## 4 The WBANs Communication Stack
### Routing Protocols
#### 4.3.1 Multipath Routing Protocols  
Multipath routing is an efficient technique to route data in wireless sensor networks (WSNs) because it can provide reliability, security and load balance, which are especially critical in the resource constrained system such as WSNs [76]. Although there are a number of advantages associated with the multipath routing mechanism such as data reliability, data security, and energy efficiency, it also has extra overheads for the evaluation of route setup time, amount of traffic, average path length, and average delay. Multipath routing protocols can be divided into three different types of routing which include infrastructure, non-infrastructure and coding-based in order to discover multiple paths and data. The major concern of the protocols within infrastructure-based routing is to construct and maintain specific multipath infrastructure by considering location and resource capabilities. Protocols, which do not build any specific infrastructure and decide the next hop based on its local knowledge, are classified as non-infrastructure based. Whereas, when the protocols use variant kinds of coding schemes to fragment the data packet at the source node and then send the chunks through discovered multiple paths it is knows as coding-based routing approach. In infrastructure-based multipath routing protocols, the main feature is the construction and maintenance of multiple paths from source to destination. An infrastructure provides reliable and fast data transmission because every intermediate data routing node has its next hop set up in advance. It also provides the protocol for reducing failure recovery time by assigning the alternative route, which is also discovered in advance [76]. Several techniques are exploited for reliable, secure and energy balanced designs this includes energy-aware and hierarchy-based multipath routing algorithms and are explained in much more detail in [76]. However, the focus here is to highlight multipath routing in general with reference to an overall overview of routing protocols. In [77] a distributed, scalable and localized multipath search protocol (called energy-efficient multipath routing EEMR) to discover multiple node-disjoint paths between the sink and source nodes was proposed. Further to that, this paper also addresses a load-balancing algorithm to distribute the traffic over the multiple paths discovered. Results shows that the proposed scheme has a higher node energy efficiency, lower average delay and control overhead than other similar protocols as explained in [78]. More details on energy-aware multipath routing protocols can be found in EEAMR [79] and MRMS [80]. Infrastructure-based multipath routing can be found in [76]. In hierarchical-based multipath routing protocols nodes establishes a hierarchical relationship to discover and maintain multipath routes and because of this hierarchy it has a much reduced overheads in comparison with infrastructure based multipath routing technique presented in [76]. In [81], a unique protocol is proposed which discovers multiple paths from each node in only one route discovery process (called N-to-1 routing protocol). For path discovery, the base station periodically broadcasts the route update message and each node receiving the update message for the first time will set the sender node as parent node. This process continues recursively until the packet reaches back to the base station. This approach will lead to a breadth first spanning tree with the base station as the root of the tree. A comparison of the main aforementioned multipath routing protocols is presented in Table 10. One of the fundamental challenges in multipath routing is how to establish the number of paths, and importantly, how to select them in an efficient way. EEMR provides high-energy efficiency by optimizing the number of paths and load balancing. However, it takes much more time in setting up various routes.