Wireless sensor network

WSNs, which can be considered as a special case of ad hoc networks with reduced or no mobility, are expected to find increasing deployment in coming years, as they enable reliable monitoring and analysis of unknown and untested environments. These networks are "data centric", i.e., unlike traditional ad hoc networks where data is requested from a specific node, data is requested based on certain attributes such as,"which area has temperature over 35°C or 95°F". Therefore a large

number of sensors need to be deployed to accurately reflect the physical attribute in a given area. Routing protocol design for WSNs is heavily influenced by many challenging factors, which must be overcome before efficient communication can be achieved. These challenges can be summarized as follows:

Ad hoc deployment - Sensor nodes are randomly deployed which requires that the system be able to cope up with the resultant distribution and form connections between the nodes. In addition, the system should be adaptive to changes in network connectivity as a result of node failure.

• Computational capabilities - Sensor nodes have limited computing power and therefore may not be able to run sophisticated network protocols leading to light weighted and simple versions of routing protocols.

• Energy consumption without losing accuracy - Sensor nodes can use up their limited energy supply carrying out computations and transmitting information in a wireless environment. As such, energyconserving forms of communication and computation are crucial as

the node lifetime shows a strong dependence on the battery lifetime. In a multi-hop WSN, nodes play a dual role as data sender and data router. Therefore, malfunctioning of some sensor nodes due to power failure can cause significant topological changes and might require rerouting of packets and reorganization of the network.

Scalability - The number of sensor nodes deployed in the sensing area may be in the order of hundreds, thousands, or more. Any routing scheme must be scalable enough to respond to events and capable of operating with such large number of sensor nodes. Most of the sensors can remain in the sleep state until an event occurs, with data from only a few remaining sensors providing a coarse quality.

• Communication range - The bandwidth of the wireless links connecting sensor nodes is often limited, hence constraining inter sensor communication. Moreover, limitations on energy forces sensor nodes to have short transmission ranges. Therefore, it is likely that a path from a source to a destination consists of multiple wireless hops

Fault tolerance - Some sensor nodes may fail or be blocked due to lack of power, physical damage, or environmental interference. If many nodes fail, MAC and routing protocols must accommodate formation of new links and routes to the data collection BSs. This may require actively adjusting transmit powers and signaling rates on the existing links to reduce energy consumption, or rerouting packets through regions of the network where more energy is available. Therefore, multiple levels of redundancy may be needed in a fault tolerant WSN.

• Connectivity - High node density in sensor networks precludes them from being completely isolated from each other. Therefore, sensor nodes are expected to be highly connected. This, however, may not prevent the network topology from varying and the network size from shrinking due to sensor nodes failures. In addition, connectivity depends on the, possibly random, distribution of nodes.

Transmission media - In a multi-hop sensor network, communicating nodes are linked by a wireless medium. Therefore, the traditional problems associated with a wireless channel (e.g., fading, high error rate) also affect the operation of the sensor network. In general, bandwidth requirements of sensor applications will be low, in the order of 1-100 kb/s. As we have seen in Chapters 4 and 5 and in the previous section, the design of the MAC protocol is also critical in terms of conserving energy in WSNs.

• QoS - In some applications (e.g., some military applications), the data should be delivered within a certain period of time from the moment it is sensed, otherwise the data will be useless. Therefore, bounded latency for data delivery is another condition for time constrained applications.

• Control Overhead - When the number of retransmissions in wireless medium increases due to collisions, the latency and energy consumption also increases. Hence, control packet overhead increases linearly with the node density. As a result, tradeoffs between energy conservation, self- configuration, and latency may exist.

• Security - Security is an important issue which does not mean physical security, but it implies that both authentication and encryption should be feasible. But, with limited resources, implementation of any complex algorithm needs to be avoided. Thus, a tradeoff exists between the security level and energy consumption in a WSN.

Protocol stack for sensor network

The protocol stack of sensor network combines power efficiency and least cost path routing. The architecture consist of

1. Physical layer

2. Data link layer

3. Network layer

4. Transport layer

5. Application layer

All these layers are backed by management plane, mobility management plane and task management plane.

Physical layer is responsible for transmitting and receiving signals.

· The data link layer consists of medium access control [MAC] which is used to prevent packet collision.

· The network layer is responsible for routing the packets

· The application layer is used for creation of packets by making use of software.

· The power management plane monitors the sensor’s power level among sensor node.

Structure of a sensor node

sensor node consist of a

1. sensing unit

2. processing unit

3. memory unit

4. self power unit

5. wireless transreceiver

· Sensing unit: it consists of a sensor and analog to digital converter [ADC]. the analog signal produced by sensor is converted to digital and is fed into processing unit. The sensing unit is responsible for collecting the data externally and interacts with central processor

· Processing and memory unit: the processing unit is responsible for performing some computations it executes some instructions which is responsible for setting up the connection with another node. The memory unit is used for storing the data.

· Self power unit: it is responsible for powering the node and keeping it alive. The main task of the sensor node is to identify events , to process data , and then to transmit the data. The power of a node is consumed mainly in the transmitter and receiver unit. The sensor node can be supplied by a self-power unit, self-power unit battery, or solar cells.

