wireless & mobile networking - old dominion...
TRANSCRIPT
Wireless & Mobile Networking
CS 752/852 - Spring 2011
Tamer Nadeem Dept. of Computer Science
Lec #3: Medium Access Control - I
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• Main Task of the data link layer:
• Provide error-free transmission over a link
Data Link Layer (DLL)
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• Framing
• The DLL translates the physical layer's raw bit stream into discrete units (messages)
called frames. How can the receiver recognize the start and end of a frame?
• Flow Control
• Flow control deals with throttling the speed of the sender to match that of the
receiver. Usually, this is a dynamic process, as the receiving speed depends on such
changing factors as the load, and availability of buffer space.
DLL Services
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• Link Management
• Allocating buffer space, control blocks, agreeing on the maximum message size, etc.
Synchronize and initialize send and receive sequence numbers with its peer at the
other end of the communications channel
• Error Control
• Error control is concerned with insuring that all frames are eventually delivered
(possibly in order) to a destination. Three items are required: Acknowledgments,
Timers, Sequence Numbers
• Error Detection and Correction
• line noise is a fact of life (e.g., signal attenuation, natural phenomenon such as
lightning, and the telephone repairman). Error Detecting Codes: Include enough
redundancy bits to detect errors and use ACKs and retransmissions to recover from
the errors. Error Correcting Codes: Include enough redundancy to detect and correct
errors. CRC Checksums
DLL Services
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• LANs first began to emerge as potential
business tools in the late 1970s
• IEEE launched Project 802 (1980,
February) to define certain LAN
standards.
• Project 802 defined network standards for
the physical components of a network (the
interface card and the cabling)
• Define the ways NICs access and transfer
data over physical media. These include
connecting, maintaining, and disconnecting
network devices.
• The IEEE 802 standards incorporated the
specifications in the bottom two OSI
layers, the physical layer and the data-link
layer.
DLL = LLC + MAC
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• More detail was needed at the data-link layer, the 802 standards
committee divided the data-link layer into two sublayers.
DLL = LLC + MAC
• Logical Link Control (LLC) Sublayer: Manages
data-link communication: establishing and
terminating links, controlling frame traffic,
sequencing frames, and acknowledging frames.
• Media Access Control (MAC) Sublayer:
Communicates directly with the NIC to provide
shared access to the physical layer: Managing
media access, delimiting frames, checking frame
errors, and recognizing frame addresses.
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DLL = LLC + MAC
Wireless LAN Standards
(IEEE 802.11)
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Introduction
• Multiple access control channels
• Each node is attached to a transmitter/receiver which
communicates via a channel shared by other nodes
• Transmission from any node is received by other nodes
Shared Channel
Node 4
Node 3
Node 2
Node 1 …
Node N
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The Channel Access Problem
• Multiple nodes share a channel
• Pairwise communication desired
• Simultaneous communication not possible
• MAC Protocols
• Suggests a scheme to schedule communication
• Maximize number of communications
• Ensure fairness among all transmitters
A C B
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The Trivial Solution
• Transmit and pray
• Plenty of collisions --> poor throughput at high load
A C B
collision
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Classification of MAC Protocols
Single Channel and Sender Initiated Protocols
• Contention-free MAC
• TDMA, FDMA, CDMA: Divides channel by time, frequency, or code
• More applicable to static networks and/or networks with centralized control
• Contention-based MAC
• Single Channel vs.
Multi-Channels
• Sender Initiated vs.
Receiver Initiated
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Carrier Sense Multiple Access
(CSMA)
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CSMA
• Listen before you talk
• Carrier sense multiple access (CSMA)
• Defer transmission when signal on channel
• Advantages
• Fairly simple to implement
• Functional scheme that works
• Disadvantages
• Can not recover from a collision
(inefficient waste of medium time)
A C B
Don’t
transmit
Can collisions still occur?
