VTU 8TH SEM CSE ADHOC NETWORKS SOLVED PAPERS OF JUNE-2014 DEC-14 & JUNE-2015

SOLVED PAPERS

OF

ADHOC NETWORKS

(JUNE-2014, DECEMBER-2014 &

JUNE-2015)

For Solved Question Papers of UGC-NET/GATE/SET/PGCET in Computer Science, visit http://victory4sure.weebly.com/

ADHOC NETWORKS SOLVED PAPER JUNE/JULY 2014

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1 a. Briefly explain the following networks with proper diagrams: i) Wireless mesh networks.

ii) Hybrid wireless-networks. (10 Marks) Ans:

i) Wireless Mesh Network (WMN) • It can be formed to provide an alternate communication-infrastructure for mobile or fixed nodes,

→ without the spectrum reuse constraint & → without the requirement of network planning of cellular-network

• It provides many alternate paths for data-transfer between source & destination, which results in quick re-configuration of path when the existing path fails due to

node-failure (Figure 5.4). • Major advantages are:

1) High scalability 2) Easy extendibility 3) Support for a high data-rate

4) Low cost/bit 5) High availability &

6) Low cost of deployment • Since the infrastructure-built is in the form of small radio relaying-devices, the investment required is much less when compared to cellular-network counterpart.

• The possible deployment scenarios include: 1) Residential zones

2) Highways 3) Business zones 4) University campuses &

5) Important civilian regions • It should be capable of self-organization and maintenance.

• It operates at license-free ISM band around 2.4 GHz & 5 GHz.



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ii) Hybrid Wireless Networks • One of the major application s in the hybrid wireless architecture such as

1) Multi-hop Cellular-network (MCN) & 2)Integrated Cellular Adhoc Relay (iCAR). • Primary concept of cellular-networks is geographical channel-reuse.

• Following techniques increase the capacity of cellular-networks: 1) Cell sectoring 2) Cell resizing & 3) Multi-tier cells • MCN combines

→ reliability & support of fixed base-station of cellular-network with → flexibility & multi-hop relaying adhoc-networks (Figure 5.6).

• Major advantages: → higher capacity than cellular-networks due to the better channel reuse

→ increased flexibility & reliability in routing

→ better coverage & connectivity in holes of a cell can be provided by means of multiple hops through intermediate-nodes in a cell



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1 b. Discuss the major issues to be considered for successful adhoc wireless internet. (10 Marks)

Ans: 1. Gateways

• They are the entry points to the wired-internet. • Generally, they are owned & operated by a service-provider. • They perform following tasks

` 1) Bandwidth management 2) Load balancing

3) Traffic shaping 4) Packet filtering &

5) Address, service and location discovery

2. Address Mobility • This problem is worse here, as the nodes operate over multiple hops.

• Possible solution: use Mobile IP 3. Routing • It is a major problem due to

→ dynamic topological changes → presence of gateways

→ multi-hop relaying &

→ hybrid character of network • Possible solution: use separate routing-protocol for the wireless part of adhoc

wireless internet. 4. Transport Layer Protocol • Several factors are to be considered here, the major one being the state-

maintenance-overhead at the gateway-nodes. 5. Load Balancing

• It is essential to distribute the load so as to avoid the situation where the gateway-nodes become bottleneck-nodes. 6. Pricing/Billing

• Since internet-bandwidth is expensive, it is very important to introduce pricing/billing strategies for the adhoc-network.

7. Provisioning of Security • Security is a prime concern, since the end-users can utilize the adhoc-network to

make e-commerce transaction. 8. QoS Support • Provisioning of QoS-support is a very important issue because of

→ widespread use of VOIP (voice over IP) & → growing multimedia applications over internet.

9. Service, Address & Location Discovery • Service-discovery refers to the activity of identifying the party which provides the

service (or resource). • Address-discovery refers to the services such as those provided by ARP or DNS

operating within the wireless-domain. • Location-discovery refers to

→ detecting location of a particular mobile-node in network or

→ detecting geographical location of nodes.



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2 a. Discuss following main issues of designing a MAC protocol. (10 Marks) i) Quality of services (QoS)

ii) Hidden and exposed node problem. Ans:

i) Quality of Service (QoS) • This is essential for supporting time-critical traffic sessions. • The protocol should have resource reservation mechanism that takes into

considerations → nature of wireless-channel and

→ mobility of nodes • Due to the inherent nature of the adhoc-network, where trades are usually mobile

most of the time, providing QoS support to data sessions in such networks is very difficult.

• Bandwidth reservation made at one point of time may become invalid once the node moves out of the region where the reservation was made. ii) Hidden & Exposed Terminal Problems

• The hidden-terminal problem refers to the collision of packets at a receiving node due to the simultaneous transmission of those nodes that are not within the direct

transmission-range of the sender but are within the transmission-range of the receiver. • Collision occurs when both nodes transmit packets at the same time without

knowing about the transmission of each other.

• In figure 6.1, S1 and S2 are hidden from each other & they transmit simultaneously

to R1 which leads to collision. • The exposed-terminal problem refers to the inability of a node, which is blocked due

to transmission by a nearby transmitting node, to transmit to another node. • If S1 is already transmitting to R1, then S3 cannot interfere with on-going transmission & it cannot transmit to R2.

• Hidden & exposed terminal problems reduce the throughput of a network when traffic load is high.



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2 b. Explain in detail the receiver initiated MAC protocol MARCH. (10 Marks) Ans:

Media Access with ReduCed Handshake protocol (MARCH) • It is a receiver-initiated protocol.

• It doesn’t require any traffic prediction mechanism. • It exploits the broadcast-nature of traffic from omni-directional-antennas to reduce the number of handshakes involved in the data transmission.

• The RTS is used only for the first packet of the stream. From the second packet onward, only the CTS is used.

• A node obtains information about the data-packet arrivals at its neighbouring nodes by overhearing the CTS-packets transmitted by them (Figure 6.13). • It then sends a CTS-packet to the concerned neighbour-node for relaying data from

that node. • The CTS-packet carries the MAC addresses of the sender and the receiver-node and

the route identification number (RTid) for that flow. • The RTid is used by nodes in order to avoid misinterpretation of CTS-packets and initiation of false CTS.

• Advantages: → The throughput of MARCH is significantly high compared to MACA.

→ Control-overhead is much less. → Average end-to-end delay in packet delivery is very low compared to MACA.

→ Less bandwidth is consumed for control traffic.



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3 a. Give brief explanation of Distributed Wireless Ordering Protocol(DWOP). (10 Marks)

Ans: Distributed Wireless Ordering Protocol (DWOP)

• Packets access the medium according to order specified by an ideal reference scheduler such as FIFO (or earliest deadline first). • In FIFO, packet priority-indices are set to the arrival-times of packets.

• Each node builds up a scheduling-table (ST) ordered according to the overheard arrival-times.

• It may not suffer due to information asymmetry (Figure 6.26). • Control-packets (RTS/CTS) are used to piggy-back priority-information regarding HOL-packets of nodes.

• Key concept: A node is made eligible to contend for the channel only if its locally queued packet has a smaller arrival-time compared to all other arrival-times in its ST.

• Following 2 additional table management techniques are used in order to keep the actual schedule close to the reference FIFO:

A) Receiver Participation Mechanism

1) When receiver finds that the source is transmitting out-of-order (i.e. the reference FIFO schedule is being violated), an out-of-order notification (OON) is

piggy-backed by the receiver on the control-packets (CTS/ACK) and it sends to the source.

2) On receiving this OON, the source goes into a back-off state after completing the transmission of its current packet. 3) The back-off period Tback-off is given by

Tback-off=R*(EIFS+DIFS+Tsuccess+CWmin) where Tsuccess=longest possible time required to transmit a

packet successfully. B) Stale Entry Elimination

1) This makes sure that the STs are free of stale entries.

2) An entry is deleted from the ST only after an ACK packet for the corresponding entry is heard by the node.



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3 b. Describe the working mechanism of the DBTMA protocol. (10 Marks) Ans:

Dual Busy Tone Multiple Access protocol (DBTMA) • The transmission-channel is divided into following 2 channels:

1) Data-channel: used for data-packet transmission 2) Control-channel: used for RTS, CTS & busy-tones(BTs).

• Use following 2 busy-tones on the control-channel:

1) BTt: indicate that it is transmitting on the data-channel 2) BTr: indicate that it is receiving on the data-channel.

• 2 busy-tone signals are 2 sine waves at different frequencies (Figure 6.9). • When a node is ready to transmit a data-packet, following events occur:

→ First, source senses the channel to determine whether BTr signal is active.

→ If there is no BTr signal, then source transmit RTS.

→ On receiving RTS, receiver checks whether the BTt tone is active. → If there is no BTt signal, receiver sends CTS and turns on the BTr signal

→ Sender receives CTS, turns on BTt signal, starts data transmission and turns off BTt signal.

→ Receiver receives data and turn off BTr signal. • DBTMA has better network utilization than RTS/CTS based protocol.



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4 a. What are the characteristics of an ideal routing-protocol for Adhoc-networks? (6 Marks)

Ans: 1. The protocol must be fully distributed as centralized routing involves high

control-overhead and hence is not scalable. 2. It must be adaptive to frequent topology changes caused by mobility of nodes.

3. Route-computation & maintenance must involve a minimum no. of nodes. 4. It must be localized, as global state maintenance involves a huge state

propagation control-overhead. 5. It must be loop-free and free from state routes. 6. It must converge to optimal routes once network-topology becomes stable.

7. It must optimally use scarce resources such as bandwidth, computing power, memory, and battery-power.

8. It should be able to provide a certain level of quality of service (QoS) as demanded by the applications. 9. The number of packet collisions must be kept to a minimum by limiting the

number of broadcasts made by each node. 10 Every node in the network should try to store information regarding the

stable local topology only.

