately setting transfer routes inside the Ph-GW automatically. This novel port management and operation that automatically confirms and identifies the access port connected to the APN terminal at the time of initial connection enables the establishment of an optical path on demand for any APN terminal connected to the APN without having to register accommodation design information into the APN controller in advance.

2.2 Control method to enable plug and play of APN terminals

Figure 1 shows an overview of a control method for automatically connecting APN terminals to the APN in a plug-and-play manner without the need for prior input of accommodation design information into the APN controller. When an APN terminal is connected, the APN controller authenticates the terminal through the exchange of control signals and registers terminal information in the database (Step i). Next, the APN controller controls the output of the client-signal light from the APN terminal (Step ii) then confirms the connection between the APN terminal and the access port of the Ph-GW (Step iii). The confirmed connection is registered in the database (Step iv). Even when multiple APN terminals are connected in a short period, the APN controller successfully confirms the access ports by sequentially controlling the main-signal light of each APN terminal [5]. Through these steps, the APN controller can set the transfer route inside the Ph-GW by referring to the connection table generated at initial connection, enabling automatic establishment of the optical path. This plug-and-play operation provides optical path establishment more on-demand, improving operational efficiency during the APN service expansion period. By applying an in-channel control scheme that wavelength-multiplexes main-signal light and control-signal light between the Ph-GW and APN terminals, plug-and-play operation is achieved even when APN terminals are placed at user sites widely distributed in the access area.

Fig. 1. Control method for enabling plug-and-play automatic connection of APN terminals.

3.1 Issues related to path design in the APN

NTT is also developing optical path design technology toward APN step3 implementation. APN step3 provides high quality and low latency by assigning optical paths per user or service. To provide optical paths on demand per user/service, rapid design of numerous optical path routes and wavelengths is necessary. Optical path routing and wavelength assignment in the APN are defined as path computation functions in the controller [6], which are essential for software-based control and automated path setup in the transport layer. For optical paths of the continuously operating optical link, shortest or near-shortest routes between two points are generally selected. One wavelength is assigned per route, requiring the same wavelength on all links along the path, which is called the wavelength continuity constraint [7]. The process involves determining the route on the basis of user demand and assigning a wavelength to each constituent link of that route. However, the method used for wavelength assignment impacts the capacity utilization efficiency. Improving this efficiency leads to reduced capital investment. Consequently, a key challenge is to establish a path design method that enables the flexible and cost-effective operation of the APN.

3.2 Path design method for considering wavelength conversion

When assigning a wavelength to an optical path in the APN, it is typically necessary to assign the same wavelength end-to-end. However, with the introduction of wavelength conversion technology, where wavelength conversion can be flexibly applied in the intermediate equipment, it becomes possible to use multiple wavelengths even within a single optical path [8]. In such cases, in addition to configuring the route and wavelength for the transport equipment, configuration information for the wavelength converters must also be generated and applied to the transport equipment (Fig. 2). Assuming wavelength conversion is applied, a method is needed to assign multiple wavelengths to a single path. One method for wavelength assignment is the traditional first-fit (FF) rule, which assigns wavelengths starting from the lowest channel number sequentially. FF can efficiently allocate wavelength resources using a simple procedure. Therefore, we are proceeding with verification, using simulation and other techniques, for a method that combines wavelength conversion and FF to assign wavelengths to a single optical path.

Fig. 2. Route and wavelength selection with wavelength conversion in the APN.

3.3 Wavelength resource optimization in the APN

Applications of wavelength conversion include connecting different network domains, such as between transport networks, or connecting different service provider networks (as shown in Fig. 3, left). These connections are often characterized by different management authorities or operational rules. When connecting these networks via a wavelength, it is necessary to negotiate the wavelength to be used. Even when mutual agreement on the same wavelength is not possible, using wavelength conversion can simplify or eliminate the need for such negotiation. This thus enhances the end-to-end connectivity of the optical path while minimizing the need for additional resources. Another application is using wavelength conversion as an alternative to equipment expansion. When the number of accommodated paths increases and the maximum wavelength number reaches a threshold, equipment expansion is needed. Using wavelength conversion to assign lower-numbered wavelengths can delay expansion timing (Fig. 3, right). The effectiveness depends on factors such as current path accommodation status, path route and length, and wavelength conversion positions. Further verification will clarify conditions under which wavelength conversion path design is especially effective.

Fig. 3. Applications of wavelength conversion.

4.1 Overview of NBI and SBI using open interfaces

NTT improves connectivity with upper-layer systems, including service orchestrators and workflow engines, and devices, by adopting interface standards that comply with open standards for its optical network controller. Regarding SBI, we are preparing for scenarios in which controlling devices through the element management system (EMS) will be necessary—covering not only direct control of devices but also considering the transition from current optical transport networks to the APN. We are thus progressing with studies based on two control methods: controlling devices directly and via EMS (Fig. 4).

Fig. 4. Overview of NBI and SBI connection configuration of the optical network controller.

Regarding NBI, we use the Transport Application Programming Interface (T-API), standardized by ONF and widely used in commercial controller products. Considering real-world operational use cases and the functions needed for APN step3 on-demand connections, we are also exploring extensions to the T-API specification. Regarding SBI, we use OpenROADM (reconfigurable optical add/drop multiplexer) for direct device control from the controller. For device control via EMS, we use T-API, which is supported by many commercial EMS systems. Even when interfaces for devices and EMS follow standard specifications, vendors might have their own interpretations. Therefore, we examine the interfaces of commercial products using actual equipment, define the requirements SBI should implement, and draft the related specifications.

On the basis of these efforts, NTT has developed interface specification documents for the optical network controller. To help understand these specifications and speed up development, we have also created a user guide summarizing design and implementation guidelines. These documents are publicly available on NTT’s website [9]. Using an optical network controller that implements these specifications, we conducted proof-of-concept experiments connecting to actual APN devices to verify the feasibility of the interface specifications.

4.2 Optical wavelength path management model with wavelength band conversion

NTT is exploring an optical wavelength path management model necessary for the optical network controller to manage end-to-end optical wavelength paths that include sections with wavelength changes, using wavelength band conversion technology—a new key technology for the APN. The current optical path management model defined in T-API assumes the use of a single wavelength for end-to-end paths (Fig. 5, upper). However, managing wavelength conversion paths requires the controller to differentiate and manage different optical signal connections. Alternatively, end-to-end network connections can be treated and managed as a single path. In line with these requirements, NTT has developed an optical wavelength path management model (Fig. 5, lower) that enables both detailed management of each optical wavelength path component by the controller and operational flexibility by handling it as a single optical path. This is achieved by separating the connections representing optical signals by wavelength and building multi-tiered end-to-end connections that span optical wavelength paths at a higher level.

Fig. 5. Comparison between the optical wavelength path management model that incorporates wavelength conversion and conventional model.

The research results related to NBI, SBI, and the optical wavelength path management model were obtained from the grant program (No. JPJ012368G60301) by National Institute of Information and Communications Technology (NICT), Japan.