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TRANSCRIPT
Introduction
2
Future Wireless Networks
Wireless Internet accessNth generation Cellular
Wireless Ad Hoc NetworksSensor Networks
Wireless EntertainmentSmart Homes/SpacesAutomated Highways
All this and more…
Ubiquitous Communication Among People and Devices
•Hard Delay Constraints•Hard Bandwidth Constraints
•Hard Energy Constraints
Exciting Developments for Wireless
o Internet and laptop use exploding
o 2G/3G wireless LANs growing rapidly
o Huge cell phone popularity worldwide
o Emerging systems such as Bluetooth, UWB, Zigbee,
and WiMAX opening new doors
o Military and security wireless needs
o Important interdisciplinary applications
Wireless and Mobile Networks
Wireless but not mobile
e.g. wireless home or office networks with stationary
workstations and large display.
Limited mobility do not require wireless links
e.g. a worker who uses a wired laptop at home, shut down the
laptop, drives to work, and attaches the laptop to the
company’s wired network.
Both wireless and mobile
e.g. a mobile user sitting in the back seat of car which travels
160 km per hour.
Wireless and Mobile Networks
o At the intersection of wireless and mobility, we willfind the most interesting technical challenges!
o The challenges posed by these networks are sodifferent from traditional wired computer networks.
(particularly at the data link and network layers)
Definitions I
Wireless hosts : the end-system run the applications.
a laptop, PDA, phone, computer, etc.
Wireless links :
o a host connects to a base station or to another wirelesshost through a wireless communication link.
o Different wireless link technologies have differenttransmission rates and can transmit over differentdistances.
Definitions II
Base station: a key part of the wireless network infrastructure.
If a wireless host is associated with a base station :
1. The host is within the wireless communication distance of the base station.
2. The host uses that base station to relay data between the host and the larger network.
e.g. Cell towers in cellular networks.
Access points in 802.11 wireless LANs.
Definitions III
When a host associated with a base station are often referred to as operating in infrastructure mode.
In ad hoc networks, wireless hosts have no suchinfrastructure with which to connect.
The host themselves must provide services such as routing,address assignment, etc.
network
infrastructure
Elements of a Wireless Network
Computer Networks: A top-down approach
wireless hostso laptop, smartphone
o run applications
o may be stationary (non-mobile) or mobileo wireless does not
always mean mobility
network
infrastructure
Elements of a Wireless Network
base stationo typically connected to
wired network
o relay - responsible forsending packets betweenwired network andwireless host(s) in its“area”
o e.g., cell towers,802.11 access points
network
infrastructure
Elements of a Wireless Network
wireless linko typically used to connect
mobile(s) to base station
o also used as backbonelink
o multiple access protocolcoordinates link access
o various data rates,transmission distance
network
infrastructure
Elements of a Wireless Network
Indoor10-30m
Outdoor50-200m
Mid-range
outdoor200m – 4 Km
Long-range
outdoor5Km – 20 Km
.056
.384
1
4
5-11
54
2G: IS-95, CDMA, GSM
2.5G: UMTS/WCDMA, CDMA2000
802.15
802.11b
802.11a,g
3G: UMTS/WCDMA-HSPDA, CDMA2000-1xEVDO
4G: LTWE WIMAX
802.11a,g point-to-point
200 802.11n
Data
rate
(M
bps)
Link Characteristics of Selected Wireless
Network Standards
infrastructure modeo base station connects
mobiles into wirednetwork
o handoff: mobilechanges base stationproviding connectioninto wired network
network
infrastructure
Elements of a Wireless Network
o ad hoc mode
o no base stations
o nodes can only transmit toother nodes within linkcoverage
o nodes organize themselvesinto a network: routeamong themselves
Elements of a Wireless Network
Wireless Networks
Classification based on the number of hops and infrastructure :
1. Single hop, infrastructure-based.all communications between a host and the base station.
2. Single hop, infrastructure-less.no base station, one node may coordinate the transmissions of
the other nodes.
3. Multi-hop, infrastructure-based.wireless mesh networks.
4. Multi-hop, infrastructure-less.mobile ad hoc networks (MANET)
vehicular ad hoc networks (VANET)
single hop multiple hops
infrastructure
(e.g., APs)
no
infrastructure
host connects to
base station (WiFi,
WiMAX, cellular)
which connects to
larger Internet
no base station, no
connection to larger
Internet (Bluetooth,
ad hoc nets)
host may have to
relay through several
wireless nodes to
connect to larger
Internet: mesh net
no base station, no
connection to larger
Internet. May have to
relay to reach other
a given wireless node
MANET, VANET
Wireless links vs. Wired links
o Decreasing signal lengtho Electromagnetic radiation attenuates as it passes through matter
(a wall, etc.).
o The signal length decreases as the distance between sender andreceiver increases (even in free spaces).
o Interference from other sourceso Radio sources transmitting in the same frequency band will
interfere with each other.
o Multipath propogations
bit errors will be more common in wireless links.
