Mobile Ad-hoc Networks
An ad-hoc network is a collection of wireless mobile hosts forming a temporary network without the aid of any stand-alone infrastructure or centralized administration.Mobile Ad-hoc networks are self-organizing and self-configuring multihop wireless networks where, the structure of the network changes dynamically. This is mainly due to the mobility of the nodes . Nodes in these networks utilize the same random access wireless channel, cooperating in a friendly manner to engaging themselves in multihop forwarding. The nodes in the network not only acts as hosts but also as routers that route data to/from other nodes in network. In mobile ad-hoc networks where there is no infrastructure support as is the case with wireless networks, and since a destination no de might be out of range of a source node transmitting packets; a routing procedure is always needed to find a path so as to forward the packets appropriately between the source and the destination. Within a cell, a base station can reach all mobile nodes without routing via broadcast in common wireless networks. In the case of ad-hoc networks, each node must be able to forward data for other nodes. This creates additional problems along with the problems of dynamic topology which is unpredictable connectivity changes.
• Problems with routing in Mobile Ad-hoc Net-works
– Asymmetric links: Most of the wired networks rely on the symmetric links which are always fixed. But this is not a case with ad-hoc networks as the nodes are mobile and constantly changing their position within network. For example consider a MANET (Mobile Ad-hoc Network) where node B sends a signal to node A but this does not tell anything about the quality of the connection in the reverse direction.
– Routing Overhead: In wireless ad hoc networks, no des often change their location within network. So, some stale routes are generated in the routing table which leads to unnecessary routing overhead.
– Interference: This is the major problem with mobile ad-hoc networks as links come and go depending on the transmission characteristics, one transmission might interfere with another one and no de might overhear transmissions of other nodes and can corrupt the total transmission.
– Dynamic Topology: This is also the major problem with ad-hoc routing since the topology is not constant. The mobile node might move or medium characteristics might change. In ad-hoc networks, routing tables must somehow reject these changes in topology and routing algorithms have to be adapted. For example in a fixed network routing table updating takes place for every 30sec. This updating frequency might be very low for ad-hoc networks.
• Classification of routing Protocols in MANET’s
Classification of routing protocols in MANET’s can be done in many ways, but most of these are done depending on routing strategy and network structure. According to the routing strategy the routing protocols can be categorized as Table-driven and source initiated, while depending on the network structure these are classified as at routing, hierarchical routing and geographic position assisted routing. Both the Table-driven and source initiated protocols come under the Flat routing.
Table-Driven routing protocols (Proactive)
These protocols are also called as proactive protocols since they maintain the routing information even before it is needed. Each and every node in the network maintains routing information to every other node in the network. Routes information is generally kept in the routing tables and is periodically updated as the network topology changes. Many of these routing protocols come from the link-state routing. There exist some differences between the protocols that come under this category depending on the routing information being updated in each routing table. Furthermore, these routing protocols maintain different number of tables. The proactive protocols are not suitable for larger networks, as they need to maintain node entries for each and every node in the routing table of every node. This causes more overhead in the routing table leading to consumption of more bandwidth.
On Demand routing protocols (Reactive)
These protocols are also called reactive protocols since they don’t maintain routing information or routing activity at the network nodes if there is no communication. If a node wants to send a packet to another node then this proto col searches for the route in an on-demand manner and establishes the connection in order to transmit and receive the packet. The route discovery usually occurs by flooding the route request packets throughout the network.
Destination Sequenced Distance Vector (DSDV) Protocol
The destination sequenced distance vector routing protocol is a proactive routing protocol which is a modification of conventional Bellman-Ford routing algorithm. This protocol adds a new attribute, sequence number, to each route table entry at each node. Routing table is maintained at each node and with this table; node transmits the packets to other nodes in the network. This protocol was motivated for the use of data exchange along changing and arbitrary paths of interconnection which may not be close to any base station.
Protocol Overview and activities
Each node in the network maintains routing table for the transmission of the packets and also for the connectivity to different stations in the network. These stations list for all the available destinations, and the number of hops required to reach each destination in the routing table. The routing entry is tagged with a sequence number which is originated by the destination station. In order to maintain the consistency, each station transmits and updates its routing table periodically. The packets being broadcasted between stations indicate which stations are accessible and how many hops are required to reach that particular station. The packets may be transmitted containing the layer 2 or layer 3 address.
