## 3 Network Architecture and System Model  
In this section, we briefly describe the network architecture model adopted by our proposed routing protocol, the communication pattern among the nodes, and the assumptions made in implementing both the routing and security algorithms. Our proposed network architecture's model is following the _single-tier clustered architecture_[1], as shown in Figure 1, deployed with heterogeneous sensor nodes where camera, audio and scalar sensors within each cluster relay data to a cluster head. The cluster head has more resources, more powerful, and it is able to perform intensive data processing. The cluster head is wirelessly connected with the sink or the gateway either directly (in the case of first-level cluster head) or through other cluster heads in multi-hop fashion. The communication among the sensor nodes both within a cluster and the communication between cluster-heads are managed by our proposed cluster-based multi-path routing protocol to efficiently handle multimedia content over the network and maintain the energy consumption of sensor nodes. Nodes within a particular cluster communicate directly with their cluster head in a certain schedule and each cluster head is responsible for performing data aggregation and fusion in order to decrease the amount of transferred data and the number of transmitted messages to the sink. A cluster head is also responsible for selecting a suitable path for each type of data, e.g., paths with good link quality or minimum delay are appropriate for multimedia content, disjoint paths are suitable for multimedia streaming, and paths with less strict QoS conditions can be used for scalar data.  
Our proposed routing protocol is based on hop count and received signal strength of the sensory message as an indication on the link quality and distance between the nodes. The level of the received power of a message in a large-scale wireless sensor network can be calculated using the propagation model at physical layer:  
\[P_{r}=P_{t}G_{t}G_{r}\left(h_{r}h_{t}\right)^{2}d^{-4}L^{-1} \tag{1}\]  
Where \(P_{t}\) and \(P_{r}\) are the power level of transmitted and received message respectively, \(G_{t}\) and \(G_{r}\) are the transmitter and receiver antenna gain respectively, \(h_{t}\) and \(h_{r}\) are antenna height for the transmitter and receiver respectively, \(d\) is the distance between the transmitter and the receiver, and \(L\) is system loss factor. Then, received signal strength (RSS) of a message can be equal to:  
\[RSS=P_{r}/P_{t}\to RSS=G_{t}G_{r}\left(h_{r}h_{t}\right)^{2}d^{-4}L^{-1} \tag{2}\]  
If we assume that antenna gain of the transmitter and receiver are equal to 1, antenna height of the transmitter and receiver are equal to 1, and system loss factor also equals 1, then RSS can be approximated as a function of distance between the transmitter and receiver as a dominating factor affecting its value: RSS = 1/\(d^{4}\). For more accurate propagation model, signal-to-noise ratio (SNR) and bit error rate (BER) should be taken into account [26] along with the received signal strength in  
Figure 1: Single-tier Clustered Architecture.  
order to consider noises (from receiver and environment) and interferences from other packets arrived simultaneously:  
\[SNR=10\log\left(\frac{P_{r}}{N_{p}+\sum_{i=1}^{n-1}P_{r}}\right) \tag{3}\]  
\[BER=0.5\times erfc\left(\sqrt{\frac{P_{r}\times BW}{N_{p}\times R}}\right) \tag{4}\]  
Where \(BW\) is the channel bandwidth, \(N_{p}\) is noise power, and \(R\) is data rate.  
With respect to our security scheme, we assume that all nodes have unique IDs and the base station is trusted node during all network operation time and thus it can keep the security keys in its memory. Also we assume that the time required for establishing secure links (at the route discovery phase) is smaller than the time needed by an adversary to compromise a node during node deployment.