Wireless sensor networks—The basics—Part II

DATA DISSEMINATION

Data dissemination is the process by which queries or data are routed in the sensor network. The data collected by sensor nodes has to be communicated to the BS or to any other node interested in the data. The node that generates data is called a source  and the information to be reported is called an event . A node which is interested in an event and seeks information about it is called a sink . Traffic models have been developed for sensor networks such as the data collection and data dissemination (diffusion) models. In the data collection model, the source sends the data it collects to a collection entity such as the BS. This could be periodic or on demand. The data is processed in the central collection entity. 

Data diffusion, on the other hand, consists of a two-step process of interest propagation and data propagation. An interest  is a descriptor for a particular kind of data or event that a node is interested in, such as temperature, intrusion, or presence of bio-agents. For every event that a sink is interested in, it broadcasts its interest to its neighbors and periodically refreshes its interest. The interest is propagated across the network, and every node maintains an interest cache of all events to be reported. This is similar to a multicast tree formation, rooted at the sink. When an event is detected, it is reported to the interested nodes after referring to the interest cache. Intermediate nodes maintain a data cache and can aggregate the data or modify the rate of reporting data. The paths used for data propagation are modified by preferring the shortest paths and deselecting the weaker or longer paths. The basic idea of diffusion is made efficient and intelligent by different algorithms for interest and data routing.

12.3.1 Flooding

In flooding, each node which receives a packet broadcasts it if the maximum hop count of the packet is not reached and the node itself is not the destination of the packet. This technique does not require complex topology maintenance or route discovery algorithms. But flooding has the following disadvantages [5]:

•  Implosion: This is the situation when duplicate messages are sent to the samefnode. This occurs when a node receives copies of the same message fromfmany of its neighbors.

•  Overlap: The same event may be sensed by more than one node due to overlappingfregions of coverage. This results in their neighbors receiving duplicatefreports of the same event.

•  Resource blindness: The flooding protocol does not consider the availablefenergy at the nodes and results in many redundant transmissions. Hence, itfreduces the network lifetime.

12.3.2 Gossiping

 Gossiping is a modified version of flooding, where the nodes do not broadcast a packet, but send it to a randomly selected neighbor. This avoids the problem of implosion, but it takes a long time for a message to propagate throughout the network. Though gossiping has considerably lower overhead than flooding, it does not guarantee that all nodes of the network will receive the message. It relies on the random neighbor selection to eventually propagate the message throughout the network.

12.3.3 Rumor Routing

 Rumor routing is an agent-based path creation algorithm [6]. Agents, or “ants,” are long-lived entities created at random by nodes. These are basically packets which are circulated in the network to establish shortest paths to events that they encounter. They can also perform path optimizations at nodes that they visit.

When an agent finds a node whose path to an event is longer than its own, it updates the node’s routing table. Figure 12.4 illustrates the working of the rumor routing algorithm. In Figure 12.4 (a), the agent has initially recorded a path of distance 2 to event E 1. Node A ’s table shows that it is at a distance 3 from event E 1 and a distance 2 from E 2. When the agent visits node A , it updates its own path state information to include the path to event E 2. The updating is with one hop greater distance than what it found in A , to account for the hop between any neighbor of A that the agent will visit next, and A . It also optimizes the path to E 1 recorded at node A  to the shorter path through node B . The updated status of the agent and node table is shown in Figure 12.4 (b).

When a query is generated at a sink, it is sent on a random walk with the hope that it will find a path (preestablished by an agent) leading to the required event. This is based on the high probability of two straight lines intersecting on a planar graph, assuming the network topology is like a planar graph, and the paths established can be approximated by straight lines owing to high density of the nodes. If a query does not find an event path, the sink times out and uses flooding as a last resort to propagate the query. For instance, as in Figure 12.4 (c), suppose a query for event E 1 is generated by node P . Through a random walk, it reaches A , where it finds the previously established path to E 1. Hence, the query is directed to E 1 through node B , as indicated by A ’s table.

12.3.4 Sequential Assignment Routing

 In [7], a set of algorithms which performs organization and mobility management in sensor networks is proposed. The sequential assignment routing (SAR) algorithm creates multiple trees, where the root of each tree is a one-hop neighbor of the sink. 

Each tree grows outward from the sink and avoids nodes with low throughput or high delay. At the end of the procedure, most nodes belong to multiple trees. An instance of tree formation is illustrated in Figure 12.5. The trees rooted at A  and B, two of the one-hop neighbors of the sink, are shown. Node C belongs to both trees, and has path lengths of 3 and 5, respectively, to the sink, using the two trees. Each sensor node records two parameters about each path through it: the available energy resources on the path and an additive QoS metric such as delay.

