## 1 Introduction  
Recent developments in Wireless Sensor Networks (WSNs) have brought this domain from merely a concept of microelectronics to a new realm of practical applications. A typical WSN comprises of a large number of low powered, low cost, memory/computationally-constrained, intelligent sensor devices. These sensors are generally involved in detecting and measuring some target phenomena. Due to its inherent energy, footprint and deployment constraints, a WSN is prone to faults and malfunctioning. These faults can be due to hardware/software failures or energy depletion. In hostile deployments, the faults may be caused by natural or human adversaries, e.g., natural disasters in calamity-struck regions or radio jamming in a battlefield [1]. Despite WSN's fault-prone characteristics, mission-critical natures of emerging WSN applications (e.g., military, healthcare, and disaster recovery applications) require that communication to/from sensors is dependable and reliable. The source to sink communication in WSNs is generally dependent on the intermediate relaying sensor nodes. Therefore the reliability of a transmission is dependent on the topology and routing techniques being deployed in the WSN. Generally, applications require either packet driven reliability (like intrusion detection applications, battlefield surveillance applications etc) or event driven reliability (like habitat monitoring applications, temperature recording applications etc). Packet driven reliability demands that all the packets must be delivered successfully from the source to the sink, whereas event driven reliability requires the event to be detected even with the help of one packet delivery. In this paper, we focus on the requirement of packet driven reliability applications. We propose to enhance WSN routing reliability using high-reliability hotline links between sensor gateways. A WSN typically contains multiple resourceful gateway nodes that provide load balancing, local cluster management and energy saving [2-4]. Since these gateways are far fewer in number than the sensor nodes, we introduce the concept of using hotlines (e.g., high speed Ethernet links) for inter-gateway communication, while traditional multi-hop techniques can be used by the sensors for intra-gateway communication. With multiple hotline-connected gateways we are able to achieve reliable source to sink communication. This paper describes this proposed topology in detail. This is followed by analytical modeling and comparison of traditional WSN communications _versus_ hotline-based approach. Our analytical results show that significant improvements in reliability can be achieved using a hotline based topology. In this paper we also introduce a Signature Based Routing (SBR) scheme, that vests intelligence to those resourceful gateway nodes. SBR is presented as a remedy to delay problems that arise in our suggested hotline assisted routing approach [5]. SBR gives an alternative routing path for all those scenarios where source and the sink are just couple of hops away. In other words, if the source to sink communication is better, in terms of end-to-end latency, via all wireless path then SBR uses that route otherwise the hotline path is used. Gateways keep the track of the latency of all the communication between the source and the sink and derive a threshold (Signature). This signature then helps to improve end-to-end latency for the subsequent communication between the source and the sink. Signature value is recomputed after every source to sink communication. Our simulation results suggest that SBR scheme helps to reduce the overall end-to-end delay. The rest of this paper is organized as follows. Section 2 outlines related work in this area. Section 3 describes our network model and assumptions and explains hotline-based reliable communication topology. Section 4 focuses on the mathematical evaluation and comparison of the proposed topology with existing WSN communications.