Hierarchical Ad-hoc Network Routing Protocol

Hierarchical State Routing Protocol(HSR) is a distributed multi-level hierarchical routing protocol that employs clustering at different levels. Clustering has various advantages in itself. A leader of the cluster is selected at each level of clustering. The benefits of this protocol are:- 

• Reduction in routing table size 
• Better scalability
• Clustering enhances resource allocation and management

There are different types of clustering in HSR as follows:

Physical Clustering

It is between nodes that have physical wireless one-hop links between them. Levels of clusters in physical clustering can be divided into two categories:

• First Level Cluster- It is done among the nodes that are reachable in a single wireless hop. 
• Next Higher Level- It is done among the nodes that have been selected as leaders.

Logical Clustering 

It is based on certain relations among the nodes rather than geographical positions. 

In the above picture, nodes 1,2,4 are the leader of the cluster. Leaders 1 and 2 themselves form a cluster at a higher level as shown in the figure. Node 4 forms a cluster at level 2 by itself. At level 3, nodes 2 and 4 combine to form a cluster. This is the way HSR hierarchically forms a cluster at higher levels. Information is exchanged between the gateway nodes and leader nodes through a path known as a virtual link.

Kinds of Nodes in Cluster

• Cluster head- It acts as a local coordinator of transmissions within the cluster. For example, in the last example as shown in the above picture, nodes 1,4,2 are cluster heads. Properties of cluster heads are allocating slot or frequency or code, call admission control, and most importantly scheduling packet transmission. 
• Gateway node- These nodes belong to two or more clusters. In the picture, the nodes which are shared by two or more clusters are gateway nodes. For example, nodes 6,7.
• Internal node- They belong to a single cluster. It is exactly the opposite of the gateway node. Nodes 5,12,8,11 are internal nodes.  

Disadvantage

• The process of exchanging information concerning all the levels of the hierarchy as well as the process of leader election in every cluster is time-consuming
• To maintain topology, the hello message needs to be transmitted periodically by the nodes to its neighboring nodes. 
• The centralization of routes through the cluster leaders leads to extra taxation of the cluster leader's power supply.