Topic%203%20-%20Fundamental%20Concepts%20in%20Wireless%20Networks

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Topic 3 - Fundamental Concepts in Wireless
Networks
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(No Transcript)
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Sensor networks are another form of
infrastructureless network, with many
similarities to ad-hock
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Fundamental concepts in wireless networks

• Sharing Resources
• Cellular concepts (reuse resources)
• WLAN (shared space)
• Adhock (shared resources)
• Sensor (shared resources, large space)

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What is a Cell?

• Cell is the Basic Union in The System
• defined as the area where radio coverage is given
  by one base station.
• A cell has one or several frequencies, depending
  on traffic load.
• Fundamental idea Frequencies are reused, but not
  in neighboring cells due to interference.

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Cell characteristics

• Implements space division multiplex base station
  covers a certain transmission area (cell)
• Mobile stations communicate only via the base
  station
• Advantages of cell structures
• higher capacity, higher number of users
• less transmission power needed
• more robust, decentralized
• base station deals with interference,
  transmission area etc. locally
• Problems
• fixed network needed for the base stations
• handover (changing from one cell to another)
  necessary
• interference with other cells
• Cell sizes from some 100 m in cities to, e.g., 35
  km on the country side (GSM) - even less for
  higher frequencies

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Different Types of Cells
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Capacity Spectrum Utilization
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Cell Planning (1/3)

• The K factor and Frequency Re-Use Distance

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Cell Planning (2/3)
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Cell Planning (3/3)

• Cell sectoring
• Directional antennas subdivide cell into 3 or 6
  sectors
• Might also increase cell capacity by factor of 3
  or 6
• Cell splitting
• Decrease transmission power in base and mobile
• Results in more and smaller cells
• Reuse frequencies in non-contiguous cell groups
• Example ½ cell radius leads 4 fold capacity
  increase

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Hierarchical Cell Structures (HCS) (1/2)

• HCS allows traffic to be directed to a preferred
  cell
• Each cell is defined in a particular layer
• The lower the layer, the higher the priority
• Mobiles will select a cell on the lowest layer as
  long as it has sufficient signal strength, even
  if higher layer cell are stronger

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WLAN Definition

• A fast-growing market introducing the flexibility
  of wireless access into office, home, or
  production environments.
• Typically restricted in their diameter to
  buildings, a campus, single rooms etc.
• The global goal of WLANs is to replace office
  cabling and, additionally, to introduce a higher
  flexibility for ad hoc communication in, e.g.,
  group meetings.

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WLAN Characteristics

• Advantages
• very flexible within radio coverage
• ad-hoc networks without previous planning
  possible
• wireless networks allow for the design of small,
  independent devices
• more robust against disasters (e.g., earthquakes,
  fire)
• Disadvantages
• typically very low bandwidth compared to wired
  networks (11 54 Mbit/s) due to limitations in
  radio transmission, higher error rates due to
  interference, and higher delay/delay variation
  due to extensive error correction and error
  detection mechanisms
• offer lower QoS
• many proprietary solutions offered by companies,
  especially for higher bit-rates, standards take
  their time (e.g., IEEE 802.11) slow
  standardization procedures
• standardized functionality plus many enhanced
  features
• these additional features only work in a
  homogeneous environment (i.e., when adapters from
  the same vendors are used for all wireless nodes)
  
• products have to follow many national
  restrictions if working wireless, it takes a very
  long time to establish global solutions

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WLAN Design goals

• global, seamless operation of WLAN products
• low power for battery use (special power saving
  modes and power management functions)
• no special permissions or licenses needed
  (license-free band)
• robust transmission technology
• simplified spontaneous cooperation at meetings
• easy to use for everyone, simple management
• protection of investment in wired networks
  (support the same data types and services)
• security no one should be able to read others
  data, privacy no one should be able to collect
  user profiles, safety low radiation
• transparency concerning applications and higher
  layer protocols, but also location awareness if
  necessary

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WLAN Technology Overview

• Core technologies (IEEE 802.1x family)
• IEEE 802.11 (Wireless LAN)
• IEEE 802.15 (Wireless PAN Bluetooth)
• IEEE 802.16 (Wireless M(etropolitan) AN) Under
  development
• Facilitating technologies
• RF-Id
• IrDA
• Home-RF

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WLAN Technology

• Can be categorized according to the transmission
  technique being used
• Infrared (IR) LANs Very limited coverage area
  (IR cant penetrate walls!)
• Spread Spectrum LANs Operate in Industrial,
  Scientific, and Medical (ISM) bands
• Narrowband Microwave LANS Operate at microwave
  frequencies but not using spread spectrum (in
  licensing or ISM bands)

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WLAN infrared vs. radio transmission

