Key Technologies Driving APN step3 Deployment

2.1 Wavelength band conversion using PPLN

NTT Science and Core Technology Laboratory Group is developing a wavelength band conversion technology based on PPLN. This technology enables direct optical connections between transmission lines with different optical specifications, or between existing networks that use a single wavelength band and APNs that use multiple bands. PPLN is a nonlinear optical material known for its high conversion efficiency and broad bandwidth. Leveraging these properties, a system configuration called the band-switchable multi-band optical cross-connect has been proposed [1]. This configuration allows for flexible switching between multiple wavelength bands to construct optical paths, significantly enhancing network flexibility and scalability. Detailed evaluations of the impact on optical signal characteristics after passing through PPLN-based converters have demonstrated that signal quality is maintained even after more than 20 conversions. Thanks to these features, wavelength signals can be connected without termination, enabling low-power operation and flexible wavelength band usage without compromising the APN’s energy efficiency.

2.2 OAO wavelength conversion technology

The OAO wavelength conversion method converts optical signals into analog electrical signals then back into optical signals, enabling conversion to arbitrary wavelengths. Unlike the conventional optical-electrical-optical (OEO) method, OAO eliminates the need for digital signal processing, offering significant advantages in terms of power consumption and latency. NTT has evaluated the performance of this OAO-based wavelength conversion technology and demonstrated that it delivers sufficient transmission performance for core networks [2]. Compared with conventional methods, it is expected to reduce power consumption by approximately 90% and latency by about 99%. This technology is key to enabling direct optical connections across multiple operators and networks. While the OEO method faces challenges such as increased delay and power consumption due to digital processing, the OAO method avoids these issues through analog processing while still achieving the required wavelength conversion. The OAO method also supports the APN’s characteristics of low latency, low power consumption, and high reliability, making it suitable for future diverse use cases. Applications are particularly expected in areas requiring real-time performance such as video transmission, remote control, and medical or industrial fields.

3.1 Ph-GW node architecture

An example of a node configuration that enables the basic functions of Ph-GW is shown in the upper part of Fig. 2. In addition to the ROADM functional module used in conventional WDM networks, it includes a remote control unit and fiber cross-connect (FXC), forming a disaggregated architecture. Control of APN-Ts is achieved through in-channel control, eliminating the need for external control via separate lines. The remote control unit multiplexes and demultiplexes control and data signals, establishing control and monitoring channels between APN-Ts distributed in the access area outside operator’s building and the APN controller. By wavelength multiplexing the control signals to the data signals, Ph-GW can accommodate various APN-Ts regardless of the protocol or transmission scheme of the data signals. FXC enables pass-through and blocking functions, allowing for optical signals to pass only when an optical path is established, and blocking signals from unauthorized APN-Ts. It also supports the turn back function, which achieves the shortest pass connection for traffic between APN-Ts accommodated in the same Ph-GW, and add/drop function, which enables intermediate processing at the Ph-GW for electrical processing. Compared with wavelength cross-connects (WXCs) used in ROADM, FXC has lower wavelength dependency, enabling it to transfer optical signals outside the C-band (1530–1565 nm) suitable for long-distance transmission. Therefore, it is expected to support short-distance use cases that adopt economical optical transceivers using O-band wavelengths (1260–1360 nm). The IOWN Global Forum defines the Open APN Functional Architecture consisting of two layers: a wavelength path layer, which provides end-to-end optical paths, and fiber path layer, which provides tunnels independent of wavelength. The node configuration shown in the upper part of Fig. 2, which achieves the basic functions of Ph-GW, conforms to this functional architecture.

Fig. 2. An example of a node configuration achieving the basic and advanced functions of Ph-GW.

3.2 Advanced functions using Ph-GW

Research and development are also underway to achieve advanced functions using Ph-GW such as optical multicast transmission and real-time optical path control. A node-configuration example for these functions is shown in the lower part of Fig. 2. In the APN, communication between two APN-Ts is typically based on a point-to-point (P2P) connection. However, to further develop the APN, it is important to support various communication schemes. For example, in public viewing scenarios where large-screen displays are used to watch live sports events across multiple locations, it is necessary to distribute high-capacity video data to multiple sites. In such cases, a point-to-multipoint connection scheme, rather than P2P, is expected to be applied. To address this, an optical multicast transmission scheme is being considered, in which optical signals are copied in their optical form using devices such as splitters placed between FXC and ROADM. Since signal duplication is not executed electrically, this scheme is expected to achieve low power consumption.

In mission-critical services, such as remote control of robots or drones over the network, stabilizing network quality becomes crucial. Therefore, real-time optical path control technology is being studied to quickly restore optical path quality in the event of degradation. The controller monitors the service quality of the established optical path and switches the path upon detecting degradation, thus continuing to provide APN services that meet the user’s quality requirements (Fig. 2, lower part). While protection and restoration mechanisms had been practically implemented, protection requires double the network resources due to redundancy, and restoration can take minutes to recover quality. Real-time optical path control, on the other hand, uses FXC capable of fast switching, enabling the controller to collect, analyze, and control the established optical path. This enables optical path switching within tens of milliseconds. This approach achieves both stabilization of established optical path quality and improved efficiency in network-resource utilization. The quality information used for analysis in real-time optical path control can also be extended to domains outside the APN through external collaboration functions. This enables quality control not only for APN services but also for end-to-end services including edge computing and wireless networks. NTT has thus far conducted demonstrations of remote robot operation in collaboration with computing resources and optical path control coordinated with wireless segment switching in cooperation with Cradio^®, a multi-radio proactive control technology [3].