Network protocol stack

Application Source coding

Packet re-ordering,
Transport congestion control (e.g., TCP)

Network Routing (e.g., IP)

Error correction, MAC,
Data-link encryption, MUX
Physical Modulation, power control,
filtering, spreading

„ Provides abstraction when designing layers

„ We'll focus on MAC, Network, Transport and Cross-
layer designs
 What is wireless communication?
„ Any form of communication that does not require the transmitter
and receiver to be in physical contact
„ Electromagnetic wave propagated through free-space
„ Radar, RF, Microwave, IR, Optical
„ Simplex: one-way communication (e.g., radio, TV)
„ Half-duplex: two-way communication but not simultaneous (e.g.,
push-to-talk radios)
„ Full-duplex: two-way communication (e.g., cellular phones)
„ Frequency-division duplex (FDD)
„ Time-division duplex (TDD): simulated full-duplex
 EM spectrum

4.3x1014
7.5x1014

1017
109
1019
1012

[imagine.gsfc.nasa.gov/docs/science/kno
w_l1/emspectrum.html]
 Why use wireless communication?

„ Provides mobility
„ A user can send or receive a message no matter where he or
she is located
„ Added convenience/reduced cost
„ Enables communication without installing an expensive
infrastructure
„ Can easily set-up temporary LANs
„ Disaster situations
„ Office moves

„ Developing nations utilize cellular telephony rather than laying

twisted-pair wires to each home
„ Only use resources when sending or receiving a signal
 How does mobility affect…
„ Hardware
„ Lighter
„ More robust
„ Lower power (battery operation)
„ Wireless communication
„ Time-varying channels
„ Network protocols
„ Name/address/location changes
„ Delay changes
„ Error rate changes
„ Fidelity
„ High fidelity may not be possible
 How does mobility affect…
„ Security
„ Lighter-weight algorithms
„ Endpoint authentication harder
„ Devices more vulnerable
„ Performance
„ Network, CPU all constrained
„ Delay and delay variability
„ Operating systems
„ New resources to track and manage: energy
„ Applications
„ Name changes
„ Changes in connectivity
„ Changes in quality of resources
„ People
„ Introduces new complexities, failures, devices
 Example changes
„ Addresses
„ Phone numbers, IP addresses
„ Network performance
„ Bandwidth, delay, bit error rates, cost, connectivity
„ Network interfaces
„ WiFi, Ethernet
„ Between applications
„ Different interfaces over phone & laptop
„ Within applications
„ Loss of bandwidth triggers change from color to B&W
„ Available resources
„ Files, printers, displays, power, even routing
 Summary
Generally, mobility stresses all resources further:
„ CPU

„ Power

„ Bandwidth

„ Delay tolerance

„ Radio spectrum

„ Human attention

„ Physical size

„ Constraints on peripherals and GUIs (modality of

interaction)
 Wireless environments

„ Differ in „ Examples
„ Mobility „ Cellular telephony
„ Type of application „ Satellite
„ Type of environment „ Metropolitan-area data
„ Media characteristics networks
„ Pervasiveness of hosts „ Local-area networks
„ Level of infrastructure „ Personal-area networks
„ Visibility of infrastructure „ Ubiquitous computing
„ Coverage environments
„ Cost „ Ad hoc networks
„ Sensor networks
 Why is wireless different than
wired?
„ Noisy, time-varying channel
„ BER varies by orders of magnitude
„ Environmental conditions affect transmission
„ Shared medium
„ Other users create interference
„ Must develop ways to share the channel
„ Bandwidth is limited
„ FCC determines how spectrum is allocated
„ ISM band for unlicensed use (902-928 MHz, 2.4-2.5 GHz and
5.725-5.875 GHz)
„ Requires intelligent SP and comm. to make efficient use of
limited bandwidth in error-prone environment
 Why is wireless different than
wired?
„ Major differences due to broadcast nature
of wireless communication
„ Transmitted signals can be received by an arbitrary (and
perhaps unknown) number of other users
„ Cannot guarantee a link from every transmitting node to
every intended receiving node
„ Each transmitted message utilizes scares resources (BW)
„ Need to provide means for fair and efficient utilization of

available bandwidth among transmitting nodes

„ Transmitted signal power important parameter
„ Require enough signal power to reach destination node

