Wireless Communication and Networks
Applications of Wireless
Communication
Wireless Communication
Technologies
Wireless Networking and
Mobile IP
Wireless Local Area
Networks
Student Presentations and
Research Papers
Wireless Medium Access
http://web.uettaxila.edu.pk/CMS/AUT2012/teWCNms/
Outline
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Medium Access Control
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CSMA
CSMA-CD
CSMA-CA
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Random Contention Access
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802.11 Multiple Access schemes
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PCF
DCF
HCF
BA and WMM
Virtual Carrier Sense
Physical Carrier Sense
Multiple Access Problems
Multi-transmitter Interference
Problem
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Similar to multi-path or noise
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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 Control
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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)
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Procedure
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Advantages

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Listen to medium and wait until it is free
(no one else is talking)
Wait a random back off time then start talking
Fairly simple to implement
Functional scheme that works
Disadvantages

Can not recover from a collision
(inefficient waste of medium time)
Medium Access Control
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Media Access Control (MAC) describes how the
media (wired or wireless) is used

Polling, token passing, contention based
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CSMA/CD is for ethernet wired networks
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CSMA/CA for wireless 802.11
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Stations using either access method must first
listen to see whether any other device is
transmitting. If another device is transmitting, the
station must wait until the medium is available.
Carrier Sense Multiple Access
with Collision Detection

CSMA-CD Procedure
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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
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Advantages

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More efficient than basic CSMA
Disadvantages
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Requires ability to detect collisions
Collision Detection Problem
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Transmit signal is MUCH stronger than
received signal
Due to high path loss in the wireless
environment (up to 100dB)
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
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CSMA-CA Procedure
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Similar to CSMA but instead of sending packets
control frames are exchanged
RTS = request to send
CTS = clear to send
DATA = actual packet
ACK = acknowledgement
Carrier Sense Multiple Access
with Collision Avoidance

Advantages


Small control frames lessen the cost of collisions
(when data is large)
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

Disadvantages
 Not as efficient as CSMA-CD
 Doesn’t solve all the problems
wireless networks
of MAC in
CSMA-CD vs CSMA-CA

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The difference is when the coast is clear
CSMA/CD node can immediately begin
transmitting.
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If a collision occurs while a CSMA/CD node is
transmitting, the collision will be detected and the
node will temporarily stop transmitting.
802.11 wireless stations are not capable of
transmitting and receiving at the same time, so
they are not capable of detecting a collision
during their transmission.

802.11 wireless networking uses CSMA/CA instead of
CSMA/CD to try to avoid collisions.
CSMA-CA
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IF CSMA/CA station sees no
transmissions it will wait a random
interval
 Keep watching the medium
 If still clear after interval, transmit
 If not, start over

Because only 1 station can use the
frequency at a time
 Half
duplex
CSMA-CA
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Standard Distributed Coordination
Function (DCF) is defined to allow access
of multiple stations
 Checks and balances to keep the
 Minimize chances of collision
line clear
CSMA-CA
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Since 802.11 stations cannot detect collisions,
how do they know when they happen?
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Every unicast frame requires an
acknowledgement
 Broadcast and
 ACK Frame

multicast don’t
When ACK Frame is received, original
station knows the frame is received
Collision Detection

Receiving device will also check the CRC
 If frame is corrupt, no ACK frame
 IF no ACK frame, sender assumes
failure
delivery
Random Contention Access

Slotted contention period
 Used
by all carrier sense variants
 Provides random access to the channel

Operation
 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)
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Fundamental method of 802.11 communication
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Mandatory access method
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Point Coordination Functions (PCF) is optional
802.11 also has Hybrid Coordination Function (HCF)
Four Main parts
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Interframe Space
Virtual Carrier Sense
Physical Carrier Sense
Random back-off timer
Pg 254
Interframe Space (IFS)
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The amount of time between transmissions
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Actual length of time depends on network speed
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Short interframe space (SIFS), highest priority
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PCF interframe space (PIFS), middle priority
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DCF interframe space (DIFS), lowest priority
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Arbitration interframe space (AIFS), used by
QoS stations
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Extended interframe space (EIFS), used with
Pg 254
retransmissions
Interframe Space (IFS)
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Only certain types of frames are sent after
certain interframe spaces
 Only
ACK and CTS after SIFS
Pg 254
802.11 DCF Example
B1 = 25
B1 = 5
wait
data
data
B2 = 20
cw = 31
cw = 31
wait
B2 = 15
B2 = 10
B1 and B2 are backoff intervals
B1 and B2 are backoff intervals
at nodes 1 and 2
at nodes 1 and 2
802.11 Contention Window