4 b. Give the classification of routing-protocols for Adhoc-networks, based on the routing information update mechanism. (6 Marks) Ans:

Figure 4.1: Classification of routing protocols based on update mechanism

i) Proactive or Table-Driven Routing Protocols • Every node maintains the topology-information in the routing-tables by periodically

exchanging routing-information. • Routing-information is generally flooded in the whole network. • Whenever a node requires a path to a destination, it runs an appropriate path-

finding algorithm. ii) Reactive or On-Demand Routing Protocols

• Do not maintain the network-topology-information. • Obtain necessary path when it is required, by using a connection-establishment process.

iii) Hybrid Routing Protocols • Combine the best features of the above 2 categories.

• Nodes within a certain distance from the node concerned, or within a particular geographical region, are said to be within the routing-zone of the given node.

• For routing within the zone, a table-driven approach is used. For nodes that are located beyond the zone, an on-demand approach is used.



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4 c. Explain in detail CGSR protocol (8 Marks). Ans:

Cluster-head Gateway Switching Routing protocol (CGSR) • It uses a hierarchical network-topology (Figure 7.9).

• It organizes nodes into clusters, with coordination among the members of each cluster. • The cluster-head is elected dynamically by employing a least cluster change (LCC)

algorithm. • LCC algorithm states that

→ A node ceases to be a cluster-head only if it comes under the range of another cluster-head, where the tie is broken either using the lowest ID or

highest connectivity algorithm • A token-based scheduling is used within a cluster for sharing the bandwidth among the members of the cluster.

• All communication passes through the cluster-head. Communication between 2 clusters takes place through the gateways.

• The gateways are common member-nodes that are members of both the cluster. • A gateway is expected to be able to listen to multiple spreading-codes that are currently in operation in the clusters.

• A gateway-conflict is said to occur when a cluster-head issues a token to a gateway over spreading-code while the gateway is tuned to another code.

• The performance of routing is influenced by → token-scheduling at cluster-heads

→ code-scheduling at gateways • Every member-node maintains a routing-table containing the destination cluster-

head for every node in the network. In addition, each node maintains a routing-table which keeps the list of next-hop nodes for reaching every destination cluster.

Advantages • It enables partial coordination between nodes by electing cluster-heads. Hence,

better bandwidth utilization is possible. • It is easy to implement priority scheduling schemes with token-scheduling and

gateway code-scheduling. Disadvantages • It increases in path-length and instability in the system at high mobility when the

rate of change of cluster-heads is high. • To avoid gateway conflicts, more resources are required.



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5 a. Explain Zone Routing-protocol (ZRP). (10 Marks) Ans:

Zone Routing Protocol (ZRP) • It combines the best features of both

1) Proactive routing-protocols & 2) Reactive routing-protocols • It uses a proactive routing scheme within a limited zone in the r-hop neighborhood

of every node. It uses a reactive routing scheme for nodes beyond the limited zone.

• Proactive routing protocol used in the limited zone is referred to as IntrA-zone Routing-Protocol (IARP). The reactive routing-protocol used beyond the limited zone is referred to as

IntEr-zone Routing-Protocol (IERP). • The routing zone of a given node is a subset of the network, within which all nodes

are reachable within less than or equal to (Figure 7.26). • Route-establishment is carried as follows: → When a node s (node 8 in the figure 7.27) has packets to be sent to a

destination= d (node 15 in fig), it checks whether node d is within its zone. → If destination belongs to its own zone, then it delivers the packets directly.

→ Otherwise, node s broadcasts the RouteRequest to its peripheral nodes (in

fig, node 8 broadcasts RouteRequest to node 2, 3, 5, 7, 9, 10, 13, 14 and 15). → If any peripheral node finds node d to be located within its routing zone, it

sends a RouteReply back to node 8 indicating the path; otherwise, the node rebroadcasts the RouteRequest packet to the peripheral nodes. • During RouteRequest propagation, every node that forwards the RouteRequest

appends its address to it. This information is used for delivering the RouteReply packet back to the source.

• The criteria for selecting the best path may be 1) Shortest path or 2) Least delay path



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• Route-maintenance is carried as follows: → When an intermediate-node in an active path detects a broken-link in the

path, it performs a local path reconfiguration in which the broken-link is bypassed by means of a short alternate path connecting ends of broken-link.

→ A path update message is then sent to the sender node.

Advantage • Reduce the control-overhead by combining the best features of proactive and

reactive protocols. Disadvantage

• Control-overhead may increase due to the large overlapping of nodes routing zones.



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5 b. Discuss the power-aware routing-metrics for adhoc-networks. (10 Marks)

Ans: Power Aware Routing protocol (PAR)

• The limitation on the availability of power for operation is a significant bottleneck. Hence, the use of routing-metrics → contributes to the efficient utilization of energy and

→ increases the lifetime of the network.

1. Minimal Energy Consumption per packet

• This metric aims at minimizing the power consumed by a packet in traversing from

source-node to the destination-node.

• The energy consumed by a packet when traversing through a path is the sum of the

energies required at every intermediate-hop in that path.

• This metric doesn’t balance the load.

• Disadvantages:

→ Selection of path with large hop-length

→ Inability to measure the power-consumption in advance

→ Inability to prevent the fast discharging of batteries at some nodes

2. Maximize Network Connectivity

• This metric attempt to balance the routing load among the cut-set (the subset of the nodes in the network, the removal of which results in network-partitions). • It is difficult to achieve a uniform battery draining rate for the cut set.

3. Maximum Variance in Node Power Levels

• This metric proposes to distribute the load among all nodes in the network so that the power-consumption pattern remains uniform across them.

• This problem is very complex when the rate and size of the data-packets vary.

4. Minimum Cost per packet

• In order to maximize the life of every node in the network, this metric is made as a

function of the state of the node’s battery.

• A node’s cost decreases with an increase in its battery change and vice versa.

• Cost of node can be easily computed.

• Advantage: congestion handling & cost calculation.

5. Minimize Maximum Node Cost

• This metric minimizes the maximum cost per node for a packet

→ after routing a number of packets or

→ after a specific period

• This delays the failure of a node, occurring due to higher discharge because of

packet forwarding.



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6 a. Explain the issues and design goals of transport-layer protocol for adhoc-networks. (10 Marks)

Ans: Issues of Transport Layer Protocol

1. Induced Traffic • In a path having multiple link, the traffic at any given link (or path) due to the traffic through neighboring links (or paths) is referred to as induced traffic.

• This is due to → broadcast-nature of the channel &

→ location-dependent contention on the channel. • This affects the throughput achieved by the protocol.

2. Induced Throughput Unfairness • This refers to the throughput unfairness at the transport-layer due to the

throughput (or delay) unfairness existing at the lower layer such as the network and MAC layers. • A transport-layer should consider these in order to provide a fair share of

throughput across contending flows 3. Separation of Congestion Control, Reliability and Flow Control

• The protocol can provide better performance if reliability, flow-control and congestion-control are handled separately. • Reliability and flow-control are end-to-end activities,

whereas congestion-control can at times be a local activity. • Objective: minimization of the additional Control-overhead generated by

them. 4. Power & Bandwidth Constraints • Nodes face resource constraints including the two most important resources:

1) Power source & 2) Bandwidth

• The performance of a protocol is significantly affected by these resource constraints.

5. Interpretation of Congestion • Interpretation of network congestion as used in traditional networks is not appropriate in adhoc-networks.

• This is because following parameters can also lead to packet-loss: → high error-rates of wireless-channel

→ location-dependent contention → hidden-terminal problem

→ packet-collisions in the network → path-breaks due to mobility of nodes and

→ node-failure due to drained battery

6. Completely Decoupled Transport Layer • Another challenge faced by transport-layer protocol is the interaction with the lower layers.

• Cross-layer interaction between the transport-layer and lower layers is important to adapt to the changing network environment.

7. Dynamic Topology • Experience rapidly changing network-topology due to mobility of nodes. • This leads to

→ frequent path-breaks → partitioning and re-merging of networks &

→ high delay in re-establishment of paths.

• Performance is affected by rapid changes in network-topology.



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Design Goals of Transport Layer Protocol 1) The protocol should maximize the throughput per connection.

2) It should provide throughput fairness across contending flows. 3) It should incur minimum connection set up and connection maintenance

overheads. 4) It should have mechanisms for congestion-control and flow control in the network.

5) It should be able to provide both reliable and unreliable connections as per the requirements of the application layer.

6) It should be able to adapt to the dynamics of the network such as rapid changes in topology. 7) It should be aware of resource-constraints such as battery-power and buffer

sizes and make efficient use of them. 8) It should make use of information from the lower layers for improving

network throughput. 9) It should have a well-defined cross-layer interaction framework. 10) It should maintain end-to-end semantics.

11) Bandwidth must be used efficiently.



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6 b. Illustrate the working of split TCP with a neat diagram. (10 Marks) Ans:

Split TCP • Major issues that affect the performance of TCP: the degradation of throughput with

increasing path-length (Figure 9.9). • Split TCP provides a unique solution to this problem by splitting the transport-layer objectives into:

→ congestion-control → end-to-end reliability

• In addition, split TCP splits a long TCP-connection into a set of short concatenated TCP-connections (called segments or zones) with a number of selected intermediate-

nodes (known as proxy-nodes) as terminating points of these short connections. • A proxy-node

→ receives the TCP packets → reads packet’s contents

→ stores the content in its local buffer and → sends Local ACKnowledgement(LACK) to the source (or the previous proxy).

• LACK does not guarantee end-to-end delivery (Figure 9.9). • The responsibility of further delivery of packets is assigned to the proxy-node.

• The no. of proxy-nodes in a session is determined by the length of the path between → source-node &

→ destination-node • Based on a distributed algorithm, the intermediate-nodes that receive packets

determine whether to act as a proxy-node or just as a simple forwarding node.

Advantages • Improved throughput.

• Improved throughput fairness. • Lessened impact of mobility.

Disadvantages • Requires modifications to TCP protocol. • End-to-end connection handling of traditional TCP is violated.

• The failure of proxy-nodes can lead to throughput degradation.



