ADHOC NETWORKS (JUNE-2014, DECEMBER-2014 & JUNE-2015)

Transcription

1 SOLVED PAPERS OF ADHOC NETWORKS (JUNE-2014, DECEMBER-2014 & JUNE-2015)

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3 ADHOC NETWORKS SOLVED PAPER JUNE/JULY a. Briefly explain the following networks with proper diagrams: i) Wireless mesh networks. ii) Hybrid wireless-networks. (10 Marks) 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. 1

4 ADHOC NETWORKS SOLVED PAPER JUNE/JULY 2014 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 2

5 ADHOC NETWORKS SOLVED PAPER JUNE/JULY b. Discuss the major issues to be considered for successful adhoc wireless internet. (10 Marks) 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 statemaintenance-overhead at the gateway-nodes. 5. Load Balancing It is essential to distribute the load so as to avoid the situation where the gatewaynodes 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. 3

6 ADHOC NETWORKS SOLVED PAPER JUNE/JULY a. Discuss following main issues of designing a MAC protocol. (10 Marks) i) Quality of services (QoS) ii) Hidden and exposed node problem. 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. 4

7 ADHOC NETWORKS SOLVED PAPER JUNE/JULY b. Explain in detail the receiver initiated MAC protocol MARCH. (10 Marks) 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 (RT id ) for that flow. The RT id 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. 5

8 ADHOC NETWORKS SOLVED PAPER JUNE/JULY a. Give brief explanation of Distributed Wireless Ordering Protocol(DWOP). (10 Marks) 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 T back-off is given by T back-off =R*(EIFS+DIFS+T success +CW min ) where T success =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. 6

9 ADHOC NETWORKS SOLVED PAPER JUNE/JULY b. Describe the working mechanism of the DBTMA protocol. (10 Marks) 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) BT t : indicate that it is transmitting on the data-channel 2) BT r : 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 BT r signal is active. If there is no BT r signal, then source transmit RTS. On receiving RTS, receiver checks whether the BT t tone is active. If there is no BT t signal, receiver sends CTS and turns on the BT r signal Sender receives CTS, turns on BT t signal, starts data transmission and turns off BT t signal. Receiver receives data and turn off BT r signal. DBTMA has better network utilization than RTS/CTS based protocol. 7

10 ADHOC NETWORKS SOLVED PAPER JUNE/JULY a. What are the characteristics of an ideal routing-protocol for Adhocnetworks? (6 Marks) 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) 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 pathfinding 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. 8

11 ADHOC NETWORKS SOLVED PAPER JUNE/JULY c. Explain in detail CGSR protocol (8 Marks). 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 clusterhead 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. 9

12 ADHOC NETWORKS SOLVED PAPER JUNE/JULY a. Explain Zone Routing-protocol (ZRP). (10 Marks) 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 10

13 ADHOC NETWORKS SOLVED PAPER JUNE/JULY 2014 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. 11

14 ADHOC NETWORKS SOLVED PAPER JUNE/JULY b. Discuss the power-aware routing-metrics for adhoc-networks. (10 Marks) 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. 12

15 ADHOC NETWORKS SOLVED PAPER JUNE/JULY a. Explain the issues and design goals of transport-layer protocol for adhoc-networks. (10 Marks) 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. 13

16 ADHOC NETWORKS SOLVED PAPER JUNE/JULY 2014 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. 14

17 ADHOC NETWORKS SOLVED PAPER JUNE/JULY b. Illustrate the working of split TCP with a neat diagram. (10 Marks) 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 intermediatenodes (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. 15

18 ADHOC NETWORKS SOLVED PAPER JUNE/JULY a. Briefly discuss the network security requirements for adhoc-networks. (5 Marks) Requirements in Security Provisioning 1. Confidentiality The data sent by the sender must be understandable only to the intendedreceiver. 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. 16

19 ADHOC NETWORKS SOLVED PAPER JUNE/JULY b. List & explain the network-layer attacks. (5 Marks) 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 pathfinding 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 authorizednodes 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 authorizednodes. 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. 17

20 ADHOC NETWORKS SOLVED PAPER JUNE/JULY c. Explain the Security-aware Adhoc Routing (SAR) protocol. (10 Marks) 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. 18

21 ADHOC NETWORKS SOLVED PAPER JUNE/JULY a. Give the layer wise classification of existing QoS solution. (5 Marks) 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 standard. The 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 19

22 ADHOC NETWORKS SOLVED PAPER JUNE/JULY b. Write a short note on: (15 Marks) i) Hard state versus soft state resource reservation. ii) Cluster TDMA 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 clusterheads 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). 20

23 ADHOC NETWORKS SOLVED PAPER JUNE/JULY 2014 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. 21

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25 ADHOC NETWORKS SOLVED PAPER DEC.2014/JAN a. Differentiate between Cellular Network and Adhoc-network. (8 Marks) 1

26 ADHOC NETWORKS SOLVED PAPER DEC.2014/JAN b. Explain wireless sensor-network. (6 Marks) 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. 2

27 ADHOC NETWORKS SOLVED PAPER DEC.2014/JAN 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. 3

28 ADHOC NETWORKS SOLVED PAPER DEC.2014/JAN c. Explain Adhoc wireless internet with a neat diagram. (6 Marks) 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 statemaintenance-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. 4

29 ADHOC NETWORKS SOLVED PAPER DEC.2014/JAN 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 adhocnetwork 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. 5

30 ADHOC NETWORKS SOLVED PAPER DEC.2014/JAN a. Discuss issues in designing MAC protocol for adhoc-networks.(10 Marks) 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. 6