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• reduces chance of collisions
• reduces the efficiency
• increases the chance for
collisions
• 1-persistant
• p-persistant
CSMA
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CSMA/CD (Collision Detection)
Collisions can still occur: Propagation delay non-zero
between transmitters
When collision: Entire packet transmission
time wasted
spatial layout of nodes
note: Role of distance & propagation delay
in determining collision probability
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CSMA/CD (Collision Detection)
• Keep listening to channel
• While transmitting
• If (Transmitted_Signal != Sensed_Signal)
Sender knows it’s a Collision
ABORT
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2 Observations on CSMA/CD
• Transmitter can send/listen concurrently
• If (Sensed - Transmitted = null)? Then success
• The signal is identical at Tx and Rx
• Non-dispersive
The TRANSMITTER can detect if and
when collision occurs
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Unfortunately …
Both observations do not hold for wireless
Because …
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Wireless Medium Access Control
A B
C D
Distance
Signal
power
CS threshold
A cannot send and listen in parallel
Signal not same at different locations
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• CMSA/CA
• The CSMA/CA algorithm is based on a basic
time unit called slot σ.
• Slot duration (σ) is equal to maximum
propagation delay.
• Time space is slotted at the boundaries of σ.
• Channel access slotted CSMA can only occur
at the boundary of σ
CSMA with Collision Avoidance (CSMA/CA)
Next Frame
Slot Time
Slotting solved collisions because of propagation delays
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Hidden Node Collisions
A B
C D
Distance
Signal
power
CS threshold
Important: D has not heard A, but can interfere at receiver B
D is the hidden node to A
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Hidden Node Collisions
A B
C D
Important: D has not heard A, but can interfere at receiver B
D is the hidden node to A
DB
D
transmitD
B
AB
A
A
B
d
PI
d
PSoI
NNoiseIceInterferen
SoIterestSignalOfInSNR
transmit
)()(
)(
DB
D
transmit
AB
A
A
B
d
PN
d
P
SNR
transmit
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Exposed Node Collisions
A B
C D
Distance
Signal
power
CS threshold
Important: X has heard A, but should not defer transmission to Y
X
Y
X is the exposed terminal to A
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So, how do we cope with
Hidden/Exposed Terminals?
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The Emergence of MACA, MACAW, & 802.11
• Wireless MAC proved to be non-trivial
• 1992 - research by Karn (MACA)
• 1994 - research by Bhargavan (MACAW)
• Led to IEEE 802.11 committee
• The standard was ratified in 1999
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• Alternative to carrier sensing, i.e. does not use CSMA
CA with Control Handshaking - (MACA)
• Multiple access with collision avoidance (MACA)
uses a three way handshake to avoid hidden
terminal problem (Karn, 90)
• When node A wants to send a packet to node B,
node A first sends a Request-to-Send (RTS) to B
• On receiving RTS, node B responds by sending
Clear-to-Send (CTS)
• All nodes within one hop of node A hear the RTS
and defer their transmissions until
corresponding CTS.
• When a node (such as D) overhears a CTS, it
keeps quiet for the duration of the transfer
• Transfer duration is included in RTS and CTS both
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30
• MACA avoids the problem of hidden terminals
• A and C want to
send to B
• A sends RTS first
• C waits after receiving
CTS from B
• MACA avoids the problem of exposed terminals
• B wants to send to A, C
to another terminal
• now C does not have
to wait for it cannot
receive CTS from A
MACA examples
A B C
RTS
CTS CTS
A B C
RTS
CTS
RTS
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MACA
• Limitations
• MACA does not provide ACK
• RTS-CTS approach does not always solve the hidden node problem
• Examples
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MACAW (MACA for Wireless)
• RTS-CTS-DS-DATA-ACK
• RTS from A to B
• CTS from B to A
• Data Sending (DS) from A to B
• Data from A to B
• ACK from B to A
• Random wait after any successful/unsuccessful transmission
• Significantly higher throughput than MACA
• Does not completely solve hidden & exposed node problems
A B C D
RTS
CTS
Data
Ack
DS
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IEEE 802.11 MAC
• Very popular wireless MAC protocol
• Two Architectures
IEEE 802.11
Medium Access Control (PCF+DCF)
FHSS DSSS Infrared OFDM
MA
C
PH
Y
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802.11 PHY Sublayers
• Physical layer convergence protocol (PLCP)
• Provides common interface for MAC
• Offers carrier sense status & CCA (Clear channel assesment)
• Performs channel synchronization / training
• Physical medium dependent sublayer (PMD)
• Functions based on underlying channel quality and characteristics
• E.g., Takes care of the wireless encoding
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PLCP
• PLCP has two structures.
