Ad Hoc Networks Routing Instructor: Carlos Pomalaza-Ráez Fall 2003 University of Oulu, FinlandAddendum
Ad Hoc Networks Routing Instructor: Carlos Pomalaza-Ráez Fall 2003 University of Oulu, Finland
Addendum
Hierarchical Routing Protocols
Traditional option when the network has a large number of nodes
Common table-driven protocols and on-demand protocols are for flat topologies and thus have a scalability problem when the network is large
For table-driven protocols there is high volume of overhead transmissions
For on-demand protocols there is large discovery latency
The experience gained in wired networks suggests the use of a hierarchical structure to address the scalability problem
The use of hierarchical routing protocol in ad-hoc networks reduces overhead traffic and discovery latency but it has drawbacks such as:
Suboptimal routes
Complex management of the network hierarchical structure due to its dynamic nature
Basic Hierarchical Approach
The basic idea is to divide the network into cluster or domains
Basic Hierarchical Approach
The mobile nodes are grouped into regions
Regions are grouped into super-regions, and so on.
A specific mobile host is chosen as the clusterhead for each region.
Hierarchical routing
Mobile nodes know how to route packets to their destination within its own region, but do not know the route outside of its own region
Clusterheads know how to reach other regions
Fisheye State Routing (FSR)
The “eye” of the fish captures with high detail the points near the focal point
As the distance from the focal point increases less details are captured
For routing this approach translates into an accurate information in the immediate neighborhood of a node and less detail as the distance increases
FSR is similar to link state (LS) routing in that each node maintains a view of the network topology with a cost for each link
In LS routing link state packets are flooded into the network whenever a node detects a topology change
In FSR nodes maintain a link state table based on the up-to-date information received from neighboring nodes and periodically exchange it with their local neighbors
For large networks in order to reduce the size of the routing update messages the FSR technique uses different exchange periods for different entries in the routing table
Relative to each node the network is divided in different scopes
0 5 1 2 4 3 0:{1}
1:{0,2,3}
2:{5,1,4}
3:{1,4}
4:{5,2,3}
5:{2,4} 1
0
1
1
2
2 LST HOP 0:{1}
1:{0,2,3}
2:{5,1,4}
3:{1,4}
4:{5,2,3}
5:{2,4} 2
1
2
0
1
2 LST HOP 0:{1}
1:{0,2,3}
2:{5,1,4}
3:{1,4}
4:{5,2,3}
5:{2,4} 2
2
1
1
0
1 LST HOP Entries in black are exchanged more frequently
FSR - Summary
Routing table entries for a given destination are updated, i.e. exchanged with the neighbors, with progressively lower frequency as distance to destination increases
The further away the destination, the less accurate the route
As a packet approaches destination, the route becomes progressively more accurate
Benefits
Scales well to large network sizes
Control traffic overhead is manageable
Problems
Route table size still grows linearly with network size
As mobility increases routes to remote destinations become less accurate
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