## 6 Signature Based Routing  
In hotline assisted routing all the packets from sender to the receiver are routed through the gateways [5]. This path may sometimes increase the end-to-end latency. The hotlines therefore provide the reliability on the cost of increased latency in certain cases. We have observed the following case where there might be chances of increased end-to-end latency:  
1. In cases where the destination node is just few hops away from the sender node and the communication takes place through hotline. The packet is sent from the sender (probably through multi hops) to its default gateway and then via the destination's default gateway  
Figure 11: A comparison of the system reliability and the associated cost. (a) Reliability of the system as a function of the number subsystems. (b) Cost of the system as a function of the number subsystems. the packet reaches the destination (probably again via multi hops). Therefore the hotline path might bring more end-to-end latency in few cases. In the following section we present the remedy for above mentioned latency issue. In order to avoid longer source to sink paths and increased latency we present Signature Based Routing (SBR) scheme. It is a known fact that general sensor nodes are mainly resource constrained devices. Putting those devices into performing additional computation (say calculating additional or optimal routing paths) would not be a good idea. On the other hand we require shorter and intelligent but reliable routing from source to the sink. As pointed out earlier, there are few cases where source to the sink direct communication is less costly as compared to the hotline path; we therefore present intelligent routing where gateways play a central role. We call this routing signature based, as the routing decision is made by the gateway depending upon comparison results with specific signatures. _Definition 7: Personal Area Network (PAN)_ is defined in accordance to IEEE 802.15.4 standard. PAN comprises of sets of low power nodes and a PAN coordinator. In our case resource rich gateways act as the PAN coordinators. In our hotline based approach we have suggested all the communication from source to the sink goes via gateways. It means all the traffic to and from a PAN goes through gateways. Moreover gateways are usually resource rich devices as compared to general sensor nodes [31]. As explained earlier, gateways share the complete network topology information in the form of gateway network tables. Therefore gateways can easily monitor the traffic, find out the source-to-sink latency of each packet and can take routing decisions based upon certain criterion [31]. In order to explain SBR we take the following example scenario. As shown in Figure 12 the source and the sink happen to be close enough that if they communicate directly, instead of communicating via hotline, it will not only decrease the latency but also will increase the reliability as it reduces number of wireless hops it would have required to reach from sender to the gateway and gateway to the sink. However, there are several other cases where source to sink communication is much more reliable with less latency, if the communication goes through the hotlines. Therefore, we vest intelligence in gateways to monitor traffic and take appropriate routing decisions (_i.e.,_ to decide if the routing is better via hotlines or directly via all wireless path). **Figure 12**. Scenario where the direct communication is better as compared to hotline routing. In above scenario the source needs to send the packet to the destination (_i.e.,_ sink), initially it follows the default route, source-gateway-gateway-destination. However, both the involved gateways keep the track of the latency of the each communication between the source and the sink. Gateways compare the latency with the signature value (we will explain signature value later in the section), if the latency is less than the signature value then the communication continue un-interrupted.