Transcript Wireless Communication Systems

Wireless Communication Systems
Background of Wireless Communication
Wireless Communication Technology
Wireless Networking and Mobile IP
Wireless Local Area Networks
Wireless Personal Area Networks
Wireless Metropolitan Area Networks
Wireless Wide Area Networks
Physical Layer
Overview
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Physical Properties of Wireless
Frequency & Public Use Bands
Free-space Path-loss
Path-loss Exponents
Multi-path Propagation
Multi-Path Effect
Digital Modulation
Examples of Digital Modulation
Multi-transmitter Interference
Symbol Rate & Bandwidth
Thermal Noise
Multi-transmitter Interference
Problem
 Medium Access Control
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CSMA, CSMA-CD, CSMA/CA
 Random Contention Access
 Distributed Coordination Function
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(DCF)
802.11 Contention Window
802.11 Adaptive Contention Window
DCF Protocol Issues
Virtual Carrier Sense
Physical Carrier Sense Mechanisms
Exposed Terminal Problem
Double Exposed Terminal Problem
Point Coordination Function (PCF)
Physical Properties of Wireless
 Makes wireless network different from wired
networks
 Should be taken into account by all layers
Wireless = Waves
 Electromagnetic radiation
 Emitted by sinusoidal current running through a wire
(transmitting antenna)
 Creates propagating sinusoidal magnetic and electric fields
according to Maxwell’s equations:
 Fields induce current in receiving antenna
Wave Propagation Example
electric
field
propagation direction
magnetic
field
Frequency & Public Use Bands
 Propagating sinusoidal wave with some
frequency/wavelength
 C (speed of light) = 3x108 m/s
f 
c
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Name
900 Mhz
2.4 Ghz
5 Ghz
Range
902 - 928
2.4 - 2.4835
5.15 - 5.35
Bandwidth
26 Mhz
83.5 Mhz
200 Mhz
Wavelength
.33m / 13.1”
.125m / 4.9”
.06 m / 2.4”
Free-space Path-loss
 Power of wireless transmission reduces with square of distance
(due to surface area increase of sphere)
 Reduction also depends on wavelength
 Long wave length (low frequency) has less loss
 Short wave length (high frequency) has more loss
 4D 
PL  
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2
Other Path-loss Exponents
 Path-Loss Exponent depends on environment:
Free space
Urban area cellular
Shadowed urban cell
In building LOS
Obstructed in building
Obstructed in factories
2
2.7 to 3.5
3 to 5
1.6 to 1.8
4 to 6
2 to 3
Multi-path Propagation
 Electromagnetic waves bounce off of conductive
(metal) objects
 Reflected waves received along with direct wave
Multi-Path Effect
 Multi-path components are delayed depending on
path length (delay spread)
 Phase shift causes frequency dependent constructive /
Amplitude
Amplitude
destructive interference
Time
Frequency
Digital Modulation
 Modulation allows the wave to carry information by
adjusting its properties in a time varying way
 Amplitude
 Frequency
 Phase
 Digital modulation using discrete “steps” so that
information can be recovered despite
noise/interference
Examples of Digital Modulation
 BPSK
 QPSK
 ODFM
 Digital modulation Practical examples
 BFSK - Mote Sensor Networks
 QPSK - 2 Mbps 802.11 & CMDA(IS-95)
Multi-transmitter Interference
 Similar to multi-path
 Two transmitting stations will
constructively/destructively interfere with each other
at the receiver
 Receiver will “hear” the sum of the two signals,
which usually means garbage
Symbol Rate & Bandwidth
 Modulation allows transmission of one of several
possible symbols
 Data stream is encoded by transmitting several
symbols in succession
 Symbol rate ≈ bandwidth
 Throughput (bits/sec)
 Spectrum usage (Hz)
 Inter-symbol interference (ISI) occurs unless delay
spread << symbol time
Thermal Noise
 Ever-present thermal noise in wireless medium
 Sums with any wireless transmission
 Potentially causes errors in reception (digital) or
degradation of quality (analog)
 Effectively limits transmission range when
transmitting signal strength falls below noise floor
Noise Limits Transmitting Distance
Short range transmission (low path loss)
+
Signal to Noise Ratio
(SNR)
=
High
Long range transmission (high path loss)
+
=
Low
Physical Channel Properties Review
 Wireless signal strength
 Transmit power
 Loss over distance (falls off by d2)
 Shadowing (e.g. absorption by walls)
 Multi-path (e.g. bouncing off of metal objects)
 Noise
 Thermal noise floor
 Environmental noise (e.g. microwave ovens)
 Channel quality
 Related to signal to noise ratio
Multi-transmitter Interference Problem
 Similar to multi-path or noise
 Two transmitting stations will
constructively/destructively interfere with each other
at the receiver
 Receiver will “hear” the sum of the two signals
(which usually means garbage)
Medium Access Methods
 The 802.11 standard ensures that all stations, both
radio-based network interface cards (NICs) and
access points, implement access methods for sharing
the air medium.