Network Transmission Basics - Analog and Digital Signals

Analog Network Signaling

An analog signal is best compared to a wave.  It has similar properties to an ocean wave, and can be described using three specific characteristics: amplitude, frequency, and wavelength.

To use the ocean wave analogy an analog signal's amplitude is like the height of a wave rolling in onto the beach. The frequency of an analog signal can be compared to how fast the waves roll in.  Wavelength can be compared to the distance between one wave and the next wave.  Wavelength is measured as the distance between the peak of one wave and the next.

 

Advantages and Disadvantages of Analog Signals

Analog signals are variable and can convey more subtly than a digital signal.  For example the human voice is analog, and has more tone than a digital representation of the same voice.  However, analog signals are very vulnerable to interference from outside forces and other waves which can cancel them out.

Digital Network Signaling

A digital signal is made up of on/off states.  Unlike the smooth curve of an analog wave, the digital signal cuts on and off, like morse code.  This happens to perfectly fit the type of communication inside a computer, which is made up of on/off states as well.

Advantages and Disadvantages of Digital Signals

Digital signals are much more reliable than analog signals because they are less vulnerable to interference and errors. However, digital equipment costs more and is much more complex.

Basic Computer Networking Concepts

Modulation

Modulation is the technique by which data is turned into an analog wave or a digital signal. There are different techniques of doing this. Amplitude modulation mixes a plain carrier wave with another data wave to change the amplitude of the carrier wave. Frequency modulation changes the frequency of the carrier wave to encode digital data into the carrier wave. A fast frequency may stand for a one whereas a normal frequency means zero. The most complicated type of modulation is analog modulation that uses phase to encode data. This changes the wave's starting point to encode data.

Analog Modulation is used in AM radio. Frequency Modulation is used in FM radio. Old broadcast television uses analog modulation for video, frequency modulation for sound, and phase modulation for color.

Digital modulation uses much less power than analog modulation, and is one of the reasons why small battery powered devices tend to use digital modulation. In digital modulation a clock is continuously sending on pulses, such that the signal looks like it is turning on and off extremely fast. The on and off signal that needs to be transmitted is mixed into these clock pulses.

Multiplexing

Multiplexing is the technique by which multiple network communications are transmitted on a single physical wire. It can be compared to a highway with multiple lanes and cars. In the physical electronic version multiple signals are combined into a single signal using a multiplexer. On the other end of the wire a demultiplexer performs the reverse operation to break the single signal into multiple signals again.

Time division multiplexing gives each signal using the wire an equal share of time to communicate over the wire. After one signal has used its share of time the next signal starts using the wire. These single shares of time are extremely short so that it appears as if all signals are using the same wire at the same time. Time division multiplexing is wasteful, however, because if one signal does not have anything to communicate it still gets a share of the wire time, which is then wasted.

Statistical multiplexing is more complicated. It measures how much each signal needs to use the wire, and signals with more data to transmit get a greater share of time using the wire. This maximizes the bandwidth by ensuring that very little time is given to signals that have very little data to transmit.

Frequency division multiplexing gives each signal is own frequency to communicate it. This allows multiple signals to communicate at the same time, rather than having to divide wire usage time into dedicated segments.

Wavelength division multiplexing is used for fiber optic multiplexing. Beams of light from different carrier waves are mixed into a single beam of light, as if by a prism. On the other end the wavelength demultiplexer works like a prism to split the light into different wavelengths again. Dense wavelength division multiplexing is an extreme version of wavelength division multiplexing, which breaks light up into 80 to 160 separate channels. It requires very high quality equipment that is extremely expensive.

Simplex, Half-Duplex, and Full-Duplex

Simplex network communication is only one direction. An example may be television, radio, or a baby monitor. You can receive communication but you can not send communication. In half-duplex network communication data can travel in both directions, but only in one direction at a time. An example may be a walkie-talkie in which you can send and receive, but not do both at the same time. In full-duplex communication you can send and receive at the same time, for example on a phone.

Point to Point versus Point to Multipoint

In point to point communication one sender is communication with one receiver. In point to multipoint communication one sender's signal is heard by many receivers. This is also called broadcast or multicast communication as opposed to non-boradcast point to point communication.

