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Ad Hoc Networks Routing. Addendum . Instructor: Carlos Pomalaza-Ráez Fall 2003 University of Oulu, Finland. Hierarchical Routing Protocols. Traditional option when the network has a large number of nodes
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Ad Hoc Networks Routing Addendum Instructor: Carlos Pomalaza-RáezFall 2003University of Oulu, Finland
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
Scopes of FSR 33 3 8 9 5 1 2 4 10 7 13 18 12 14 19 6 21 11 22 15 36 23 16 17 20 29 35 24 27 25 26 28 34 30 32 1 hop 31 2 hops 3 or more hops
Message Reduction in FSR LST HOP 0 LST HOP 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 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 1 3 LST HOP Entries in black are exchanged more frequently 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 2 4 5
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