Sensors_**2010**, _10_, 8920-8937; doi:10.3390/s101008920  
_Series of_  
**ISSN 1424-8220**  
www.mdpi.com/journal/sensors  
_Article_  
**ZERO: Probabilistic Routing for Deploy and Forget Wireless Sensor Networks**  
**[PERSON] \({}^{1,\star}\), [PERSON] \({}^{1}\), [PERSON] \({}^{2}\), [PERSON] \({}^{2}\), [PERSON] \({}^{1}\), [PERSON] \({}^{3}\) and [PERSON] \({}^{3}\)**  
\({}^{1}\) Universitat Oberta de Catalunya, Rambla Poblenou 156 08018 Barcelona, Spain;  
E-Mails: [EMAIL_ADDRESS] (J.L. ); [EMAIL_ADDRESS] (A.A.)  
\({}^{2}\) Universitata Politecnica de Catalunya, Jordi Girona, 08034 Barcelona, Spain;  
E-Mails: [EMAIL_ADDRESS] (J.C.P. ); [EMAIL_ADDRESS] (I.V.) \({}^{3}\) Universitat Autonoma de Barcelona, 08193 Bellaterra (Barcelona), Spain;  
E-Mails: [EMAIL_ADDRESS] (J.L.V. ); [EMAIL_ADDRESS] (A.M.)  
\({}^{\star}\) Author to whom correspondence should be addressed; E-Mail: [EMAIL_ADDRESS];  
Tel. : +34-93-326-37-07. _Received: 10 August 2010; in revised form: 18 September 2010 / Accepted: 20 September 2010 / Published: 29 September 2010_  
**Abstract:** As Wireless Sensor Networks are being adopted by industry and agriculture for large-scale and unattended deployments, the need for reliable and energy-conservative protocols become critical. Physical and Link layer efforts for energy conservation are not mostly considered by routing protocols that put their efforts on maintaining reliability and throughput. Gradient-based routing protocols route data through most reliable links aiming to ensure 99% packet delivery. However, they suffer from the so-called \"hot spot\" problem. Most reliable routes waste their energy fast, thus partitioning the network and reducing the area monitored. To cope with this \"hot spot\" problem we propose ZERO a combined approach at Network and Link layers to increase network lifespan while conserving reliability levels by means of probabilistic load balancing techniques. **Keywords:** energy efficiency; probabilistic routing; collection-tree-protocol; gradient routing; wireless sensor networks; agriculture and industry Introduction  
As Wireless Sensor Networks (WSN) are being adopted by industry and agriculture and their use is being widespread out of academical environments the need for long lifespan (The lifetime of a long-lived WSN is typically expected to be on the order of years with node duty cycles less than 1%. e.g., less than 1 sample per minute), robustness and throughput are becoming critical points for the success of any application. Industry addresses real scenarios usually with well defined requirements and constraints which do not always fit with the research carried out at the university. Whilst numerous protocols and techniques have been developed to ensure energy efficiency, 99.9% delivery ratio and efficient routing protocols, more work is still needed to provide a robust implementation to attain the deploy and forget wireless sensor networks. Due to the heterogeneity of industrial scenarios and possible applications of WSN, the combination of techniques does not always provide a good candidate solution. In this paper we address those scenarios where WSN have to be deployed to collect data for very long periods of time and, therefore, information lost becomes critical (e.g., natural hazard monitoring, smart city applications, metering, _etc._). We aim to implement an energy efficient routing protocol combined with an improvement of the reliability of data transmission whilst keeping a balanced energy consumption with respect to delivery ratio. Besides a key objective is to minimize protocol configuration and its dependency to the addressed scenario. Energy consumption is the key limitation of long lived WSN which is strictly dependant to transmission rates. Gradient routing protocols [1] usually overload best paths to the sink, _i.e._, nodes with particularly favourable links are thus likely to have a heavier workload than their neighbours as they are chosen to relay traffic that they do not generate.