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7 a. Briefly discuss the network security requirements for adhoc-networks. (5 Marks)

Ans: Requirements in Security Provisioning

1. Confidentiality • The data sent by the sender must be understandable only to the intended-receiver.

• Though an intruder might get hold of the data being sent, he must not be able to derive any useful information out of the data.

• Data encryption can be used to ensure confidentiality. 2. Integrity • The data sent by the source-node should reach the destination-node without

being altered. • It should not be possible for any malicious-node to tamper with the data

during transmission 3. Availability • The network should remain operational all the time.

• The network must be → robust enough to tolerate link-failures &

→ capable of surviving various attacks mounted on it. • The network should be able to provide guaranteed services whenever an

authorized-user requires them. 4. Non Repudiation

• It is a mechanism to guarantee → that the sender of a message cannot later deny having sent the message

→ that the recipient cannot deny having received the message • Digital signatures can be used to ensure non-repudiation.



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7 b. List & explain the network-layer attacks. (5 Marks) Ans:

1. Wormhole Attack • An attackers

→ receive packets at one location in the network & → tunnel them to another location in the network, where the packets are

resent into the network. This tunnel between 2 colliding attackers is referred to as a wormhole.

• If proper mechanisms are not employed to defend the network against wormhole attacks, existing routing-protocols for adhoc-networks may fail to find valid routes. 2. Blackhole Attack

• A malicious-node falsely advertises good paths to destination-node during path-finding process.

• The intention of malicious-node could be → to hinder the path-finding process or

→ to intercept all data-packets being sent to the destination-node 3. Byzantine Attack

• A set of compromised-nodes work in collusion & carries out attack such as → creating routing loops

→ routing packets on non-optimal paths & → selectively dropping packets

4. Information Disclosure • A compromised-node may leak confidential information to unauthorized-nodes in

the network. 5. Resource Consumption Attack • A malicious-node tries to consume (or waste) resources of other nodes present in

the network. • The resources targeted are:

1) Bandwidth 2) Battery-power 3) Computational-power 6. Routing Attacks

i. Routing-table Overflow

• Adversary-node advertises routes to non-existent nodes, to the authorized-nodes present in the network.

• The main objective is to cause an overflow of routing-tables, which would in turn prevent the creation of entries corresponding to new routes to authorized-nodes.

ii. Routing-table Poisoning • The compromised-nodes

→ send wrong routing updates or → modify genuine route update packets

• This may result in → sub-optimal routing

→ congestion in network or → even make some parts of network inaccessible

iii. Packet Replication • An adversary-node would replicate state packets.

iv. Route Cache Poisoning • Similar to routing-table poisoning, an adversary can also poison the route

cache to achieve similar activities. v. Rushing Attack • On-demand routing-protocols that use duplicate suppression during the route

discovery process are vulnerable to this attack.



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7 c. Explain the Security-aware Adhoc Routing (SAR) protocol. (10 Marks) Ans:

Security-aware Adhoc Routing protocol (SAR) • This uses security as one of the key metrics in path finding.

• In adhoc-networks, communication between end-nodes through possibly multiple intermediate-nodes is based on the fact that the two end-nodes trust the intermediate-nodes.

• This defines level of trust → as a metric for routing &

→ as one of the attributes for security to be taken into consideration while routing

• Two paths exist between the two officers O1 and O2 who want to communicate with each other (Figure: 9.14).

• One of these paths is a shorter path which runs through private-nodes whose trust levels are very low. • Hence, the protocol chooses a longer but secure path which passes through other

secure-nodes. • Nodes of equal levels of trust distribute a common key among themselves and with

those nodes having higher levels of trust. • This could be incorporated into both on-demand and table-driven routing-protocols

• This allows the application to choose the level of security it requires. But the protocol requires different keys for different levels of security.

This tends to increase number of keys required when the number of

security levels used increase.



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8 a. Give the layer wise classification of existing QoS solution. (5 Marks) Ans:

1. MAC Layer Solutions • The existing MAC protocols for adhoc-networks use

→ channel sensing and → random back-off schemes. This make them suitable for best-effort data traffic

• In most cases, adhoc-networks share a common radio-channel operating in the ISM band2 or in military bands (Figure 10.13).

• The most widely deployed medium access technology is the IEEE 802.11 standard. •The 802.11 standard has two modes of operation:

1. Distributed Coordination Function (DCF) mode: provides best-effort

service. 2. Point Coordination Function (PCF) mode: has been designed to provide

real-time traffic support in infrastructure-based wireless-network configurations.

2. Network Layer Solutions

• The bandwidth reservation and real-time traffic support capability of MAC proto-cols can ensure reservation at the link level only, hence the network-layer support for

ensuring end-to-end resource negotiation, reservation, and reconfiguration is very essential. 3. QoS Frameworks

•A framework for QoS is a complete system that attempts to provide required/promised services to each user or application.

• All components within this system co-operate in providing the required services. • The key component of any QoS framework is the QoS service model which defines

the way user requirements are met. • The key design issue here is whether to serve users → on a per session-basis or

→ on a per class-basis



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8 b. Write a short note on: (15 Marks) i) Hard state versus soft state resource reservation.

ii) Cluster TDMA Ans:

i) Hard-state Resource Reservation • Resources are reserved at all intermediate-nodes along the path from the source to the destination throughout the duration of the QoS session.

• If the path is broken due to network dynamics, the reserved resources have to be explicitly released by a de-allocation mechanism.

• This mechanism → introduces additional control-overhead

→ may also fail to release resources. • The hard state schemes reserve resources explicitly and hence, at high

network loads, the call blocking ratio will be high. Soft-state Resource Reservation

• This maintains reservations only for small time intervals.

• The reservations get refreshed if packets belonging to the same flow are received before the time-out period.

• The timeout period can be equal to → packet inter-arrival time or

→ a multiple of the packet inter-arrival time. • If no data packets are received for the specified time interval, the resources

are de-allocated in a decentralized manner without incurring any additional control-overhead. Thus no explicit teardown is required for a flow. • The soft state schemes provide high call acceptance at a gracefully degraded

fashion. ii) Cluster TDMA

• It is used for supporting real-time traffic in adhoc-networks. • In bandwidth-constrained adhoc-networks, the limited resources available need to be managed efficiently. To achieve this goal, cluster TDMA is used.

• Nodes are split into different groups. • Each group has a cluster-head which acts as

→ a regional broadcast-node and → a local coordinator to enhance the channel throughput.

• Every node within a cluster is one hop away from the cluster-head. • The formation of clusters and selection of cluster-heads are done in a distributed

manner. • Clustering algorithms split the nodes into clusters so that they are interconnected and cover all the nodes.

• Three algorithms are used: 1. Lowest-ID Algorithm

• A node becomes a cluster-head if it has lowest ID among all its neighbors. 2. Highest-degree Algorithm

• A node with a degree greater than the degrees of all its neighbors becomes the cluster-head. 3. Least Cluster Change (LCC) Algorithm

• Cluster-head change occurs only if a change in network causes two cluster- heads to come into one cluster or one of the nodes moves out of the range of

all the cluster-heads. (degree refers to number of neighbors which are within transmission-range of a node) • The TDMA scheme is used within a cluster for controlling access to the channel.

• Further, it is possible for multiple sessions to share a given TDMA slot via code division multiple access (CDMA).



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• Across clusters, following methods can be used to reduce the effect of inter-cluster interference

→ spatial reuse of the time-slots or → different spreading-codes

• A synchronous time division frame is defined to → support TDMA access within a cluster and

→ exchange control-information.

• Each synchronous time division frame is divided into slots. • Slots and frames are synchronized throughout the network. • A frame is split into 2 phases:

1. Control Phase: Here, following control-functions are done: → routing

→ clustering → code assignment

→ power management

→ virtual circuit (VC) setup → frame and slot synchronization

2. Data Phase: This supports both real-time and best-effort traffic.



ADHOC NETWORKS SOLVED PAPER DEC.2014/JAN.2015

1

1 a. Differentiate between Cellular Network and Adhoc-network. (8 Marks) Ans:



2

1 b. Explain wireless sensor-network. (6 Marks) Ans:

Wireless Sensor Networks • These are used to provide a wireless-communication among the sensors used in a

specific application domain. • Sensor-nodes are tiny devices that have capability to

→ sense physical parameters

→ process the gathered-data &

→ communicate to the monitoring-system. • The issues that make sensor-network a distinct category of adhoc-network are the

following: 1. Node Mobility • Mobility of nodes is not a mandatory-requirement in sensor-networks.

• For example: → node used for periodic monitoring of soil property is not required to be

mobile → node fitted on body of a patient is designed to support partial mobility

2. Network Size • The number of nodes in sensor-network can be much larger than that in a

typical adhoc-network. 3. Density of Deployment • The density of nodes varies with the domain of application.

• For example: → military applications require high availability of network, which makes

redundancy a high priority. 4. Power Constraints

• The power-constraints in sensor-networks are much more severe than those in adhoc-networks. This is because → the nodes are expected to operate in harsh environmental conditions,

with minimum human supervision & maintenance. • In certain case, the recharging of the energy-source is impossible.

• Running a sensor-network demands very efficient protocol at → network-layer

→ data link layer → physical layer

• Classification of power-sources: 1. Replenishable Power-source

• Power-source can be replaced when existing source is fully drained. 2. Non-replenishable Power-source

• Power-source cannot be replenished once the n/w has been deployed. • The replacement of node is the only solution. 3. Regenerative Power-source

• Power-source have the capability of regenerating power from the physical parameter under measurement.

5. Data/Information Fusion • Data-fusion refers to aggregation of multiple packets into one before relaying it.

• Data-fusion is used → to reduce bandwidth consumed by redundant headers of packets &

→ to reduce delay involved in transmitting multiple packets. • Information-fusion is used

→ to process sensed-data at intermediate-nodes & → to relay the outcome to the monitoring-system.



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6. Traffic Distribution • The communication traffic pattern varies with the domain of application.