• Infrared
• uses IR diodes, diffuse light, multiple
  reflections (walls, furniture etc.)
• Advantages
• simple, cheap, available in many mobile devices
• no licenses needed
• simple shielding possible
• Disadvantages
• interference by sunlight, heat sources etc.
• many things shield or absorb IR light
• low bandwidth
• Example
• IrDA (Infrared Data Association) interface
  available everywhere

• Radio
• typically using the license free ISM band at 2.4
  GHz
• Advantages
• experience from wireless WAN and mobile phones
  can be used
• coverage of larger areas possible (radio can
  penetrate walls, furniture etc.)
• Disadvantages
• very limited license free frequency bands
• shielding more difficult, interference with other
  electrical devices
• Example
• WaveLAN, HIPERLAN, Bluetooth

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WLAN Spread Spectrum

• Most popular category!
• Spread Spectrum Communications
• Developed initially for military and intelligence
  requirements
• Essential idea Spread the information signal
  over a wider bandwidth to make jamming and
  interception more difficult
• Frequency hopping
• Direct sequence spread spectrum

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WLAN infrastructure vs. ad-hoc networks
infrastructure network
AP Access Point
AP
AP
wired network
AP
ad-hoc network
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WLAN Infrastructure-based networks

• Infrastructure networks provide access to other
  networks.
• Communication typically takes place only between
  the wireless nodes and the access point, but not
  directly between the wireless nodes.
• The access point does not just control medium
  access, but also acts as a bridge to other
  wireless or wired networks.
• Several wireless networks may form one logical
  wireless network
• The access points together with the fixed network
  in between can connect several wireless networks
  to form a larger network beyond actual radio
  coverage.
• Network functionality lies within the access
  point (controls network flow), whereas the
  wireless clients can remain quite simple.
• Use different access schemes with or without
  collision.
• Collisions may occur if medium access of the
  wireless nodes and the access point is not
  coordinated.
• If only the access point controls medium access,
  no collisions are possible.
• Useful for quality of service guarantees (e.g.,
  minimum bandwidth for certain nodes)
• The access point may poll the single wireless
  nodes to ensure the data rate.
• Infrastructure-based wireless networks lose some
  of the flexibility wireless networks can offer in
  general
• They cannot be used for disaster relief in cases
  where no infrastructure is left.

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WLAN ad-hoc networks

• No need of any infrastructure to work
• greatest possible flexibility
• Each node communicate with other nodes, so no
  access point controlling medium access is
  necessary.
• The complexity of each node is higher
• implement medium access mechanisms, forwarding
  data
• Nodes within an ad-hoc network can only
  communicate if they can reach each other
  physically
• if they are within each others radio range
• if other nodes can forward the message

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WLAN Standards
WirelessLAN
2.4 GHz
5 GHz
802.11(2 Mbps)
802.11b(11 Mbps)
802.11g(22-54 Mbps)
HiSWANa(54 Mbps)
802.11a(54 Mbps)
HiperLAN2(54 Mbps)
HomeRF 2.0(10 Mbps)
Bluetooth(1 Mbps)
HomeRF 1.0(2 Mbps)
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WLAN Standards (ii)

• IEEE 802.11 and HiperLAN2 are typically
  infrastructure-based networks, which additionally
  support ad-hoc networking
• Bluetooth is a typical wireless ad-hoc network
• IEEE 802.11b offering 11 Mbit/s at 2.4 GHz
• The same radio spectrum is used by Bluetooth
• A short-range technology to set-up wireless
  personal area networks with gross data rates less
  than 1 Mbit/s
• IEEE released a new WLAN standard, 802.11a,
  operating at 5 GHz and offering gross data rates
  of 54 Mbit/s
• Shading is much more severe compared to 2.4 GHz
• Depending on the SNR, propagation conditions and
  the distance between sender and receiver, data
  rates may drop fast
• uses the same physical layer as HiperLAN2 does
• HiperLAN2 tries to give QoS guarantees
• IEEE 802.11g offering up to 54 Mbit/s at 2.4 GHz.
• Benefits from the better propagation
  characteristics at 2.4 GHz compared to 5 GHz
• Backward compatible to 802.11b
• IEEE 802.11e MAC enhancements for providing some
  QoS

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Ad Hoc Networks Definition

• A network made up exclusively of wireless nodes
  without any access points operating in
  peer-to-peer configuration, grouped together in a
  temporary manner.

