In this section, a corridor maintenance strategy is introduced that is used to adapt the corridor structure to node movements. Using unipath routing, a new path has to be found every time one ore more links within the path break and are no longer available. This reconstruction procedure requires a large number of resources to be spent for signaling overhead. In the worst case, a completely new path has to be found. Instead of following a reactive approach, i.e, acting only after some link is already broken, a preventive strategy is proposed to maintain the corridor structure and to reduce wasteful overhead.
Node movements change the link qualities within the corridor. This effect is tracked by the nodes based on the regular exchange of hello messages. A significant SNR drop of a link may require an adaptation of the structure to avoid a link breakage before it takes place. A local maintenance strategy at a fixed time interval is proposed that includes a potential update concerning:
1. the number of clusters in the corridor,
2. the master node of each cluster,
3. the remaining cluster nodes.
Due to the movement of source and destination, it might be possible to reduce the number of clusters in the corridor or it might be necessary to increase the number of
3.4 Structure Maintenance 27
clusters. Furthermore, a new master node for each cluster is assigned according to the changed network conditions. Finally, the additional cluster nodes might be replaced by other nodes that provide better conditions.
For some steps of the maintenance process, Conditions 1 & 2 are used again. In other steps, Condition 2 is replaced by the following condition:
Condition 2a : ¯γ ≥ γmin,cor0 with γmin,cor0 < γmin,cor,
where γmin,cor0 is a minimum link SNR requirement that is slightly relaxed compared to Condition 2 and is used for the update of the master node of a given cluster. The relaxation should prevent an excessive increase in the number of clusters between source and destination. Since the initial composition of the unipath aims at minimizing the number of hops under Conditions 1 & 2, the links between master nodes tend to only marginally exceed Condition 2, especially in dense networks. Therefore, small movements could lead to the need for additional hops with regard to Condition 2. By replacing Condition 2 by Condition 2a, a small margin is introduced for SNR fluctuations between the master nodes. For which cases, Condition 2a is used instead of Condition 2 is explained in detail in the following.
For the structure maintenance process, each master node executes Algorithm 2 to deal with the number of required clusters in the corridor. Of course, it is always desirable to reduce the number of clusters in the corridor if this is possible. Therefore, at first, the master node checks if there is an appropriate link between its adjacent master nodes such that the intermediate cluster can be removed. To this end, the node checks if Conditions 1 & 2 are fulfilled by the link between the adjacent master nodes. If the cluster cannot be removed, next, the master node determines the best suited node to become the new master for its own cluster out of the set of its neighbors and itself, i.e., it determines the node that provides the highest minimum LLT concerning the adjacent master nodes and that fulfills Conditions 1 & 2a concerning adjacent masters. Here, the slightly relaxed Condition 2a is used in order to avoid a hasty addition of clusters. Since the maintenance is done cluster by cluster, the link conditions concerning a certain cluster can improve after the master node of this cluster has executed the maintenance procedure due to an update of the following cluster. If there is no suitable candidate that fulfills Conditions 1 & 2a, the number of clusters need to be increased. Therefore, the master node tries to find two nodes out of its neighborhood that build a path between the adjacent master nodes. All links of this path have to fulfill Conditions 1 & 2. If there are multiple node pairs that fulfill these
28 Chapter 3: Corridor Construction and Maintenance
Algorithm 2 Corridor maintenance procedure (update of master nodes)
Require: Average SNR + estimated LLT concerning 1- and 2-hop neighbors of the master node
if SNR between previous and next master node fulfills Conditions 1 and 2 then Remove current cluster
else
Store all nodes that fulfill Conditions 1 and 2a concerning adjacent master nodes in set S (candidates)
if S 6= {} then
Out of set S, appoint node with highest minimum LLT concerning adjacent master nodes as new master of current cluster
else
Out of set S, determine all node pairs that can build a path between previous and next master that fulfills Conditions 1 and 2 concerning all affected links; Appoint the pair that provides the highest minimum LLT concerning these links as two new master nodes
end if end if
conditions, the node pair with the highest minimum LLT concerning these links is selected as two new master nodes which means that one cluster is replaced by two new clusters.
After Algorithm 2 has been executed by a master node, Algorithm 1 is used to update the cluster nodes. Due to the node movements, cluster nodes might need to be replaced by other nodes that provide better link conditions.