## 2 Related Work  
The growing interest in WSNs and the continual emergence of new techniques inspired some efforts to design communication protocols for this area. Communication protocols take the task of data transmission in the large scale network and are important to achieve possible better performance. Normally, current routing can be typically classified into four main categories, namely data-centric protocols, hierarchical protocols, location-based protocols and flow-based and QoS-aware protocols [9]. Of course, there are also some hybrid protocols that fit under more than one category. The typical data-centric routing protocols proposed for WSNs include SPIN [10] and Directed Diffusion [11], which are obviously different from traditional address-based routing; location-based protocols such as MECN [12], GAF [13] and GEAR [14] require location information for sensor nodes, which are energy-aware. Hierarchical protocols are scalable for a larger number of sensors covering a wider region of interest, which overcome the defects of single-gateway architecture. LEACH is one of the first hierarchical routing approaches for WSNs [15]. Although the above three categories are promising in term of energy efficiency, more attentions should be paid to address the issues of network flow and QoS posed by real-time applications[16]. One of the protocols for WSNs that includes some notions for QoS in its routing decisions is the SequentialAssignment Routing (SAR) [17]. The SAR protocol creates trees routed from one-hop neighbor of the sink by taking into account the QoS metric, the energy resource on each path and the priority level of each data packet. By using created trees, multiple paths from sink to sensors are formed. One of these paths is selected according to the energy resources and achievable QoS on each path. [PERSON] _et al_ extend SAR by selecting a path from a list of candidate paths that meet the end-to-end delay requirement and maximizing the throughput for best effort traffic [18]. Their protocol does not require sensors involved in route setup so that the overhead problems in SAR approach can be avoided. Minimum cost forwarding protocol is a kind of flow-based routing protocol [19]. It aims at finding the minimum cost path in a large scale sensor networks, which will be simple and scalable. The data flows over the minimum cost path and resources on the nodes are updated after each flow. [PERSON] _et al_ also propose a cost field based protocol to Minimize Cost forwarding Routing (MCR) [2]. In the design, they present a novel backoff-based cost field setup algorithm that finds the optimal cost of all nodes to the sink with one single message overhead at each node. Once the field is established, the message, carrying dynamic cost information, flows along the minimum cost path in the cost field. Each intermediate node forwards the message only if it finds itself to be on the optimal path, based on dynamic cost states. [PERSON] _et al_ improve the cost field based protocol and proposed minimum hop field based routing (MHFR) protocol [1]. This protocol can be used in large scale sensor networks and ensure that the data is forwarded along shortest path and message number is least. Simulation experiments validate the effectiveness of the design. The proposed protocol in this paper is inspired by the approach in [2] and [1]. In the proposed protocol, a connectivity graph based on hop count field is built and the concept of dynamic window (\(m,k\)) is introduced to well estimate link quality before making routing decisions. The idea of link estimation has been studied by [PERSON] _et al_. in [20]. They designed and evaluated different link estimators and find WMEWMA (Window Mean with Exponentially Weighed Moving Average) is the best one for WSNs. However, it might be too complicated for resource-constrained sensor nodes to be used for routing decision.