Introduction to Mobile Ad-Hoc Network (MANET)

Introduction

• Mobility support such as Mobile IP, DHCP, Cellular phone networks relies on existence of at least some infrastructure.

• For example cellular phone network requires base stations, infrastructure networks etc.

• Ad-hoc network is used in situations where users of a network cannot rely on infrastructure, it is expensive, or there is none at all.

• In these situations mobile ad-hoc network is used.

• Mobile ad-hoc networks should be mobile and use wireless communications.

• Examples• Single-hop: All partners max. one hop apart

• Bluetooth piconet, PDAs in a room, gaming devices.

• Multi-hop: Cover larger distances, circumvent obstacles• Bluetooth scatternet,

TETRA police network, car-to-car networks.

Applications

• Instant infrastructure: Unplanned meetings, spontaneous interpersonal communication.

• Disaster relief: infrastructure break down in disaster areas, hurricanes cut phone and power lines. Flood destroy base stations. In these cases extremely fast and reliable ad-hoc network can be set-up.

• Remote areas: can be used where infrastructure is expensive to set up ex. Sparsely populated area

Mobile Ad-hoc NetworkingAnd mobile IP

FixedNetwork

MobileDevices

MobileRouter

Manet

Mobile IP, DHCP

Router End system

Routing

• Routing of data is most difficult issues in ad-hoc networks.

• In wireless network with infrastructure support a base station always reaches mobile nodes, but in ad-hoc network this is not the case.

• Destination node may be out of range of a source node transmitting packets.

• In ad-hoc network each node must be able to forward data for other nodes.

• This create additional problems, such as Link don't have same characteristics in both direction

•

good linkweak link

time = t1 time = t2

N1

N4

N2

N5

N3

N1

N4

N2

N5

N3

Difference between wired networks and ad-hoc wireless networks.

1. Asymmetric links: Quality of connection in both direction may be different

2. Redundant links: Many redundant links

3. Interference:

4. Dynamic topology: frequent changes of connections, connection quality, participants

Difficulties in design of ad-hoc routing protocols

• Traditional algorithm know for wired network may not work efficiently or fail completely because these are not design with highly dynamic topology, asymmetric links, or interference in mind.

• Additional information such as connectivity or interference is needed for finding best path.

• Centralized approach cannot be used.

Routing Algorithms 1.) Destination sequence distance vector(DSDV)

• Enhanced version of distance vector routing for ad-hoc network.

• DVR is used as routing information protocol in wired network.

• In DVR each node exchanges its routing table periodically with its neighbor, this strategy is not used in case of wireless ad-hoc networks, due to rapidly changing topology.

• DSVR adds two things to distance vector routing algorithm:

1. Sequence number :2. Damping :Part of routing table for DSDV

Destination

Next hop

Metric

Sequence no.

Instal time

N1 N1 0 S1–321 T4–001

N2 N2 1 S2–218 T4–001

N3 N2 2 S3–043 T4–002

N4 N4 1 S4–092 T4–001

N5 N4 2 S5–163 T4–002

2. Dynamic source routing

• DSR divide task of routing into two separate problems 1. 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

2. 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

Working of DSR• 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.

1. If the node has already received the request (which is identified using the unique identifier), it drops the request packet.

2. If the node recognizes its own address as the destination, the request has reached its target.

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.

• 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.

• The destination may receive several lists containing different paths from the initiator. It could return the best path, the first path, or several paths to offer the initiator a choice.

Example Applying route discovery to the example in fo a

route from N1 to N3 at time t1 results in the following.

N1N2

N3

good linkweak link

time = t1

N4N5

Example

1. N1 broadcasts the request ((N1), id = 42, target = N3), N2 and N4 receive this request.

2. 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.

3. 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).

4. N4 drops N5’s broadcast, N3 recognizes (N1, N2, N5) as an alternate, but longer route.

5. 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.

The assumption of bi-directional links holds for many ad-hoc networks. However, 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. 1. N3 has to broadcast a route request ((N3), id = 17, target =

N1). Only N5 receives this request.2. N5 now broadcasts ((N3, N5), id = 17, target = N1), N3

and N4 receive the broadcast.3. 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.

