Routing in mobile ad-hoc networks (MANETs)

1. Routing in mobile ad-hoc networks (MANETs)

2. 1. WHAT IS A MANET ? A MANET can be defined as a system of autonomous mobile nodes that communicate over wireless links without any preinstalled infrastructure. 2. WHAT ARE THE APPLICATIONS OF MANETs? MANETs are useful in places that have no communications infrastructure or when that infrastructure is severely damaged. Typical applications are: • emergency rescue operations. • disaster relief efforts. • law enforcement. • military operations.

3. 3. HOW MANETs DIFERE FROM OTHER NETWORKS ? MANETs differ from wired networks in that: • fast and unpredictable topology changes due to nodes mobility. • no dedicated routers to do routing; every node works as a router and a host. • changing channel capacity due to environmental effects. MANETs differ from mobile cellular networks in that: • due to the absence of base stations, MANETs use multihop approach to deliver data. 4. WHAT MAKES ROUTING IN MANETs CHALLANGING ? Routing in MANETs is a dynamic optimization task aiming at providing paths that are :: • optimum in terms of some criterion (e.g. minimum distance, maximum bandwidth, shortest delay).. • satisfying some constraints ( e.g. limited power of mobile nodes , limited capacity of wireless links)..

4. Types of MANET Routing Algorithms: 1 Based on the information used to build routing tables : • Shortest distance algorithms : algorithms that use distance information to build routing tables.(like the RIP, a distance vector protocol) • Link state algorithms : algorithms that use connectivity information to build a topology graph that is used to build routing tables. (like the OSPF, a link state protocol) 2 Based on when routing tables are built: • Proactive algorithms : maintain routes to destinations even if they are not needed . (DSDV) • Reactive algorithms : maintain routes to destinations only when they are needed . (DSR) • Hybrid algorithms : maintain routes to nearby nodes even if they are not needed and maintain routes to far away nodes only when needed . (ZRP)

5. EXAMPLE PROACTIVE ROUTING ALGORITHMS : 1- Destination Sequenced Distance Vector ( DSDV ). The procedure of DSDV : • each node maintains a table with an entry for every possible destination. each entry specifies : • the destination identifier. • the next hop on the route to the destination. • the distance ( in terms of hops ) to the destination. • a sequence number that specifies how fresh the route is. • nodes exchange their routing tables with their neighbors periodically. • based on the received tables , nodes update their routing tables. Disadvantages : • Large routing overhead. • Uses only bidirectional links. • Suffers from count to infinity problem.

6. Assume that MH X receives a routing information from Y about a route to MH Z • Let S(X) and S(Y) denote the destination sequence number for MH Z as stored at MH X, and as sent by MH Y with its routing table to node X, respectively

7. MH X takes the following steps: • If S(Y) < S(X) (means that routing entry in Y is stale), then X ignores the routing information received from Y • If S(Y) = S(X), and cost of going through Y is smaller than the route known to X, then X sets Y as the next hop to Z • If S(Y) > S(X), then X sets Y as the next hop to Z, and S(X) is updated to equal S(Y)

8. 2- Global State Routing (GSR). It is based on the link state routing approach in which every node sends the identities of all nodes connected to it to all other nodes using flooding. link state routing approach (LSR) : • Every node (router) sends the identities of all nodes connected to it to all others using flooding. • Based on the connectivity information, every node builds a topology map. • Every node uses the topology map to build the routing table. Allows for multiple paths GSR Does not use flooding: • Every node node sends the identities of nodes connected to it only to its • neighbors to reduce the routing overhead. Disadvantages : • As the number of nodes increase the size of the routing table increases, i.e., not scalable (Common to all table proactive algorithms)

9. 3- Fisheye State Routing (FSR) • Based on the GSR. • Entries of nearby nodes in the routing table are updated and exchanged with neighbors more frequently (to reduce the update message size). • the accuracy of route increases as packets gets closer to the destination. • FSR is capable to scale to large networks. Disadvantages : • As the mobility of remote nodes increases the accuracy of the routing information decreases.