31 ADHOC NETWORKS SOLVED PAPER DEC.2014/JAN 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. 7

32 ADHOC NETWORKS SOLVED PAPER DEC.2014/JAN b. Define Five-phase Reservation protocol and explain five-phase reservation protocol. (10 Marks) 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 destinationnode. 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. 8

33 ADHOC NETWORKS SOLVED PAPER DEC.2014/JAN a. Explain directional busy-tone-based MAC protocol in detail. (6 Marks) 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 BT r 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 BT t 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 BT r in the direction towards source. 4) On receiving the CTS, the source turns on busy-tone BT t in the direction towards receiver. 5) Once the packet transmission is over, the source turns off the BT t signal. 6) After receiving the Data-packet, the receiver turns off the BT r signal. (For a uni-cast transmission, only a single antenna element is used. For broadcast transmission, all the N antenna elements transmit simultaneously). 9

34 ADHOC NETWORKS SOLVED PAPER DEC.2014/JAN b. Explain interleaved carrier-sense multiple access. (6 Marks) Interleaved-Carrier Sense Multiple Access protocol (I-CSMA) It efficiently overcomes the exposed-terminal problem. Consider figure 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 ongoing transmission. If they receive any RTS on channel-2, they would not refrain from sending CTS on channel-1 for the received RTS. 10

35 ADHOC NETWORKS SOLVED PAPER DEC.2014/JAN c. Write a note on multi-channel MAC protocol. (8 Marks) 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. 11

36 ADHOC NETWORKS SOLVED PAPER DEC.2014/JAN.2015 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. 12

37 ADHOC NETWORKS SOLVED PAPER DEC.2014/JAN a. Discuss the issues in designing a routing-protocol for Adhoc-network. (8 Marks) 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. 13

38 ADHOC NETWORKS SOLVED PAPER DEC.2014/JAN Error-prone Shared Broadcast Radio-channel The broadcast-nature of the radio-channel poses a unique challenge in adhocnetworks. 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) 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) 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. 14

39 ADHOC NETWORKS SOLVED PAPER DEC.2014/JAN a. Briefly explain the core extraction distribution Adhoc routing-protocol (CEDAR) by mentioning its advantages and disadvantages. (12 Marks) 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 pathbreak 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 15

40 ADHOC NETWORKS SOLVED PAPER DEC.2014/JAN.2015 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) For answer, refer Solved Paper June/July 2014 Q.No.5b. 16

41 ADHOC NETWORKS SOLVED PAPER DEC.2014/JAN a. Discuss briefly why the TCP does not perform well in Adhoc-networks? (10 Marks) 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. 17

42 ADHOC NETWORKS SOLVED PAPER DEC.2014/JAN 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) For answer, refer Solved Paper June/July 2014 Q.No.6a. 6 c. List the advantages and disadvantages of Adhoc TCP. (4 Marks) 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) For answer, refer Solved Paper June/July 2014 Q.No.7b. 18

43 ADHOC NETWORKS SOLVED PAPER DEC.2014/JAN b. Explain the following: (12 Marks) i) Network security requirements ii) Key management approaches iii) Requirements of a secure routing-protocol for Adhoc-networks. 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. 19

44 ADHOC NETWORKS SOLVED PAPER DEC.2014/JAN.2015 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. 20

45 ADHOC NETWORKS SOLVED PAPER DEC.2014/JAN a. Briefly explain the issues and challenges in providing QoS in Adhocnetworks. (10 Marks) 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 networktopology 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. 21

46 ADHOC NETWORKS SOLVED PAPER DEC.2014/JAN b. Briefly explain the on-demand QoS routing-protocol by mentioning its advantages and disadvantages. (10 Marks) 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. 22

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48 ADHOC NETWORKS SOLVED PAPER JUNE/JULY a. List and explain different applications of Ad-hoc network. (5 Marks) 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) For answer, refer Solved Paper Dec.2014/Jan.2015 Q.No.1b. 1

49 ADHOC NETWORKS SOLVED PAPER JUNE/JULY c. Discuss the major issues, one need to consider while designing a MAC protocol for Adhoc-network. (10 Marks) 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. 2

50 ADHOC NETWORKS SOLVED PAPER JUNE/JULY a. What are the design goals to be met while designing a MAC protocol for Adhoc-networks? (6 Marks) 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. 3

51 ADHOC NETWORKS SOLVED PAPER JUNE/JULY b. Explain the classification of MAC protocols. (4 Marks) 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) For answer, refer Solved Paper Dec.2014/Jan.2015 Q.No.2b. 4

52 ADHOC NETWORKS SOLVED PAPER JUNE/JULY a. Mention the factors to be considered while making scheduling decisions. (4 Marks) 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) 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 piggybacked 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 piggybacks 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) 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) For answer, refer Solved Paper June/July 2014 Q.No.4a. 5

53 ADHOC NETWORKS SOLVED PAPER JUNE/JULY b. Explain the classification of routing-protocols. (8 Marks) 6

54 ADHOC NETWORKS SOLVED PAPER JUNE/JULY 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 connectionestablishment 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 geographicalinformation available. 7

55 ADHOC NETWORKS SOLVED PAPER JUNE/JULY c. Explain working principle of wireless routing-protocol (WRP). (8 Marks) 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. 8

56 ADHOC NETWORKS SOLVED PAPER JUNE/JULY 2015 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 9

57 ADHOC NETWORKS SOLVED PAPER JUNE/JULY a. Explain zone based hierarchical link-state protocol (ZHLS) with example. (10 Marks) 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. 10