AD HOC WIRELESS NETWORKS

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AD HOC WIRELESS NETWORKS

• BY Radwa Bayoumy

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OUTLINE

• Introduction
• Application
• Design Principles and Challenges
• Protocol Layers
• Cross-Layer Design
• Network Capacity Limits
• Energy-Constrained Networks

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Introduction

• An ad-hoc (or "spontaneous") network is a local
  area wireless or temporary plug-in connections.
• In Latin, ad hoc literally means "for this,"
  further meaning "for this purpose only,"

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Applications

• An ad-hoc network has been applied to future
  office or home networks in which new devices can
  be quickly added, using, for example, Bluetooth
  technology in which devices communicate with the
  computer and perhaps other devices using wireless
  transmission.

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Examples of applications

• Data Networks
• Home Networks
• Device Networks
• Sensor Networks
• Distributed Control Systems

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Data Networks

• These types of networks support data exchange
  between laptops, palmtops, personal digital
  assistance (PDA), and other information devices.
  According to the area being covered, the category
  of the network is being identified weather its
  LANs, MANs, and WANs.

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• Although the wireless LANs has a good
  performance at low cost, the ad hoc have some
  advantages over it. Only one access point is
  needed for communication with an already wired
  network and in certain cases no access point is
  needed.
• Its inefficient if a node had to go through an
  access point or a base station to exchange
  information for example between two PDAs right
  next to each other.

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• In wireless MANs the multihop routing is a
  necessary as they cover a large area. For the
  high mobility users (example military programs)
  it complicated to communicate with the lake of
  centralized network control. The ad hoc network
  had offered the solution for such case with a
  limited success.

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• In Wireless WANs its used where network infra
  structure cannot be developed and network that
  must be build up and torn out quickly as in
  military application.

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Home Networks

• All the house electronics equipment can be
  linked together by the help of the ad hoc
  network. Such a network could enable a smart room
  from the light adjustment to the fire alarm that
  is connected to the fire department. These types
  of application are mostly used by the people with
  certain disabilities.

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Device Networks

• Short range wireless connection between devices
  is supported by device Networks. Usually such
  connection is wired. Devices as cell phones,
  modems, headsets, PDAs, computers, printers,
  projectors, network access point, the main
  technology drivers for a network between them can
  be one at low cost, low power radio with network
  capabilities such as Bluetooth, Zigbee, and UWB.

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Sensor Networks

• These networks consist of small nodes with
  sensing, computation, and wireless network.
  Sensor array can be deployed and used for remote
  sensing in nuclear power plants, mines, and
  military applications.

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Distributed control systems

• Distributed Control applications, with remote
  plants, sensors, and actuator linked together by
  wireless communication channel are enable by ad
  hoc wireless networks. The ad hoc wireless
  networks can be used to support coordinated
  control of multiple vehicles in an automated
  highway system.

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Design Principles and Challenges

• The most design principles and challenges of an
  ad hoc network come from the fact of its lack of
  infrastructure. For wireless networks there is no
  peer to peer communication.
• On the other side ad hoc wireless network has
  peer to peer communication, networking and
  control functions that are distributed among all
  nodes, and routing that can exploit intermediate
  nodes are relays.

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• Ad hoc wireless networks may create structure
  to improve network performance, although its not
  a fundamental design requirement of the network.
  Ad hoc wireless networks can form a backbone
  infrastructure from a subset of nodes in a
  network or some nodes may be chosen to perform as
  base stations for neighboring nodes. If a node in
  this backbone subset leaves the network, the
  backbone can be reconfigured.

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• One of the biggest challenges in ad hoc wireless
  networks design is energy constrain. These
  constrains arise in wireless network nodes power
  by batteries that cannot be recharged. Therefore
  energy consumption must be optimized over all
  aspects of the network design.

