## 4 The WBANs Communication Stack
### Routing Protocols
#### 4.3.3 Cluster-based Routing Protocols  
Cluster-based routing is an important class of routing protocols used in WBANs. Different methods are exploited to select a cluster head where data is transferred to the coordinator nodes through these heads to reduce the direct communication between the sensing nodes and the coordinator. The cluster head nodes are more powerful than the rest of the nodes in terms of available resources. The following sections give the overview of some of the important cluster-based routing protocols for WBANs. Self-organization for wireless multi-hop systems can be based on reactive on-demand solutions, which are adapted to low-energy low-traffic WBANs. The authors in (AnyBody [94]) have shown that, despite the relative high cost to build and maintain a topology, a cluster-based approach is particularly suited for Body Area Networks. There are five steps of AnyBody protocol that includes: neighbor discovery, density calculation, constructing cluster head, setting up the backbone, and setting up the routing path. AnyBody uses the similar principle of Low Energy Adaptive Clustering Hierarchy (LEACH) [95], which selects the cluster head at regular time intervals to balance the energy consumption, and cluster head aggregates the data and sends to the remote station. In LEACH it is assumed that all nodes are within the range of remote base station, whereas, AnyBody addresses this problem by using a density-based cluster head selection method and using these cluster heads to build a backbone network. The main advantage of AnyBody [94] is that the number of clusters remains almost constant even when the number of nodes in the network are increased, whereas, in LEACH as the number of nodes increases, the number of clusters has to increase as well. This results in a bigger cluster size of AnyBody as compared to LEACH. Furthermore, AnyBody also reduces the cost of setting up the cluster. Tree based algorithms have been used in solving many computational complex problems. Using this structure, the authors in [96] proposed a secure cluster-based multipath routing protocol for multimedia content. The multimedia (which is the essential for the specific critical and rescue applications) requires a better QoS and in order to achieve that, the nodes in the network have to process the data frequently and which will causes faster depletion of energy at nodes. To achieve the data reliability and to provide a faster data computation, the protocol uses a hierarchical structure of multiple paths based on clusters. The multiple paths are discovered by message broadcasting and each cluster head node is connected to the sink either directly or via other cluster head node. Hybrid Indirect Transmission (HIT) [97] is another cluster-based data gathering hybrid architecture protocol. HIT utilizes parallel processing to minimize energy consumption and network delay for both inter-clustered and intra clustered communication. Results show that it performs better (both in terms of energy and delay) when it is compared with Low Energy Adaptive Clustering Hierarchy(LEACH) [95] and Power Efficient Gathering for Sensor Information System (PEGASIS) [98] under direct transmission for small number of nodes, however it consumes more energy in dense networks. PEGASIS significantly reduces the overhead of dynamic cluster formation by providing an opportunity to all the nodes (turns-by-turn) to transmit the fused data to the sink node and as a result to improve the network lifetime. It considered variable (small-to-medium) size network and optimize the sole metric, _i.e._, global energy consumption. However, it does not consider other performance metrics to analyze the complete performance. HIT results in low packet delivery delay and energy consumption and considerable packet delivery ratio. It also considered security as additional metric and provides high throughput. On the other hand, AnyBody improves significantly the packet delivery ratio for large network.