• All 802.11b systems have to support Long preamble.
• Short preamble option is provided to improve efficiency when trasnmitting
voice, VoIP, streaming video.
• PLCP Frame format
• PLCP preamble
• SFD: start frame delimiter
• PLCP header
• 8-bit signal or data rate (DR) indicates how fast data will be transmitted
• 8-bit service field reserved for future
• 16-bit length field indicating the length of the ensuing MAC PDU (MAC sublayer’s
Protocol Data Unit)
• 16-bit Cyclic Redundancy Code
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PLCP (802.11b)
long
preamble
192us
short
preamble
96us
(VoIP, video)
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IEEE 802.11 MAC
• Two modes:
• DCF (distributed coordination function)
• PCF (point coordination function)
• IEEE 802.11 DCF is based on CSMA/CA
• Physical Carrier Sense
• Explicit ACK from receiver (for unicast transmission)
• RTS/CTS reservation frames (Virtual Carrier Sensing)
• Retry Counters
• Different Timing Intervals for priorities
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• RTS-CTS used for frames longer than a Threshold
• RTS-CTS overhead not efficient for short frames
• Some environments may not find RTS-CTS useful, e.g. many infrastructure
networks
• Threshold variable can be tuned
• Virtual carrier sensing
• Duration field in all frames, including RTS and CTS, monitored by every station
• Duration field used to construct a network access vector (NAV)
• Inhibits transmission, even if no carrier detected
IEEE 802.11 DCF Basics – RTS/CTS & Virtual Carrier Sense
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• Counter and timer for each frame
• Short or long retry counter
• Lifetime timer
• Retry counter
• Incremented for each transmission attempt
• Use of short versus long retry counter based on Threshold variable
• Threshold limit
• ShortRetryLimit for short retry counter ‰
• LongRetryLimit for long retry counter ‰
• If threshold exceeded, frame is discarded and upper layer is notified via
MAC interface
IEEE 802.11 DCF Basics – Retry Counters
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• Timing intervals are defined that control a station’s access to the medium
• Slot time (SlotTime)
• Specific value depends on PMD layer
• Derived from propagation delay, transmitter delay, etc. (20micro-sec for DSSS and
50 for FHSS)
• Basic unit of time for MAC, e.g. for backoff time is a multiple of slot time
• Short Inter-Frame Space (SIFS)
• Shortest interval -- SIF < SlotTime e.g. 10 microsec for FHSS
• Used for highest priority access to the medium, e.g., for ACK and CTS
• Allows Data-ACK and RTS-CST to be atomic transactions
IEEE 802.11 DCF Basics – Timing Intervals
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• Priority (or PCF) Inter-Frame Space (PIFS)
• PIFS = SIFS + SlotTime
• Used for Point Coordination Function (PCF) access to the medium
• Allows priority based access to the medium after ACKs but before contention based
access
• Distributed (or DCF) Inter-Frame Space (DIFS)
• DIFS = SIFS + 2×SlotTime
• Used for Distributed Control Function (DCF) access to the medium
• Results in lower priority access than using SIFS or PIFS
• Extended Inter-Frame Space (EIFS)
• EIFS = SIFS + (8×ACK) + PreambleLength + PLCPHeaderLength + DIFS
• Used in the event that the MAC receives a frame with an error
• Provides an opportunity for a fast retransmit of the error frame
• In summary …
• SIFS < SlotTime < PIFS < DIFS << EIFS
IEEE 802.11 DCF Basics – Timing Intervals
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• When a sender has a data to transmit, it picks a random wait period. The
wait period is decremented if the channel is idle
• When this period expires, the node tries to acquire the channel by
sending a RTS packet
• The Receiving node (destination) responds with a CTS packet indicating
that its ready to receive the data
• The sender then completes the packet transmission
• If the packet is received without errors, the destination node responds
with an ACK
• If an ACK is not received, the packet is assumed to be lost and the packet
is retransmitted
802.11 DCF Mode Principles
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• If RTS fails, the node attempts to resolve the collision by doubling the
wait period. (This is known as binary exponential back-off (BEB)).
