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Topic 3 - Fundamental Concepts in Wireless Networks


Topic 3 - Fundamental Concepts in Wireless Networks Fundamental concepts in wireless networks Sharing Resources Cellular concepts (reuse resources) WLAN (shared space ... – PowerPoint PPT presentation

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Title: Topic 3 - Fundamental Concepts in Wireless Networks

Topic 3 - Fundamental Concepts in Wireless
(No Transcript)
Sensor networks are another form of
infrastructureless network, with many
similarities to ad-hock
Fundamental concepts in wireless networks
  • Sharing Resources
  • Cellular concepts (reuse resources)
  • WLAN (shared space)
  • Adhock (shared resources)
  • Sensor (shared resources, large space)

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.

Cell characteristics
  • Implements space division multiplex base station
    covers a certain transmission area (cell)
  • Mobile stations communicate only via the base
  • 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)
  • 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

Different Types of Cells
Capacity Spectrum Utilization
Cell Planning (1/3)
  • The K factor and Frequency Re-Use Distance

Cell Planning (2/3)
Cell Planning (3/3)
  • Cell sectoring
  • Directional antennas subdivide cell into 3 or 6
  • 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

Hierarchical Cell Structures (HCS) (1/2)
  • HCS allows traffic to be directed to a preferred
  • 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

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.

WLAN Characteristics
  • Advantages
  • very flexible within radio coverage
  • ad-hoc networks without previous planning
  • wireless networks allow for the design of small,
    independent devices
  • more robust against disasters (e.g., earthquakes,
  • 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
  • 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

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

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
  • Facilitating technologies
  • RF-Id
  • IrDA
  • Home-RF

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)

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
  • 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

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

WLAN infrastructure vs. ad-hoc networks
infrastructure network
AP Access Point
wired network
ad-hoc network
WLAN Infrastructure-based networks
  • Infrastructure networks provide access to other
  • 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
  • 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
  • Collisions may occur if medium access of the
    wireless nodes and the access point is not
  • 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
  • They cannot be used for disaster relief in cases
    where no infrastructure is left.

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
  • The complexity of each node is higher
  • implement medium access mechanisms, forwarding
  • Nodes within an ad-hoc network can only
    communicate if they can reach each other
  • if they are within each others radio range
  • if other nodes can forward the message

WLAN Standards
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)
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

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.

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

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

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

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

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

Ad Hoc Networks Broadcast (ii)
5 forwarding nodes 4 hop time
6 forwarding nodes 3 hop time
Be notified
Be shortest
Ad Hoc Networks Routing
  • Table Driven vs. On Demand
  • Hierarchical and Hybrid
  • ZONE
  • Specific assumption
  • Unidirectional link, Directional antenna, GPS
  • QoS-aware
  • Power, Delay, Bandwidth

Ad Hoc Networks Multicast
  • Parameter
  • The delay to send a packet to each destination
  • The number of nodes that is concerned in
  • The number of forwarding nodes

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

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
  • Sensor nodes can collect, process, analyze and
    disseminate data in order to provide access to
    information anytime and anywhere.

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

Sensor Networks Operation
  • Sensors work in clusters
  • Each cluster assigns a cluster head to manage its
  • 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.

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

Sensor Networks Challenges (ii)
  • Sensors use a broadcast paradigm
  • Most networks are based on point to point
  • 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

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

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.

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.

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