The all-optical network structure based on ROADM and what is CDC-F ROADM?

In the past two years, 5G has become the focus of the world. It is characterized by high speed, wide connectivity and low latency. Wireless communication technology has achieved the first two characteristics of 5G. However, the delay of 5G communication is related to the optical fiber network supporting wireless base stations. The high speed and wide connection of terminal equipment exhausts the bandwidth of the optical fiber communication system and causes more time delay. The optical fiber network needs to be upgraded, and the focus is on the upgrade of the metropolitan area network. Based on cost considerations, the existing metropolitan area network is mainly based on CWDM and FOADM (Fixed Optical Branching Multiplexer) technology. In order to upgrade the network, DWDM and ROADM (Reconfigurable Optical Branching Multiplexing) previously used in the backbone network (Device) technology is expected to sink to the metropolitan area network.

All-optical network structure

In order to improve the efficiency and operating cost of the optical fiber network, the new generation of all-optical network AON should have SDN (software-defined network) functions. The SDN network can be reconfigured through software settings to avoid manual operations. ROADM is the key equipment to realize SDN network, as shown in Figure 1. The all-optical network based on ROADM includes a three-layer architecture: long-distance network, metropolitan area network and access network. The long-distance network realizes the connection between big cities, usually built into a MESH (mesh network) structure. Metropolitan area networks usually use fiber-optic ring network structures. With the diversification and complexity of telecommunication services, metropolitan area networks have evolved into a multi-ring structure, including a core ring network and multiple edge ring networks. The access network is supported by the metropolitan area ring network and extends to the vicinity of end users. The final connection between the access network and users includes FTTx (fiber to business buildings, schools and homes, etc.) and wireless base stations.

figure 1.All-optical network structure

What is CDC-F ROADM?

Each ROADM node contains a network node interface (NNI) and a user network interface (UNI). NNI interconnects DWDM signals from/to multiple transmission directions, and these DWDM signals switch between transmission directions with wavelength granularity. UNI downloads the signal of the destination node with wavelength granularity, and uploads the signal from the node. In order to achieve non-blocking wavelength switching and uploading/downloading, the new generation of ROADM nodes are required to have the characteristics of colorless, non-directional and non-competitive (CDC ROADM).

Consider an 8-dimensional ROADM node, each transmission direction has 80 DWDM channels, so the total number of wavelengths that need to be processed at the node is 8×80=640. However, according to statistics, the number of wavelengths that need to be uploaded/downloaded on the UNI side of each ROADM node is generally less than 20% of the total, and most wavelengths are only exchanged on the NNI side. Therefore, it is sufficient to configure 640×20%=128 upload/download ports on the UNI side. However, only 20% of the upload/download ports are prepared, and each port is required to be a generalist, which means that each upload or download port can upload or download different wavelengths to or from different directions according to the arrangement of the control system (colorless And non-directional), and at the same time require the UNI side to be able to download the same wavelength from different directions at the same time (no competition).

The signals transmitted in the optical fiber may have different bit rates. In high-speed transmission systems, sidebands are generated due to modulation, and signals of different bit rates require different channel widths. As shown in Figure 2, signals with bit rates of 100G, 400G, and 1T require channel widths of 50 GHz, 75 GHz, and 150 GHz, respectively, which are very different from low-speed signals (≤ 25G). The low-speed signal usually occupies a channel width of 50GHz or 100GHz, which depends on the design of the DWDM system, and is not limited by the modulation rate of the signal.

figure 2.The required channel width for signals of different bit rates

In order to adapt to the upcoming high-speed transmission, the DWDM system should have a super channel function, and the channel width should be variable, which can be dynamically adjusted to 50GHz, 75GHz, 100GHz, 150GHz, etc. as needed. The super channel function is the system design language, and optical module designers usually use another word “flexible bandwidth” to express the same meaning.

A ROADM node with colorless, non-directional and non-competitive functions is called CDC ROADM. If the node further supports flexible bandwidth functions, it is called CDC-F ROADM.

The upcoming 5G applications promote the upgrade of all-optical networks. As a key part of all-optical networks, the ROADM market is expected to usher in rapid growth, especially in the application of metropolitan area networks.