Routing information is advertised by broadcasting or multicasting the packets which are transmitted periodically as when the nodes move within the network. The DSDV protocol requires that each mobile station in the network must constantly, advertise to each of its neighbors, its own routing table. Since, the entries in the table my change very quickly, the advertisement should be made frequently to ensure that every node can locate its neighbours in the network. This agreement is placed, to ensure the shortest number of hops for a route to a destination; in this way the node can exchange its data even if there is no direct communication link.
The data broadcast by each node will contain its new sequence number and the following information for each new route:
– The destination address
– The number of hops required to reach the destination and
– The new sequence number, originally stamped by the destination
The transmitted routing tables will also contain the hardware address, network address of the mobile host transmitting them. The routing tables will contain the sequence number created by the transmitter and hence the most new destination sequence number is preferred as the basis for making forwarding decisions. This new sequence number is also updated to all the hosts in the network which may decide on how to maintain the routing entry for that originating mobile host. After receiving the route information, receiving node increments the metric and transmits information by broadcasting. Incrementing metric is done before transmission because, incoming packet will have to travel one more hop to reach its destination. Time between broadcasting the routing information packets is the other important factor to be considered. When the new information is received by the mobile host it will be retransmitted soon effecting the most rapid possible dissemination of routing information among all the co-operating mobile hosts. The mobile host cause broken links as they move from place to place within the network. The broken link may be detected by the layer2 protocol, which may be described as infinity. When the route is broken in a network, then immediately that metric is assigned an infinity metric there by determining that there is no hop and the sequence number is updated. Sequence numbers originating from the mobile hosts are defined to be even number and the sequence numbers generated to indicate infinity metrics are odd numbers. The broadcasting of the information in the DSDV protocol is of two types namely:
Full dump and incremental dump. Full dump broadcasting will carry all the routing information while the incremental dump will carry only information that has changed since last full dump. Irrespective of the two types, broadcasting is done in network protocol data units (NPDU). Full dump requires multiple NPDU’s while incremental requires only one NPDU to fit in all the information. When an information packet is received from another node, it compares the sequence number with the available sequence number for that entry. If the sequence number is larger, then it will update the routing information with the new sequence number else if the information arrives with the same sequence number it looks for the metric entry and if the number of hops is less than the previous entry the new information is updated (if information is same or metric is more then it will discard the information). While the nodes information is being updated the metric is increased by 1 and the sequence number is also increased by 2. Similarly, if a new node enters the network, it will announce itself in the network and the nodes in the network update their routing information with a new entry for the new node.
During broadcasting, the mobile hosts will transmit their routing tables periodically but due to the frequent movements by the hosts in the networks, this will lead to continuous burst of new routes transmissions upon every new sequence number from that destination. The solution for this is to delay the advertisement of such routes until it shows up a better metric.
Operation at Layer2
Address stored in the routing table at the mobile hosts will correspond to the layer at which the DSDV protocol is operated. Layer3 will use network layer addresses for the next hop and destination addresses and layer 2 will use the MAC address for its operation. A difficulty is arise at the layer 3 operation and a way must be provided to resolve these layer-3 addresses into MAC addresses. Otherwise, problems like broadcast address resolution would be needed and loss of bandwidth would be observed. This loss could be substantial because such mechanisms will require retransmission by every mobile node. The solution here is to provide layer3 protocol information along with the layer2 information at the layer 2 operation. Each mobile node would advertise, reachability, information about the layer3 protocols at that destination.
Advantages of DSDV
– DSDV proto col guarantees loop free paths.
– Count to infinity problem is reduced in DSDV.
– We can avoid extra traffic with incremental updates instead of full dump updates.
– Path Selection: DSDV maintains only the best path instead of maintaining multiple paths to every destination. With this, the amount of space in routing table is reduced.
Limitations of DSDV
– Wastage of bandwidth due to unnecessary advertising of routing information even If there is no change in the network topology.
– DSDV doesn’t support Multi path Routing.
– It is di cult to determine a time delay for the advertisement of routes.
– It is di cult to maintain the routing table’s advertisement for larger network. Each and every host in the network should maintain a routing table for advertising. But for larger network this would lead to overhead, which consumes more bandwidth.
