## 2 GFZ-Wireless Sensing Unit (GFZ-WSU): Hardware And Communications
### Software  
The main software operating on the WSU currently consists of the following:  
OpenWRT: The operating system for the WRAP boards [20] with Linux kernel 2.6.22 [21]. OpenWRT is an open-source freely available and highly configurable distribution. By default, it contains only the minimum that is required to run Linux, so it can also run on very size-limited systems. Moreover, it provides an environment for building your own Linux distribution for several platforms, including our x86 er target platform for the WRAP boards.  
Data-provider: The program that handles the data streams from the digitizer board, and then archives them _via_ SeedLink (SeisComP).  
SeedLink (SeisComP): The Seismological Communication Processor (SeisComP) is an open-source software package and concept for near real-time seismic data distribution, ([[http://geofon.gfz-potsdam.de/geofon/SeisComP/seedlink.html](http://geofon.gfz-potsdam.de/geofon/SeisComP/seedlink.html)]([http://geofon.gfz-potsdam.de/geofon/SeisComP/seedlink.html](http://geofon.gfz-potsdam.de/geofon/SeisComP/seedlink.html))) developed by the GFZ for a networked seismographic system. In particular, in a seismic network SeisComP is responsible of the following tasks: data acquisition, data recording, monitoring and controlling, real-time communication, user access, and automatic (near-)real-time data processing (quality control, event detection and location). The SeedLink program is part of SeisComP, and is the system devoted to the near real-time seismic data distribution, that is, a server protocol based on Transmission Control Protocol (TCP) [16]. In particular, _via_ SeedLink the data are sent in the form of 512-byte Mini-SEED packets with a 8-byte SeedLink header. The header contains the packet sequence number, which allows the unit to resume transmission where it left off (_i.e._, the recovering of the connection in the event of network errors, and the support of non-permanent connections in \"dialup mode\"). It has client-server architecture and is capable of many tasks (data acquisition, data recording, monitoring and controlling, real-time communications, user access, near-real-time data processing). In the WSU, the SeedLink server stores the data in a ring buffer of configurable size on the Compact Flash card. The data in the ring buffer will be kept for the order of 20 days. If more storage is found to be necessary, then it is simply a case of using a larger Compact Flash card. Actually, SeedLink is the only component of SeisComP used by the GFZ-WISE. Nevertheless, in order to retrieve the data from the system, at the moment every user must install SeisComP on its computer.  
Optimized Link State Routing [17]: OLSR is a table-driven pro-active routing protocol currently chosen for the wireless mesh network ([[http://www.olsr.org](http://www.olsr.org)]([http://www.olsr.org](http://www.olsr.org))). As a proactive protocol it periodically assesses and maintains the network topology by flooding information about its direct neighborhood throughout the whole network. OLSR has proven that it is capable of operating with hundreds of nodes, and it is also widely accepted by several mesh networking communities, _i.e._, Freifunk ([[http://www.freifunk.net](http://www.freifunk.net)]([http://www.freifunk.net](http://www.freifunk.net))) and the Funkfeuer ([[http://www.funkfeuer.at](http://www.funkfeuer.at)]([http://www.funkfeuer.at](http://www.funkfeuer.at))) projects.