 When installing wireless LANs (WLAN), most
people don't give much thought to these mechanisms.
 A solid understanding of 802.11's medium access
methods, will enable us to deal more effectively with
issues such as radio frequency interference, denial of
services attacks and throughput issues.
Medium Access Control (MAC)
 Protocol required to coordinate access
 i.e. transmitters must take turns
 Similar to talking in a crowded room
 Also similar to hub based Ethernet
Carrier Sense Multiple Access (CSMA)
 Procedure
 Listen to medium and wait until it is free (no one
else is talking)
 Wait a random back off time then start talking
 Advantages
 Fairly simple to implement
 Functional scheme that works
 Disadvantages
 Can not recover from a collision
(inefficient waste of medium time)
Carrier Sense Multiple Access
with Collision Detection (CSMA-CD)
 Procedure
 Listen to medium and wait until it is free
 Then start talking, but listen to see if someone else starts
talking too
 If a collision occurs, stop and then start talking after a
random back off time
 This scheme is used for hub based Ethernet
 Advantages
 More efficient than basic CSMA
 Disadvantages
 Requires ability to detect collisions
Collision Detection Problem
 Transmit signal is MUCH stronger than received
signal
 Due to high path loss in the wireless environment
 Impossible to “listen” while transmitting because you
will drown out anything you hear
 Also transmitter may not even have much of a signal
to detect due to geometry
Carrier Sense Multiple Access
with Collision Avoidance (CSMA-CA)
 Procedure
 Similar to CSMA but instead of sending packets
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control frames are exchanged
RTS = request to send
CTS = clear to send
DATA = actual packet
ACK = acknowledgement
Carrier Sense Multiple Access
with Collision Avoidance (CSMA-CA)
 Advantages
 Small control frames lessen the cost of collisions
(when data is large)
 RTS + CTS provide “virtual” carrier sense protects
against hidden terminal collisions (where A can’t
hear B)
A
B
Carrier Sense Multiple Access
with Collision Avoidance (CSMA-CA)
 Disadvantages
 Not as efficient as CSMA-CD
 Doesn’t solve all the problems of MAC in wireless
networks
Random Contention Access
 Slotted contention period
 Used by all carrier sense variants
 Provides random access to the channel
 Operation
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Each node selects a random back off number
Waits that number of slots monitoring the channel
If channel stays idle and reaches zero then transmit
If channel becomes active wait until transmission
is over then start counting again
Distributed Coordination Function (DCF)
 The 802.11 standard makes it mandatory that all
stations implement the DCF, a form of carrier sense
multiple access with collision avoidance
(CSMA/CA).
 CSMA is a contention-based protocol making certain
that all stations first sense the medium before
transmitting.
 The main goal is to avoid having stations transmit at
the same time, which results in collisions and
corresponding retransmissions.