Ad-hoc On-Demand Distance Vector (AODV) Protocol
AODV is a very simple, efficient, and effective routing protocol for Mobile Ad-hoc Networks which do not have fixed topology. This algorithm was motivated by the limited bandwidth that is available in the media that are used for wireless communications. It borrows most of the advantageous concepts from DSR and DSDV algorithms. The on demand route discovery and route maintenance from DSR and hop-by-hop routing, usage of node sequence numbers from DSDV make the algorithm cope up with topology and routing information. Obtaining the routes purely on-demand makes AODV a very
useful and desired algorithm for MANETs.
Working of AODV
Each mobile host in the network acts as a specialized router and routes are obtained as needed, thus making the network self-starting. Each node in the network maintains a routing table with the routing information entries to its neighbouring no des, and two separate counters: a node sequence number and a broadcast-id. When a node (say, source node ‘S’) has to communicate with another (say, destination node ‘D’), it increments its broadcast-id and initiates path discovery by broadcasting a route request packet RREQ to its neighbours. The RREQ contains the following fields:
– source-sequence# -to maintain freshness info about the route to the source.
– dest-sequence# – specifies how fresh a route to the destination must be before it is accepted by the source.
The (source-addr, broadcase-id) pair is used to identify the RREQ uniquely. Then the dynamic route table entry establishment begins at all the nodes in the network that are on the path from S to D. As RREQ travels from node to node, it automatically sets up the reverse path from all these no des back to the source. Each no de that receives this packet records the address of the node from which it was received. This is called Reverse Path Setup. The nodes maintain this info for enough time for the RREQ to traverse the network and produce a reply to the sender and time depends on network size. If an intermediate no de has a route entry for the desired destination in its routing table, it compares the destination sequence number in its routing table with that in the RREQ. If the destination sequence number in its routing table is less than that in the RREQ, it rebroadcasts the RREQ to its neighbours. Otherwise, it unicasts a route reply packet to its neighbour from which it was received the RREQ if the same request was not processed previously (this is identified using the broad case-id and source-addr). Once the RREP is generated, it travels back to the source, based on the reverse path that it has set in it until travelled to this node. As the RREP travels back to source, each node along this path sets a forward pointer to the node from where it is receiving the RREP and records the latest destination sequence number to the request destination. This is called Forward Path Setup. If an intermediate node receives another RREP after propagating the first RREP towards source it checks for destination sequence number of new RREP. The intermediate node updates routing information and propagates new RREP only,
– If the Destination sequence number is greater, OR
– If the new sequence number is same and hop count is small, OR
Otherwise, it just skips the new RREP. This ensures that algorithm is loop-free and only the most effective route is used.
Route Table Management
Each mobile node in the network maintains a route table entry for each destination of interest in its route table. Each entry contains the following info:
– Next hop
– Number of hops
– Destination sequence number
– Active neighbours for this route
– Expiration time for the route table entry
The other useful information contained in the entries along with source and destination sequence numbers is called soft-state information associated to the route entry. The info about the active neighbours for this route is maintained so that all active source nodes can be notified when a link along a path to the destination breaks. And the purpose of route request time expiration timer is to purge the reverse path routing entries from all the nodes that do not lie on the active route.
Interesting concepts of AODV
The concepts of AODV that make it desirable for MANETs with limited bandwidth include the following:
– Minimal space complexity: The algorithm makes sure that the nodes that are not in the active path do not maintain information about this route. After a node receives the RREQ and sets a reverse path in its routing table and propagates the RREQ to its neighbours, if it does not receive any RREP from its neighbours for this request, it deletes the routing info that it has recorded.
Advanced uses of AODV
– Maximum utilization of the bandwidth: This can be considered the major achievement of the algorithm. As the protocol does not require periodic global advertisements, the demand on the available bandwidth is less. And a monotonically increased sequence number counter is maintained by each node in order to supersede any stale cached routes. All the intermediate nodes in an active path updating their routing tables also make sure of maximum utilization of the bandwidth. Since, these routing tables will be used repeatedly if that intermediate node receives any RREQ from another source for same destination. Also, any RREPs that are received by the nodes are compared with the RREP that was propagated last using the destination sequence numbers and are discarded if they are not better than the already propagated RREPs.
