Mobile Ad Hoc Networks: Introduction MANET Mobile Ad Hoc Networks: Introduction

Mobile Ad hoc Networks (MANET) This presentation is about ways of information exchange in a network of mobile and wireless nodes without any infrastructural support. Such networks are often called ad hoc networks to emphasize that they do not depend on infrastructural support. The purpose of an ad hoc network is to set up (possibly) a short-lived network for a collection of nodes.

However, most of these protocols do not work well for ad hoc networks. Most network operations involve exchange of information among the computers participating in a network. There are numerous protocols for networks with infrastructural support , starting from LANs, Ethernets and global networks like the Internet. However, most of these protocols do not work well for ad hoc networks.

Routing in an Ad hoc Network If all the wireless nodes are within the transmission range of each other, routing is easy. Every node can listen to all transmissions. However, this is not true in most situations, due to short transmission range. Hence, most ad hoc neworks are multi-hop. A message from a source node must go through intermediate nodes to reach its destination.

Routing in multi-hop networks All nodes cooperate in delivering messages across the network. The nodes must collect local neighbourhood information in order to make global routing decisions. This situation is quite different from wired networks where the routing decisions are made based on the infrastructure.

Bluetooth Mobility becomes much more important when there is a need to network between several PANs. Bluetooth is a short-range radio technology for wireless connectivity of PDAs and other similar devices. If each device is equipped with a Bluetooth radio, it is possible to connect upto 8 such devices into a piconet.

BAN – Body Area Networks For wearable computers (see Xybernaut) Other types BAN – Body Area Networks For wearable computers (see Xybernaut) Not yet implemented UWB – Ultra Wide Band Deemed a probable successor of Bluetooth However, the present commercial implementations were not successful, and some UWB chip makers closed down their facilities (inclusive the leader – Tzero Technologies)

MANET Routing Protocols We will discuss routing protocols for mobile ad hoc networks (MANET). Routing protocols for MANETs can be classified as either reactive or proactive. This classification is based on the way a protocol tries to find a route to a destination.

Proactive Protocols Proactive protocols are based on periodic exchange of control messages and maintaining routing tables. Each node maintains complete information about the network topology locally. This information is collected through proactive exchange of partial routing tables stored at each node.

Proactive Protocols Since each node knows the complete topology, a node can immediately find the best route to a destination. However, a proactive protocol generates large volume of control messages and this may take up a large part of the available bandwidth. The control messages may consume almost the entire bandwidth with a large number of nodes and increased mobility.

Reactive Protocols In a reactive protocol, a route is discovered only when it is necessary. In other words, the protocol tries to discover a route only on-demand, when it is necessary. These protocols generate much less control traffic at the cost of latency, i.e., it usually takes more time to find a route compared to a proactive protocol.

Topology based protocols Some examples of proactive protocols are : Destination Sequenced Distance Vector (DSDV) STAR Some examples of reactive protcols are : Dynamic Source Routing (DSR) Ad hoc On-demand Distance Vector (AODV) Temporally Ordered Routing Algorithm (TORA) Some examples of hybrid protocols are: Cluster Zone Routing Protocol (ZRP)

Position based protocols Protocols based on location services are: Distance Routing Algorithm for Mobility (DREAM) Quorum-based Geographic Location Services (GLS) Home zone, etc. Protocols based on forwarding strategy are: Greedy Gready Perimeter Stateless Routing (GPSR) RDF Hierarchical. etc

The Dynamic Source Routing Protocol - DSR An Overview of the DSR Protocol

Nodes in the network may move about, join or leave. The DSR Protocol (I) Nodes in the network may move about, join or leave. All routing is automatically determined by the protocol. The number and sequence of intermediate hops needed to reach any destination may change dynamically. Hence the network topology may be quite complex.

DSR is an on-demand or reactive routing protocol. The DSR Protocol (II) DSR is an on-demand or reactive routing protocol. When a source node S wants to send a message to a destination node D, the process starts with a route discovery phase. The message is sent once a route has been discovered and S knows about the discovered route.