4. 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.

5. N3 and N5 drop the request packet

• Now N3 holds the list for a path from N1 to N3, (N1, N2, N3), and N1 knows the path from N3 to N1, (N3, N5, N4, N1). But N1 still does not know how to send data to N3! The only solution is to send the list (N1, N2, N3) with the broadcasts initiated by N3 in the reverse direction. This example shows clearly how much simpler routing can be if links are symmetrical.

Route maintenance is DSR• After a route has been discovered, it has to be

maintained for as long as the node sends packets along this route. Depending on layer two mechanisms, different approaches can be taken:

1. If the link layer uses an acknowledgement (as, for example, IEEE 802.11) the node can interpret this acknowledgement as an intact route.

2. If possible, the node could also listen to the next node forwarding the packet, so getting a passive acknowledgement

3. A node could request an explicit acknowledgement.

Conclusion of DSR

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.

Overview of ad-hoc routing protocols

• Hong (2002) separates routing algorithms into three categories:

1. flat routing, 2. hierarchical routing, 3. geographic-position-assisted routing.

Flat ad-hoc routing

• Flat ad-hoc routing protocols comprise those protocols that do not set up hierarchies with clusters of nodes, special nodes acting as the head of a cluster, or different routing algorithms inside or outside certain regions.

• All nodes in this approach play an equal role in routing.

• The addressing scheme is flat.• This category again falls into two subcategories:

proactive and reactive protocols.

• Proactive type of protocols maintains fresh lists of destinations and their routes by periodically distributing routing tables throughout the network.

• The main disadvantages of such algorithms are:1. Respective amount of data for maintenance.2. Slow reaction on restructuring and failures.• Examples of proactive algorithms are:1. Optimized Link State Routing Protocol (OLSRP)2. Destination Sequence Distance Vector (DSDV)

• reactive protocols finds a route on demand by flooding the network with Route Request packets.

• The main disadvantages of such algorithms are:1. High latency time in route finding.2. Excessive flooding can lead to network clogging.• Examples of on-demand algorithms are:1. Ad hoc On-demand Distance Vector(AODV) 2. Dynamic Source Routing 3. Flow State in the Dynamic Source Routing

Hierarchical ad-hoc routing

• Algorithms such as DSDV, AODV, and DSR only work for a smaller number of nodes and depend heavily on the mobility of nodes.

• For larger networks, clustering of nodes and using different routing algorithms between and within clusters can be a scalable and efficient solution.

• The advantage is that if the topology within a cluster changes, only nodes of the cluster have to be informed. Nodes of other clusters only need to know how to reach the cluster.

Clustering of ad-hoc networksInternet

Super cluster

Cluster

Base stationCluster head

• Clusters can be combined to form super clusters etc., building up a larger hierarchy.

• Using this approach, one or more nodes can act as cluster heads, representing a router for all traffic to/from the cluster.

• All nodes within the cluster and all other cluster heads use these as gateway for the cluster.

• Figure shows an ad-hoc network with interconnection to the internet via a base station. This base station transfers data to and from the cluster heads. In this example, one cluster head also acts as head of the super cluster, routing traffic to and from the super cluster.

• Different routing protocols may be used inside and outside clusters.

• Some are1. CGSR – Clusterhead-Gateway Switch

Routing2. HSR – Hierarchical State Routing3. LANMAR – Landmark Ad Hoc Routing4. ZRP – Zone Routing Protocol

Geographic-position-assisted ad-hoc routing

• If mobile nodes know their geographical position this can be used for routing purposes.

• This improves the overall performance of routing algorithms .• One way to acquire position information is via the global

positioning system (GPS). • Mauve (2001) gives an overview of several position-based ad-

hoc routing protocols.• some of them are1. DREAM – Distance Routing Effect Algorithm for Mobility2. GeoCast – Geographic Addressing and Routing3. GPSR – Greedy Perimeter Stateless Routing4. LAR – Location-Aided Routing