„ Want to limit signal power to minimize interference and

max battery life

 Network topologies
„ Types of network topologies
„ Centralized
„ De-centralized (peer-to-peer)
„ Hybrid
„ Centralized (hub-and-spoke) topology
„ Communication from one node to another goes through hub
(base station)
„ Hub station controls nodes and monitors transmissions from
each node
„ Hub manages access by nodes to network’s allocated
bandwidth
„ Configuration for cellular systems and many WLAN networks
 Decentralized topologies
„ Decentralized (peer-to-peer) topology
„ Fully-connected network
„ All nodes can communicate directly
„ Requires nodes to be co-located
„ Multi-hop network
„ If nodes cannot directly reach destination,
intermediate nodes must relay messages to
destination
„ Widely used in ad-hoc networks where cannot
guarantee connectivity of all nodes
 Centralized topology
„ Advantages
„ Efficient use of transmit power
„ Compared with fully-connected peer-to-peer, nodes can reach
other nodes twice the distance with same signal power (since BS
high power)
„ Hub/BS can be appropriately placed to minimize obstruction
„ Hub/BS provides connection to backbone network Æ reason
many WLANs have centralized topology
„ Nodes can be made simple and BS complex
„ Helpful for power control Æ a central point can determine
required power for nodes to minimize interference and conserve
battery
„ Hub can provide common timing reference
 Centralized topology (cont.)
„ Disadvantages
„ Single point of failure
„ Delay due to multiple transmit/receive operations
„ Cannot deal with unpredictable propagation
environments
„ Cannot cover wide areas where connections exceed
range of single link
„ Not suitable for ad-hoc networks
„ Requires significant infrastructure setup
 Fully connected peer-to-peer
„ Advantages
„ No single point of failure
„ No store-and-forward delay
„ No routing so complexity of nodes reduced
„ Can provide a node that is a gateway to backbone network
„ Disadvantages
„ Performance degradation in large networks
„ Near-far problem
„ Transmitters operating at high power levels (to reach far
station) will interfere with unintended receivers in close
proximity
 Multi-hop peer-to-peer
„ Advantages
„ Power efficiency if Tx power dominates over Rx power
„ Only solution if no infrastructure available
„ Widely used in military applications
„ Gaining popularity in other types of wireless networks
„ Ad hoc networks
„ Sensor networks
„ Disadvantages
„ Complex algorithms for efficient message routing and control
„ Multiple store-and-forwards Æ increase delay for users
separated by multiple hops
„ Overhead to set up efficient routes
„ No central timing or power control authority
 Types of networks
„ WiFi / 802.11
„ Two modes
„ Centralized: wireless local area data network
„ Peer-to-peer: MAC/PHY for ad hoc networks
„ Ad hoc networks
„ Multi-hop peer-to-peer networks
„ Hybrid networks
„ Unicast, multicast and broadcast networks
„ Wireless sensor networks
„ Different applications uses different network topologies
„ Converge-cast, unicast, broadcast and multicast networks
 Mobile ad hoc networks
„ Definition
„ A collection of wireless mobile hosts forming a temporary network
without the aid of any centralized administration or standard support
services
„ Often ad-hoc network topology is dynamic—nodes enter and
leave the network continuously
„ No centralized control or fixed infrastructure to support network
configuration or reconfiguration
„ Example scenarios for MANETs
„ Meetings
„ Emergency or disaster relief situations
„ Military communications
„ Wearable computers
„ Sensor networks
 MANETs (cont.)
„ Mobile nodes have limited communication range
„ Reduces battery drain
„ Enables spatial reuse of limited bandwidth Æ increased
network capacity
„ To connect all nodes in the network, each node is a
„ Packet source
„ Packet sink
„ Router
„ Nodes must route packets for other nodes to keep the
network fully connected
„ In MANETs, a big problem is how to determine where a
destination node is located relative to a sending node
 MANETs (cont.)
„ Route-finding is a current area of much research
„ Want to determine an “optimal” way to find “optimal”
routes
„ Dynamic links
„ Broken links must be updated when a node moves
out of communication range with another node
„ New links must be formed when a node moves into
communication range with another node
„ Based on this new information, routes must be
modified
„ Frequency of route changes a function of node
mobility
 Issues in MANETs
„ Routing performance
„ Routes change over time due to node mobility
„ Would like to avoid long delays when sending packets
„ But would like to avoid lots of route maintenance overhead
„ Want as many participating nodes as possible for greater
aggregate throughput, shorter paths, and smaller chance of
partition
„ MAC
„ Broadcast communication channel
„ Neighbor nodes change over time
„ Nodes sleep to reduce energy drain
„ No coordination/cooperation among nodes
 Issues in MANETs
„ Quality of service
„ Link variability Æ route variability
„ Collisions
„ Congestion
„ Security - interesting new vulnerabilities and
complexities
„ Routing denial of service
„ Nodes may agree to route packets
„ Nodes may then fail to do so
„ Broken, malicious, selfish nodes
„ Key distribution and trust issues
 Wireless sensor networks
„ Microsensors
„ Low power, cheap sensors

