Redundant multi format data acquisition
What is CAPS?
The Common Acquisition Protocol Server (CAPS) was developed to fulfill the needs to transfer multi-sensor data from the station to the data center. As nowadays more and more stations with co-located sensors like broadband seismometers, accelerometers, CGPS, temperature sensors, video cameras, etc. are build up, a acquisition protocol is required, which can efficiently handle low- and high-sampled data through one unified protocol. Such a flexible acquisition protocol is available with CAPS. CAPS is also a backbone for secure and redundant data centers.
The focus on early warning in the last decade gave the timeliness of data. This made it necessary to allow backfilling of data (send most recent data first) as well as reducing the size of the data records. These requirements for data acquisition cannot be fulfilled by many standard software packages. For example SeedLink, the de facto standard for real-time data transmission widely used for transmitting seismic data, can neither handle backfilled data nor does it support a variable record size. CAPS fills this gap as it is format independent allowing to handle low and high sampled data in parallel while keeping track of the timely order of packets.
In the last years one topic became a focal point of interest - redundancy. CAPS allows to fetch data packets through several connections in parallel, for example through a satellite line and Internet. Once the data is transmitted to CAPS, it is merged with the already available data set where duplicates are dropped. This allows full redundancy in data transmission and the only single point of failure left is the equipment at the station itself.
One of the main problem for institutions handling massive data is the downtime during reconfiguration. CAPS reduces the downtime to an absolute minimum and at the same time takes care that no gaps or redundant data occur. This is realized through wildcard requests by clients. Wildcard request means, that CAPS can request all data available at the source and doesn’t need a specific list of stations. Once the source adds a station, the data is directly transmitted to the target without the need to reconfigure or restart CAPS. CAPS furthermore supports the live reconfiguration of access control parameters without service interruption.
Figure Architecture shows the architecture of CAPS. The central component is the server, which receives data from sensors or other data centers, stores it into an archive and provides it to connected clients. The connection between a data provider and CAPS is made through a plugin.
Plugins are independent applications which, similar to clients, use a network socket to communicate with the server. The advantages of this loose coupling are:
Figure Deployment illustrates a possible deployment of CAPS and its plugins. The acquisition of data from other data centers is most likely done through a public interface reachable over the Internet. For this center-to-center communication a plugin is typically launched on the receiving site to feed the local CAPS server.
For the direct acquisition of data from a sensor the plugin can run on the sensor station. This plugin will send the data either directly to the data center or to a local CAPS instance. The advantage of the second approach is:
The ability to connect different CAPS instances simplifies sharing of data. One protocol and one implementation is used for the sensor-to-center and center-to-center communication. In the same way multiple CAPS instances may be operated in one data center on different hardware to create backups, establish redundancy or balance the server load.
CAPS ships with a read-only web interface which lists the available data streams, shows waveforms and allows downloading which provides server statistics in separate perspectives.
CAPS is developed as a standard SeisComP application. It uses the SeisComP infrastructure for startup and configuration. A GUI application simplifies the setup and configuration tasks. In addition we offer a full featured web based configuration interface allowing you to control your installation from anywhere using any device running a web browser.
Since CAPS entered the market many more plugins have been developed for transferring data from multiple sources and in different formats reaching beyond the available SeedLink plugins . For new data sources and format plugins can be easily developed and added. The most import existing plugins are: