Hierarchical Routing

Hierarchical routing protocols consist of a hierarchical topology to organize the network and routing information. Multiple layers and levels are introduced in a network. Each layer may be assigned a different responsibility like forwarding packets, maintaining routing tables, etc. HRPs are valuable for large networks, as they provide the capability of organizing network information and reducing the amount of routing information that should be exchanged between nodes. Hence, HRPs demonstrate significant scalability and fault tolerance. This is attributed to their hierarchical structure, which provides redundancy and facilitates the efficient distribution of routing data throughout the network.

Differences Between Hierarchical and Flat Routing Protocol

Basis	Hierarchical Routing Protocol	Flat Routing Protocol
Topology	Hierarchical topology used	Single-level topology
Network	Suitable for large networks	Suitable for small networks
Routing Tables	Uses multiple routing tables to organize network information	The single routing table is used.
Scalability	Highly scalable, able to handle expansive networks with multiple layers	Limited scalability, can get congested and wasteful as the network grows
Complexity	More complex to set up and maintain	Simpler in comparison
Optimality	Simple but non-optimal	This can be made optimal by making it more complex
Scheduling	It is a channel reservation-based scheduling	It is a contention-based scheduling
Collisions	Collisions are avoided	Collisions may occur frequently
Allocation	Ensures fair channel allocation	The allocation might not be fair most of the time
Energy	Energy dissipation is constant	Energy Dissipation depends on patterns in traffic

If one has to choose between the two, the choice might depend on the nature of the network that requires routing and its needs and characteristics. For further information, visit this article.

Advantages of HRP

• Scalability: Hierarchical routing protocols exhibit excellent scalability by partitioning the network into smaller segments or areas. This division reduces the demand for routing tables and updates on each router, enhancing network efficiency and decreasing overall network traffic.
• Better Traffic Control: Hierarchical routing protocols demonstrate superior traffic management compared to flat routing protocols. The hierarchical framework enables more efficient traffic control, mitigating the need for unnecessary routing updates and preventing loops in the network.
• Easy to Manage: The organisational framework in these protocols facilitates simplified management and maintenance. Segmentation of the network into manageable sections enhances the ease of troubleshooting and diagnosing issues.

Disadvantages of HRP

• Complexity: Hierarchical routing protocols tend to be more intricate compared to flat routing protocols. The presence of additional layers and segments necessitates more extensive configuration, posing potential challenges in implementation.
• Latency: Latency may be introduced into the network due to the presence of additional layers and segments. Such delays in data transmission can pose challenges, particularly for real-time applications.

Protocols of Hierarchical Routing

There are two well known hierarchical routing protocols:

• Hierarchical State Routing Protocol
• Fisheye State Routing Protocol

Hierarchical State Routing Protocol

The hierarchical state routing protocol (HSR) is a multi-level and distributed routing protocol. It makes use of clustering, present on different levels. Each level of cluster has the potential to manage its members efficiently. This improves resource allocation and management. Leaders are elected in each cluster, which form the members of the immediate higher level.

Various clustering algorithms are employed for electing leaders in each level. There can be two types of clustering: physical and logical. One level of physical clustering is done among nodes that are available in a single wireless hop. The other level is made among nodes that act as cluster heads of each of the first-level clusters. Logical clustering scheme of HSR is based on relationships among nodes rather than their geographical locations.

Nodes have complete details about how to route packets to destinations within its own cluster. But it does not have any information on the internal structure of other regions.

Multilayer Clustering in HSR Protocol

At the lowest level, there are three clusters. Nodes 1, 3 and 6 are classified as cluster leaders, or gateway nodes. A cluster leader is entrusted with responsibilities such as slot/frequency/code allocation, call admission control, scheduling of packet transmissions, exchange of routing information, and handling route breaks. The higher level nodes are further organized into clusters.

Working of HSRP

• Each node maintains information about its neighboring node and their link status
• The information regarding the cluster is broadcast in the network at regular intervals.
• The job of the cluster leader is to exchange topology and link state routing information among other cluster leaders of neighborhood clusters.
• The exchange of link state information is carried out over multiple hops that consist of gateway nodes and cluster-heads.
• The path between two cluster-heads which is formed by multiple wireless links is called virtual link.
• The link status for the virtual link(otherwise called tunnel) is obtained from the link status parameters of the wireless links that constitute the virtual link.
• After obtaining information from its peers, the cluster head floods the information to the lower levels.
• Hierarchical addressing in HSR reduces routing information compared to link-state routing. HSR’s HID and node ID structure simplifies addressing and topology management.
• HSR tables update with received routing packets, maintaining accurate hierarchy information.

Kinds of Nodes in a Cluster

• Cluster Head – This node acts as a local coordinator of transmissions within the cluster. Cluster head performs the function of allocating slot or frequency or code, call admission control, and most importantly scheduling packet transmission.
• Gateway Node – These nodes are associated with more than two clusters.
• Internal Node – These are the members of the clusters at the most root level

Virtual Link – Information is exchanged between the gateway nodes and leader nodes through this path.

