VTU Computer Networks 2 – 6th Sem CSE Notes

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  • Unit 1 [Packet Switching Networks] Notes, Unit 2 [Packet Switching Networks – 2] and  Unit 7 [Multimedia Networking] Notes By Prof.K B Sulekha, Acharya Institue of Technology, Bangalore
  • Unit 3 [TCP / IP-1] Notes and Unit 4 [TCP/IP-2] Notes By Dr. H S Ramesh Babu, AIT, Bangalore
  • Unit 8 [Mobile AdHoc Networks and Wireless Sensor Networks], Unit 5 [Applications, Network Management, Network Security] and Unit 6 [QoS, VPNs, Tunneling, Overlay Networks] By Dr. T G Basavaraju, SKSJTI, Bangalore and Prof. C B Yogesh, Acharya Institue of Technology Bangalore

Notes Credit – ELearning

See Computer Networks 2 Syllabus / 6th Semester Computer Science Syllabus

UNIT 8:- Mobile Ad-Hoc Networks  [Sample Notes]

Wireless Sensor Networks
a Mobile Ad hoc NETwork (MANET) is one that comes together as needed, not necessarily with any support from the existing infrastructure or any other kind of fixed stations. We can formalize this statement by defining an ad hoc (ad-hoc or adhoc) network as an autonomous system of mobile hosts (MHs) (also serving as routers) connected by wireless links, the union of which forms a communication network modeled in the form of an arbitrary communication graph. This is in contrast to the wellknown single hop cellular network model that supports the needs of wireless communication by installing base stations (BSs) as access points. In these cellular networks, communications between two mobile nodes completely rely on the wired backbone and the fixed BSs. In a MANET, no such infrastructure exists and the network topology may dynamically change in an unpredictable manner since nodes are free to move.

Important characteristics of a MANET Characteristics:
Dynamic Topologies Nodes are free to move arbitrarily with different speeds; thus,the network topology may change randomly and at unpredictable times.
Energy-constrained Operation Some or all of the nodes in an ad hoc network may rely on batteries or other exhaustible means for their energy. For these nodes, the most important system design optimization criteria may be energy conservation.

Limited Bandwidth: Wireless links continue to have significantly lower capacity than infra structured networks. In addition, the realized throughput of wireless communications – after accounting for the effects of multiple access, fading, noise, and interference conditions, etc., is often much less than a radio’s maximum transmission rate

Security Threats : Mobile wireless networks are generally more prone to physical security threats than fixed-cable nets. The increased possibility of eavesdropping, spoofing, and minimization of denial-of service type attacks should be carefully considered.

Applications of MANETs
Collaborative Work – For some business scenarios, the need for collaborative computing might be more important outside office environments than inside a building. After all, it is often the case where people do need to have outside meetings to cooperate and exchange information on a given project;

• Crisis-management Applications – These arise, for example, as a result of natural disasters where the entire communications infrastructure is in disarray (for example, Tsunamis, hurricanes, etc.).
Restoring communications quickly is essential. By using ad hoc networks, an infrastructure could be set up in hours instead of days/weeks required for wire-line communications;

Personal Area Networking – A personal area network (PAN) is a short-range, localized network where nodes are usually associated with a given person. These nodes could be attached to someone’s cell phone, pulse watch, belt, and so on. In these scenarios, mobilityis only a major consideration when interaction among several PANs is necessary, illustrating the case where, for instance, people meet in real life. Bluetooth [Haarstenl998] is an example of a technology aimed at, among other things, supporting PANs by eliminating the need of wires between devices such as printers, cell phones, PDAs, laptop computers, headsets, and so on, and is discussed later in this book. Other standards under the IEEE 802.15 working group for wireless PANs are also described.

Classification of routing protocols
Ad-hoc Routing protocols can be categorized as table-driven or source initiated. Table-driven or proactive ,routing protocols finds routes to all possible destinations ahead of time. The routes are recorded in the nodes’ routing tables and are updated within the predefined intervals. Proactive routing protocols are faster in decision making ,but cause problems if the topology of the network continually changes.
These protocols require every node to maintain one or more tables to store updated routing information from every node to all other nodes.