10. 4- Distance Routing Effect Algorithm For Mobility (DREAM). Uses directed flooding mechanism to deliver messages to destinations. The direction of the flood is controlled by the location information. New concepts introduced : • nodes keep track of location information of each other this info. takes small update packets and stored in smaller tables. • location information update rate is proportional to : • the distance between nodes; nearby nodes inform each other about their location info. more frequently. • the mobility rate; highly mobile nodes inform others about their locations more frequently while fixed nodes do not send updates • scalable and requires little computations. the direction of the flood is calculated only when needed (reactive behavior). Disadvantages : • Requires the use of GPS equipments to get the location information

11. Costs associated with inaccurate position information at remote MHs can be reduced since greater the distance separating two MHs is, slower they appear to be moving with respect to each other • For example, from MH A’s perspective, the change in direction will be greater for MH B than for MH C

12. 5- Multimedia support in Mobile Wireless Networks (MMWN). A hierarchal routing algorithm that takes the QoS requirements into account in route selection. It mimics the cellular phone systems by selecting few nodes to work as dedicated router for others. • nodes arrange themselves in groups (cells). • each group selects a cell-head and a location manager LM. • cell heads arrange themselves in higher-level groups and select new heads • starting at the highest level, members of the same group exchange the link state information (including QoS info.) with each other. • after getting link state information from its peers, every cell head informs the heads of lower level groups belonging to it. So, the heads of level-0 groups can create complete topology map. • LM nodes are the only nodes that know about the affiliation of other nodes .

14. To start a session : • the source node tells the head of its cell about the destination ID. • the cell-head consults its LM about the ID of the cell-head of the destination. • using the ID of the destination cell-head and the topology map the source cell-head can find a route to the destination. • after getting link state information from its peers, every cell head informs the heads of lower level groups belonging to it. So, the heads of level-0 groups can create complete topology map. • LM nodes are the only nodes that know about the affiliation of other nodes . • Disadvantages : • the location management is performed in hierarchal manner; so node movement may trigger a series of updates that may cause inconsistency. • the cluster maintenance process might consume a lot of resources.

15. 6- Cluster-head Gateway Switch Routing (CGSR). • A hieratical routing algorithm that differs from MMWN in : • There is only one level of clustering. • Cluster-heads use distance information to select routes ( it uses DSDV to route messages between cluster-heads through gateways). • Gateways are nodes that are in the communication range of more than one cluster-head.

16. CGSR works as follow: • periodically, every nodes sends a hello message containing its ID and a monotonically increasing sequence number. • using these messages, every cluster-head maintains a table containing the IDs of nodes belonging to it and their most recent seq numbers. • cluster-heads exchange these tables with each other through gateways; eventually, each node will have an entry in the affiliation table of each cluster-head. This entry shows the node’s ID & cluster-head of that node. • each cluster-head and each gateway maintains a routing table with an entry for every cluster-head that shows the next gateway on the shortest path to that cluster head . • Disadvantages : • the same disadvantage common to all hierarchal algorithms related to cluster formation and maintenance.

17. 7- Optimized Link State Routing (OLSR). An improved version of the traditional link state algorithm. OSLR uses controlled flood to disseminate the link state information of each node. • every node creates a list of its one hop neighbors • neighbor nodes exchange their lists with each other. • based on the received lists, each node creates its MPR. The multipoint relays of each node, (MPR), is the minimal set of 1-hop nodes that covers all 2-hop points. • the members of the MPR are the only nodes that can retransmit the link state information in an attempt to limit the flood.

18. The MPR nodes

19. Summary of the proactive routing algorithms : • in general, flat routing algorithms are not scalable due to the large routing overhead they introduce to the network. • OLSR and DREAM are the most scalable flat routing algorithms. • hierarchal routing algorithms more scalable than flat ones. However, in highly mobile networks they introduce processing over head related to group formation and maintenance.

20. A need to send to G EXAMPLE REACTIVE ROUTING ALGORITHMS : 1- Dynamic Source Routing (DSR). Route Discovery: at source A Look up cache for route A to G Route found? Start Route Discovery Protocol No Buffer packet Wait Yes Route Discovery finished Yes Write route in packet header Send packet to next-hop Done

21. Route Discovery: At an intermediate node Accept route request packet <src-id> in recently seen requests list? Discard route request yes My- add = target Add my-addr to partial route No yes Store <src-id> in list Send route reply packet Broadcast packet Done

22. Source broadcasts a route packet with the address of the source and destination S A B A H A-B H D A H A H H C G A neighbor that receives the request looks up its route cache to look for a route to destination. If not found it appends its address into the packet and re-broadcasts it D A -C H A -D H F E node discards packets having been seen

23. Source broadcasts a route packet with the address of the source and destination S A B D H C G D A-D-E -F H node discards packets having been seen E F A-D-E H

24. reply packet follows the reverse path of the route request packet recorded in broadcast packet S A B A-B-G-H D H A-B-G-H A-B-G-H A-B-G-H C G D E F Disadvantages : since the whole route is embedded in every packet of the message; the routing overhead is large in large networks .