UNIT 2 WIRELESS NETWORKS - MANET and DSDV
- 2. There may be several situations where users of a network cannot rely on an infrastructure, it is too expensive, or there is none at all. In these situations mobile ad-hoc networks are the only choice. These networks should be mobile and use wireless communications. Mobile ad-hoc networks Instant infrastructure Disaster relief Remote areas Effectiveness Fig. MANETs and mobile IP
- 3. Routing At a certain time t1 the network topology might look as illustrated on the left side of the figure. Five nodes, N1 to N5, are connected depending on the current transmission characteristics between them. In this snapshot of the network, N4 can receive N1 over a good link, but N1 receives N4 only via a weak link. Links do not necessarily have the same characteristics in both directions. The reasons for this are ,e.g., different antenna characteristics or transmit power. N1 cannot receive N2 at all, N2 receives a signal from N1. Fig. ad-hoc network
- 4. Difference between wired networks and ad-hoc network (a) Asymmetric links: Node A receives a signal from node B. But this does not tell us anything about the quality of the connection in reverse. However, many routing algorithms for wired networks rely on a symmetric scenario (b) Redundant links: Wired networks, too, have redundant links to survive link failures. However, there is only some redundancy in wired networks, which, additionally, are controlled by a network administrator. A high redundancy can cause a large computational overhead for routing table updates (c) Interference Interference creates new problems by ‘unplanned’ links between nodes: if two close-by nodes forward two transmissions, they might interfere and destroy each other.
- 5. (d) Dynamic topology: The greatest problem for routing arises from the highly dynamic topology. This results in frequent changes in topology, so snapshots are valid only for a very short period of time. Routing table updates in fixed networks, for example, take place every 30 seconds. This updating frequency might be too low to be useful for ad-hoc networks. Ad-hoc networks using mobile nodes face additional problems due to hardware limitations. Using the standard routing protocols with periodic updates wastes battery power without sending any user data and disables sleep modes. Periodic updates waste bandwidth and these resources are already scarce for wireless links. An additional problem is interference between two or more transmissions that do not use the same nodes for forwarding. If, for example, a second transmission from node N4 to N5 takes place at the same time as the transmission from N1 to N3, they could interfere.
- 6. Additional difficulties in comparison to wired networks Traditional routing algorithms known from wired networks will not work efficiently Centralized approaches will not really work, because it takes too long to collect the current status and disseminate it again. Many nodes need routing capabilities. While there might be some without, at least one router has to be within the range of each node. Algorithms have to consider the limited battery power of these nodes. Nodes have to make local decisions for forwarding and send packets roughly toward the final destination. A last alternative to forward a packet across an unknown topology is flooding. The maximum number of hops, should be known.
- 7. Destination sequence distance vector Destination sequence distance vector (DSDV) routing is an enhancement to distance vector routing for ad-hoc networks. Distance vector routing is used as routing information protocol (RIP) in wired networks. It performs extremely poorly with certain network changes due to the count-to-infinity problem. The strategies used in fixed networks do not help in the case of wireless ad-hoc networks, due to the rapidly changing topology. DSDV now adds two things to the distance vector algorithm: Sequence numbers: Each routing advertisement comes with a sequence number. Within ad-hoc networks, advertisements may propagate along many paths. Sequence numbers help to apply the advertisements in correct order.
- 8. Damping: Transient changes in topology that are of short duration should not destabilize the routing mechanisms. A node waits with dissemination if these changes are probably unstable. Waiting time depends on the time between the first and the best announcement of a path to a certain destination. Dynamic source routing In an ad-hoc network where nodes exchange packets from time to time. These algorithms maintain routes between all nodes, although there may currently be no data exchange at all. This causes unnecessary traffic and prevents nodes from saving battery power. Dynamic source routing (DSR), therefore, divides the task of routing into two separate problems
- 9. Route discovery: A node only tries to discover a route to a destination if it has to send something to this destination and there is currently no known route. Route maintenance: If a node is continuously sending packets via a route, it has to make sure that the route is held upright. As soon as a node detects problems with the current route, it has to find an alternative. Dynamic source routing eliminates all periodic routing updates and works as follows. If a node needs to discover a route, it broadcasts a route request with a unique identifier and the destination address as parameters. Any node that receives a route request does the following.
- 10. Case 1 : If the node has already received the request (which is identified using the unique identifier), it drops the request packet. Case 2 : If the node recognizes its own address as the destination, the request has reached its target. Case 3 : Otherwise, the node appends its own address to a list of traversed hops in the packet and broadcasts this updated route request. Using this approach, the route request collects a list of addresses representing a possible path on its way towards the destination. As soon as the request reaches the destination, it can return the request packet containing the list to the receiver using this list in reverse order. One condition for this is that the links work bi-directionally. If this is not the case, and the destination node does not currently maintain a route back to the initiator of the request, it has to start a route discovery by itself.
- 11. Applying route discovery to the example in for a route from N1 to N3 at time t1 results in the following N1 broadcasts the request ((N1), id = 42, target = N3), N2 and N4 receive this request. N2 then broadcasts ((N1, N2), id = 42, target = N3), N4 broadcasts ((N1, N4), id = 42, target = N3). N3 and N5 receive N2’s broadcast, N1, N2, and N5 receive N4’s broadcast. N3 recognizes itself as target, N5 broadcasts ((N1, N2, N5), id = 42, target = N3). N3 and N4 receive N5’s broadcast. N1, N2, and N5 drop N4’s broadcast packet, because they all recognize an already received route request (and N2’s broadcast reached N5 before N4’s did). N4 drops N5’s broadcast, N3 recognizes (N1, N2, N5) as an alternate, but longer route. N3 now has to return the path (N1, N2, N3) to N1. This is simple assuming symmetric links working in both directions. N3 can forward the information using the list in reverse order.
- 12. If links are not bi-directional, the scenario gets more complicated. The algorithm has to be applied again, in the reverse direction if the target does not maintain a current path to the source of the route request. N3 has to broadcast a route request ((N3), id = 17, target = N1). Only N5 receives this request. N5 now broadcasts ((N3, N5), id = 17, target = N1), N3 and N4 receive the broadcast. N3 drops the request because it recognizes an already known id. N4 broadcasts ((N3, N5, N4), id = 17, target = N1), N5, N2, and N1 receive the broadcast. N5 drops the request packet, N1 recognizes itself as target, and N2 broadcasts ((N3, N5, N4, N2), id = 17, target = N1). N3 and N5 receive N2’s broadcast. N3 and N5 drop the request packet.
- 13. The basic algorithm for route discovery can be optimized in many ways. To avoid too many broadcasts, each route request could contain a counter. Every node rebroadcasting the request increments the counter by one. Knowing the maximum network diameter (take the number of nodes if nothing else is known), nodes can drop a request if the counter reaches this number. Drawback: Although dynamic source routing offers benefits compared to other algorithms by being much more bandwidth efficient, problems arise if the topology is highly dynamic and links are asymmetrical.