Source-initiated routing protocols:
Source-initiated, or reactive, routing protocols are on-demand procedures and create routes only when requested to do so by source nodes. A route request initiates a route-discover process in the network and is completed once a route is discovered. If it exists, at the time of request, a route is maintained by a route-maintenance procedure until either the destination node becomes irrelevant to the source or the route is no longer needed.
Control overhead of packets is smaller than of proactive protocols.
Table driven / proactive
* Destination sequenced distance vector [DSDV]: The DSDV is table driven based routing algorithm. DSDV is improved version of Bellman Ford routing algorithm.
* Each DSDV node maintain two routing tables: – table for forwarding packets, and table for advertising incremental updates. The nodes will maintain a routing table that consists
of a sequence number. The routing table periodically exchanged so that every node will have latest information.
* DSDV is suitable for small networks.

The algorithm works as follows
* A node or a mobile device will make an update in its routing table and send the information to its neighbor upon receiving the updated information and make an update in its own routing table.
* The update is made by comparing the sequence number received is greater than present sequence number than the new one will be used.
* If there is a link failure in one of the node will change the metric value to infinity and broadcast the message.

Cluster head gateway switch router [CGSR]
CGSR is also a table driven routing protocol. In this algorithm the mobile devices will be grouped to form a cluster the grouping is based on the range and each cluster is controlled by cluster head. All the mobile devices will maintain 2 tables cluster member table and routing table.
The cluster member table will have the information about the cluster head for each destination the routing table will have routing information. In this protocol the packet cannot be directly sent to the destination instead cluster heads are used for routing.
CGSR routing involves cluster routing, where a node finds the best route over cluster heads from the cluster member table.

Wireless routing protocol [WRP]
WRP is also based on table driven approach this protocol makes use of 4 tables
1. Distance table :- Which contains information like destination, next hop, distance
2. Routing table: – Which contains routing information.
3. Link cost table:- Which contains cost information to each neighbor
4. Message retransmission list table: – this table provides sequence number of the message, a retransmission counter, acknowledgements and list of updates sent in update message.
Whenever there is a change in the network an update will be made which will be broadcasted to other nodes.
Other nodes upon receiving the updated information will make an update in their table.
If there is no update in the network a hello message should be sent.

Source initiated / reactive protocol
* Dynamic source routing [DSR]: DSR is a source initiated or on demand routing protocol in which source finds unexpired route to the destination to send the packet. It is used in the network where mobile nodes move with moderate speed.
* Overhead is significantly reduced, since nodes do not exchange routing table information it has 2 phases.
1. Route discovery
2. Route maintenance
The source which wants to send the information to the destination will create a route request message by adding its own identification number and broadcasts them in the network. The intermediate nodes will continue the broadcast but adding their own identification number.
When the destination is reached a route reply message is generated which will be sent back to the source. The source can receive multiple route replies indicating the presence of multiple paths.
The source will pick up one of the path and will use for transmission. If there is a link failure one of the node will detect and will create a route error message which will be sent back to the source in this case the path has to be re-established for further transmission.

