where S is the average size of a packet and B is a link’s bandwidth. It expresses the expected time necessary to successfully transmit a packet at the MAC layer. To capture packet loss as a routing metric, the Expected Transmission Count (ETX) can be used, which is defined as the number of transmissions necessary to successfully deliver a packet over a wireless link (Couto et al. 2003). Very often multiple QoS metrics (e.g., end-to-end latency and packet loss rate) are combined, for example, the bandwidth-delay product refers to the product of a link’s bandwidth and its end-to-end delay. Which metrics are chosen affects the design of the network at different levels, including the network (routing) and MAC layers. Most WSNs must strike a balance between satisfying the application-specific QoS requirements and the goal of energy efficiency in the network as whole.