25-08-2014, 01:05 PM
AD HOC NETWORKS ON SEMINAR REPORT
AD HOC NETWORKS.pptx (Size: 714.65 KB / Downloads: 227)
Introduction
An Ad-hoc network is a local area network or some other small network, especially one with wireless (or temporary plug in connections), in which some of the network devices are the part of the network only for the duration of a communications session.
Allows new network devices to be quickly added.
Each user has a unique network address that is recognized as the part of the network.
Collection of nodes that do not rely on a predefined infrastructure
Auto-configurable network and Self organizing
Nodes are mobile and hence have dynamic network topology.
Nodes in ad-hoc networks play both the roles of routers and terminals.
Routing protocol required
Examples
Classroom
Ad hoc network between student PDAs and workstation of the instructor
Large IT campus
Employees of a company moving within a large campus with PDAs, laptops, and cell phones
Moving soldiers with wearable computers
Eavesdropping, denial-of-service and impersonation attacks can be launched
Shopping mall, restaurant, coffee shops
Customers spend part of the day in a networked mall of specialty shops, coffee shops, and restaurants
MANET
A Mobile Ad-hoc Network (MANET) is a collection of autonomous nodes or terminals which communicate with each other by forming a multi-hop radio network and maintaining connectivity in a decentralized manner over relatively bandwidth constrained wireless links..
Each device in a MANET is free to move independently in any direction, and will therefore change its links to other devices frequently.
The topology is highly dynamic and frequent changes in the topology may be hard to predict.
Criteria – node to be part of a network
To be connected to a network, a node should must be within the area of influence of at least one node on the network.
A node with no remaining power, or one that is off, is not currently a part of the network.
Even if the source and the destination nodes are not within each other’s communication range, data packets are forwarded to the destination by relaying transmission through other nodes that exist between the two nodes.
MANET Constraints and Issues
Lack of a centralized entity
Network topology changes frequently and unpredictably
Routing and Mobility Management
Channel access/Bandwidth availability
Hidden/Exposed station problem
Lack of symmetrical links
Physical security is limited due to the wireless transmission.
Affected by higher loss rates, and can experience higher delays and jitter than fixed networks due to the wireless transmission.
As nodes are battery operated (power constraint), energy savings are an important system design criterion.
Network architecture
MANETs is formed by set of mobile nodes such as laptops, mobile phones etc.
Mobile ad hoc networks are based on wireless links(air).
It can use single hop or multi hop communication.
In single hop communication, all hosts are in one coverage area and hence communication is direct from host to host
In multi hop communication, host communicate using intermediate hosts as many coverage area intersects with each other.
Classifications of MAC Protocols
Contention-based protocols
Sender-initiated protocols: Packet transmissions are initiated by the sender node.
Single-channel sender-initiated protocols: A node that wins the contention to the channel can make use of the entire bandwidth.
Multichannel sender-initiated protocols: The available bandwidth is divided into multiple channels.
Receiver-initiated protocols: The receiver node initiates the contention resolution protocol.
Contention-based protocols with reservation mechanisms
Synchronous protocols: All nodes need to be synchronized. Global time synchronization is difficult to achieve.
Asynchronous protocols: These protocols use relative time information for effecting reservations.
PAMAS (Power aware medium access control with signaling)
RTS-CTS exchanges over a signaling channeling
Data transmission over a separate data channel
Receiver sends out a busy tone, while receiving a data packet over the signaling channel
Nodes listen to the signaling channel to determine when it is optimal to power down transceivers
A node powers itself off if it has nothing to transmit and its neighbor is transmitting
A node powers off if at least one neighbor is transmitting and another is receiving
Use of ACK and transmission of multiple packets can enhance performance
Radio transceiver turnaround time was not considered
Conventional Routing Protocols ?
Not designed for highly dynamic, low bandwidth networks
“Count-to-infinity” problem and slow convergence
Loop formation during temporary node failures and network partitions
Protocols that use flooding techniques create excessive traffic and control overhead
Table Driven Routing Protocol
Send periodic updates of the routes.
Each node uses routing information to store the location information of other nodes in the network and this information is then used to move data among different nodes in the network.
Have lower latency since routes are maintained at all times
Topology changes
Broken links assigned a metric of ∞
Any route through a hop with a broken link is also assigned a metric of ∞
“∞ routes” are assigned new sequence numbers by any host and immediately broadcast via a triggered update
If a node has an equal/later sequence number with a finite metric for an “∞ route”, a route update is triggered
DSR Optimization: Route Caching
Each node caches a new route it learns by any means
When node S finds route [S,E,F,J,D] to node D, node S also learns route [S,E,F] to node F
When node K receives Route Request [S,C,G] destined for node, node K learns route [K,G,C,S] to node S
When node F forwards Route Reply RREP [S,E,F,J,D], node F learns route [F,J,D] to node D
When node E forwards Data [S,E,F,J,D] it learns route [E,F,J,D] to node D
A node may also learn a route when it overhears Data
Problem: Stale caches may increase overheads
AODV Forward path setup
RREQ arrives at a node that has current route to the destination ( larger/same sequence number)
unicast request reply (RREP)<source_addr, dest_addr, dest_sequence_#, hop_cnt,lifetime> to neighbor
RREP travels back to the source along reverse path
each upstream node updates dest_sequence_#, sets up a forward pointer to the neighbor who transmit the RREP
Link Failure
A neighbor of node X is considered active for a routing table entry if the neighbor sent a packet within active_route_timeout interval which was forwarded using that entry
Neighboring nodes periodically exchange hello message
When the next hop link in a routing table entry breaks, all active neighbors are informed
Link failures are propagated by means of Route Error (RERR) messages, which also update destination sequence numbers
Denial of service attacks
A Denial of service attack is an explicit attempt to prevent the legitimate user of a service or data.
The common method of attack involves overloading the target system with requests, such that it cannot respond to legitimate traffic.
it makes the system or service unavailable for the user.
The basic types of attack are: consumption of bandwidth or consumption of processor time, obstructing the communication between two machines, disruption of service to a specific system or person, disruption of routing information, disruption of physical components etc.
If the sensor network encounters DoS attacks, the attack gradually reduces the functionality as well as the overall performance of the wireless sensor network.
Key Benefits of Mobile Ad-hoc Networks
No expensive infrastructure must be installed
Use of unlicensed frequency spectrum
Quick distribution of information around sender
Use of ad-hoc networks can increase mobility and flexibility, as ad-hoc networks can be brought up and torn down in a very short time.
Ad-hoc networks can be more economical in some cases, as they eliminate fixed infrastructure costs and reduce power consumption at mobile nodes.
Because of multi-hop support in ad-hoc networks, communication beyond the Line of Sight (LOS) is possible at high frequencies