„ Sensor module (e.g., acoustic, seismic, image)

„ A digital processor for sig. proc. and network protocol functions

„ Radio for communication

„ Battery-operated

„ Sensors monitor environment

„ Cameras, microphones, physiological sensors, etc.

„ Gather data for some purpose

„ Microsensor data limited in range and accuracy

„ Each node can only gather data from a limited physical area

„ Data may be noisy

„ Data aggregation enables higher quality (less noisy) data, gives

information about larger physical area than individual data
 WSNs (cont.)
„ Hundreds or thousands of nodes scattered throughout an
environment
„ New wireless networking paradigm
„ Requires autonomous operation
„ Highly dynamic environments
„ Sensor nodes added/fail
„ Events in the environment
„ Distributed computation and communication protocols required
„ Microsensor network applications
„ Home security
„ Machine failure diagnosis
„ Chemical/biological detection
„ Medical monitoring
„ Surveillance and reconnaissance
„ Animal/plant monitoring (e.g., for research)
 WSNs (cont.)
„ Networking sensors enables
„ Extended range of sensing Æ improved quality
„ Fault tolerance due to redundancy in data from different
sensors
„ Distributed processing of large amounts of sensor data
„ Scalability: quality can be traded for system lifetime
„ “Team-work”: nodes can help each perform a larger sensing
task
 MANETs vs. WSNs
MANETs WSNs
Unreliable communication Unreliable communication
Require self-configuration Require self-configuration
Constrained energy and Very constrained energy and
bandwidth bandwidth
Small-scale Large-scale
Typically mobile Typically immobile
Competitive Cooperative
Address-centric Data-centric
QoS: delay, etc Application-specific QoS
 Discussion
„ What potential do you see for wireless networks?
„ What do you see as the hardest things for us to
address?
„ If you could wish for one key piece of technology
to come true, what would it be?
„ What applications can you envision if we had
pervasive ad hoc networks?
„ What everyday uses can you think of for wireless
sensor networks?
„ What are user-level issues in wireless
communication that need to be addressed?
 Discussion
„ What is your biggest complaint about current
wireless technology?
„ In what application areas do you see wireless
networks succeeding?
„ In what application areas do you see wireless
networks failing?
„ What do you see as the motivating factors for
using wireless as opposed to wired networks?
„ What do you see as the most pressing research
need to improve wireless networking?