• Station trying to send an ACK is given preference over a station that is
acquiring a channel (Different waiting intervals are specified)
• A node needs to sense channel for Distributed Inter- Frame Space (DIFS)
interval before making an RTS attempt and a Short Inter Frame Space
(SIFS) interval before sending an ACK packet
802.11 DCF Mode Principles
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DIFS
RTS
CTS
DATA
ACK
NAV (CTS)
NAV (RTS)
SIFS SIFS SIFS
B
A
C
Contention
Window
DIFS
802.11 DCF Mode
RTS
Deferred CW
A B
C
D
D
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• Because SIFS is shorter than the DIFS interval, the station sending an
ACK attempts transmission before a station sending a data packet
• In addition to physical channel sensing, virtual carrier sensing is
achieved due to NAV (Network allocation vector) field in the packet
• NAV indicates the duration of current transmission
• Nodes listening to RTS, or CTS messages back off NAV amount of time
before sensing the channel again
• Several papers describe this protocol and even suggest enhancements.
802.11 DCF Mode Notes
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802.11 Contention Window
• Random number selected from [0,cw]
• If transmission was successful, set CW = CWmin
• If transmission fails (i.e., no ACK), CW = min{2(CW+1)-1,
CWmax}
• 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
• 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
• Project Idea: • Evaluate literature for CW calculation schemes under different scenarios
• Enhance/New adaptive CW scheme
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Physical Carrier Sense Mechanisms
• Energy detection threshold
• Monitors channel during “idle” times between packets to measure floor noise
• Energy levels above this floor noise 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
• Carrier can be sensed at lower levels than
packets can be received
• Results in larger carrier sense range than transmission range
• More than double the range in NS2 802.11 simulations
Receive Range
Carrier Sense Range
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• RTS/CTS & Carrier Sense • When RTS/CTS is useful?
• Should Carrier Sensing replace RTS/CTS?
• Interference Range vs. Carrier Sense Range • How effective CSMA carrier sense?
• BER & Date rate and Transmission Range (data rate affect the SNR threshold and
hence the transmission range but not the physical CS)
• Contention Window Size
• Is ACK necessary? • MACA said no ACKs. Let TCP recover from losses
On 802.11 Issues
The search for the best MAC protocol is still on. However, 802.11 is being optimized too.
Thus, wireless MAC research still alive
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On RTS/CTS & Carrier Sense
• Does RTS/CTS (Virtual CS) solve hidden terminals ?
• Assuming carrier sensing zone = communication zone
C F
A B
E
D
CTS RTS
E does not receive CTS successfully Can later initiate transmission to D.
Hidden terminal problem remains.
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On RTS/CTS & Carrier Sense
• Hidden Terminal: How about increasing Physical Carrier Sense range ??
• E will defer on sensing carrier no collision !!!
C B D Data
A
E
CTS
RTS F
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On RTS/CTS & Carrier Sense
• Exposed Terminal: B should be able to transmit to A
• Carrier sensing makes the situation worse
C A B
E
D
CTS
RTS
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On RTS/CTS & Carrier Sense
• 802.11 does not solve HT/ET completely
• Only alleviates the problem through RTS/CTS and recommends larger CS zone
• Large CS zone aggravates exposed terminals
• Spatial reuse reduces A tradeoff
• RTS/CTS packets also consume bandwidth
• Moreover, backing off mechanism is also wasteful
• Carrier sense relies on channel measurements at the sender to infer the probability of reception at the receiver!
• Project Idea: • Evaluation of the benefits and drawbacks of carrier sense
• Scheme to intelligently choose a Carrier sensing threshold
• Evaluate tracking correlation between channel conditions at the sender and at
the receiver.
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On Contention Window Size
• Optimal CW for known number of contenders &
know packet size
• 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 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
• Project Idea: • Evaluate literature for CW calculation schemes under different scenarios
• Enhance/New adaptive CW scheme
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• 802.11 physical layer (e.g., Direct Sequence Spread Spectrum (DSSS)
used in 802.11b)
Capture effect: two transmissions received by the same receiver, the signals of the
stronger transmission will capture the receiver radio, and signals of the weaker
transmission will be rejected as noise.
