## 4 Network Communication
### Channel Characterization
#### 4.1.5 Routing Layer  
Although MAC protocols can solve many problems in BSNs, they do not cover addressing and end-to-end package delivery problems which rely on routing protocols. In BSNs, the development of routing protocols needs to consider the following issues: firstly, the available power of each node is very limited, and power consumption is not balanced among nodes. Along with the continuous power consumption when running, part of the nodes consume more power while others consume less, which will result in an unbalanced remaining power distribution and reduce the life of the entire network. Some researchers focus on resolving this issue of unbalanced energy distribution. Reference [32] presents a heuristic self-adaptive routing algorithm for rerouting connections among nodes in an energy constrained on-body network. It can select the reachable parent node automatically to balance energy distribution. This protocol can help disconnected nodes reconnect again and minimize network transmission delays. Secondly, the topological partition problem is another issue that BSN routing protocols must take into consideration. The body movements will cause frequent partitioning or disconnection in BSN topologies. In addition, this situation gets aggravated by the ultra-short wireless range due to its wearable design [123]. To solve the problem of topological partition, [PERSON] _et al._, have put forward a probabilistic packet routing mechanism in Reference [89]. LLF, a stochastic metric called link likelihood fact, is calculated in each node using the history of the link quality between nodes in the network. This metric which could reflect the postural trend of the human body, can be used for decision-making in forwarding the packets to neighbors. In fact, the routing goal is to reduce end-to-end delays by choosing the highest likelihood links. Another method to solve the topological partition problem is to employ routing protocols based on replication which make several copies of a packet to send to several proper nodes and thus increase the chances of arrival. These protocols will definitely cause more serious network congestion and delay, due to excessive packet transmissions. For ultra-resource-constrained BSN, such overheads are not acceptable. [PERSON] _et al._, have proposed a store-and-forward data routing method by adopting a gossiping strategy to improve the performance of replication-based protocol [124]. Different from the random method, a predefined part of the data is selected to be sent, which offer a better solution to the frequent topological partition problem in large scale BSNs. Finally, WSNs almost never consider temperature factors. However, the effect of sensor temperature in BSNs cannot be ignored, since the implantable sensors usually transmit biomedical data to neighboring nodes from time to time which may cause damage to the human body due to high sensor temperatures. At present, a lot of researchers study algorithms by means of thermal-aware routing to deal with temperature factors. Reference [125] proposes a Thermal-Aware RoutingAlgorithm (TARA) protocol defining hotspots as areas where nodes have a relatively high temperature. In order not to cause a temperature rise of these areas any longer, TARA attempts to establish another route to detour around the hotspots using a withdrawal strategy. Unlike TARA, the Least Temperature Routing (LTR) protocol always chooses neighboring node which has the lowest temperature as its next stop [126]. This protocol algorithm is very simple, since LTR does not initially schedule the route of packets and just choose nodes with the lowest temperature instead and the packets will basically detour to the destinations. In fact, LTR is a greedy approach, which may be locally optimal, and probably cannot become globally optimal. One variant of LTR protocol is the Adaptive Least Temperature Routing protocol (ALTR) [126]. Different from LTR, ALTR keeps the number of hops of each packet.