Associated based routing [ABR]: ABR is an efficient on-demand or source initiated routing protocol. In ABR, the destination node decides the best route, using node associativity. ABR is suitable for small networks, as it provides fast route discovery and creates shortest paths through associativity.
Each node keeps track of associativity information by sending messages periodically.
If the associativity value is more means nodes mobility is less.
If the associativity value is less means nodes mobility is
In ABR the source which wants to send the packet to the destination will create a query packet and broadcast in the network. Query packet generation is required for discovering the route.
The broadcast continues as long as destination is reached once the destination is reached it creates the reply packet and sends back to the source.
The query packet will have the following information.
1. Source id
2. Destination id
3. All intermediate node id
4. Sequence number
5. CRC and
6. Time to live [TTL]
A node sends an update packet to the neighbors and waits for the reply if update is received back, then associative tick will be incremented high then it means mobile device is still a part of the network otherwise it might not be.
Adhoc on demand distance vector [AODV]
It is a source initiated routing protocol in mobile adhoc networks.
The algorithm consist of 2 phases
1. Route discovery phase
2. Route maintenance phase
In route discovery phase the path from source to destination is identified by broadcasting route request packet [RREQ]. When the intermediate node receive RREQ they will create a backward pointer and continue the broadcast when the route request packet reaches the destination a route reply would be generated [RREP]. The route reply will have information about the path that can be chosen for the packet transmission.
The route request packet can have following information.
1. Source id
2. Destination id
3. Sequence number
4. Backward pointer information
5. CRC and
6. Time to live[TTL]
In the above network the RREQ will be broadcasted by the source node 1 to its neighbor and neighbors will check whether RREQ is already processed. If it is already processed the packet will be discarded.
If it is not processed a backward pointer is created and the broad cast continues.
When the packet is reached at destination a route reply is created [RREP] in the above network the first RREP is sent to the source can have the path information as 1-2-4-6-8.
When the source receives this information it will be stored in the routing table. Mean while the destination can create one more RREP which can have the information as 1-3-7-8  the destination will send this RREP to the source and will also ask the source to discard old path as the new path is having minimum number of hops.

Route maintainence phase
The nodes in the network periodically exchange hello messages to inform that they are still a part of network and the path is valid. Whenever there is a link failure detected. A route error packet [RERR] will be sent to the source indicating the path is no more valid.

Temporary ordered routing algorithm [TORA]
It is also a source initiated routing algorithm, creates multiple routes for any source/ destination pair. The advantage of multiple routes is that route discovery is not required for
every alteration in the network topology.
TORA consists of three phases,
1. Route Creation/discovery
2. Route maintenance
3. Route erasure

TORA uses three types of packets: Query Packets for route creation, Update Packets for both creation and maintenance
The route will be discovered from the source to destination only when a request is made for the transmission. In this algorithm the source will generate a query packet which will be broadcasted in the network this continues as long as a node that is directly connected to the destination is identified.
When the destination is identified an update packet will be generated and sent back to the source. The update packet will have the path information if there are more than one
update packet received by the source, it means there are multiple paths to the destination, the source has to choose best path available.

Security in adhoc networks
The following are the security threat in adhoc network.
1. Limited computational capabilities : the nodes in the mobile adhoc network are modular, independent and will have limited computational capability.
It becomes a source of vulnerability when they handle public key cryptography.
2. Limited power supply : since nodes have limited power supply attacker can exhaust batteries by giving excessive computations to be carried out.
3. Challenging key management : the key management becomes extremely difficult as the mobile devices will be under movement.

Types of attack in adhoc network
The attack can be classified into 2 types
1. Passive 2. Active
In passive attack, the normal operation of routing protocol is not interrupted. The attacker just tries to gather the information
In active attack, the attacker can insert some arbitrary packets and therefore might affect the normal operation of network
Attack can also be one of the following types
1. Pin attack : with the pin attack, an unauthorized node pretends to have shortest path to the destination
The attacker can listen to path setup phase and become the part of network.
2. Location disclosure attack : by knowing the locations of intermediate nodes, the attacker can find out the location of target node
3. Routing table overflow : the attacker can create some routes whose destination do not exist. It will have major impact on proactive based routing
4. Energy exhaustion attack : the attacker tries to forward unwanted packets or send unwanted requests which can conserve the battery of the nodes

Criteria for a secure routing protocol
The attack in adhoc network can be prevented by using a securing routing protocol. It should have following properties
1. Authenticity: when a routing table is updated, it must verify whether updates were provided by authenticated node.
2. Integrity of information : when a routing table is updated the information must be verified whether it is modified or not
3. In order updates: sequence numbers or some mechanism must be used to maintain updates in order.
4. Maximum update time: updates in routing table must be done as quickly as possible.
5. Authorization: only authorized nodes must be able to send update packets.

