## 4 Local Transmission Control: Redundancy-Based Transmission
### Local Route Establishment
#### 4.1.2 Local Route Reply  
The destination node begins the route-reply process by sending a route-reply (RR) packet. A RR packet is passed back to the source node along the intermediate node through which the RR packet passed. Figure 4 shows the route-reply process with a redundant-route setup. After \(H_{11}\) selects the path \(H_{1}\)-\(N_{3}\)-\(N_{6}\)-\(N_{12}\)-\(N_{13}\)-\(H_{11}\) as the best local path, \(H_{11}\) sends a RR packet to \(N_{13}\). \(N_{13}\) increases the hop distance by one, updates the route information for the destination, and then sends the RR packet to \(N_{12}\). \(N_{13}\) also has a redundant upstream node \(N_{14}\); therefore, it sends a redundant-route-reply (RRR) packet to \(N_{14}\). The RRR packet is used to establish redundant paths around the primary local path. The RRR packet from \(N_{13}\) is forwarded to \(N_{6}\) and \(N_{12}\). Then, \(N_{6}\) and \(N_{12}\) generate the  
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1:Case 1: If the receiving node has the destination address of the receiving RR packet, it saves the address of the node that delivered the RR packet to the address of the next hop for the destination in the routing table. The receiving node then increments the hop distance of the receiving packet by 1 and sends the updated packet to the upstream node that was previously recorded during the route setup process. If there is a spare upstream node, an RRR packet is generated and sent to the redundant neighbor node. 2:Case 2: If the receiving node is different from the destination address of the receiving RR packet, the receiving node drops the packet. ```  
**Algorithm 4** Local route reply process. ```
1:Case 1: If the receiving node is on a primary path, then it generates a redundant-routing table (RRT) entry. The next hop field of the redundant-routing table entry stores the address of the node that delivered the RRR packet. 2:Case 2: If the receiving node is on the redundant path, it saves the address of the node that delivered the RRR packet to the address of the next hop of the RRT entry. Then, the receiving node forwards the packet to the upstream node. 3:Case 3: If the receiving node is on a redundant path and the same RRR packet has already been received, the receiving node drops the packet. 4:Case 4: If the receiving node is different from the destination address of the receiving packet, the receiving node drops the packet. ```  
**Algorithm 5** Local redundant link setup process. The redundant link can be established as either a backup link or a relayed cooperative link, depending on the link status and link-synchronization, as in Algorithm 6. For example, for a primary link \(N_{12}\)-\(N_{13}\), if the redundant link \(N_{12}\)-\(N_{14}\)-\(N_{13}\) is set up as a relay link, the data are sent to \(N_{13}\) through \(N_{12}\)-\(N_{13}\) and \(N_{12}\)-\(N_{14}\)-\(N_{13}\) at the same time. On the other hand, if the redundant link is set up as a backup link, the data are sent to \(N13\) through \(N_{12}\)-\(N_{14}\)-\(N_{13}\) when the primary link \(N_{12}\)-\(N_{13}\) is broken. Regarding relay link setup, the \(N_{13}\) can recover better signal quality using receiving diversity and maximal ratio combining (MRC) [14]. The proper relay scheme should be clarified: amplify-and-forwarding (AF), decode-and-forwarding (DF), or compress-and-forwarding (CF) [15], according to the redundant link status. ```
1:Case 1: If the primary link is weak, the redundant link acts as a relayed cooperative link. 2:Case 2: If the primary link is normal and the redundant link is link-synchronized, the redundant link acts as a backup link. 3:Case 3: If the primary link is normal and the redundant link is not link-synchronized, the redundant link acts as a relayed cooperative link. 4:Case 4: If the primary link is strong and the redundant link is link-synchronized, the redundant link acts as a backup link. 5:Case 5: If the primary link is strong and the redundant link is not link-synchronized, the redundant link is ignored.