The DSR Protocol (IV) Each node maintains a route cache to remember routes that it has learnt about. One of the main advantages of DSR as opposed to a table driven protocol like DSDV is that the number of control messages is much smaller. Hence DSR is more energy-efficient and does not congest the network with too many control messages.

An Example when a Route is Discovered Completely B AB A C ABC ABCD E A is trying to find a route to E Each intermediate node appends its ID at the end. E knows the reverse route and sends a route reply.

An Example when a Route is Discovered Partially B C D E A AB A is trying to find a route to E C already has a route to E in its route cache. C sends back the complete route to A.

Assumptions All nodes wishing to communicate with other nodes in the ad hoc network participate fully in running the DSR protocol. The diameter of the network is more than 1, but usually a small number. In other words, a message usually goes through only a small number of hops.

Assumptions The nodes may move at any time without notice and may even move continuously. However, we assume that the speed with which the nodes move is moderate with respect to the packet transmission latency or wireless transmission range of the network. If the nodes move extremely fast, the only possible protocol is flooding.

This is necessary for assigning a unique ID for each node. Assumptions Each node selects a single IP address by which it is known in the network. This is necessary for assigning a unique ID for each node. Each node may or may not work in the promiscuous mode. However, most wireless interface cards support this.

Overview of the DSR Protocol Route discovery and route maintenance operate entirely on-demand. There is no need to broadcast periodically to update routing information in individual nodes. DSDV requires such periodic broadcasts. The number of overhead packets is much smaller in DSR. The number of overhead packets drop to zero when the nodes are static and all routes have been discovered.

Overview of the DSR Protocol When nodes are mobile and/or communication pattern changes, the number of overhead packets increase proportionately. It is necessary to discover new routes in these situations and hence the new route discovery packets are the overhead packets. Note that, a node may receive multiple routes to a destination in response to a route discovery request.

Overview of the DSR Protocol A node may store multiple routes to a destination in its route cache. A node can react to changes in network topology much more rapidly by taking advantage of cached routes. For example, if one route to a destination is broken, the source node can choose another route to the destination from its route cache.

DSR Route Discovery Consider the case when a source node S wants to send a packet to a destination node D. In a ‘good situation’, S already knows a route to D from its route cache. In this case, S will add the sequence of hops to D in the header of the packet. Then S will send the packet to the first node in this sequence.

In the ‘bad case’ S will not find any route to D in its route cache. DSR Route Discovery In the ‘bad case’ S will not find any route to D in its route cache. S will initiate the route discovery protocol. In this case, we call S the initiator and D the target of this protocol.

Node S is trying to discover a route to node D. DSR Route Discovery Node S is trying to discover a route to node D. S broadcasts a route request message to its neighbours. This message is received by all nodes within the transmission range of S. Each route request message contains the initiator and target of the route discovery. Also, each route request is stamped with a unique ID assigned by the initiator.

An Example Route Discovery AB ABCD B ABC C A E

How a Node Deals with a Route Request Message B D If B has a route to D in its route cache, B sends back that route to S. S B D If B does not have a route to D, B broadcasts the route request message.

DSR Route Request When a Route Request message reaches a node, the accumulated route indicates the nodes through which it has passed. This accumulated route is used by a node to send a Route Reply message back to the initiator. The Route Reply message can be sent either by the destination node or by an intermediate node that finds a route to the destination in its route cache.

How Often Should a Node Initiate a Route Discovery? Suppose S is the initiator of a route discovery message to a node D. If S does not receive a route reply message, S would like to initiate another route request message. However, S should not send route request messages frequently, because D may be unreachable at the moment.

How Often Should a Node Initiate a Route Discovery? P S P S may have a better chance of finding a route to D later. S should not flood the network with route requests.

