## 2 GFZ-Wireless Sensing Unit (GFZ-WSU): Hardware And Communications
### Communication  
The communication of seismic data and information amongst the GFZ-WSU is based on the routing concept. The term routing refers to the procedure of selecting within a network the paths along which data can be sent from a source to a sink. Routing activities within a wireless network are made more complicated by the fact that all nodes act contemporarily as sources, sink and routers of data. GFZ-WSUs rely on the OLSR (Optimized Link State Routing) protocols as the routing strategy (see above for the OLSR description), and are designed to form a self-organizing ad-hoc wireless mesh network (WMN). The use of WMN protocols allows a network of WSUs to continuously adapt to changing circumstances (addition or removal of nodes, interference in communications, loss of sensors due to unforeseen malfunctioning _etc._) in order to maintain optimal communications [14]. The main advantages of WMN for seismic noise array surveys in urban context are (1) the system is free from cable usage, thus, allowing improved array geometries, and azimuthal coverage, (2) in the case of large arrays, data also can be transferred to a user by multi-hop communications from those instruments that are not in light-of-sight with the user itself (remote stations).  
The OLSR that the GFZ-WISE employs is a _proactive_ routing protocol, where every node has a map of the complete network topology, allowing data to be immediately sent along the optimal path towards the users or the gateways. This leads to each node having a routing table that describes the most efficient way to reach each other node. It makes use of advanced metrics, _i.e._, measurement methods, for the evaluation of a multi-hop path within the network.  
In order to make the transmission of messages as efficient as possible, particularly in limiting duplicate transmissions, a MultiPoint Relays (MPR) communication schema is adopted. This approach is important, especially when the network is done for monitoring purposes. Roughly speaking, each unit periodically broadcasts \"Hello\" messages to its direct neighborhood. These messages include the list of known neighbors, combined with the status of the quality of the connection to them. By knowing its two K-hop neighborhood, every node independently chooses a subset of the one K-hop neighborhood by which the complete two K-hop neighborhood is reachable. This results in certain nodes being designated as a MPR, which allows for a reduction in transmissions when flooding the network as only the MPRs need to rebroadcast a message to reach the complete 2 K-hop neighborhood (Figure 3). Every node also announces its chosen MPR, so that each node knows if it is a MPR or not. Nodes selected as MPR regularly flood the network with topology control (TC) messages at a defined interval (less frequently than \"Hello\" messages). These messages contain the link states of the nodes that selected this node as a MPR (the MPR selectors). By receiving these messages, a node therefore has enough information to locally reproduce the complete topology of the network. This enables a node to compute optimal paths to all known destinations, which in OLSR is done using [PERSON]'s shortest path algorithm [25].  
Within the GFZ-WISE, seismic data are transferred among stations using a multi-hops strategy by means of the SeedLink protocol (_i.e._, by 512-byte Mini-SEED data packets), with a rate of up to 54 Mbps in both the 2.4 GHz and 5 GHz unlicensed bands. In the case of a low signal-to-noise ratio in the communications, the WLAN cards driver can automatically decrease the rate of transmission. Tests performed within the framework of seismic early-warning activities in Istanbul, Turkey, using instruments with the same software and a similar hardware configuration showed that WLAN communications between line-of-sight stations equipped with omni-directional antenna is possible until a distance of ca. 250 m within an urban context [14].