Heterogeneity of nodes in Wireless Sensor Networks (WSN) is one of the challenges that can negatively affect the overall network capability due to potential mismatches in the data format and structures exchanged between nodes. All nodes in a network must be able to communicate similar data structures and also confirm to the same data exchanging model. However, WSN nodes have very limited resources (e.g. storage, power) that require delegating concerns other than data collection and transmission to a different type of nodes that has more resources and capabilities. In this paper, we propose Service based Mid-tier Component (SMC) as middleware framework for wireless sensor networks that is based on SOA paradigm where every component of it represents a service. The paper describes the architectural design of the proposed middleware and discusses its key characteristics as compared to those found in the literature.