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Ad Hoc Networks Some Features

• Lack of a centralized entity
• All the communication is carried over the
  wireless medium
• Rapid mobile host movements
• Limited wireless bandwidth
• Limited battery power
• Multi-hop routing

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Ad Hoc Networks Operation

• Assumption
• Unidirectional link
• Adjustable power level
• Directional antenna
• GPS
• Operation
• Broadcasting
• Routing
• Multicasting

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Ad Hoc Networks Challenges (i)

• Hidden terminal problem
• A transmits to B
• C wants transmits to B
• C does not hear As transmission
• Collision
• Exposed terminal problem
• B transmits to A
• C wants to transmit to D
• C hear Bs transmission
• Unnecessarily deferred

A
B
C
B
C
D
A
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Ad Hoc Networks Challenges (ii)

• Challenges
• Mobility
• Scalability
• Power
• Minimizing power consumption during the idle time
  
• Minimizing power consumption during communication
• QoS
• End to End delay
• Bandwidth management
• Probability of packet loss

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Ad Hoc Networks Broadcast (i)

• Objective
• paging a particular host
• sending an alarm signal
• finding a route to a particular host
• Two types
• Be notified -gt topology change
• Be shortest -gt finding route
• A simple mechanism Flooding
• Suffer from broadcast storm

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Ad Hoc Networks Broadcast (ii)
5 forwarding nodes 4 hop time
6 forwarding nodes 3 hop time
source
source
Be notified
Be shortest
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Ad Hoc Networks Routing

• Table Driven vs. On Demand
• DSDV, TORA, DSR, AODV
• Hierarchical and Hybrid
• ZONE
• Specific assumption
• Unidirectional link, Directional antenna, GPS
• QoS-aware
• Power, Delay, Bandwidth

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Ad Hoc Networks Multicast

• Parameter
• The delay to send a packet to each destination
• The number of nodes that is concerned in
  multicast
• The number of forwarding nodes

D
D
D
s
s
s
D
D
D
D
D
D
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Ad Hoc Networks Recommended Introductory Reading

• M. Frodigh, et al, "Wireless Ad Hoc Networking
  The Art of Networking without a Network,"
  Ericsson Review, No. 4, 2000.
• F. Baker, "An outsider's view of MANET," Internet
  Engineering Task Force document, 17 March 2002.
• IEEE tutorial

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Sensor Networks Definition

• A sensor network is a collection of collaborating
  sensor nodes (ad hoc tiny nodes with sensor
  capabilities) forming a temporary network without
  the aid of any central administration or support
  services.
• Sensor nodes can collect, process, analyze and
  disseminate data in order to provide access to
  information anytime and anywhere.

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Sensor Networks Some Features

• Large number of sensors
• Low energy use
• Efficient use of the small memory
• Data aggregation
• Network self-organization
• Collaborative signal processing
• Querying ability

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Sensor Networks Operation

• Sensors work in clusters
• Each cluster assigns a cluster head to manage its
  sensors
• Three layers
• Services layer
• Data layer
• Physical Layer
• To compensate for hardware limitations (e.g.
  memory, battery, computational power)
• Applications deploy a large number of sensor
  nodes in the targeted region.

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Sensor Networks Challenges (i)

• Hardware design
• Communication protocols
• Applications design
• Extending the lifetime of a sensor network
• Building an intelligent data collecting system
• Topology changes very frequently
• Sensors are very limited in power
• Sensors are very prone to failures

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Sensor Networks Challenges (ii)

• Sensors use a broadcast paradigm
• Most networks are based on point to point
  communication
• Sensors may not have a global identification (ID)
• Very large overhead
• Dynamic environmental conditions require the
  system to adapt over time to changing
  connectivity and system stimuli

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Sensor Networks Aggregation

• Some sensor nodes are designed to aggregate data
  received from their neighbors.
• Aggregator nodes cache, process and filter data
  to more meaningful information.
• Aggregation is useful because
• Increased circle of knowledge
• Increased accuracy level
• Data redundancy
• To compensate for sensor nodes failing

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Sensor Networks Dissemination

• Two ways for data dissemination
• Query driven sink broadcasts one query and
  sensor nodes send back a report in response
• Continuous update sink node broadcasts one query
  and receives continuous updates in response (more
  energy consuming but more accurate)
• Problems
• Intermediate nodes failing to forward a message
• Finding the shortest path (a routing protocol)
• Redundancy a sensor may receive the same data
  packet more than once.

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Sensor Networks Advantages

• Coverage of a very large area through the
  scattering of thousands of sensors.
• Failure of individual sensors has no major impact
  on the overall network.
• Minimize human intervention and management.
• Work in hostile and unattended environments.
• Dynamically react to changing network conditions.
• E.g. Maintain connectivity in case of unexpected
  movement of the sensor nodes.

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Sensor Networks Recommended Introductory Reading

• I. F. Akyildiz, W. Su, Y. Sankarasubramaniam, E.
  Cyirci, A survey on Sensor Networks, Computer
  Networks, 38(4)393-422, March 2002.
• Chee-Yong Chong, S. P. Kumar, Sensor networks
  evolution, opportunities, and challenges,
  Proceedings of IEEE, pp 1247-1256, August 2003.
• IEEE tutorial