Routing and Transport in Wireless Sensor Networks
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30-10-2009, 04:09 PM
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presented by:Ragul G R.
Routing and Transport in Wireless Sensor Networks
Wireless Sensor Networks (WSN) are fast emerging as a new sensing paradigm based on the collection of large number of sensors deployed close to or inside the phenomenon to be observed, and have the potential of providing diverse services to numerous applications. The realization of WSNs require intensive technical research efforts especially in power aware scalable wireless ad hoc communications protocols due to their unusual application requirements and unique constraints. This seminar and presentation is a study of the design of various routing and transport protocols for WSNs. The design of routing and transport protocols become very important in the sense that the sensors are deployed in very harsh environments with limited power resources. These con¬straints make it necessary to have an application specific protocol architecture than a common traditional layered protocol architecture.
1.1 What is a Wireless Sensor Network?
" A Wireless Sensor Network is an autonomous, ad hoc system consisting of a collection of networked sensor nodes designed to intercommunicate via wireless radio ". A sensor node is a device that produces a measurable response to a change in physical condition. A sensor node is equipped with a power aware CPU with processor speed of a few MHz, a program and data memory of a few KB and a small embedded OS like Tiny OS, Embedded Linux or Windows CE. Wireless Sensor Networks use low power wireless communication of around 100 MHz radio.
1.2 Why specialized protocols are required?
An important challenge in the design of WSNs is that two key resources - communication bandwidth and energy, are significantly more limited than in a tethered network environment. The processing power and storage is also very low. The communication channels are unreliable due to battery depletion and harsh environment. Data aggregation and removal of redundant data become very important in a WSN. Mostly we find that routing and transport in a WSN are intertwined. These constraints require innovative design techniques to use the available bandwidth and energy efficiently.
1.3 The Protocol design goals
• Low energy consumption: To achieve this the communication should be minimal and data aggregation in the network is required. Low node duty cycle is desired and this can be achieved by minimizing individual node responsibility, traffic spreading or load sharing and shut down the nodes whenever possible.
• Robust: The WSN should be able to adapt to unpredictable environment without in¬tervention.
• Scalable: The routing and transport should rely on localized algorithms and no central¬ized control should be there.
• Small Footprint: Must run on hardware with severe memory and computational power constraints.
1.4 The Protocol Stack
• Physical Layer: The physical layer deals with frequency selection, carrier frequency generation, signal detection and modulation. Binary and M-ary modulation schemes are generally used out of which the Binary modulation scheme is more energy efficient.
• Data Link Layer: This layer deals with multiplexing of data streams, data frame de¬tection, medium access and error control. The error control has two modes - forward error correction with decoding complexities and Automatic Repeat Request (ARQ) with additional retransmission energy cost and overhead. The best solution is to have simple error control codes with low complexity encoding and decoding.
Application Layer: Sensor Management Protocol (SMP) makes the hardware and soft¬ware of the lower layers transparent to the sensor network management applications. Sys¬tem administrators interact with sensor networks using SMP. Task Assignment and Data Advertisement Protocol (TADAP) provides the user software with efficient interfaces for interest dissemination. Sensor Query and Data Dissemination Protocol (SQDDP) pro¬vides user applications with interfaces to issue queries respond to queries and collect incoming replies.
Network Layer: The major tasks are to find energy efficient routes and data aggregation. The routing protocol can be based on the following techniques
— Clustering and Cellular
— Energy Aware
Transport Layer: Reliability in transport is dealt in a wireless network using TCP. The link error rates in a wireless network is around 30% more compared to wired networks. Due to the fact that WSNs have very less power ratios, retransmissions reduce the lifetime of the sensors. The transport protocols are very much application specific and we do not have single solution like TCP.
2 Routing Protocols
2.1 Flooding Based Approaches
• Flooding: In flooding, each node receiving a data or management packet repeats it by broadcasting.
• Gossiping: In gossiping, each node sends the incoming packets to a randomly selected neighbor.
Three deficiencies of these classical approaches render them inadequate as a protocol for wireless sensor networks
• Implosion: In classic flooding, a node always sends data to its neighbors, regardless of whether or not the neighbour has already received the data from another source. This leads to the implosion problem. In figure 3, node A starts out by flooding data to its neighbours, B and C. These nodes store the data from A and send a copy to their neighbour D. The protocol thus wastes resources by sending two copies of data to D.
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