# Source Anonymity against Global Adversary in WSNs Using Dummy Packet Injections: A Survey
## 3 Dummy Packets Techniques
### Optimal-Cluster-Based Source Anonymity Protocol in Delay-Sensitive Wireless Sensor Networks (OSAP)  
Techniques such as _ConstRate_ and _FitProbRate_ provide source anonymity but they are costly since they inject the network with a large number of fake messages. Further, all nodes in the network transmit traffic towards the sink that causes the network to be imbalanced as nodes closer to the sink consume more power than nodes far from the sink, which leads to a shorter lifetime of the network. To overcome the high overhead in _ConstRate_ and _FitProbRate_, and latency in _PFS_ and _TFS_, _OSAP_[40] was developed. _OSAP_ is based on _FitProbRate_ and [PERSON], [PERSON] et al. [40] argue that the use of unequally clustering mechanism will reduce the traffic overhead, improve the network balance, and decrease the latency of the real event. The unequally clustering mechanism is achieved by adjusting the transmission rate and the radius of unequal clusters. As a result, this mechanism fixes the overhead issue by transforming the issue into a mathematical programming problem that is solved by mathematical methods. If a node has a real event to report, it becomes a source that generates some real packets, unlike _FitProbRate_, which allows all nodes to transmit packets whether they are real or fake. In addition, authors of _OSAP_ assumed that the sink is placed in the center of the network and works as the data collector for all events. Each packet has a source _ID_, event description, event time, and packet type (real or fake). Each node determines the number of hops to the sink by using the following formula: \((2n-1)\pi r^{2}\theta\) where \(n\) is the number of hops, \(r\) is the transmission range and \(\theta\) is the network density of nodes distribution. After a node knows its hops count to the sink, the sensor network is divided into uneven clusters. Clusters, which are close to the center of the network are larger than the ones  
Figure 5: An example of PFS process. that have a further distance from the center, as shown in Figure 6. The sink node has the largest cluster and nodes at edges of the network have the smallest clusters. Nodes are categorized into cluster heads and cluster members. The purpose of the cluster head is to filter the incoming fake packets and forward the real packets of the cluster members to the sink. The cluster head of the cluster with radius \(R_{1}\) is the sink and all nodes within this cluster transmit their packets directly to the sink. All nodes with distance \(R_{2}\), \(R_{3}\),..., \(R_{n}\) are cluster head candidates that have cluster radius of \(R_{2}-R_{1}\), \(R3-2R_{2}+R_{1}\),..., \(R_{n}-R_{n-1}-radius_{n-1}\), respectively. After cluster head nodes are selected, they broadcast BEACON packets with \(TTL\), which includes the radius of the clusters they belong to. Member nodes select their cluster head based on the least communication cost, which is decided by the received BEACON packets. Then, each member node notifies its selected cluster head by a BEACON response. Therefore, the network will consist of rings and each ring consists of clusters. The distance between cluster head nodes and the sink is \(R_{2}\), \(R_{3}\),..., \(R_{n}\). This technique assumes that each member node shares a pairwise key with its cluster head using a keying scheme. Each cluster head shares a key with its neighboring cluster head nodes. Once the member node detects an event, it sends the event to its cluster head using a multi-hop routing protocol. The member node delays the real packet to the next interval. Therefore, the adversary cannot distinguish the real packets from the fake ones using time analysis. Then, the cluster head decrypts the message and forwards it to the next cluster head encrypted by the shared key between them. To satisfy source anonymity, the transmitting time intervals follow the exponential distribution as exhibited by the _FitProbRate_ technique. When a fake packet is received by the head cluster, it is discarded.