For example: → Environmental sensing application generates short packets indicating

status of environmental parameter. This kind of traffic requires low bandwidth.

→ Military applications generally carry user traffic such as digitized voice stream. This kind of traffic requires high bandwidth.



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1 c. Explain Adhoc wireless internet with a neat diagram. (6 Marks) Ans:

Adhoc Wireless Internet • Adhoc wireless internet extends the services of the internet to the end-users over

an adhoc-network (Figure 5.7). • Some applications are:

1) Wireless mesh network

2) Temporary internet-services to major conference/sport venues 3) Temporary military settlements in battlefields &

4) Broadband internet-services in rural regions

Figure 5.7 Schematic diagram of adhoc wireless internet

• The major issues to be considered are:

1. Gateways • They are the entry points to the wired-internet.

• Generally, they are owned & operated by a service-provider. • They perform following tasks:

` 1) Bandwidth management 2) Load balancing

3) Traffic shaping 4) Packet filtering 5) Address, service & location discovery

2. Address Mobility • This problem is worse here, as the nodes operate over multiple hops. • Possible solution: use Mobile IP

3. Transport Layer Protocol • Several factors are to be considered here, the major one being the state-

maintenance-overhead at the gateway-nodes. 4. Load Balancing

• It is essential to distribute the load so as to avoid the situation where the

gateway-nodes become bottleneck-nodes.



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5. Routing • It is a major problem due to

→ dynamic topological changes → presence of gateways

→ multi-hop relaying &

→ hybrid character of network • Possible solution: use separate routing-protocol for the wireless part of adhoc

wireless internet. 6. Pricing/Billing

• Since internet-bandwidth is expensive, it is very important to introduce

pricing/billing strategies for the adhoc-network. 7. Provisioning of Security

• Security is a prime concern, since the end-users can utilize the adhoc-network to make e-commerce transaction.

8. QoS Support

• Provisioning of QoS-support is a very important issue because of → Widespread use of VOIP (voice over IP) &

→ Growing multimedia applications over internet. 9. Service, Address & Location Discovery

• Service-discovery refers to the activity of identifying the party which provides the service (or resource).

• Address-discovery refers to the services such as those provided by ARP or DNS operating within the wireless-domain. • Location-discovery refers to

→ detecting location of a particular mobile-node in network or → detecting geographical location of nodes.



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2 a. Discuss issues in designing MAC protocol for adhoc-networks.(10 Marks) Ans:

1. Bandwidth Efficiency • It is defined as the ratio of the bandwidth utilized for data transmission to the total

available bandwidth. • Bandwidth must be utilized in efficient manner. • Control-overhead must be kept as minimal as possible.

2. Quality of Service support • This is essential for supporting time-critical traffic-sessions.

• The protocol should have resource reservation mechanism that takes into considerations

1) Nature of wireless-channel and

2) Mobility of nodes 3. Synchronization

• This is very important for bandwidth (time-slot) reservation by nodes. • The protocol must consider synchronization between nodes in the network. • Exchange of control-packets may be required for achieving time-synchronization

among nodes. 4. Hidden and Exposed Terminal Problems

• The hidden-terminal problem refers to the collision of packets at a receiving-node due to the simultaneous transmission of those nodes that are not within the direct

transmission-range of the sender but are within the transmission-range of the receiver. • Collision occurs when both nodes transmit packets at the same time without

knowing about the transmission of each other.

• In figure 6.1, S1 and S2 are hidden from each other & they transmit simultaneously

to R1 which leads to collision. • The exposed-terminal problem refers to the inability of a node, which is blocked due to transmission by a nearby transmitting node, to transmit to another node.

• If S1 is already transmitting to R1, then S3 cannot interfere with on-going transmission & it cannot transmit to R2.

• Hidden & exposed-terminal problems reduce the throughput of a network when traffic load is high.



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5. Error-prone Shared Broadcast Channel • When a node is receiving data, no other node in its neighborhood (apart from the

sender) should transmit. • A node should get access to the shared medium only when its transmission do not

affect any ongoing session. • The protocol should grant channel access to nodes in such a manner that collisions are minimized.

• Protocol should ensure fair bandwidth allocation. 6. Distributed Nature

• There is no central point of coordination due to the mobility of the nodes. • Nodes must be scheduled in a distributed fashion for gaining access to the channel. 7. Mobility of Nodes

• Nodes are mobile most of the time • The protocol design must take this mobility factor into consideration so that the

performance of the system is not affected due to node mobility.



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2 b. Define Five-phase Reservation protocol and explain five-phase reservation protocol. (10 Marks)

Ans: Five Phase Reservation Protocol (FPRP)

• It is a single-channel TDMA-based broadcast scheduling-protocol. • The protocol is fully distributed i.e. multiple reservations can be simultaneously made throughout the network (Figure 6.21).

• Time is divided into 2 frames:1) Reservation frame (RF) & 2) Information frame (IF) • Each RF has N reservation-slots (RS).

Each IF has N information-slots (IS). Each RS has M reservation-cycles (RCs).

• In order to reserve an IS, a node needs to contend during the corresponding RS. • Based on these contentions, a TDMA schedule is generated in the RF and is used in the subsequent IFs until the next RF.

• During the corresponding IS, a node would be in one of the 3 states: 1. Transmit (T) 2. Receive (R) 3. Blocked (B)

• The reservation takes place in following 5 phases: 1. Reservation Request Phase • A source-node sends reservation-request (RR) packet to the destination-

node. 2. Collision Report Phase

• If a collision is detected by any node during the reservation-request phase, then that node broadcasts a collision-report (CR) packet. 3. Reservation Confirmation Phase

• A source-node is said to have won the contention for a slot if it does not receive any CR messages in the previous phase.

• Then, the source-node transmits a reservation-confirmation (RC) message to the destination-node.

4. Reservation Acknowledgment Phase • The destination-node acknowledges reception of the RC by sending back a reservation acknowledgment (RA) message to the source-node.

• The hidden nodes that receive this message defer their transmissions during the reserved slot.

5. Packing & Elimination (P/E) Phase • Two types of packets are: 1) Packing packet (PP) & 2) Elimination packet • A PP is sent by each node that is located within 2 hops from a TN, and that

had made a reservation since the previous P/E phase. • A node receiving a PP understands that there has been a recent success in

slot reservation 3 hops away from it.



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3 a. Explain directional busy-tone-based MAC protocol in detail. (6 Marks) Ans:

Directional Busy Tone based MAC Protocol • The nodes use directional-antennas for transmitting & receiving data-packets,

thereby reducing their interference to other neighbor-nodes. This leads to an increase in the throughput of the system. • The purpose of the busy-tones (BTs) is as follows:

1) Before transmitting an RTS, the source makes sure that the BTr tone is not active in its neighborhood, so that its transmissions do not interfere with

packets being received at a neighboring receiver. Similarly, a receiver, before transmitting CTS, verifies that a BTt is not active in its neighborhood.

2) The directional busy-tones can permit simultaneous transmissions in the neighborhood of a source or a receiver.

• The protocol works as follows (Figure 6.30): 1) A source transmits an RTS addressed to the receiver on all its antennas (omni-directional transmission).

2) On receiving this RTS, the receiver determines the antenna-element on which the RTS is received with maximum gain.

3) The receiver then sends back a directional-CTS (DCTS) to the source using the selected antenna-element. It also turns on busy-tone BTr in the direction

towards source. 4) On receiving the CTS, the source turns on busy-tone BTt in the direction towards receiver.

5) Once the packet transmission is over, the source turns off the BTt signal. 6) After receiving the Data-packet, the receiver turns off the BTr signal.

(For a uni-cast transmission, only a single antenna element is used. For broadcast transmission, all the N antenna elements transmit simultaneously).



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3 b. Explain interleaved carrier-sense multiple access. (6 Marks) Ans:

Interleaved-Carrier Sense Multiple Access protocol (I-CSMA) • It efficiently overcomes the exposed-terminal problem.

• Consider figure 6.38. Here, when a transmission is going on from node A to node B, nodes C and F would not be permitted to transmit to nodes D and E respectively. • Node C is called a sender-exposed node, and node E is called a receiver-exposed

node. • The total available bandwidth is split into two equal channels (say, channel-1 and

channel-2). • The handshaking process is interleaved between the two channels, hence the name interleaved carrier-sense multiple access.

• The protocol works as follows (Figure 6.39): 1) The source transmits the RTS on channel-1.

2) On receiving RTS, the receiver checks its E-NAV and finds out whether free time-slots are available. It sends the CTS only if free slots are available. (Each node maintains a data structure called extended network allocation vector).

3) On receiving the CTS, the source transmits the DATA on channel-1. 4) The receiver responds with the ACK on channel-2.

• The performance improvement is attributed to the following facts: 1) Nodes that hear RTS in a particular channel (say channel-1) and do not hear

the corresponding CTS on the other channel (channel-2) conclude that they are only sender-exposed in channel-1. Therefore, if they have packets to send, they can use channel-1 to transmit RTS to other nodes.

2) Nodes that hear only the CTS in a particular channel (say channel-1) and had not heard the corresponding RTS on the other complementary channel

(channel-2) realize that they are only receiver-exposed on channel-1 to the on-going transmission. If they receive any RTS on channel-2, they would not refrain from sending CTS on channel-1 for the received RTS.



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3 c. Write a note on multi-channel MAC protocol. (8 Marks) Ans:

Multi-channel MAC protocol (MMAC) • Each node maintains a data-structure called PCL (PreferableChannelList).

• PCL contains the usage of the channels within the transmission-range of the node. • Based on channel-usage, the channels can be classified into 3 types:

1) High Preference Channel (HIGH)

• The channel has been selected by the current-node and is being used by the node in the current beacon-interval.

2) Medium Preference Channel (MID) • The channel is free and is not being currently used in the transmission-range of the node.

3) Low Preference Channel (LOW) • The channel is already being used in the transmission-range of the node by

other neighboring nodes. • A counter is associated with each LOW state channel.

• Time is divided into beacon-intervals.

