## 4 Routing Protocols for HCWSNs
The results showed that TinyADMR achieved a PDR greater than 80% when the data rate was less than 20 packets per second for a network transmission that spanned 2 to 4 hops [29]. For the mobility experiments, ADMR showed robustness to node movement for typical mobility rates and did not suffer a significant drop in PDR. In experiments to measure the packet latency for transmissions across multiple hops, the end-to-end message delay was less than 200 ms in all scenarios. Considering that the latency was measured for scenarios with up to 7 hops, this result is very impressive. _Modified CTP_[19]: MEDiSN adopts a modified version of the Collection Tree Protocol (CTP) [30] to forward messages through the routing tree structure. CTP's design is composed of a routing engine and a forwarding engine. The routing engine is used to discover and maintain reliable multi-hop paths between the source and destination nodes. The forwarding engine controls and monitors all transmissions between the RPs and PMs. Specifically, it determines the maximum number of retransmissions required to make the transmission of a packet successful. Results from the literature have indicated that the routing engine of CTP performs well in clinical environments [31]. However, CTP's forwarding engine incurs high packet loss when the network traffic load is high because CTP allows a high number of retransmissions, which can eventually congest a RP's packet queue. As a result, [PERSON] _et al._[19] modify CTP's forwarding engine to implement an algorithm that dynamically adjusts the maximum number of retransmissions as a function of the RP's queue size. When the queue size of a RP is above a certain threshold, the number of permissible retransmissions is reduced by half. If the queue size is below a certain threshold and the maximum number of retransmissions has been issued, the maximum number of retransmissions is increased by one. Another modification made to CTP's forwarding engine is the dynamic adjustment of the inter-packet interval (IPI) that CTP uses. By default, a sender in CTP waits in between 16 ms and 31 ms to send data, which inevitably causes congestion in MEDiSN when high-rate data is transmitted. To counter this problem, the IPI is calculated as an exponentially weighted moving average of all the previous times that a node waited before it successfully transmitted a packet. Unlike RPs, PMs do not use CTP because doing so would force PMs to act as relay nodes and possibly cause expensive routing tree reconfigurations. Instead, patient information packets (PIP) are created at RPs and periodically propagated to the gateway. The PIP contains a list of all PMs that have a connection to a particular RP. When the gateway receives the PIP, it can select the appropriate routing path for transmitting data to the targeted PM. MEDiSN was evaluated in realistic hospital environments and the results showed that MEDiSN succeeded in consistently monitoring the patients even when they were mobile. For example, when MEDiSN was deployed at the University of Maryland Shock Trauma Center, the routing mechanism allowed for a PDR of 98.3% [19]. Furthermore, in the Johns Hopkins Hospital Emergency Room, data was transmitted with a PDR of 95.4% [19]. In this section we have reviewed the characteristics and operation of the RMCP, TinyADMR, and modified CTP routing protocols as they pertain to their respective HCWsNs. A summary of the techniques used by the reviewed routing protocols to address reliability, scalability, latency, and robustness to mobility is provided in Table 3. Of the three routing protocols, RMCP's performance decreases drastically as the data rate of the source and mobility of the nodes in the network increase. On a positive note, RMCP introduces network aggregation techniques that can be used to support a large number of devices and thus improve scalability. Both TinyADMR and modified CTP have already been integrated into HCWSNs and tested in realistic environments. TinyADMR was evaluated in CodeBlue as part of a 30 node sensor network test bed.