Communication energy model

IEEE standards as 802.11a, b, and g provide a wide range of data rates: 54,48,36,24,18,12,9 and 6 mb/s. this range reflects the trade off between the transmission range and data rate intrinsic in a wireless communication channel. An accurate energy model is crucial for the development of energy efficient clustering and routing protocols. The energy consumption, E for all components of the watts is summarized as

E=theta+ηwdn

Where Ѳ is the distance independent term that accounts for the overhead of the radio electronics and digital processing, and ηwd^n is the distance dependent term in which η represents the amplifier inefficiency factor w is the free space path loss d is the distance and n in the environmental factor. Based on an environmental condition, n can be any number between 2 and 4 and η specifies the inefficiency of the transmitter when generating maximum power wd^n at the antenna. Clearly the distance dependent portal of total energy consumption depends on the real-world Tranreceiver parameters, Ѳ, η and the path attenuation wd^n. if the value of Ѳ overshadows ηwd^n, the reduction in the transmission distances through the use of multihop communication is not effective.

Clustering protocol

Decentralized energy efficient propagation protocol [DEEP]: DEEP is used for identifying a head and the members.

The algorithm works as follows

1. Initialize: when network is created one of the node in the network will be made as cluster head. The cluster head sends a signal known as cluster head declaration signal to all the nodes which are in the range ’ .

This is used for identifying the members of the cluster. The cluster head sends cluster head exploration to all the nodes which are in the range dr1 and dr2. This is done to identify a new cluster head.

2. Repeat : even though many nodes receive the cluster head exploration only one node fro which the equation ERC1 < ER < ERC2 can become the candidate of the cluster head. ‘ER’ is the energy of cluster head exploration and

‘ERC!’=Pout – ηwd 1

‘ERC2’=Pout–ηwd 2

η, w, n are dependent on environment factors.

The new candidate sends cluster head declaration for all the nodes in the range dr. if a node receives multiple cluster head declaration which ever signal is having higher energy will be chosen as cluster head. The other candidates will be eliminated.

3. Conclusion: if there are not enough members in a cluster member exploration/search signal will be generated atleast if a node does not receive any signal member exploration or search signal will be generated.

Low energy adaptive clustering hierarchy protocol [LEACH]:

LEACH is a clustering protocol used for identifying the cluster head and the members. It consist of 2 phases

1. Clustering phase

2. Steady state phase

Both these phases are repeated in round. The algorithm works as follows. A node will pick up a number between 0 and 1 and compares the value with if is greater it becomes the candidate for cluster head.

P is the ratio of cluster heads to total number of nodes r-> is the round

g-> number of nodes who has not got during the first round will be equivalent to P and node can become the cluster head.

Similarly when the value of ‘r’ reaches close to (1-p) will be equivalent to P and node can become a cluster head

The cluster head candidates begin to send a signal to other nodes which ever candidate is having a higher signal will become cluster head.

Routing protocol

Routing protocol in sensor network: In sensor network the routing of information can happen within a cluster or between nodes of different clusters.

If the routing is happening within a cluster. Then the protocol is called as intra cluster routing.

If the routing is happening between the nodes of different clusters it is called as inter cluster routing.

Intracluster routing

In intracluster routing, the packets are transmitter with in a cluster It can be of two types.

1. Direct routing algorithm

2. Multihop routing algorithm

In direct routing, the cluster head as the destination for all cluster nodes. The cluster nodes can communicate directly with cluster nodes.

In multihop the destination is reached through multiple hops. If there are many paths. Then only the path which is energy efficient will be considered. In multihop routing, a node might have to under go multiple hops before it reaches the destination. The sensor node will be at different distances apart from other nodes. A packet from a node is routed to a neighbor node that exhibits high energy. The number in the node indicates the remaining energy in the node

Inter cluster routing [ICR]: It is a destination initiated reactive routing algorithm. The destination is called as local base station [LBS] it will start the route discovery by creating interest signal and following them. ICR works in two phases, Route discovery and data acquisition.

In Route Discovery Phase, the LBS initiates route discovery by sending an interest signal

within the range Ri,

1. All the nodes which are in the range Ri will receive the interest signal.

2. Upon receiving the interest signal it will be stored and flooding continues.

3. If an intermediate node receive already processed interest signal it will be discarded

4. Before flooding the interest signal the cost value will be updated. The format of the interest signal and the formula for the cost is as given below.

The type field indicates message format. The period indicates how often interest signal has to

be sent. The source address field is used for sorting address of the source node. The cost field indicates number of hops required to reach the source. In formula for the cost α and β are normalization factor based on environment.

Write a note on Zigbee technology?

· ZigBee is one of the newest technologies enabling Wireless Personal Area Networks (WPAN).

· ZigBee is an established set of specifications for wireless personal area networking (WPAN), i.e. digital radio connections between computers and related devices.

· WPAN Low Rate or ZigBee provides specifications for devices that have low data rates, consume very low power and are thus characterized by long battery life. ZigBee makes possible completely networked homes where all devices are able to communicate and be controlled by a single unit.

· The IEEE 802.15.4 standard and Zigbee wireless technology are designed to satisfy the market's need for a low-cost, standard-based and flexible wireless network technology, which offers low power consumption, reliability, interoperability and security for control and monitoring applications with low to moderate data rates.

· The data which gets transmitted includes temperature reading on or off state of a switch keystroke of a keyboard etc.

· The Bluetooth technology which is used in mobile phones, laptops, runs on zigbee.

Zigbee is an IEEE 802.15.4 standard. Zigbee operates in a frequency range 900MHz- 2.5 GHz.

· The technology can be used for transmitting the data within the range of 20mts.

· It can also be used for transmission of data within a range greater than 20mts. This is possible through the intermediate nodes.