AB
C
Hidden terminal problemo B, A hear each other
o B, C hear each other
o A, C can not hear each othermeans A, C unaware of theirinterference at B
A B C
A’s signal
strength
space
C’s signal
strength
Signal attenuation:o B, A hear each other
o B, C hear each other
o A, C can not hear each otherinterfering at B
Hidden Terminal Problem
More ProblemsAmbiguous collisions : Prevents A from overhearing transmissions from B.
Receiver collisions : The node cannot tell the packet is received.
Multi-transmitter Interference Problem
o Similar to multi-path or noise
o Two transmitting stations will
constructively/destructively interfere with each
other at the receiver
o Receiver will “hear” the sum of the two signals
(which usually means garbage)
Medium Access Control
o Protocol required to coordinate access
transmitters must take turns
o Similar to talking in a crowded room
o Also similar to hub based Ethernet
Carrier Sense Multiple Access (CSMA)
o Procedure
o Listen to medium and wait until it is free (no one else is talking)
o Wait a random back off time then start talking
o Advantages
o Fairly simple to implement
o Functional scheme that works
o Disadvantages
o Can not recover from a collision
o (inefficient waste of medium time)
Carrier Sense Multiple Access
with Collision Detection (CSMA-CD)o Procedure
o Listen to medium and wait until it is free
o Then start talking, but listen to see if someone else starts talking
too
o If a collision occurs, stop and then start talking after a random
back off time
o This scheme is used for hub based Ethernet
o Advantages
o More efficient than basic CSMA
o Disadvantages
o Requires ability to detect collisions
25
CSMA/CD Does Not Work
Collision detection
problems
Relevant contention
at the receiver, not
sender
Hidden terminal
Exposed terminal
Hard to build a radio
that can transmit and
receive at same time
A
B
C
A
B
C
D
Hidden Exposed
26
Hidden Terminal Effect
o Hidden terminals: A, C cannot hear each other
o Obstacles, signal attenuation
o Collisions at B
o Collision if 2 or more nodes transmit at same time
o CSMA makes sense:o Get all the bandwidth if you’re the only one
transmitting
o Shouldn’t cause a collision if you sense another transmission
o Collision detection doesn’t work
o CSMA/CA: CSMA with Collision Avoidance
Carrier Sense Multiple Access
with Collision Avoidance (CSMA-CA)
o Procedure
o Similar to CSMA but instead of sending packets
control frames are exchanged
o RTS = request to send
o CTS = clear to send
o DATA = actual packet
o ACK = acknowledgement
CSMA/CA
29
IEEE 802.11 MAC Protocol:
CSMA/CA802.11 CSMA: sender
o If sense channel idle for DIFS (Distributed Inter Frame Space) then transmit entire frame (no collision detection)
o If sense channel busythen binary backoff
o 802.11 CSMA: receiver
o If received OKreturn ACK after SIFS --Short IFS (ACK is needed due to hidden terminal problem)
30
Collision Avoidance Mechanisms
o Problem:
o Two nodes, hidden from each other, transmit
complete frames to base station
o Wasted bandwidth for long duration!
o Solution:
o Small reservation packets: RTS+CTS
o Nodes track reservation interval with internal
“network allocation vector” (NAV)
31
Collision Avoidance: RTS-CTS Exchange
o Explicit channel reservationo Sender: send short RTS: request
to send
o Receiver: reply with short CTS: clear to send
o CTS reserves channel for sender, notifying (possibly hidden) stations
o RTS and CTS short:o collisions less likely, of shorter
duration
o end result similar to collision detection
o Avoid hidden station collisions
o Not widely used/implementedo Consider typical traffic patterns
Carrier Sense Multiple Access
with Collision Avoidance (CSMA-CA)
o Advantages
o Small control frames lessen the cost of collisions
(when data is large)
o RTS + CTS provide “virtual” carrier sense which
protects against hidden terminal collisions (where
A can’t hear B)
A B
Carrier Sense Multiple Access
with Collision Avoidance (CSMA-CA)
o Disadvantages
o Not as efficient as CSMA-CD
o Doesn’t solve all the problems of MAC in wireless
networks
Exposed Terminal Problem
R1
S1 S2
R2
The sender mistakenly think themedium is in use, so that itunnecessarily defers thetransmission.
