Optical Cross-Connect Technology and Application

ROADM can implement multi-degree large-capacity wavelength-level scheduling to meet the networking demands of backbone, metro and data center interconnection (DCI). However, as the degrees of a ROADM grow, the number of fiber connections inside the ROADM site increases dramatically, which makes the service provisioning and maintenance process time-consuming, prone to human errors and increases the footprint and power consumption. Optical cross-connect (OXC) addresses these problems by using the all-optical backplane in combination with the highly integrated optical line boards and optical add/drop boards. Since 2018, OXC has been widely used by Chinese operators.

Composition of OXC and Key Technologies

A 20-degree ROADM requires three cabinets, more than 100 boards, and 400 fibers inside the site. It occupies a large area and has high power consumption and complicated fiber connection, making service provisioning and maintenance difficult. As the number of degree increases to 32, the footprint, power consumption, and the number of fiber connections will increase dramatically, and it will be difficult to locate problems due to a large workload of service provisioning and maintenance. Compared with ROADM, OXC uses highly integrated boards and optical backplane to reduce the footprint and power consumption and simplify internal fiber connections (Fig. 1). A 20-degree OXC needs only one cabinet to reduce the footprint by 2/3 and about 30 boards to reduce the number of boards by 2/3 while also reducing the corresponding power consumption. The optical backplane connects all fibers inside the site to achieve automatic fiber connection, which improves the provisioning efficiency and reduces the maintenance costs.

An OXC is mainly composed of a optical backplane, optical line boards and optical add/drop boards, and involves key technologies like flexible optical backplane, high-density optical connector, 1×N wavelength selective switch (WSS) and M×N WSS. The optical backplane includes a flexible optical backplane and high-density connectors. The optical add/drop boards have two types: with colorless, directionless, flexgrid (CDF) capability or with colorless, directionless, contentionless and flexgrid (CDC-F) capability. The first type employs TWIN 1×N WSS and does not support contentionless functionality. It integrates the WSS and the optical amplifier, and occupies one slot. It can add/drop 32 wavelengths and schedule a service to any optical direction through the high-density connectors and fiber connections on the optical backplane. The latter type employs M×N WSS and occupies two slots. It supports contentionless add/drop of 48 wavelengths in 8/16 degrees. The optical line board highly integrates the OA, OP, OSC and OTDR function modules, and one slot corresponds to one direction. One optical line board occupies one slot and corresponds to one direction. The optical line board is connected with the optical backplane through the high-density connectors and can schedule a group of wavelengths to any optical direction or any optical add/drop board for service add/drop.

—Optical backplane: The optical backplane technology is used to convert internal fibers between optical interfaces of the ROADM board into high-density interconnected fibers on the optical backplane. Internal fibers are divided into multiple groups, deployed through the fiber cabling machine and encapsulated into a flexible plate to form a flexible optical backplane that supports non-blocking fiber connections.

—High-density connector: The optical backplane is connected with the optical line board and optical add/drop board through the high-density connector. The optical connector must have high density to ensure full interconnection of all optical boards inside the OXC site and also support blind insertion with features like high interconnection precision and reliability of multiple plugging/unplugging.

—WSS: The core components of optical add/drop boards and optical line boards are 1×N WSS and M×N WSS. The related technologies mainly include micro-electro mechanical system (MEMS) and liquid crystal on silicon (LCoS).

OXC Application and Development

OXC products have the following advantages:

—High integration to save equipment room space: A ROADM site needs a large number of separate boards and so needs several cabinets. OXC optimizes it through high integration of board functions. One cabinet can implement 32-degree optical cross-connect scheduling, saving the footprint by 2/3.

—Energy saving and convenient O&M with low OAM costs: Compared with ROADM, the OXC equipment has a lower power consumption and requires a fewer number of power supply terminals. The optical backplane is used to solve the problems of a large number of fiber connections inside the ROADM, low provisioning efficiency, and difficult maintenance. OXC also supports optical-layer OAM to facilitate fault location and reduce the OAM cost.

—Support for ultra-large-capacity optical cross-connect and low latency: OXC products support CE, C++ and L++ bands. One cabinet supports 32 degrees and 1024T optical cross-connect capacity to meet the requirements of backbone and metro networks. OXC nodes are only connected through fibers, so that the latency is almost zero.

—Intelligent network: As a physical-layer device, OXC supports the CDC feature, so it can solve wavelength conflicts, increase the flexibility of optical cross-connect scheduling, and improve the utilization of network resources. It supports flexgrid and can dynamically adjust the bandwidth of transmission pipes and intelligently schedule 100G and B100G wavelengths. It supports optical domain balancing and automatic optical power optimization to reduce inter-WSS crosstalk, improve the system transmission performance, increase available routes, and improve network survivability. The application of OXC in backbone and metro networks, especially in high-degree networks, provides guarantee for network intelligence.

Since 2018, manufacturers in China have gradually launched 16/20/32-degree OXCs to replace ROADM in the backbone and metro networks of China Mobile, China Telecom, and China Unicom. Backbone networks with a long transmission distance (from hundreds to thousands of kilometers) generally use a star typology and have a small number of physical links and low degrees. However, the node traffic is heavy, especially at the core node. Traffic add/drop occupies a large number of OXC degrees. The 16-degree, 20-degree and 32-degree OXCs have 16, 20 and 32 slots, respectively. One slot can be inserted with an optical line board to support one degree or an optical add/drop board for the add/drop of 32 wavelengths. The network design needs to consider the degrees and add/drop traffic capacity in the current stage while reserving slots for the expansion of line degrees and add/drop traffic capacity. When the OXC slots are insufficient, the local add/drop board that has two extension ports (a port for add/drop of 32 wavelengths) can work with the WSS board on the transmission subrack to add/drop 96 wavelengths on a single OXC slot.

Metro networks with a short transmission distance (generally hundreds of kilometers) use mesh networking and have a large number of physical links and line degrees. Core nodes have a large number of line degrees and heavy add/drop traffic. Non-core nodes have fewer line degrees and small expansion potential. Similarly, the OXC degrees need to be determined in accordance with the current and future requirements of line degrees and add/drop traffic capacity. Generally, 32-degree OXCs are used at core nodes and 16-degree OXCs are used at non-core nodes of metro networks in China.

With the network development, the number of optical layer scheduling degrees at core nodes is increasing, and the add/drop traffic at core nodes is also increasing. In the northwestern ring network of China Telecom, the Taiyuan hub site has grown to be 57 degrees. Currently, the commercially deployed 1×N WSS can maximally support 32 degrees, which cannot meet the high-degree requirements of core nodes in the existing network. With further network development, there will be increasing demands for high degrees. Therefore, 64-degree or 128-degree OXCs will become a development trend.

In high-degree scenarios, contentions could occur if services using the same wavelength from different degrees are added or dropped at the same add/drop structure, so the CDC capability has become an important requirement of OXC. Currently, the commercial M×N WSS has 8/16 degrees, and usually 24 add/drop ports, which cannot meet the requirements of high degrees and heavy add/drop traffic. In addition, M×N WSS does not support L++ band. Therefore, higher degrees, a higher port count and band extension are also development directions of OXC.