5G is the focus of the world in recent years, which is characterized by high speed, large capacity and low delay. The wireless technologies have enabled the former two. However, the delay of 5G is related to the optical fiber network supporting the base stations. The high speed and wide connection of terminals exhausts the bandwidth of optical fiber communication and results in more delay. The optical fiber network is required to be upgraded and the focus is on metro network. Based on cost consideration, the current metro network is mainly based on CWDM and FOADM (fixed optical add/drop multiplexer) technologies. The DWDM and ROADM technologies for long haul network are expected to sink to metro network.
Structure of All Optical Network
In order to improve the efficiency and reduce the OPEX (operation expense) cost of the optical fiber network, the new generation of all optical network (AON) is required to be software defined network (SDN). The SDN network can be reconfigured based on software setting, exempting from manual operation. ROADM is the key equipment enabling SDN network, as shown in Fig.1. The ROADM-based AON includes three level of networks: long haul, metro and access network. The long haul network connects big cities and is usually constructed as a mesh network. The metro network usually employs fiber ring structure. As the telecom services become diverse and complicated, the metro network extends to be a multiple-ring network, including a core ring and many edge rings. The access network is fed by the metro rings and extends to the vicinity of the end users. The final links between the access network and the users include FTTx (to the business buildings, schools, homes, etc.) and wireless base stations.
Fig.1 Structure of all optical network
What’s CDC-F ROADM?
A ROADM node has a network node interface (NNI) and a user network interface (UNI). The NNI interconnects DWDM signals from/to multiple directions. The DWDM signals are switched between different directions in wavelength granularity. The UNI downloads signals designated to the node and uploads signals from the node in wavelength granularity. In order to realize non-blocking switching and adding/dropping of wavelengths, the new generation of ROADM nodes are required to be colorless, directionless and contentionless (CDC ROADM).
Considering an 8-dimentional (8D) ROADM node with 80-channel DWDM in each direction, the total wavelengths to be handled in the node is 8×80=640. However, the wavelengths to be added/dropped by the UNI of the node is usually less than 20% based on statics. Most of the wavelengths (more than 80%) are just switched by the NNI of the ROADM node. Thus 640×20%=128 add/drop ports are enough for the UNI. However, 20% reserve of add/drop ports requires each port to be versatile, which means that each add/drop port can add/drop different wavelengths from different directions (colorless and directionless) according to allocation by the control system. Meanwhile, the UNI needs to be capable to simultaneously drop the same wavelengths from different directions (contentionless).
The signals transmitting in the optical fiber may have different bit rates. In high-speed transmission system, signals with different bit rates need different channel width due to the sideband originating from modulation. As shown in Fig.2, signals with bit rates of 100G, 400G and 1T need channel width of 50GHz, 75GHz and 150GHz, which is quite different from low-speed (≤25G) signals. Low-speed signals usually occupy channel width of 50GHz or 100GHz, depending on the design of DWDM system.
Fig.2 Channel width requirement for signal of different bit rate
In order to accommodate the coming high-speed transmission, the DWDM system need to provide super-channel function. The channel width should be flexible. It can be dynamically allocated as 50GHz, 75GHz, 100GHz,150GHz, etc., according to demand. Super-channel is a term for system designer. The module designer uses another term “flexgrid” for the same meaning.
A ROADM capable of colorless, directionless and contentionless functions is called CDC ROADM. It can be further defined as a CDC-F ROADM when flexgrid function is also supported.
The coming 5G application promotes the upgrading of AON. As the key part for AON, ROADM market is expected to increase rapidly, especially in metro network application.