Frame 2
Frame 1
Received Frame
Frame 2 Frame 1
Received Frame
• Simple and widely accepted model:
• Capturing stronger signal ≠ Capturing stronger frame
On Interference Range vs. Carrier Sense Range
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Inefficiency:
• Interference Range:
R
I 1 2
I
C
d
• Power path loss model:
• Capture model:
Given:
R=250m, C=550, l =2, α=5
On Interference Range vs. Carrier Sense Range
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On Interference Range vs. Carrier Sense Range
• Project Idea: • How to estimate interference range (distance)
• Propagation Delay?
• Interference Aware MAC Scheme
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On Transmission Date rate
Floor Noise Data Rate
Received Power
Channel Bandwidth
• Bit error (p) for BPSK and QPSK :
SNR
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On ACKnowledgment
• APs typically backlogged with traffic
• Persistent traffic possibility of optimization
• Use implicit ACK optimization
• Piggyback the CTS with ACK for previous dialog
802.11
Implicit
ACK
Gain
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On ACKnowledgment
• The optimization timeline
T R
RTS
CTS
Data
ACK
RTS
CTS
Data
ACK
T R
RTS
CTS
Data
RTS
CTS +ACK
Data
T R
RTS
CTS
Data
Poll +ACK
Data
RTS
CTS +ACK
Ba
cko
ff
Ba
cko
ff
Ba
cko
ff
Ba
cko
ff
Poll +ACK
Data
Ba
cko
ff
Ba
cko
ff
802.11 Implicit ACK Hybrid Channel Access
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Performance Analysis of the
IEEE 802.11 Distributed
Coordination Function (Giuseppe Bianchi)
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802.11 DCF Throughput Analysis (Bianchi)
• Objective:
• Analytical Evaluation of Saturation Throughput
• Assumptions:
• Fixed number of stations having packet for transmission
• Each packet collide with constant and independent probability
• Model bi-dimensional process {s(t) , b(t)} with discrete-time
Markov chain
• Analysis divided into two parts:
• Study the behavior of single station with a Markov model
• Study the events that occur within a generic slot time & expressed
throughput for both Basic & RTS/CTS access method
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• Closed form solution for Markov chain
Markov Chain Model
• Stationary Probability
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Markov Chain Model
• Probability τ that a station transmits in randomly chosen slot
time
• When m =0 no exponential backoff is considered probability τ
results independent of p
• In general τ depends on conditional collision probability p
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Throughput Analysis
• Normalized system throughput S
• Probability of transmission Ptr
• Probability of successful transmission Ps
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Normalized system throughput
Throughput Analysis
Specify Ts and Tc to compute throughput for DCF access mechanism
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• Considering System via Basic Access mechanism
• Packet header H = PHYhrd +MAChrd
• Propagation delay δ
Throughput Analysis
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• Packet transmission via RTS/CTS Access mechanism
Throughput Analysis
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Model Validation
• Compared analytical results with that obtained by means of
simulation
• Analytical model extremely accurate
• Analytical results (lines) coincide with simulation results
(symbols) in both Basic Access & RTS/CTS cases
Saturation throughput analysis vs. simulation
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Maximum Saturation Throughput
• τ depends on n, W, and m
• Analytical model determines maximum achievable saturation
throughput
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Performance Evaluation
Saturation throughput vs. initial window
size for Basic Access mechanism
• Greater the network size lower is the throughput for basic access
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• Throughput of Basic Access mechanism depends on W
• W depends on number of terminals
• High value of W gives
excellent throughput
performance
Performance Evaluation
Saturation throughput vs. initial window
size for Basic Access mechanism
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• Throughput obtained with RTS/CTS mechanism
• Independent of value of W
Performance Evaluation
Saturation throughput vs. initial window
size for RTS/CTS mechanism
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• Number of transmissions per packet increases as W reduces
& network size n increases.
Performance Evaluation
Average number of transmissions
per packet