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Protocol Layer

• A protocol is a set of rules that govern the
  operation of functional units to achieve
  communication. There is an international standard
  called OSI (open system interconnection) model
  that was developed as a framework for protocol
  layer in data networks. From this standard model,
  different models were derived as the TCO
  (transport control protocol) and IP (internet
  protocol). The TCP and IP protocol consist of
  five layer model.

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Application
Transport
Network
Access
Physical
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Physical Layer Design

• The Physical Layer which is also referred to as
  the link layer, deals primarily with transmitting
  bits over a point-to-point wireless link
• In ad hoc networks bits are pocketsize for
  transmission. The link packets error rate (PER)
  is determined by physical layer along with the
  channel and interference conditions.

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• A multiple antennas can be used with ad hoc
  wireless network. In fact it can increase the
  data rate on the link, by providing diversity to
  fading so that average BER is reduced leading to
  a fewer retransmission. It also can provide
  directionality to reduce fading and the
  interference of signal causes to anther.
  furthermore multiplexing will increase the link
  rate, which reduces overcrowding and delay in
  the link and benefits all multiple routes using
  that link.

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• Considering the transmitted power of all nodes
  in the network, it must be optimized with respect
  to all layers that it impacts. As increasing
  transmit power at the physical layer reduce PER
  and that decrease the retransmission required at
  the access layer, but a high transmit power from
  one node of the network can cause significant
  interference to the other nodes. Therefore SINR
  drives the performance in an ad hoc wireless.
• The transmit power coupled with adaptive
  modulation and coding for a given node defines
  its local neighborhood- the collection of
  nodes that it can reach in a single hop - and
  defines the context in which access, routing, and
  other higher layer protocols operate.

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Access Layer Design

• Access layer is the layer that controls how
  different users share the available spectrum and
  ensures successful reception of packets
  transmitted over this shared spectrum.
• There are two types of access either multiple
  access or random access.

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• Multiple access divides the signaling
  dimensions into dedicated channel.
• The common used methods of multiplexing are
  TDMA, FDMA, and CDMA.
• In random access, channels are assigned to
  achieve user dynamically, and in multihop
  networks these protocols must content with hidden
  and expose terminals.

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• The power control across the network is also a
  part of the access layer functions. The main role
  of power control is to insure that SINR targets
  can be met on all links in the network.

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• Assume an ad hoc wireless network with K nodes
  and N links between different transmitter-receiver
  pairs of these nodes. The SINR link K is given
  by

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• The access layer is also responsible for
  retransmitting of packet received in error over
  the wireless link.
• The data packets have an error detection code
  that is used by the receiver to determine if one
  or more bits in the packets were corrupted and
  cannot be corrected. For such packet
• the receiver will discard the corrupted
  packets and inform the transmitter by a feedback
  that the packet must be retransmitted.
• or
• The access layer can save it and use a form of
  diversity to combine the corrupted packet with
  the retransmission packets for a higher
  probability of correct packet reception.
• or
• An alternative retransmitting the original
  packets in its entirety the transmitter can just
  send some additional coded bits to provide a
  stronger error correction capability for the
  original packets to correct for its corrupted
  bits.

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Network Layer Function
Neighbor discovery
Routing
Dynamic Resource Allocation
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How does neighbor discover occurs?
A group of neighboring nodes with some initial
transmit power
If not discovered
The transmit power is increased
Until all nodes connections established or the
maximum power is reached
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Routing
Flooding Routing
Proactive Routing
Reactive Routing
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Flooding
A packet is broadcast to all nodes within
receiving range
Nodes also broadcast the packet
Forward continues until the packet reaches it
destination
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Advantages of Flooding
Highly flexible to changing network topologies
Requires little routing overhead
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Disadvantages of Flooding
Wasting Bandwidth
Battery power of transmitting nodes
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Centralization

