## 5 Signal Oriented--Event Driven Approach
### Protocols in Signal Oriented-Event Driven Environment  
Popular protocols can be used in these scenarios, in conjunction with a WUR, in order to provide an efficient WBAN network for medical applications. The WUR can be used for a quick transition between the sensor states (stand-by, monitoring, _etc._) in the network, without the need for the main transceiver to be on. The signal approach can be realised in higher OSI layers of the protocol, and this can be protocol independent. The protocols do not need any modifications in order to be efficient with a WUR. The WUR signals have different modulation and they can operate in a different frequency band (although this is not recommended due to antenna and matching issues). Ideally, there can be a frequency band reserved for wake up packets, since the rate of occurrence of these packets is relatively low and WUR sensitivity is not high.  
The signal approach is realised in the application layer. The application layer of the sensor contains the services that the sensor can provide. This should be protocol independent. During network forming,which is done by a standardised method for each communication protocol, this list of services is obtained from all of the sensors within a network, and depending on the required scenario, the sensors are given the task of measuring certain values. In order to avoid a situation where the sensor would be constantly in the disconnected mode, searching for a network, which is power consuming in most of the described protocols, we consider a broadcast wake up packet, which is used for the sensors in off mode and not assigned to any network to wake up in a search for Master Node using the pairing process defined by the protocol. If the sensor is not paired with a MN after some time out period, it would return to the off mode, leaving only the WUR to be active.  
After this initial process, the sensors go into the required state and they can change states as illustrated in Figure 3. The transition between states are:  
* INIT--Transition from the unattached state to being connected. It is done according to the protocol.
* WUP--Transition initiated by the Wake Up Radio, followed by a command for transition.
* COMM--Transition initiated by the Master Node using the regular receiver while acknowledging the packet or in a beacon.
* RESET--Transition initiated by the Master Node using the WUR or regular communication, or initiated by an external reset.
* T/O--Transition to Off after a certain time out period.  
The wake up packet can be a network address assigned during network forming, or a hardware MAC identifier as defined in a protocol, for better robustness and better reliability in the case of the multiple BANs operating in the vicinity. In the case of Bluetooth, a MAC-48 identifier can be used, or in case of ZigBee, EUI-64. The wake up packets can optionally contain some other defined commands and identifiers, if there is a need to address multiple sensors at once, but one has to note that if higher  
Figure 3: Chart of the sensor states for an event driven environment with possible transitions between them.  
sensitivity is required, lower data rates must be used for the WUR (1-5 kb/s), so the wake up packets can be longer in time compared to other WBAN communications.  
As mentioned, this mode of operation can be realised in higher layers of the communication protocols, and can be MAC protocol independent. The WUR is used to instantly transition the sensor node (with very low latency) from one mode of operation to another. This can be performed in a more energy-efficient manner with a WUR than using just the regular protocol, because most of the communication protocols, if required to work with low latency, require significant amount of power to be spent on beacon or connection events.