RTS
CTS
DATA
R1
time
S1
S2
R2
Find medium in use
1 2 3
Wait until medium is clear
Exposed Terminal Problem
o When a node hears an RTS from aneighboring node, but not thecorresponding CTS, that node candeduce that it is an exposed terminaland is permitted to transmit to otherneighboring nodes.
R1 R2
S1 S2
Wireless Networks
Ad hoc networksmobile ad hoc networks (MANETs)vehicular ad hoc networks (VANETs)
Wireless Sensor Networks (WSN)
Mobile Ad hoc Networks (MANETs)
Self configuring network of mobile nodes connected
by wireless links
Provide communication in the absence of a fixed
infrastructure
Dynamic topology due to mobility
Attractive for many applications
disaster recovery operations
military applications
Vehicular Ad hoc Networks (VANETs)
o Ad hoc network composed of vehicles
o Individual nodes different from traditional wirelessnodes
o No power constraint
o Nodes mostly mobile
o Complements existing infrastructure
o Extends existing infrastructure
Applications
o Traffic controlo Automatic speed limit enforcement
o Rerouting in traffic congestion
o Safetyo Notification of accident up ahead
o Decentralized 911 service
o Prioritized over non-safety applications
o Extended communication
o Will become the largest ad hoc network.
VANET
Two type of communication:
o V2V : vehicle to vehicle
o V2I : vehicle to infrastructure
Basic Components I
OBU : on-board unit
A device inside the vehicle
o Processes the data collected from varioussensors fitted inside the vehicle and givesinformation about the condition of thevehicle
o Responsible for communication with theoutside network (other vehicles,infrastructure)
Basic Components II
RSU : roadside unit
o RSU acts similar to a wireless LAN accesspoint and can provide communications withinfrastructure.
Wireless Sensor Network
“A wireless sensor network (WSN) is a wireless network consisting of spatially distributed autonomous devices using
sensors to cooperatively monitor physical or environmental conditions, such as
temperature, sound, vibration, pressure, motion or pollutants, at different locations.”
- Wikipedia
Mote
A very low cost low power computer
Monitors one or more sensors
A Radio Link to the outside world
Are the building blocks of Wireless Sensor Networks (WSN)
External Memory
Dig
ital
I/O
po
rts
Radio Transceiver
An
alo
g I
/O P
ort
s
Microcontroller
A/D
D/A
Sensor
Sensor
Wireless Sensor Networks Formed by hundreds or thousands of motes that
communicate with each other and pass data along from one to another
Research done in this area focus mostly on energy aware computing and distributed computing
Super Node
Links to Other networks or
Similar Super Nodes
Motes
WSN Applications
Environmental/Habitat monitoring
Acoustic detection
Seismic Detection
Military surveillance
Inventory tracking
Medical monitoring
Smart spaces
Process Monitoring
slides from Nuwan Gajaweera
Habitat Monitoring on Great Duck Island
http://www.greatduckisland.net/
Intel Research Laboratory at Berkeley initiated a collaboration with the College of the Atlantic in Bar Harbor and the University of California at Berkeley to deploy wireless sensor networks on Great Duck Island, Maine (in 2002)
Monitor the microclimates in and around nesting burrows used by the Leach's Storm Petrel
Goal : habitat monitoring kit for researchers worldwide
FireBug
Wildfire Instrumentation System Using Networked Sensors
Allows predictive analysis of evolving fire behavior
Firebugs: GPS-enabled, wireless thermal sensor motesbased on TinyOS that self-organize into networks for collecting real time data in wild fire environments
Software architecture: Several interacting layers (Sensors, Processing of sensor data, Command center)
A project by University of California, Berkeley CA.
Preventive Maintenance on an Oil Tanker in the
North Sea: The BP Experiment
Collaboration of Intel & BP
Use of sensor networks to support preventive maintenance on board an oil tanker in the North Sea.
A sensor network deployment onboard the ship
System gathered data reliably and recovered from errors when they occurred.
The project was recognized by InfoWorld as one of the top 100 IT projects in 2004,
50
Internet protocol stack
o application: supporting network applicationso FTP, SMTP
o transport: host-host data transfero TCP, UDP
o network: routing of datagrams from source to destinationo IP, routing protocols, mobile IP, DHCP
o link: data transfer between neighboring network elementso PPP, Ethernet, WLAN
o physical: bits “on the wire”
application
transport
network
link
physical
Reading
Chapter 6
‘Computer Networks: A top-down approach’, by
Kurose and Ross, 6th Edition, Addison-Wesley