• Every node is synchronized by periodic beacon transmissions. • At the start of every beacon-interval, there exists a time interval called the adhoc

traffic indication messages (ATIM) window. • ATIM window is used by the nodes to negotiate for channels for transmission during

the current beacon-interval. • The protocol works as follows (Figure 6.34):

1) A source sends an ATIM to the intended receiver. The ATIM carries the PCL

of the source. 2) On receiving the ATIM, the receiver uses the PCL carried on the ATIM and its

own PCL to select a channel. It includes this channel information in the ATIM-ACK packet & sends to the source. 3) Then, source determines whether it can transmit on the channel indicated in

the ATIM-ACK message. If so, it responds by sending the receiver an ATIM-RES(reservation) packet.

4) At the end of the ATIM window, the source and receiver switch to the agreed-upon channel and start communicating by exchanging RTS/CTS.



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• If a receiver R receives an ATIM packet from a source S, it selects a channel as below:

1) If there exists a HIGH state channel in the node R's PCL, then that channel is selected.

2) If there exists a HIGH state channel in the PCL of node S, then that channel is selected. 3) If there exists a common MID state channel in the PCLs of both node S and

node R, then that channel is selected. 4) If there exists a channel which is in the MID state at only one of the two

nodes, then that channel is chosen. 5) If all channels in both PCLs are in the LOW state, the counters of the corresponding channels at nodes S and R are added, and the channel with the

least count is selected.



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4 a. Discuss the issues in designing a routing-protocol for Adhoc-network. (8 Marks)

Ans: 1. Mobility

• Network-topology is highly dynamic due to movement of nodes. Hence, an ongoing session suffers frequent path-breaks.

• Disruption occurs due to the movement of either

→ intermediate nodes in the path or

→ end-nodes.

• Wired-network routing-protocols cannot be used in adhoc-networks because

→ nodes are here are not stationary and

→ convergence is very slow in wired-networks.

• Mobility of nodes results in frequently changing network topologies

• Routing-protocols must be able to perform efficient and effective mobility

management. 2. Bandwidth Constraint • Abundant bandwidth is available in wired-networks due to

→ advent of fiber optics and

→ exploitation of wavelength division multiplexing (WDM) technologies.

• In a wireless-network, the radio-band is limited, and hence the data-rates it can offer are much less than what a wired-network can offer.

• This requires that the routing-protocols use the bandwidth optimally by keeping the overhead as low as possible.

3. Hidden & Exposed Terminal Problems • The hidden terminal problem refers to the collision of packets at a receiving-node due to the simultaneous transmission of those nodes that are not within the direct

transmission-range of the receiver, but are within the transmission-range of the receiver (Figure 6.1).

• Collision occurs when both nodes transmit packets at the same time without knowing about the transmission of each other.



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4. Error-prone Shared Broadcast Radio-channel • The broadcast-nature of the radio-channel poses a unique challenge in adhoc-networks.

• The wireless-links have time-varying characteristics in terms of

1) Link capacity and

2) Link-error probability

• This requires that the routing-protocol interact with the MAC layer to find alternate routes through better quality links.

• Transmissions in adhoc-networks result in collisions of data- and control-packets.

• Therefore, it is required that routing-protocols find paths with less congestion. 5. Resource Constraints • Two essential and limited resources are battery life and processing power.

• Devices require portability, and hence they also have size and weight constraints along with the restrictions on the power-source.

• Increasing the battery-power and processing ability makes the nodes bulky and less portable.

4 b. List the characteristics of an ideal routing-protocol for Adhoc-networks. (8 Marks)

Ans: For answer, refer Solved Paper June/July 2014 Q.No.4a.

4 c. List the advantages and disadvantages of on-demand routing-protocol. (4 Marks) Ans:

Advantages • Uses a reactive approach which eliminates the need to periodically flood the network

with table update messages.

• Route is established only when required.

• Reduce control-overhead. • Connection setup delay is less.

• Works well when topology is highly dynamic. Disadvantages

• Route-maintenance mechanism does not locally repair a broken link.

• Stale route cache information could result in inconsistencies during route

construction phase.

• Performance degrades rapidly with increasing mobility.

• Routing overhead is more & directly proportional to path length.

• Periodic beaconing leads to unnecessary bandwidth consumption.



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5 a. Briefly explain the core extraction distribution Adhoc routing-protocol (CEDAR) by mentioning its advantages and disadvantages. (12 Marks)

Ans: Core Extraction Distribution Adhoc Routing protocol (CEDAR)

• It integrates

1) Routing & 2) Support for QoS.

• It is based on extracting core-nodes (also called as Dominator nodes) in the

network.

• Core-nodes together approximate the minimum Dominating-Set (DS).

• A DS of a graph is defined as a set of nodes such that every node in the graph is either

→ present in the DS or

→ neighbor of some node present in the DS.

• There exists at least one core-node within every 3 hops.

• It employs a distributed algorithm to select core-nodes.

• Route-establishment is carried out in two phases: (Figure 7.24).

1) The first phase finds a core-path from source to destination.

• The core-path is defined as the path from dominator of the source-node (source-core) to the dominator of the destination-node (destination core).

• A node initiates a RouteRequest if the destination is not in the local topology

table of its core-node; otherwise the path is immediately established.

• For establishing a route, the source-core initiates a core-broadcast in which

RouteRequest is sent to all neighboring core-nodes which in turn forwards it.

• A core-node which has the destination-node as its core member replies to the

source-core.

2) In the second phase, once the core-path is established, a path with the

requested QoS support is then chosen.

• Route-maintenance is done as follows (Figure 7.25): 1) Route-maintenance attempts to repair a broken route locally when a path-

break occurs 2) A node after which the break occurred:

→ sends a notification of failure

→ begins to find a new path from it to the destination

→ rejects every received packet till the moment it finds the new path to

destination



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Advantages • Performs both routing and QoS path computation very efficiently with the help of

core-nodes.

• Utilization of core-nodes reduces traffic-overhead.

• Core-broadcasts provide a reliable mechanism for establishing paths with QoS

support. Disadvantages

• Since route-establishment is carried out at core-nodes, the movement of core-nodes adversely affects the performance of the protocol.

• Core-node update information causes control-overhead. 5 b. Explain Power-Aware routing-protocol. (8 Marks)

Ans: For answer, refer Solved Paper June/July 2014 Q.No.5b.



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6 a. Discuss briefly why the TCP does not perform well in Adhoc-networks? (10 Marks)

Ans: 1. Misinterpretation of Packet Loss

• In traditional TCP design, packet-loss is mainly attributed to network congestion. • Adhoc-network experience a much higher packets loss due to

1) High bit rate &

2) Increased Collections 2. Frequent Path Breaks

• If the route re-establishment time is greater than the RTO period of sender, then the sender

→ assumes congestion in the network

→ retransmits lost packets and

→ initiates congestion-control algorithm. • This leads to wastage of: 1) Bandwidth and 2) Battery-power

3. Effect of Path Length • As path length increases, the throughput decreases (Figure: 9.3 & 9.4).

4. Misinterpretation of Congestion Window

• When there are frequent path-breaks, the congestion window may not reflect the maximum transmission-rate acceptable to the network and the receiver. 5. Asymmetric Link Behavior

• Radio-channel has different properties such as location dependent contention, directional properties etc leading to asymmetric links.

• This can lead to TCP invoking the congestion-control algorithm and several retransmissions.

6. Network Partitioning & Remerging • Figure 9.5 illustrate the effect of network-partitions.



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7. Uni-directional Path • TCP relies on end-to-end ACK for ensuring reliability.

• Path-break on an entirely different reverse path can affect the performance of the network as much as a path-breaks in the forward path.

8. Multipath Routing • For TCP, multipath routing leads to significant amount of out-of-order packets, when intern generates a set of duplicate acknowledgement(DUPACKs), which cause

additional power-consumption and invocation of congestion-control.

6 b. Write a note on “Issues considered in designing transport-layer protocol for Adhoc-networks“. (6 Marks) Ans: For answer, refer Solved Paper June/July 2014 Q.No.6a.

6 c. List the advantages and disadvantages of Adhoc TCP. (4 Marks)

Ans: Advantages • It maintains the end-to-end semantics of TCP.

• It is compatible with traditional TCP. • Improves throughput of TCP in adhoc-network.

Disadvantages • Dependency on network-layer protocol to detect the route changes and partitions.

• Addition of thin ATCP layer to TCP/IP requires changes in the interface functions. 7 a. Briefly discuss network-layer attacks. (8 Marks)

Ans: For answer, refer Solved Paper June/July 2014 Q.No.7b.



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7 b. Explain the following: (12 Marks) i) Network security requirements

ii) Key management approaches iii) Requirements of a secure routing-protocol for Adhoc-networks.

Ans: i) Network Security Requirements: For answer, refer June/July 2014 Q.No.7a. ii) Key Management Approaches:

1. Key Predistribution • This involves distributing key to all interested parties before the start of

communication. • This method involves much less communication & computation, but all participants must be known a priori, during the initial configuration.

• Once deployed, there is no mechanism to include new members in the group or to change the key.

2. Key Transport • One of the communicating-entities generates keys & transports them to the other members.

• The simplest scheme assumes that a shared key already exists among the participating-members. This shared key is used to encrypt a new key & is

transmitted to all corresponding nodes. • Only those nodes which have the prior shared key can decrypt it.

• An interesting method for key transport is the shamir’s three-pass protocol. (Figure: 9.13).

Figure 9.13: Shamir’s three-pass protocol.

3. Key Arbitration • This uses a central arbitrator to create & distribute keys among all

participants. Hence, they are a class of key transport schemes. • The problem with arbitrated protocols is that the arbitrator has to be powered

on at all times to be accessible to all nodes. • This leads to a power drain on that particular node. 4. Key Agreement

• Key agreement protocols are used to establish a secure context over which a session can be run, starting with many parties who wish to communicate & an

insecure channel. • In group key agreement schemes, each participant contributes a part to the

secret key. • The most popular key agreement schemes use the Diffie-Hellman exchange algorithm.