Advantages

• Reduction in Table Size: HSR protocol reduces the routing table size by making use of hierarchy information. The storage space required is of O(N x M) whereas in flat topology the space required is O(NM). Here, N is the average number of nodes in a cluster and m is the number of levels.

Disadvantages

• Overhead Involved: The overhead for exchanging packets containing information about multiple levels of hierarchy and cluster head appointment process makes the protocol expensive from the ad hoc wireless network point of view.
• Scalability: The number of nodes involved in an ad hoc network does not grow to the dimensions of the number of nodes in the Internet which is more suited for hierarchical routing.
• Time Consuming: Sharing information across all levels of the hierarchy and conducting leader elections within each cluster is a time-consuming process.
• Regular Hello Messages: To preserve the network’s topology, nodes must regularly send hello messages to their neighboring nodes, which contributes to overhead.
• Power Supply: The concentration of routes through cluster leaders imposes additional strain on the power supply of these leaders.

Fisheye State Routing Protocol

Fisheye State Routing (FSR) is an implicit hierarchical routing protocol most meant for mobile ad hoc networks. This protocol makes use of the fisheye technique to reduce information required to represent graphical data, in order to reduce routing overhead. It is based on the property of a fish’s eye that can capture pixel information with greater accuracy near its eye’s focal point. As the distance from the centre of the focal point decreases, accuracy decreases. Similarly, in FSR, accurate information about nodes in its local topology is maintained, and not-so-accurate information about far-away nodes is recorded. Hence, we can say accuracy of network information decreases with increasing distance.

Fisheye State Routing

Topology of the network is maintained at every node, but this information is not flooded in the entire network, which is mostly what happens in link state routing protocols. Instead, a node shares topology information only with its neighbours.

Working of FSRP

• A sequence number is used to identify recent topology changes.
• A hybrid approach consisting of a combination of link-level information exchange of distance vector protocols and the complete topology information exchange of link state protocols is used.
• Using the complete topology information stored at each node, the desired shortest paths are computed as required.
• The topology information exchange takes place periodically rather than when an event occurs, as instability of wireless links may cause excessive control overhead when event-driven updates are used.

Routing Scope – The set of nodes that are reachable in a specific number of hops. The scope of node at two hops is the set of nodes that can be reached in two hops. Link state information for nodes belonging to the smallest scope is exchanged at the highest frequency. This frequency decreases with an increase in scope.

This keeps the immediate neighbourhood topology information maintained at a node more precise compared to the information about nodes farther away from it. Thus the message size for a typical topology information update packet is significantly reduced due to the removal of topology information regarding the far-away nodes.

This ensures that a node maintains more accurate topology information about its immediate neighbourhood than the information pertaining to nodes situated at a greater distance. Consequently, the message size for a standard topology information update packet experiences a notable reduction due to the exclusion of information regarding distant nodes. This way Fisheye exhibits a trade-off between the accuracy of the routing function and the overhead due to the generation of control messages by routing protocols.

Advantages

• Bandwidth Optimization: The notion of multi-level scopes employed by FSR significantly reduces the bandwidth consumed by link state update packets. This makes FRS suitable for large and mobile wireless networks.
• Reduced Overhead: The routing overhead is significantly reduced by adopting different frequencies of updates for nodes belonging to different scopes.
• Scalability: FSR scales well because of the reduction in routing overhead, where varying frequencies of updates are used.

Disadvantages

• Number of Hops: The performance of the protocol at various mobility levels is notably impacted by the selection of the number of hops assigned to each scope level. Therefore, this parameter must be chosen with careful consideration.
• Loop Creations: The occurrence of temporary loops is possible when nodes or links fail, particularly when Hello messages are transmitted more frequently. In the event of a node failure, its neighboring nodes detect the broken link well in advance of other nodes. Subsequently, they recalculate their routing tables, a process that may clash with the routing tables of other nodes and result in the creation of loops.

FAQs on Hierarchical Routing

1. What is fisheye state routing in mobile ad hoc networks?

Fisheye State Routing (FSR) is a table driven implicit hierarchical routing protocol.

2. How does Fisheye State Routing Protocol (FSR) optimize bandwidth in wireless networks?

FSR employs multi-level scopes to reduce link state update packet bandwidth. It optimizes routing overhead by using varying update frequencies for nodes in different scopes, making it suitable for large and mobile wireless networks.

3. What distinguishes Hierarchical State Routing Protocol (HSR) from other hierarchical protocols?

HSR is a multi-level, distributed protocol employing clustering. It uses physical and logical clustering for leader election. The protocol reduces routing table size, enhances resource management, and maintains accurate hierarchy information through periodic updates.

4. What are the advantages of Hierarchical Routing Protocols (HRPs) over Flat Routing Protocols?

HRPs offer superior scalability for large networks by organizing them into smaller segments. They provide better traffic control, simplify management, and reduce unnecessary routing updates, enhancing overall network efficiency.