Wireless sensor network
WSNs, which can be considered as a special case of ad hoc networks with reduced or no mobility, are expected to find increasing deployment in coming years, as they enable reliable monitoring and analysis of unknown and untested environments. These networks are “data centric”, i.e., unlike traditional ad hoc networks where data is requested from a specific node, data is requested based on certain attributes such as,”which area has temperature over 35°C or 95°F”. Therefore a large number of sensors need to be deployed to accurately reflect the physical attribute in a given area.
Routing protocol design for WSNs is heavily influenced by many challenging factors, which must be overcome before efficient communication can be achieved. These challenges can be summarized as follows:
Ad hoc deployment – Sensor nodes are randomly deployed which requires that the system be able to cope up with the resultant distribution and form connections between the nodes. In addition, the system should be adaptive to changes in network connectivity as a result of node failure.
• Computational capabilities – Sensor nodes have limited computing power and therefore may not be able to run sophisticated network protocols leading to light weighted and simple versions of routing protocols.
• Energy consumption without losing accuracy – Sensor nodes can use up their limited energy supply carrying out computations and transmitting information in a wireless environment. As such, energyconserving forms of communication and computation are crucial as the node lifetime shows a strong dependence on the battery lifetime. In a multi-hop WSN, nodes play a dual role as data sender and data router. Therefore, malfunctioning of some sensor nodes due to power failure can cause significant topological changes and might require rerouting of packets and reorganization of the network.

Scalability – The number of sensor nodes deployed in the sensing area may be in the order of hundreds, thousands, or more. Any routing scheme must be scalable enough to respond to events and capable of operating with such large number of sensor nodes. Most of the sensors can remain in the sleep state until an event occurs, with data from only a few remaining sensors providing a coarse quality.
• Communication range – The bandwidth of the wireless links connecting sensor nodes is often limited, hence constraining inter sensor communication. Moreover, limitations on energy forces sensor nodes to have short transmission ranges. Therefore, it is likely that a path from a source to a destination consists of multiple wireless hops
Fault tolerance – Some sensor nodes may fail or be blocked due to lack of power, physical damage, or environmental interference. If many nodes fail, MAC and routing protocols must accommodate formation of new links and routes to the data collection BSs. This may require actively adjusting transmit powers and signaling rates on the existing links to reduce energy consumption, or rerouting packets through regions of the network where more energy is available. Therefore, multiple levels of redundancy may be needed in a fault tolerant WSN.
• Connectivity – High node density in sensor networks precludes them from being completely isolated from each other. Therefore, sensor nodes are expected to be highly connected. This, however, may not prevent the network topology from varying and the network size from shrinking due to sensor nodes failures. In addition, connectivity depends on the, possibly random, distribution of nodes.

Transmission media – In a multi-hop sensor network, communicating nodes are linked by a wireless medium. Therefore, the traditional problems associated with a wireless channel (e.g., fading, high error rate) also affect the operation of the sensor network. In general, bandwidth requirements of sensor applications will be low, in the order of 1-100 kb/s. As we have seen in Chapters 4 and 5 and in the previous section, the design of the MAC protocol is also critical in terms of conserving energy in WSNs.
• QoS – In some applications (e.g., some military applications), the data should be delivered within a certain period of time from the moment it is sensed, otherwise the data will be useless. Therefore, bounded latency for data delivery is another condition for time constrained applications.
• Control Overhead – When the number of retransmissions in wireless medium increases due to collisions, the latency and energy consumption also increases. Hence, control packet overhead increases linearly with the node density. As a result, tradeoffs between energy conservation, selfconfiguration, and latency may exist.
• Security – Security is an important issue which does not mean physical security, but it implies that both authentication and encryption should be feasible. But, with limited resources, implementation of any complex algorithm needs to be avoided. Thus, a tradeoff exists between the security level and energy consumption in a WSN.

Protocol stack for sensor network
The protocol stack of sensor network combines power efficiency and least cost path routing.
The architecture consist of
1. Physical layer
2. Data link layer
3. Network layer
4. Transport layer
5. Application layer
All these layers are backed by management plane, mobility management plane and task management plane.
Physical layer is responsible for transmitting and receiving signals.
* The data link layer consists of medium access control [MAC] which is used to prevent packet collision.
* The network layer is responsible for routing the packets
* The application layer is used for creation of packets by making use of software.
* The power management plane monitors the sensor’s power level among sensor node.

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