• approach information about channel conditions and
  network topology

forwarded to a centralized location that computes
the routing tables for all nodes in the network
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Advantages
minimum average delay
minimum number of hops
minimum network congestion.
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Disadvantages
cannot adapt to fast change in the channel
condition and network topology
requires much over head for periodically
collecting local node information
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Reactive routing
source node initiates a route-discovery process
when it has data to send
It will determine if one or more route are
available to the destination.
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Advantages
can be obtained with relatively little overhead
Disadvantages
significant initial delay
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Advantages
can be obtained with relatively little overhead
Disadvantages
significant initial delay
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Resource Allocation and flow Control
Identify the route a packet should follow from a
source to its destination
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How are Routing, resource allocation flow
control related?
Routing based on minimum delay
Delay is a function of the link data rate or
capacity
The higher the capacity, the more data that can
flow with min. delay
The link capacity depends on the resources
allocated to the link
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Fij
Dij
Cij- Fij

• Fij traffic flow
• Dij delay on a link
• Cij capacity

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Link Utilization Formula
Fij
Dij
Cij
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Transport Layer Functions(end to end functions)
Transport layer
Error recovery
Retransmission
Reordering
Flow control
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Application Layer Functions
Generates the data to be sent over the network
Processes the corresponding data received over
the network
Provides compression of the application data
along with error correction and cover up
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MDC
Multiple description coding
A form of compression
Multiple descriptions of the data are generated
The original data can be reconstructed from any
of these descriptions with some loss
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Cross Layer Design

• The layering approach to wireless network
  design, where each layer of the protocol stack is
  unaware to the design and operation of other
  layers, has not worked well in general
• Cross-layer design clearly requires
  information exchange between layers,
• adaptively to this information at each
  layer,
• diversity built into each layer

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• interaction between layers
• Ease of modifying the functionality of one of
  the layers
• May need to generate a new protocol stack each
  time a small change is modified
• Long term survivability of such architectures?

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Network Capacity Limits

• Capacity is the set of maximum data rates
  possible between all nodes.
• The capacity region has dimension of K(K - 1)
• For a large K,
• the per-node rate is 1/vK logK ,
• and the throughput is vK/logK

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Energy-ConstrainedNetworks
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Batteries in nodes and devices
Nodes are powered by batteries with a limited
life time
Devices with rechargeable batteries must conserve
energy to max. time between recharges
Some devices can not be recharges
Some operate solely form the environment
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Energy constraints associated with node operation
Hardware operation
Transmit power
Signal Processing
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Design consideration with energy constrained
nodes
Modulation Coding
MIMO and Cooperative MIMO
Access, Routing, Sleeping
Cross Layer Design under Energy Constraints
Capacity Per Unit Energy
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Modulation coding are based on
Required transmit power
Data rate
BER
Complexity
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Modulation
The design choice should be based on the total
energy consumption
Circuit energy consumption increases with
transmition time
Decrease transmition time putting nodes to sleep
Example between M-ary Modulation and binary
modulation
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Coding
Reduces the required transmit energy per bit for
a given BER target
Some coding schemes encode bits into a codeword
that is longer than the original bit
sequence Such as block codes Convolution
codes
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continue
The total transmit energy required for the
codeword to be sent , a longer transmission time
consumes more circuit energy, and band width
expansion
MQAM is more efficient then MFSK
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MIMO
Multiple Inputs Multiple outputs
Functions
Multiplexing gain Provides a higher data rate
Diversity gain Provides a lower BER in fading
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Continue
Provide energy savings over a single antenna
system for most transmition distances
Why?
The reason is that MIMO systems can support a
higher data rate for a given energy per bit. So,
it transmits the bits quicker and then it shuts
down
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Cooperative MIMO
Small nodes that can not support multiple
antennas they are grouped together to form a
transmitter while others form a receiver
Distance between the nodes is small so the energy
associated is small
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Cooperative MINO
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AccessHow to be more energy efficient?
Minimizing collision
Optimizing transmit power
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Routing
Routing is affected by energy consumption
distributed across all node
Routing optimization to minimize end to end
consumption
How
By applying the Standard optimization procedure
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Sleep
Nodes consume power even in stand by mode
How to solve?
By scheduling sleep periods for nodes
Each node to only listen during a certain period
of time