Distributed Coordination Function (DCF)
 If a station wanting to send a frame senses energy above a
specific threshold on the medium (which could mean the
transmission of another station), the station wanting access
will wait until the medium is idle before transmitting the
frame.
 The collision avoidance aspect of the protocol pertains to the
use of acknowledgements that a receiving station send to the
sending station to verify error-free reception.
 Think of this process of accessing the medium as a meeting
where everyone is polite and each person only speaks when
no one else is talking.
 In addition, everyone who understands what the person is
saying nods their head in agreement.
Distributed Coordination Function (DCF)
 The DCF protocol is somewhat more complex than this,
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though.
For example, an 802.11 station utilizes information it gains
from other frames that stations are sending over the wireless
network.
In the control field of each frame, there is a duration field that
a sending station places a value in, to indicate how long the
station will require the medium.
As part of making a decision on whether to transmit a frame, a
station must see that the time associated with the duration
value of the last frame sent has expired, as well as sense that
no physical transmission is taking place.
The duration field enables stations to reserve the medium for
subsequent frames of some specific 802.11-defined frame
exchanges (e.g. RTS/CTS)
802.11 DCF Example with Backoff
B1 = 25
B1 = 5
wait
data
data
B2 = 20
cw = 31
wait
B2 = 15
B2 = 10
B1 and B2 are backoff intervals
at nodes 1 and 2
802.11 Contention Window
 Random number selected from [0,cw]
 Small value for cw
 Less wasted idle slots time
 Large number of collisions with multiple senders (two or
more stations reach zero at once)
 Optimal cw for known number of contenders & know packet
size
 Tricky to implement because number of contenders is
difficult to estimate and can be VERY dynamic
802.11 Adaptive Contention Window
 802.11 adaptively sets cw
 Starts with cw = 31
 If no CTS or ACK then increase to 2*cw+1 (63, 127, 255)
 Reset to 31 on successful transmission
 802.11 adaptive scheme is unfair
 Under contention, unlucky nodes will use larger cw than
lucky nodes (due to straight reset after a success)
 Lucky nodes may be able to transmit several packets while
unlucky nodes are counting down for access
 Fair schemes should use same cw for all contending nodes
(better for high congestion too)
802.11 DCF (CSMA-CA)
 Full exchange with “virtual” carrier sense
(called the Network Allocation Vector)
A
B
Sender
Sender
Receiver
A
B
RTS
Receiver
DATA
CTS
ACK
NAV (RTS)
NAV (CTS)
802.11 DCF (CSMA-CA)
 Because of its nature, DCF supports the transmission of
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asynchronous signals.
A distinguishing factor of asynchronous signaling is that there
are no timing requirements between data carrying frames.
For example, the DCF protocol doesn't make any attempt to
deliver a series of data frames within any timeframe or at any
instant in time.
As a result, there is a random amount of delay between each
data frame transmission.
This form of synchronization is effective for network
applications, such as e-mail, Web browsing and VPN access
to corporate applications.
DCF Protocol Issues
 The DCF protocol is the heart of many WLAN troubles.
 RF Interference is probably the biggest problem.
 If a source of RF interference (e.g., cordless phone or other
WLAN) is present, the DCF can block stations from
transmitting for as long as the interfering signal is present.
 The stations sense enough energy on the medium and wait
patiently, in most cases for just a few seconds or minutes.
 This causes the throughput of the network to drop
significantly.
 That's why you should perform an RF site survey in the
facility before installing a WLAN.
DCF Protocol Issues
 Similar to the impact of typical RF interference,
someone could implement a denial of service attack,
which is a deliberate action to instill RF interference
at a level high enough to block a majority of the
stations from transmitting.
 Again, all of the stations will not transmit because
they respectfully follow the DCF protocol.
DCF Protocol Issues
 Similar to the impact of typical RF interference, someone
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could implement a denial of service attack, which is a
deliberate action to instill RF interference at a level high
enough to block a majority of the stations from transmitting.