How Fast Should a Node Send a Route Reply? A node can send a route reply if it finds a route to the target in its route cache. However, if many nodes try to send route reply for the same destination, it may result in a route reply storm. Too many replies for the same route request may flood the network.

Route Reply Storm A E D B C F G Each of the nodes B, C, D, E and F knows a route to G.

Waiting Before Replying Simultaneous replies from all these nodes will result in network congestion and packet collisions. Each node should wait for a random amount of time and listen to the traffic. If there is a better (shorter) route reply, or the initiator (node A in this example) starts using another route, there is no need to reply.

Caching Overheard Routing Information An important aspect of DSR is to maintain an up-to-date route cache. A good and current route cache helps a node to find (i) routes for itself faster, and (ii) reply fast to route request messages. If the nodes overhear other messages, they should analyse these messages and update their own route cache if necessary.

Caching Overheard Routing Information B C D E P P overhears the route A-B-C-D-E when B is sending a packet to C. This packet was originally sent by A. P stores this route in its route cache for future use. If P receives a route request for any of the destinations C,D or E, P can use this information.

Route Request Hop Limit Sometime it is not good to propagate a route request message throughout the network. Suppose S is the source of a route request message for a destination D. In case D is in the neighbourhood of S, the route request message from S should not propagate too far away.

Route Request Hop Limit If D is near S, propagating the route request message too far will result in too many unnecessary route reply messages in future. D S

Restricted Propagation of Route Request A better strategy is to propagate route request messages with increasing hop count. Initially, send the route request to a distance of 2 hops. If no route reply is received after sometime, send the route request to a distance of 4 hops and so on. This reduces network congestion by reducing the number of route reply messages.

Route maintenance is important for correct delivery of messages. DSR Route Maintenance Route maintenance is important for correct delivery of messages. When forwarding a packet, each node should ensure that the packet reaches the next hop. A packet reaches its correct destination if each node ensures this correct delivery along the path.

Ensuring Correct Delivery A B C D E Node A is sending a packet to E, using the route A-B-C-D-E. A is responsible for the correct delivery of the packet to B B is responsible for the correct delivery of the packet to C and so on.

Active or Passive Acknowledgment The correct delivery can be ensured through an active acknowledgment. An active acknowledgment may be part of the MAC protocol in use. IEEE 802.11 standards provide such link level active acknowledgment. In a passive acknowledgment, node A may overhear the forwarding of the packet by node B and knows that the packet has been received by B.

Route Error Message A B C D E When A initiates a message to E, there are two parts in the message. The route from A to E and the actual message. Each intermediate node tries to forward the mesage by looking up the next hop from the route.

When a message cannot be forwarded A node like C tries to forward the message and waits for acknowledgment. C will retransmit the message a fixed number of times if no acknowledgment arrives. After that, C will initiate a route error message.

Route Error back to the Initiator D E In this example, C will initiate a route error message back to A indicating that the link to D is currently broken. A will remove this route from its route cache and try another route to E, if it has one. Or, A may start a new route discovery.

Packet Salvaging After sending a route error message, a node may try to send the packet that caused the route error. In the previous example, C may try to find a route to E from its own route cache. If C can find a route to E, it will replace the previous route by the new route and send the packet to E. C should also indicate that this packet has been salvaged, so that other nodes do not try to salvage it.

Automatic Route Shortening Some routes may become unnecessarily long when nodes move around. C A G E B F C overhears the transmission when A sends the packet to B. Recall that A sends the complete route to B. C informs A that B,F,G can be removed from the route.

Spreading of Route Error Message B C D E When A receives a route error message from C, A knows that the link C-D is broken. A removes this route from its route cache. In future, A may try a new route or try to discover a new route. A piggybacks the route error message so that other nodes know that the link C-D is broken.

Advantages and Disadvantages DSR is a simple and efficient routing protocol with low overhead of control messages. However, DSR has relatively high latency in finding routes. DSR is not very scalable since packet size increases with increasing hop numbers in a route.