Random number selected from [0,cw]
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Small value for cw
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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
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Computed by minimizing expected time wastage (by both
collisions and empty slots)
Tricky to implement because number of contenders is difficult
to estimate and can be VERY dynamic
802.11 Adaptive Contention
Window
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802.11 adaptively sets cw
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802.11 adaptive scheme is unfair
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Starts with cw = 31
If no CTS or ACK then increase to 2*cw+1 (63, 127, 255)
Reset to 31 on successful transmission
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 PCF (CSMA-CA)
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Optional access control method
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AP is point coordinator
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Polling
Only work in a BSS
Not in ad-hoc/IBSS
Both AP and client station must support PCF
AP will switch between DCF and PCF

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PCF time is contention free period (CFP)
DCF is contention period (CP)
802.11 DCF (CSMA-CA)

Full exchange with “virtual” carrier sense
(called the Network Allocation Vector)
A
B
Sender
Sender
Receiver
A
B
B
RTS
Receiver
DATA
CTS
ACK
NAV (RTS)
NAV (CTS)
Hybrid Coordination Function (HCF)
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Added in 802.11e
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Enhanced Distributed Channel Access (EDCA)
HCF Controlled Channel Access (HCCA)
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DCF and PCF require contention for each frame
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HCF defines ability to send multiple frames
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Transmit Opportunity (TXOP)
During TXOP period, client station can send a frame
burst
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Uses Short Interframe Space (SIFS)
Pg 259
Enhanced Distributed Channel Access (EDCA)
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Provides differentiated access using eight user
priority levels
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Uses frame tags similar to 802.1D standard
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Extension of DCF
QoS standard at MAC (layer 2) level
Define priority values
With priority queuing
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Data waiting in higher priority queues transmits
before lower priority queues
Pg 259
Enhanced Distributed Channel Access (EDCA)
Pg 259
Enhanced Distributed Channel Access (EDCA)
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Defines four access categories base on User
Priority level
From lowest to highest priority access category:
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AC_BK (Background)
AC_BE (Best Effort)
AC_VI (Video)
AC_VO (Voice
For each category, Enhanced Distributed
Cannel Access Function (EDCAF) is used

Frames with higher category have lower back off
values and are more likely to get a TXOP
Pg 259
Block Acknowledgement (BA)
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Part of 802.11e
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Allows for a single acknowledgement for
multiple frames
 Reduces

overhead
Two Types
 Immediate
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For low latency traffic
 Delayed
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For latency tolerant traffic
Pg 261
Block Acknowledgement (BA)-Immediate
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Originator sends a block of QoS data
frames to station

Originator requests acknowledgement of
all outstanding QoS data
 Block

AckReq frame
Recipient can send a single ack frame for all
received frames
 Can
request a single frame from block be
retransmitter
Pg 261
Block Acknowledgement (BA)-Immediate
Pg 261
W i-Fi Multimedia (WMM)
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For latency sensitive data
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Voice, video, audio, have less tolerance of
latency (cumulative delay)
802.11e had layer 2 MAC methods to meet QoS
requirements

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Real time voice and video
WiFi Alliance created WMM
Because WMM is based on EDCA mechanisms,
802.1D priority tags from the Ethernet side are
used to direct traffic to four access-category
priority queues. The WMM certification
provides for traffic prioritization via four access
categories
Pg 262
W i-Fi Multimedia (WMM)
Pg 262
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

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DSSS correlation threshold
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Monitors channel during “idle” times between packets to
measure the noise floor
Energy levels above the noise floor by a threshold trigger carrier
sense
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
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Receive Range
Carrier Sense Range
Results in larger carrier sense
range than transmission range
More than double the range in
NS2 802.11 simulations
Long carrier sense range helps
protect from interference
Hidden Terminal Revisited

Virtual carrier sense no longer needed in
this situation
RTS
CTS
A
B
C
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
Q&A
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?