Again, all of the stations will not transmit because they
respectfully follow the DCF protocol.
Instead of lasting for only a few seconds, a denial of service
attack could be planned in a way to corrupt the network for
hours or days until the jamming source is found.
This type of attack will generally cause the network to be
useless (i.e., throughput equal to zero).
In order to reduce this impact, maximize the use of directional
antennas to minimize the reception of RF signals from outside
the facility where someone could attack them with a highpowered jamming device
Virtual Carrier Sense
 Provided by RTS & CTS
 Designed to protect against hidden terminal collisions
(when C can’t receive from A and might start
transmitting)
 However this is unnecessary most of the time due to
physical carrier sense
RTS
CTS
A
B
C
Physical Carrier Sense Mechanisms
 Energy detection threshold
 Monitors channel during “idle” times between packets to
measure the noise floor
 Energy levels above the noise floor by a threshold trigger
carrier sense
 DSSS correlation threshold
 Monitors the channel for Direct Sequence Spread Spectrum
(DSSS) coded signal
 Triggers carrier sense if the correlation peak is above a
threshold
 More sensitive than energy detection (but only works for
802.11 transmissions)
 High BER disrupts transmission but not detection
Physical Carrier Sense Range
 Carrier can be sensed at lower
levels than packets can be
received
 Results in larger carrier sense
range than transmission range
 Long carrier sense range helps
Receive Range
Carrier Sense Range
protect from interference
Hidden Terminal Revisited
 Virtual carrier sense no longer needed in this situation
RTS
CTS
A
B
C
Physical Carrier Sense
RTS CTS Still Useful Sometimes
 Obstructed hidden terminal situation
A
B
 Fast collision resolution for long data packets
Exposed Terminal Problem
 Hidden terminal is not the only challenge for a
distributed wireless MAC protocol
 A blocks B, and C doesn’t know what is happening
(B is exposed)
D
A
B
C
Double Exposure Problem
 If A and C are out of phase, there is NO time D can
transmit without causing a collision
A
B
D
C
Point Coordination Function (PCF)
 As an optional access method, the 802.11 standard defines the
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PCF, which enables the transmission of time-sensitive
information.
With PCF, a point coordinator within the access point controls
which stations can transmit during any give period of time.
Within a time period called the contention free period, the
point coordinator will step through all stations operating in
PCF mode and poll them one at a time.
For example, the point coordinator may first poll station A,
and during a specific period of time station A can transmit
data frames (and no other station can send anything).
The point coordinator will then poll the next station and
continue down the polling list, while letting each station to
have a chance to send data.
Point Coordination Function (PCF)
 Thus, PCF is a contention-free protocol and enables stations
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to transmit data frames synchronously, with regular time
delays between data frame transmissions.
This makes it possible to more effectively support information
flows, such as video and control mechanisms, having stiffer
synchronization requirements.
Timing mechanisms within the 802.11 protocol ensure that
stations on the WLAN alternate between the use of DCF and
PCF.
As a result, the WLAN can support both asynchronous and
synchronous information flows.
For a period of time, stations will access channel by using
CSMA. For the following time period, the stations will wait
for a poll from the point coordinator before sending data
frames.
Point Coordination Function (PCF)
 The big name vendors, such as Cisco, Proxim, and Symbol,
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don't include PCF mode in their devices as a standard.
Some chipsets have PCF functionality embedded somewhere
in the firmware, but access point vendors seem to be reluctant
to activate it, even though PCF has been part of the 802.11
standard since its inception in 1997.
The problem is that the 802.11 standard is fairly vague in
defining portions of the PCF protocol.
As a result, you need to use the same vendor for the access
points and radio cards to make it work properly.
The Wi-Fi Alliance does not include PCF functionality in their
interoperability standard.
Q&A
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Assignment #1
 Explain the terms highlighted in GREEN color
throughout this lecture slide
 Send your assignments in Word Document Format to
[email protected] or [email protected]
 Last date of submission of assignment is 24th March
2009.