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iii) Requirements of a Secure Routing Protocol 1. Detection of Malicious Nodes

• A routing-protocol → should be able to detect the presence of any malicious-node in the

network & → should avoid the participation of such nodes in the routing process.

2. Guarantee of Correct Route Discovery • If a route between the source & destination-node exist, the protocol

→ should be able to find the route & → should also ensure the correctness of the selected route.

3. Confidentiality of Network Topology • Once the network-topology is known, the attacker may try to study the traffic

pattern in the network. • If some of the nodes are found to be more active compared to others, the attacker may try to mount attacks.

• This may ultimately affect the ongoing routing process. Hence, confidentiality of network-topology is important.

4. Stability against Attacks • The protocols must be self-stable in the sense that it must be able to revert to its normal operating state within a finite amount of time after passive or an

active attack.



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8 a. Briefly explain the issues and challenges in providing QoS in Adhoc-networks. (10 Marks)

Ans: 1. Dynamically varying Network Topology

• Since the nodes in an adhoc-network do not have any restriction on mobility, the network-topology changes dynamically. • Hence, the admitted QoS sessions may suffer due to frequent path-breaks, thereby

requiring such sessions to be re-established over new paths. • The delay incurred in re-establishing a QoS session may cause some of the packets

belonging to that session to miss their delay targets/deadlines. This is not acceptable for applications that have severe QoS requirements. 2. Imprecise State Information

• In most cases, the nodes maintain both 1) Link-specific state-information and 2) Flow-specific state-information

• The link-specific state-information includes: 1) Bandwidth 2) Delay 3) Jitter 4) Packet loss rate 5) Error rate 6) Cost

• The flow-specific information includes: 1) Session ID 2) Source-address

3) Destination-address 4) QoS requirements of the flow • The state-information is inherently imprecise due to dynamic changes in network-

topology and channel-characteristics. • Hence, routing-decisions may not be accurate, resulting in some of the real-time packets missing their deadlines.

3. Lack of Central Coordination • Unlike wireless LANs and cellular-networks, adhoc-networks do not have central

controllers to co-ordinate the activity of nodes. • This further complicates QoS provisioning in the networks. 4. Error-prone Shared Radio Channel

• The radio-channel is a broadcast medium by nature. • During propagation through the wireless-medium, the radio-waves suffer from

several impairments such as 1) Attenuation 2) Multipath propagation & 3) Interference 5. Hidden Terminal Problem

• This problem occurs when packets originating from two or more sender-nodes, which are not within the direct transmission-range of each other, collide at a common

receiver-node. • It necessitates the re-transmission of the packets, which may not be acceptable for flows that have stringent QoS requirements.

6. Limited Resource Availability • Following resources are limited:

1) Bandwidth 2) Battery-life 3) Storage-space & 4) Processing capability • Out of these, bandwidth and battery life are critical resources, the availability of which significantly affects the performance of the QoS provisioning mechanism.

• Hence, efficient resource management mechanisms are required for optimal utilization of these limited resources.

7. Insecure Medium • Due to the broadcast-nature of the wireless-medium, communication through a wireless-channel is highly insecure. .’. Security is an important issue, especially for

military applications. • Adhoc-networks are susceptible to following attacks:

1) Eavesdropping 2) Spoofing 3) Denial of service(DoS) & 4) Impersonation • Without sophisticated security mechanisms, it is very difficult to provide secure communication guarantees.



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8 b. Briefly explain the on-demand QoS routing-protocol by mentioning its advantages and disadvantages. (10 Marks)

Ans: On-demand QoS Routing protocol (OQR)

• This is used to guarantee bandwidth for real-time applications. • The protocol is explained below. 1. Route Discovery

• During the route discovery process, the source-node floods a QoS route request (QRREQ) packet.

• A QRREQ packet contains the following fields: 1) Source ID 2) Destination ID 3) Sequence number 4) TTL

5) Route-list 6) Slot array list • The pair {source ID, sequence number} is used to uniquely identify a packet.

• The route-list records the nodes that have been visited by the QRREQ packet, whereas the slot array list records free slots available at each of these nodes. • The TTL field limits the maximum length of the path to be found.

• A node N receiving a QRREQ packet performs the following operations: 1) If a QRREQ with the same {source ID, sequence number} had been received

already, this QRREQ packet gets discarded. 2) Otherwise, the route-list field is checked for the address of this node N. If it

is present, node N discards this QRREQ packet. 3) Otherwise, node N decrements TTL by one. If TTL counts down to zero, it discards this QRREQ packet.

2. Bandwidth Reservation • The destination-node may receive one or more QRREQ packets, each giving a

feasible QoS path for the connection-request. • The destination-node selects the least-cost path among them. • Then it copies the fields {route-list, slot array list} from the corresponding QRREQ

packet to the QoS Route Reply (QRREP) packet and sends the QRREP packet to the source along the path recorded in the route-list.

3. Reservation Failure • The reservation of bandwidth may fail, either → due to route breaks or

→ because the free slots that are recorded in the slot array list get occupied

by some other connection(s). • All nodes on the path from the interrupted node to the destination free the reserved slots for this connection on receiving the ReserveFail packet.

• If no connection could be set up due to non-availability of feasible-paths, the destination broadcasts a NoRoute packet to notify the source.

4. Route-maintenance • When a route gets broken, the nodes detecting the link break send a RouteBroken packet to the source and the destination-nodes.

• After receiving the RouteBroken packet, the source restarts the route discovery process in order to reestablish the connection over a new path, while the destination

releases resources reserved for that connection. Advantages & Disadvantages • It uses an on-demand resource reservation scheme and hence produces lower

control-overhead. • Since it uses the CDMA-over-TDMA channel model, the network needs to be fully

synchronized. • Further, the on-demand nature of route discovery process leads to higher connection setup time.




1

1 a. List and explain different applications of Ad-hoc network. (5 Marks) Ans:

1. Military Application • Adhoc-networks can be used to establish communication among a group of soldiers

for tactical operations. • Setting up of a fixed infrastructure for communication in enemy territories may not be possible. In such a case, adhoc-networks can be used.

• Main requirements are: → reliability

→ efficiency → secure communication &

→ support for multicast routing

2. Collaborative & Distributed Computing • Adhoc-network can be used to establish communication among a group of people in a conference.

• In distributed file-sharing application, the main requirement is reliability which can be provided by adhoc-network.

• Here, devices used can be: → PDA (Personal Digital Assistant)

→ mobile-devices with high processing-power & → laptops with add-on wireless-interface cards

3. Emergency Operations • Adhoc-networks can be used in emergency operations such as

→ search & rescue → crowd control &

→ commando operations • Major factors are:

→ freedom & flexibility of mobility → independent of fixed infrastructure &

→ self-configuration of system with minimal overhead 4. Wireless Mesh Network (WMN)

• It can be formed to provide an alternate communication-infrastructure for mobile or fixed nodes,

→ without the spectrum reuse constraint & → without the requirement of network planning of cellular-network

• It provides many alternate paths for data-transfer between source & destination, which results in quick re-configuration of path when the existing path fails due to

node-failure • Major advantages are: 1) High scalability 2) Easy extendibility 3) Support for a high data-rate

4) Low cost/bit 5) High availability 6) Low cost of deployment • The possible deployment scenarios include:

1) Residential zones 2) Highways 3) Business zones 4) University campuses 5) Important civilian regions

1 b. Explain the issues that makes wireless sensor-network a distinct category of ad-hoc wireless-networks. (5 Marks)

Ans: For answer, refer Solved Paper Dec.2014/Jan.2015 Q.No.1b.



2

1 c. Discuss the major issues, one need to consider while designing a MAC protocol for Adhoc-network. (10 Marks)

Ans: 1. Distributed Operation

• The adhoc-networks need to operate in environments where no centralized coordination is possible. • The protocol-design should be fully distributed involving minimum control-overhead.

2. Synchronization • The protocol-design should take into account requirement of time-synchronization.

• Synchronization is mandatory for TDMA-based systems for management of transmission- & reception-slots. 3. Hidden Terminals

• Hidden-terminals are nodes that are hidden (or not reachable) from the sender of a data-transmission session, but are reachable to the receiver of the session.

4. Exposed Terminals • Exposed-terminals, the nodes that are in the transmission-range of the sender of an on-going session, are prevented from making a transmission.

5. Throughput • The protocol should attempt to maximize the throughput of the system, which can

be done by → minimizing the occurrence of collisions

→ maximizing channel utilization & → minimizing control-overhead

6. Access Delay • This refers to the average delay that any packet experiences to get transmitted.

• The protocol should attempt to minimize the delay. 7. Fairness • This refers to the ability of the protocol to provide an equal share (or weighted

share) of the bandwidth to all competing nodes. • Fairness can be either node-based or flow-based.

8. Resource Reservation • The provisioning of QoS requires reservation of resources such as

1) Bandwidth 2) Buffer space & 3) Processing power 9. Ability to Measure Resource Availability • The protocol should be able to provide an estimation of resource availability at

every node in order → to handle the resources efficiently and

→ to perform call admission-control. • This can also be used for making congestion-control decisions.

10. Capability for Power Control • The transmission-power-control

→ reduces the energy-consumption at the nodes → causes a decrease in interference at neighboring-nodes &

→ increases frequency-reuse. 11. Adaptive Rate Control

• This refers to the variation in the data-rate achieved over a channel. • The protocol should → make use of a high data-rate when the sender and receiver are nearby &

→ adaptively reduce the data-rate as they move away from each other.

12. Use of Directional Antennas • This has following advantages:

→ increased spectrum reuse

→ reduction in interference &

→ reduced power-consumption.



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2 a. What are the design goals to be met while designing a MAC protocol for Adhoc-networks? (6 Marks)

Ans: 1. The available bandwidth must be utilized efficiently.