– Simple: It is simple with each no de behaving as a router, maintaining a simple routing table, and the source node initiating path discovery request, making the network self-starting.
– Most effective routing info: After propagating an RREP, if a node finds receives an RREP with smaller hop-count, it updates its routing info with this better path and propagates it.
– Most current routing info: The route info is obtained on demand. Also, after propagating an RREP, if a no de finds receives an RREP with greater destination sequence number, it updates its routing info with this latest path and propagates it.
– Loop-free routes: The algorithm maintains loop free routes by using the simple logic of nodes discarding non better packets for same broadcast-id.
– Coping up with dynamic topology and broken links: When the nodes in the network move from their places and the topology is changed or the links in the active path are broken, the intermediate node that discovers this link breakage propagates an RERR packet. And the source node re-initializes the path discovery if it still desires the route. This ensures quick response to broken links.
– Highly Scalable: The algorithm is highly scalable because of the minimum space complexity and broadcasts avoided when it compared with DSDV.
Advanced uses of AODV
– Because of its reactive nature, AODV can handle highly dynamic behaviour of Vehicle Ad-hoc networks.
– Used for both unicasts and multicasts using the ‘J’ (Join multicast group) flag in the packets.
Limitations/Disadvantages of AODV
– Requirement on broadcast medium: The algorithm expects/requires that the no des in the broadcast medium can detect each other’s’ broadcasts.
– Overhead on the bandwidth: Overhead on bandwidth will be occurred compared to DSR, when an RREQ travels from node to node in the pro cess of discovering the route info on demand, it sets up the reverse path in itself with the addresses of all the no des through which it is passing and it carries all this info all its way.
– No reuse of routing info: AODV lacks an efficient route maintenance technique. The routing info is always obtained on demand, including for common case traffic.
– It is vulnerable to misuse: The messages can be misused for insider attacks including route disruption, route invasion, node isolation, and resource consumption.
– AODV lacks support for high throughput routing metrics: AODV is designed to support the shortest hop count metric. This metric favours long, low bandwidth links over short, high-bandwidth links.
– High route discovery latency: AODV is a reactive routing protocol. This means that AODV does not discover a route until a flow is initiated. This route discovery latency result can be high in large-scale mesh networks.
Discussion and Conclusion
After reviewing the concept of wireless ad-hoc networks and two routing protocols namely, AODV and DSDV. We would like to make a comparative discussion of both the protocols with their pro’s and con’s. Most of the discussion being made is based on previous studies and implementations.
– DSDV is a proactive routing proto col, which maintains routes to each and every node in the network, while AODV is a reactive routing protocol which finds the path on demand or whenever the route is required.
– Broadcasting in DSDV is done periodically to maintain routing updates and in AODV, only hello messages are propagated to its neighbours to maintain local connectivity.
– DSDV routing algorithm maintains a sequence number concept for updating the latest information for a route. Even, the same concept is adapted by AODV routing protocol.
– Due to the periodic updates being broadcasted in DSDV, bandwidth is wasted when the no des are stationary. But, this is not the case with AODV, as it propagates only hello messages to its neighbours.
– For sending data to a particular destination, there is no need to find a route as DSDV routing protocol maintains all the routes in the routing tables for each node. While, AODV has to find a route before sending a data.
– Overhead in DSDV is more when the network is large and it becomes hard to maintain the routing tables at every node. But, in AODV overhead is less as it maintains small tables to maintain local connectivity.
– DSDV cannot handle mobility at high speeds due to lack of alternative routes hence routes in routing table is stale. While in AODV this is the other way, as it find the routes on demand.
– Throughput decreases comparatively in DSDV as it needs to advertise periodic updates and even-driven updates. If the node mobility is high then occurrence of event driven updates are more. But in AODV it doesn’t advertise any routing updates and hence the throughput is stable.
The study reveals that, DSDV routing protocol consumes more bandwidth, because of the frequent broadcasting of routing updates. While the AODV is better than DSDV as it doesn’t maintain any routing tables at nodes which results in less overhead and more bandwidth. From the above, it can be assumed that DSDV routing protocols works better for smaller networks but not for larger networks. So, my conclusion is that, AODV routing protocol is best suited for general mobile ad-hoc networks as it consumes less bandwidth and lower overhead when compared with DSDV routing protocol.