Throughput versus Bandwidth

These two terms are often used interchangeably, but technically they mean two different things. Bandwidth refers to the range of frequencies used on a channel. This range is closely correlated with the possible speed of the communication. Throughput is the actual measure of how many bits you can transmit per second. Because of the close relationship between bandwidth and throughput the term bandwidth is often used to mean throughput.

Throughput is reduced by transmission flaws such as noise, which may result in transmission errors that have to be corrected; and latency, which slows communication times.

Baseband versus Broadband

Baseband is a transmission form in which there is only one communication channel. Broadband has multiple channels. An example of this might be cable television.

Transmission Flaws

Noise is caused in a communication by electromagnetic interface, cross talk between wires, radio frequency interface. Noise causes analog waves to no longer be smoothly curved.

In a digital communication noise causes the square on and off state of the network to appear slightly jagged. A analog signal is extremely hard to clean of noise. Amplifiers to boost the analog signal may even exaggerate the noise. With a digital signal repeaters can effectively clean noise from the signal completely. Noise can also be limited by shielding wires with metal conduit, or choosing network mediums which are more resistant to noise.

Attenuation is the loss of a signal's strength. The further the signal gets from the source the less distinct it becomes. Amplifiers for analog signals and repeaters for digital signals boost signals to remove attenuation. Amplifiers and repeaters are simple devices that fit in the physical layer of the OSI model. They have not understanding of what they are amplifying or repeating.

Latency is a third transmission flaw. The farther away from each other two network hosts are the more latency there will be. Latency depends on the type of network used. Fiber optic will have less latency than copper wire networks, which will in turn have less latency than a wireless network. Latency can also be caused by bottlenecks and slow hardware on a network.

BASIC CONCEPTS OF COMPUTER NETWORKS

Definition of Computer Networks: A computer network is a Devices – printer, scanner collection of computers Communication devicesand devices NIC Router connected together via Hub ?? ??communication devices Transmission Mediaand transmission media.For examples it may Physical connect computers, Infrared printers and scanners. Radiowave Satellite

Definition of Communication Communication describes a process in which two or more computer or devices transfer data, instructions and information.

The Importance/Advantage of Computer Networkss Sharing of devices such as printer and scanner a Sharing of program/software a Sharing of files, Sharing of dataa Sharing of information, Sharing of single high-speed internet connectiond Can access server centered database B Better communication using internet services such as email, mailing list and Internet Relat Chat (IRC)

Types of Computer Networks: A local area network is a network that connects Local Area Network (LAN) computers and device in a limited geographical area such as a home, school computer laboratory, office building.

A metropolitan area network (MAN) is a high speed Metropolitan Area network that connects local area networks in a Network (MAN) metropolitan area such as city or town and handles bulk of communication activity across the region A MAN typically includes one or more LAN but covers a smaller geographic area than a WAN.

A wide area network is a network that covers a large: Wide Area Network geographical area such country or the world (WAN) WAN combines many types of media such as telephone lines, cables and radio wave. A WAN can be one large network or can consist of two or more LANs connected together The internet is the worlds largest WAN.

Differentiate between the types of Computer Networks Different LAN MAN WAN Cost Low optic High Higher Network Size Small Larger Largest Speed Fastest Slower Slowest Transmission Twisted-pair Twisted-pair Fiber optic Media Fibre-optic cables Radio wave Satellite Number of Smallest Large Largest Computers

Network Architecture Network architecture is the overall design of a computer network that describes how a computer network is configured and what strategies are being used. It is mainly focuses on the function of the networks. It is also known as network model or network design. Two main network architecture:

A server is a computer that provides services to clients and controls access to hardware, software and other resources A client is a computer that request services from a server computer.

On a client/server network, one computer act as a server Client/Server that provides services and the other computers (client) on the network request services from the server. A server is a computer that controls access to the hardware, software and other resources on the network and provides a centralized storage area for program. A client is a computer that request services from a server computer. Peer-to-peer is a simple, inexpensive network that Peer-to-Peer typically connects fewer than 10 computers. All computers in the network have equal capabilities to use the resources (hardware, software, data and file) available on the network. With peer-to-peer networks, there is no central server.

The Differences between Client/Server and Peer-to-Peer Client/Server Peer-to-Peer1) Server has to control ability 1) All computers have equal while client’s don’t ability

2) Higher cabling cost 2) Cheaper cabling cost3) It is used in small and large 3) Normally used in small networks networks with less than 104) Easy to manage computers5) Install software only in the 4) Hard to manage server while the clients share 5) Install software to every the software computer6) One powerful computer 6) No server is needed acting as server