23-05-2012, 05:45 PM
wireless sensor networks
Wireless Sensor Networks.pdf (Size: 423.09 KB / Downloads: 70)
Introduction
A wireless sensor network is a collection of nodes organized into a cooperative network [10]. Each node
consists of processing capability (one or more microcontrollers, CPUs or DSP chips), may contain multiple
types of memory (program, data and flash memories), have a RF transceiver (usually with a single omni-
directional antenna), have a power source (e.g., batteries and solar cells), and accommodate various sensors
and actuators. The nodes communicate wirelessly and often self-organize after being deployed in an ad hoc
fashion. Systems of 1000s or even 10,000 nodes are anticipated. Such systems can revolutionize the way we
live and work.
MAC
A medium access control (MAC) protocol coordinates actions over a shared channel. The most commonly
used solutions are contention-based. One general contention-based strategy is for a node which has a message
to transmit to test the channel to see if it is busy, if not busy then it transmits, else if busy it waits and
tries again later. After colliding, nodes wait random amounts of time trying to avoid re-colliding. If two or
more nodes transmit at the same time there is a collision and all the nodes colliding try again later. Many
wireless MAC protocols also have a doze mode where nodes not involved with sending or receiving a packet
in a given timeframe go into sleep mode to save energy. Many variations exist on this basic scheme.
Routing
Multihop routing is a critical service required for WSN. Because of this, there has been a large amount of
work on this topic. Internet and MANET routing techniques do not perform well in WSN. Internet routing
assumes highly reliable wired connections so packet errors are rare; this is not true in WSN. Many MANET
routing solutions depend on symmetric links (i.e., if node A can reliably reach node B, then B can reach A)
between neighbors; this is too often not true for WSN. These differences have necessitated the invention and
deployment of new solutions.
ROUTING
semantics may be that a single node closest to the geographic destination is to be the unicast node. Second,
the semantics could be that all nodes within some area around the destination address should receive the
message. This is an area multicast. Third, it may only be necessary for any node, called anycast, in the
destination area to receive the message. The SPEED [5] protocol supports these 3 types of semantics. There
is also often a need to flood (multicast) to the entire network. Many routing schemes exist for supporting
efficient flooding.
Node Localization
Node localization is the problem of determining the geographical location of each node in the system.
Localization is one of the most fundamental and difficult problems that must be solved for WSN. Localization
is a function of many parameters and requirements potentially making it very complex. For example, issues
to consider include: the cost of extra localization hardware, do beacons (nodes which know their locations)
exist and if so, how many and what are their communication ranges, what degree of location accuracy is
required, is the system indoors/outdoors, is there line of sight among the nodes, is it a 2D or 3D localization
problem, what is the energy budget (number of messages), how long should it take to localize, are clocks
synchronized, does the system reside in hostile or friendly territory, what error assumptions are being made,
and is the system subject to security attacks?
Wireless Sensor Networks.pdf (Size: 423.09 KB / Downloads: 70)
Introduction
A wireless sensor network is a collection of nodes organized into a cooperative network [10]. Each node
consists of processing capability (one or more microcontrollers, CPUs or DSP chips), may contain multiple
types of memory (program, data and flash memories), have a RF transceiver (usually with a single omni-
directional antenna), have a power source (e.g., batteries and solar cells), and accommodate various sensors
and actuators. The nodes communicate wirelessly and often self-organize after being deployed in an ad hoc
fashion. Systems of 1000s or even 10,000 nodes are anticipated. Such systems can revolutionize the way we
live and work.
MAC
A medium access control (MAC) protocol coordinates actions over a shared channel. The most commonly
used solutions are contention-based. One general contention-based strategy is for a node which has a message
to transmit to test the channel to see if it is busy, if not busy then it transmits, else if busy it waits and
tries again later. After colliding, nodes wait random amounts of time trying to avoid re-colliding. If two or
more nodes transmit at the same time there is a collision and all the nodes colliding try again later. Many
wireless MAC protocols also have a doze mode where nodes not involved with sending or receiving a packet
in a given timeframe go into sleep mode to save energy. Many variations exist on this basic scheme.
Routing
Multihop routing is a critical service required for WSN. Because of this, there has been a large amount of
work on this topic. Internet and MANET routing techniques do not perform well in WSN. Internet routing
assumes highly reliable wired connections so packet errors are rare; this is not true in WSN. Many MANET
routing solutions depend on symmetric links (i.e., if node A can reliably reach node B, then B can reach A)
between neighbors; this is too often not true for WSN. These differences have necessitated the invention and
deployment of new solutions.
ROUTING
semantics may be that a single node closest to the geographic destination is to be the unicast node. Second,
the semantics could be that all nodes within some area around the destination address should receive the
message. This is an area multicast. Third, it may only be necessary for any node, called anycast, in the
destination area to receive the message. The SPEED [5] protocol supports these 3 types of semantics. There
is also often a need to flood (multicast) to the entire network. Many routing schemes exist for supporting
efficient flooding.
Node Localization
Node localization is the problem of determining the geographical location of each node in the system.
Localization is one of the most fundamental and difficult problems that must be solved for WSN. Localization
is a function of many parameters and requirements potentially making it very complex. For example, issues
to consider include: the cost of extra localization hardware, do beacons (nodes which know their locations)
exist and if so, how many and what are their communication ranges, what degree of location accuracy is
required, is the system indoors/outdoors, is there line of sight among the nodes, is it a 2D or 3D localization
problem, what is the energy budget (number of messages), how long should it take to localize, are clocks
synchronized, does the system reside in hostile or friendly territory, what error assumptions are being made,
and is the system subject to security attacks?