2. Control-overhead must be kept as low as possible. 3. The operation of a protocol should be distributed. 4. The access-delay must be kept low. (Access-delay refers to the average

delay experienced by any packet to get transmitted). 5. The protocol should provide QoS support for real-time traffic.

6. The protocol should minimize the effects of hidden and exposed-terminal problems. 7. The protocol should provide time-synchronization among nodes.

8. The protocol should ensure fair allocation of bandwidth to nodes. 9. The protocol must be scalable to large networks.

10. The protocol should have power-control mechanisms in order to efficiently manage energy-consumption of the nodes. 11. The protocol should have mechanisms for adaptive data-rate control.

12. The protocol should try to use directional-antennas which can provide advantages such as

→ reduced interference → increased spectrum reuse &

→ reduced power-consumption.



4

2 b. Explain the classification of MAC protocols. (4 Marks) Ans:

Figure 2.1: Classification of MAC protocols

1. Contention based Protocol • Here, the channel access policy is based on competition (Figure 2.1). • Whenever a node needs to send a packet, it tries to get access to the channel.

• It cannot provide QoS, since access to the n/w cannot be guaranteed beforehand. • It can be classified into following:

i) Sender-initiated Protocol • Packet transmissions are initiated by the sender-node.

a) Single-channel Sender-initiated Protocol

• A node that wins the contention to the channel can make use of the entire bandwidth.

b) Multi-channel Sender-initiated Protocol • The available bandwidth is divided into multiple-channels. • Many nodes can simultaneously perform data-transmission using

multiple-channels. ii) Receiver-initiated Protocol

• The receiver-node initiates the contention resolution protocol. 2. Contention based Protocol with Reservation Mechanism • It provides bandwidth reservation ahead. .’. It can provide QoS support.

• It can be classified into following: i) Synchronous Protocol

• There is time-synchronization among all nodes in the network. • The nodes in the neighborhood are informed of the reservations. ii) Asynchronous Protocol

• No global synchronization is needed. • Relative time is used for the reservations.

3. Contention based Protocol with Scheduling Mechanism • There can be

→ packet-scheduling at the nodes or

→ node-scheduling for access to the channel.

• It considers battery-power in their node-scheduling. 2 c. Explain with example, a working principle of Five Phase Reservation

Protocol (FPRP) (10 Marks) Ans: For answer, refer Solved Paper Dec.2014/Jan.2015 Q.No.2b.



5

3 a. Mention the factors to be considered while making scheduling decisions. (4 Marks)

Ans: 1) Delay targets of packets 2) Laxities of packets

3) Traffic load at nodes 4) Remaining battery-power at nodes 3 b. Explain the operation of DPS protocol. (8 Marks)

Ans: Distributed Priority Scheduling protocol (DPS)

• It uses the basic RTS-CTS-DATA-ACK packet-exchange mechanism. • The protocol works as follows (Figure 6.25): i) When source transmits a RTS, priority-tag of current DATA is piggy-backed on RTS

ii) On receiving RTS, the receiver responds with CTS. iii) The receiver copies priority-tag from the received-RTS and piggy-backs it along.

iv) Neighbors → receive the RTS or CTS

→ retrieve the piggy-backed information & → make a corresponding entry in their scheduling-tables.

(STs contain information about packets, which were originally piggy-backed on control and data packets).

I) When source transmits a DATA, its head-of-line(HOL) packet information is piggy-backed on DATA. (HOL packet of a node refers to the packet to be transmitted next by the node).

II) On receiving DATA, the receiver responds with ACK. III) The receiver copies the HOL-information from the received-DATA and piggy-

backs it along. IV) Neighbors

→ receive the DATA or ACK

→ retrieve the piggy-backed information &

→ make a corresponding entry in their STs. V) When a node hears an ACK, it removes from its ST any entry made earlier for the

corresponding DATA.

3 c. Explain the working of multi-channel MAC (MMAC) protocol. (8 Marks)

Ans: For answer, refer Solved Paper Dec.2014/Jan.2015 Q.No.3c.

4 a. What are the characteristics of an ideal routing-protocol ? (4 Marks)

Ans: For answer, refer Solved Paper June/July 2014 Q.No.4a.



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4 b. Explain the classification of routing-protocols. (8 Marks) Ans:



7

1. Based on the Routing-information Update Mechanism i) Proactive or Table-driven Routing Protocols

• Every node maintains the network-topology-information in the routing-tables by periodically exchanging routing-information (Figure 7.4).

• Routing-information is generally flooded in the whole network. • Whenever a node requires a path to a destination, it runs an appropriate path-finding algorithm.

ii) Reactive or On-demand Routing Protocols • Do not maintain the network-topology-information.

• Obtain the necessary path when it is required, by using a connection-establishment process. iii) Hybrid Routing Protocols

• Combine the best features of the above 2 categories. • Nodes within a certain distance from the node concerned, or within a

particular geographical region, are said to be within the routing-zone of the given node. • For routing within the zone, a table-driven approach is used.

For nodes that are located beyond the zone, an on-demand approach is used. 2. Based on the use of Temporal Information for Routing

i) Routing Protocols using past Temporal Information • Use information about

→ past status of the links or → status of links at the time of routing.

ii) Routing Protocols that use future Temporal Information • Use information about the expected future status of the wireless-links to make approximate routing-decisions.

• Apart from the lifetime of wireless-links, the future status information also includes information regarding

• The future status information includes: → lifetime of wireless-links

→ lifetime of node → prediction of location &

→ prediction of link-availability 3. Based on the Routing Topology

i) Flat Topology Routing Protocols • Make use of a flat addressing scheme.

• It assumes the presence of a globally unique addressing mechanism for nodes in the network. ii) Hierarchical Topology Routing-protocols

• Make use of a logical hierarchy in the network and an associated addressing scheme.

• The hierarchy can be based on → geographical-information or

→ hop distance 4. Based on the Utilization of Specific Resources

i) Power-aware Routing • Aims at minimizing the consumption of battery-power. • The routing-decisions are based on minimizing the power-consumption either

logically or globally in the network. ii) Geographical Information-assisted Routing

• Improve the performance of routing. • Reduce the control-overhead by effectively utilizing the geographical-

information available.



8

4 c. Explain working principle of wireless routing-protocol (WRP). (8 Marks) Ans:

Wireless Routing Protocol (WRP) • It inherits the properties of the distributed bellman-ford algorithm.

• To counter the count-to-infinity problem, it employs a unique method of maintaining information regarding

→ shortest distance to every destination-node in the network &

→ penultimate hop node on the path to every destination-node.

• It maintains an up-to-date view of the network, every node has a readily available route to every destination-node in the network. • Each node has following tables:

1) Distance Table (DT) • It contains the network view of the neighbors of a node.

• It contains a matrix where each element contains → distance &

→ penultimate node reported by neighbor for a particular destination 2) Routing-Table (RT)

• It contains the up-to-date view of the network for all known destinations. • It keeps following information: → shortest distance

→ predecessor/penultimate node

→ successor node & → a flag indicating the status of the path

• The path status may be → a simplest (correct) path or

→ a loop (error) or → destination-node not marked (null)

3) Link cost Table (LCT) • It contains the cost of relaying messages through each link.

• The cost of broken-link is ∞. • It also contains the number of update periods passed since the last successful

update was received from that link. 4) Message Retransmission List (MRL) • It contains an entry for every update message that is to be retransmitted.

• It maintains a counter for each entry.



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• After receiving the update message, the node → updates the distance for transmitted neighbors &

→ checks other neighbors’ distance. Hence convergence is much faster than DSDV.

• Consider the example shown in figure 7.7: → The source of the route is node 1 and destination is node 15.

→ From the routing-table shown, the route from node 1 to node 15 has the next node as node 2.

→ The predecessor node of 15 corresponding to this route is route 12. The predecessor information helps WRP to converge quickly during link breaks.

→ When a node detects a link break, it sends an update message to its neighbors with the link cost of the broken-link set to ∞.

→ After receiving the update message, all affected nodes update their minimum distances to the corresponding nodes.

→ The node that initiated the update message then finds an alternative route, if available from its DT. Figure 7.8 shows route-maintenance in WRP.

Advantages • It has faster convergence. • It involves fewer table updates.

Disadvantages • The complexity of maintenance of multiple tables demands:

→ larger memory from nodes & → greater processing power from nodes

• It is not suitable for → highly dynamic networks &

→ very large networks



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5 a. Explain zone based hierarchical link-state protocol (ZHLS) with example. (10 Marks)

Ans: Zone-based Hierarchical Link State protocol (ZHLS)

• It uses the geographical-information of the nodes to form non-overlapping zones. • A hierarchical addressing consists of 1) Zone ID & 2) Node ID • It employs

→ proactive approach inside the geographical-zone and → reactive approach behind the zone.

• Every node requires GPS-support for obtaining its own geographical-location. • The assignment of zone addresses to geographical areas is done during a phase

called the network design phase( or network deployment phase). • Each node maintains 2 link-state packets (LSP):

1) Node level LSP: list of connected-neighbors. 2) Zone LSP: list of connected-zones.

• Route-establishment is done as follows (Figure 7.28 & Table 7.1):

→ If a source-node src wants to communicate with a destination-node dest, src checks whether dest resides in its own zone.

→ If dest belongs to same zone, then packets are delivered to the dest as per the intra-Zone routing-table.

→ If dest does not belong to same zone, then src originates a location-request packet containing the sender’s and destination’s information.

→ The gateway-node of a zone at which the location-request packet is received verifies its routing-table for the destination-node.

→ The gateway-node that finds destination-node originates a location-response packet containing the zone-information to the sender.

• Route-maintenance is done as follows: → If a given gateway-node away causing a zone level connection failure,

routing can still take place with the help of the other gateway-nodes. →This is due to hierarchical addressing that makes use of zone ID and node ID.

Advantages • Reduce storage requirements and common overhead.

• Robust and resilient to path-breaks. • Non overlapping zones.

Disadvantages • Additional overhead incurred in creation of zone level topology. • Path to destination is suboptimal.

• Geographical information may not be available in all environments.



