## 3 Alternative Paths for Message Streams in Cluster-Tree Wireless Sensor Networks
### ARounD Protocol Mechanisms
#### 3.4.1 Synchronisation and Configuration of the Involved Nodes  
The setup of a new ARounD communication path is triggered by the source node using an _ARounD request command_ frame (ARounD req). The _ARounD req_ frame is transmitted during the CAP period towards the PAN coordinator. Figure 11 illustrates the format of this frame. The MAC header fields contain the frame control, sequence number and addressing field. The _Addressing field_ contains the _destination Cluster Identifier_ and _destination address_ fields, which corresponds to PAN coordinator. In turn, the _source Cluster Identifier_ and _source address_ fields correspond to the source node that is requesting the ARounD communication path. The 16-bit short address is used as addressing type. In order to identify the _ARounD req_ frame, the value of the _command frame identifier_ field is defined as _0x0a_, which is a value not used by IEEE 802.15.4. The _ARounD req payload_ identifies the cluster and address of the destination node. Within this context, after receiving an _ARounD req_ frame and performing the ARounD algorithms, the PAN coordinator uses an _ARounD configuration command_ frame (ARounD conf) to establish/maintain/close an ARounD communication path. Figure 12 illustrates the format of this frame. The _Addressing field_ contains the _destination Cluster Identifier_ and _destination address_ fields, which correspond to the RP node to be configured. In turn, the _source Cluster Identifier_ and _source address_ fields correspond to the PAN coordinator. In order to identify the _ARounD conf_ frame, the value of the _command frame identifier_ field is defined as _0x0b_, which is a value not used by IEEE 802.15.4. Figure 11: Format of the _ARounD-req_ frame. The _ARounD configuration payload_ contains the configuration fields of the RP node specified in the addressing field. The _Alternative path identifier_ field defines the alternative path to be configured, which allows to set up to 32 message streams at the same time. The _ACK_ field is set to 1 if the ARounD communication supports acknowledgements; otherwise, this field is set to 0. The _Tx_ and _Rx_ fields are set to 1 if the involved node is a receiver or/and transmitter, respectively; otherwise, these fields should be set to 0. Note that, if the _Rx_ field is set to 1, the _Rx StartTime_ field should be configured to the time (_MS_Offset_) when the node will wake up in receiver mode. In turn, if the _Tx_ field is set to 1, the _Tx StartTime_ field should be configured to the time (_MS_Offset_) when the node will wake up to send data (transmitter mode). In addition, the _Keep alive_ field specifies the period (seconds) during which the alternative path should be kept active. The PAN coordinator will send the _ARounD conf_ frames along the cluster coordinators until reaching all the involved nodes, using indirect communication mechanisms. After receiving the _ARounD conf_ frame, the involved node sends an acknowledgement frame to inform the PAN coordinator, using the _ARounD ack command_ frame. Figure 13 illustrates the format of this frame. In order to identify the _ARounD ack_ frame, it is used the value _0x0c_ in the _command frame identifier_ field, which is a value not used by the IEEE 802.15.4 standard. The _Addressing field_, _destination Cluster Identifier_ and _destination address_ fields correspond to the PAN coordinator. In turn, the _source Cluster Identifier_ and _source address_ fields correspond to the configured node. The 16-bit short address is used for addressing purposes. The _ARounD ACK payload_ field is composed of the _alternative path identify_ field (5-bits) to identify the alternative path and the _ACK type_ field to specify the _acknowledgement frame type_. Table 1 summarises the _ACK frame_ types used in the ARounD communication scheme. Figure 12: Format of the _ARounD-conf_ frame. Figure 13: Format of the _ARounD acknowledgement command_ frame.