11

5 b. Explain operation of Fisheye State Routing-protocol (FSRP). (10 Marks) Ans:

Fisheye State Routing Protocol (FSRP) • It uses fisheye technique to reduce the routing-overhead.

• Principle: → Property of a fish’s eye that can capture pixel information with greater

accuracy near its eye’s focal point. → This accuracy decreases with an increase in the distance from the center of

the focal point. → This property is translated to routing by a node.

• Each node maintains accurate information about near nodes. • Nodes exchange topology-information only with their neighbors. • A sequence numbering scheme is used to identify the recent topology changes.

• It consists of → link-level information exchange of distance-vector protocols and

→ complete topology-information exchange of link-state protocols. • It defines routing scope, which is the set of nodes that are reachable in a specific

no. of hops. • The scope of a node at 2 hops is the set of nodes that can be reached in 2 hops.

• Figure 7.32 shows scope of node 5 with one hop and 2 hops.

• The link-state information for the nodes belonging to the smallest scope is

exchanged at the highest frequency. • Frequency of exchanges decreases with an increase in scope.

• Figure 7.33 illustrates an example showing the topology-information maintained at nodes in a network. • The routing-information for the nodes that are one hop away from a node are

exchanged more frequently than the routing-information about nodes that are more than one hop away.

• Information regarding nodes that are more than one hop away from the current-node are listed below the dotted line in the topology table. Advantages

• Reduce bandwidth-consumption by link-state update packets. • Suitable for large and highly mobile adhoc-network.

Disadvantages • Very poor performance in small adhoc-networks.



12

6 a. Explain the issues in designing transport-layer protocols for adhoc-networks. (10 Marks)

Ans: 1. Induced Traffic

• In a path having multiple-link, the traffic at any given link (or path) due to the traffic through neighbouring-links (or paths) is referred to as induced traffic. • This is due to

→ broadcast-nature of the channel & → location-dependent contention on the channel

• This affects the throughput achieved by the protocol. 2. Induced Throughput Unfairness

• This refers to the throughput unfairness at the transport-layer due to the throughput (or delay) unfairness existing at the lower layer (such as the network- and

MAC-layers). The transport-layer should consider this issue in order to provide a fair share of throughput across contending flows. 3. Separation of Congestion-control, Reliability and Flow Control

• The protocol can provide better performance if reliability, flow-control and congestion-control are handled separately.

• Reliability and flow-control are end-to-end activities whereas congestion-control can at times be a local activity.

• Objective: Minimization of the additional control-overhead generated by them.

4. Power & Bandwidth Constraints • Nodes face resource-constraints including the two most important resources:

1) Power-source & 2) Bandwidth

• The performance of a protocol is significantly affected by these resource-constraints.

5. Interpretation of Congestion • Interpretation of network-congestion as used in traditional networks is not

appropriate in adhoc-networks. • This is because following parameters can also lead to packet-loss:

→ high error-rates of wireless-channel → location-dependent contention

→ hidden-terminal problem → packet collisions in the network

→ path-breaks due to mobility of nodes & → node-failure due to drained battery

6. Completely Decoupled Transport-layer • Another challenge faced by transport-layer protocol is the interaction with the lower

layers. • Cross-layer interaction between the transport-layer and lower layers is important to

adapt to the changing network-environment. 7. Dynamic Topology • Experience rapidly changing network-topology due to mobility of nodes.

• This leads to → frequent path-breaks

→ partitioning and remerging of networks & → high delay in re-establishment of paths.

• Performance is affected by rapid changes in network-topology.



13

6 b. With a neat diagram, explain the operation of adhoc-TCP (ATCP) protocol. (10 Marks)

Ans: Adhoc TCP (ATCP)

• Based on feedback information received from the intermediate-nodes, the sender changes its state to the

1) Persist state 2) Congestion-control state or 3) Retransmission state

• When an intermediate-node finds that the network is partitioned, then the sender-state is changed to the persist-state where it avoids unnecessary retransmissions.

• Figure 9.8 shows the thin layer implementation of ATCP between 1) Traditional TCP layer & 2) IP layer(network-layer) • ATCP layer does not require changes in the existing TCP protocol.

• ATCP layer is active only at the TCP sender.

• Major function of the ATCP layer is that it monitors the:

→ packet sent & received by TCP sender

→ state of the TCP-sender → state of the network

• The 4 states in the ATCP are: 1. NORMAL. 2. CONGESTED 3. LOSS 4. DISCONN

• When a TCP-connection is established, the sender-state is in NORMAL, here ATCP does not interfere with the operation of TCP and it remains invisible.

Table 9.1. The actions taken by ATCP

Event Action

Packet-loss due to high BER Retransmits the lost packets without reducing congestion window

Route re-computation delay Makes the TCP sender go to persist-state and stop transmission until new route has been found

Transient partitions Makes the TCP sender go to persist-state and stop transmission until new route has been found

Out-of-order packet delivery due to multipath routing

Maintains TCP sender unaware of this and retransmits the packets from TCP buffer

Change in route Re-computes the congestion window

Advantages

• It maintains the end-to-end semantics of TCP. • It is compatible with traditional TCP. • Improves throughput of TCP in adhoc-network.

Disadvantages • Dependency on network-layer protocol to detect the route changes and partitions.

• Addition of thin ATCP layer to TCP/IP requires changes in the interface functions.



14

7 a. Discuss the requirements and challenges in security provisioning for adhoc-networks. (10 Marks)

Ans: Requirements in Security Provisioning: For answer, refer June/July 2014 Q.No.7a

Challenges in Security Provisioning

1. Shared Broadcast Radio Channel

• The radio-channel used for communication → is broadcast in nature &

→ is shared by all nodes within its direct transmission-range. • Data transmitted by a node is received by all nodes within its direct

transmission-range. So, a malicious-node could easily obtain transmitted-data in the network.

• This problem can be minimized to a certain extent by using directional-antennas. 2. Limited Resource Availability

• Following resources are limited in adhoc-networks: 1) Bandwidth

2) Battery-power & 3) Computational-power • Hence, it is difficult to implement complex cryptography-based security

mechanisms in networks. 3. Insecure Operational Environment

• The operating environments where adhoc network is used may not always be secure. • One important application of such networks is in battlefields.

4. Lack of Central Authority • In wired-networks & infrastructure-based wireless-networks, it would be

possible to → monitor the traffic on network through certain important central points &

→ implement security mechanisms at such points. • Since adhoc-networks do not have central points, these mechanisms cannot

be applied. 5. Physical Vulnerability

• Nodes in the networks are usually compact & hand-held in nature. • Nodes could get damaged easily. • Nodes are also vulnerable to theft.

6. Lack of Associations • Since the networks are dynamic in nature, a node can join or leave the

network at any point of time. • If no proper authentication mechanism is used for associating nodes in a network, an intruder would be able to

→ join into the network quite easily & → carry out his attacks.



15

7 b. What is key management? Explain symmetric key algorithm. (10 Marks) Ans:

• The secure administration of cryptographic keys is called key management. Symmetric Key Algorithms

• These rely on the presence of shared key at both the sender & receiver, which has been exchanged by some previous arrangement. • There are 2 kinds of symmetric-key algorithms:

1) Block-ciphers & 2) Stream-ciphers Block Cipher

• A block-cipher is an encryption scheme in which plaintext is broken into fixed-length segments called blocks. • The blocks are encrypted one at a time.

• Two of the simple block-ciphers are: 1) Substitution & 2) Transposition • In substitution, each alphabet of plaintext is substituted by another in the cipher

text. The table mapping of the original & the substituted alphabet is available at both the sender & receiver (Figure 9.12). • A transposition cipher permutes the alphabet in plaintext to produce the cipher

text.

Figure 9.12 Substitution and transposition.

Stream Cipher

• A stream-cipher is, in effect, a block-cipher of block length 1. • One of the simple stream-ciphers is vernam cipher, which uses a key of same length as plaintext for encryption.

• For example: If plaintext = 10010100 & key = 01011001

then cipher text = 11001101 which is obtained by XOR of the plaintext & key. The plaintext is again recovered by XOR-ing the cipher text with the same key.

8 a. Explain the issues in provisioning QoS in adhoc-network. (10 Marks) Ans: For answer, refer Solved Paper Dec.2014/Jan.2015 Q.No.8a.



16

8 b. Discuss working principle of Ticket QoS routing-protocol. (10 Marks) Ans:

Ticket Based QoS Protocol (TBP) • This is a distributed QoS routing-protocol for adhoc-networks.

• This protocol has the following features: 1) It can tolerate imprecise state-information during QoS route-computation. • It exhibits good performance even when the degree of imprecision is high.

2) It probes multiple paths in parallel for finding a QoS feasible-path. • This increases the chance of finding such a path.

3) The optimality of a path among several feasible-paths is explored. • A low-cost path that uses minimum resources is preferred when multiple feasible-paths are available.

4) A primary-backup-based fault-tolerant technique is used to reduce service disruption during path-breaks that occur quite frequently in adhoc-networks.

Protocol Overview • The basic idea: the source-node → issues a certain number of tickets and

→ sends these tickets in probe packets for finding a QoS feasible-path.

• Each probe packet carries one or more tickets. • Each ticket corresponds to one instance of the probe. • For example, when the source-node issues 3 tickets, it means that a maximum of 3

paths can be probed in parallel. • The number of tickets generated is based on

→ precision of state-information available at the source-node & → QoS requirements of the connection-request

• More tickets are issued in order to improve the chances of finding a feasible-path in following 2 cases:

→ If the available state-information is not precise or → If the QoS requirements are very severe

• If the QoS requirements are not severe and can be met easily, fewer tickets are issued in order to reduce the level of search, which in turn reduces the control-

overhead. • There exists a trade-off here between → performance of the QoS routing-protocol &

→ control-overhead


Engineering

VTU 8TH SEM CSE ADHOC NETWORKS SOLVED PAPERS OF JUNE-2014 DEC-14 & JUNE-2015