Performance Evaluation Study for IEEE 802.11 WLAN Protocol.

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Performance Evaluation Study for IEEE 802.11 WLAN
Protocol.

• Submitted to
• Dr. Ashraf S Hasan Mahmoud
•  
• Submitted by
• Mohammad Abdur Razzaque(22030
• Khawar Saeed Khan (220514)

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Objectives

• To provide an overview of IEEE 802.11 standard
  for WLAN including its details of MAC layer
  specifically different access schemes, its
  variants etc.
• Performance Evaluation through Simulation

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Why do we need Standard?

• Multi vendors Compatibility
• Protects Customers investment
• High volumes reduce prices

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Overview of IEE 802.11

• In recent years, much interest has been involved
  in the design of wireless networks for the local
  area communications.
• Study group 802.11 was formed under IEEE Project
  802 to recommend an international standard for
  Wireless Local Area Networks.
• IEEE has standardized the 802.11 for wireless
  local area networks in 1990 .

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Features of IEEE802.11

•     Support of asynchronous and time-bounded
  delivery service
•   Continuity of service within extended areas via
  a distribution system, such as Ethernet.
• Accommodation of transmission rates of 1 and 2-
  Mbps
•       Multicast (including broadcast) services
•       Network management services
•      Registration and Authentication services

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Target environments

• Inside buildings, such as offices, banks, shops,
  malls, hospitals, manufacturing plants, and
  residences.
• Outdoor areas, such as parking lots, campuses,
  building complexes, and out-door plants

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IEEE 802.11 Requirements 

• To be IEEE 802.11 standard compatible a device
  has fulfill the following requirements
• Single MAC supporting multiple PHYs
• Mechanism to allow multiple overlapping networks
  in the same area
• Provisions to handle the interface from other
• ISM band radios and microwave ovens
• Mechanism to handle hidden terminal problem
• Options to support time-bounded services
• Provisions to handle privacy and access security

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IEEE 802.11 Topology

• The 802.11 standard supports the following two
  topologies
• Independent Basic Service Set (IBSS) networks
• Extended Service Set (ESS) networks
• Independent Basic Service Set (IBSS) networks
• smallest building block of a wireless LAN
• stand-alone BSS, no backbone infrastructure and
  consists of at least two wireless stations
• often referred to as an ad hoc network

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IBSS

• A basic service set may be isolated or it may
  connect to a backbone distribution system through
  an access point

IBSS
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ESS

• An extended service set (ESS) consists of two or
  more basic service sets inter-connected by
  Distribution system
• Typically, the distribution system is a wired
  backbone LAN.

ESS
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IEEE 802.11 services  
Services are divided into two groups those that
are part of every station, and those that are
part of a DS The service provided by stations is
known as the station service. a)
Authentication b) Deauthentication c) Privacy d)
MSDU delivery
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IEEE 802.11 servicescont..
The service provided by the DS is known as the
distribution system service. a) Association b)
Disassociation c) Distribution d) Integration e)
Reassociation
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802.11 operation modes

• Two operation modes defined in IEEE 802.11
  Infrastructure Mode and Ad Hoc Mode
• Infrastructure Mode
• At least one access point (AP) connected to the
  wired network infrastructure and a set of
  wireless end stations
• An AP bridge or route the wireless traffic to a
  wired Ethernet network

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802.11 operation modesCont..
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802.11 operation modesCont..

• Ad-Hoc mode
• A set of 802.11 wireless stations that
  communicate directly with each other without
  using an access point or any connection to a
  wired network.
• Ad-Hoc Mode is also called peer-to-peer mode or
  an Independent Basic Service Set (IBSS).

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Variants of 802.11

• Up to present following are the variants of
  802.11 and they all are same in MAC layer view
  but different PHYs.
• IEEE802.11a
• OFDM _at_ 5 GHz U-NII bands same as HIPERLAN-2
• Rates up to 54 Mb/s
• IEEE802.11b
• CCK _at_ 2.4GHz
• Rates up to 5.5 and 11 Mb/s
• Same PLCP as IEEE802.11 DSSS

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Variants of 802.11cont

• 802.11g
• Supporting PBCC (packet binary convolution
  coding, a single carrier technique) and OFDM.
• Speed extension for the 802.11b standard.
• Data rates up to 54 Mbps in the 2.4 GHz band.
• 802.11d
• The IEEE 802.11d TG d describes a protocol that
  will allow a 802.11 device to receive the
  regulatory information required to configure
  itself properly to operate anywhere on earth.
• 802.11e
• The 802.11e TG e is proceeding to build improved
  support for quality of service (for example for
  voice transmission).

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IEEE 802.11 Logical Architecture

• The logical architecture defines the networks
  operation
• 802.11 only defines the MAC and PHY layers
• MAC layer is divided into MAC sublayer and MAC
  management sublayer entities
• station management sublayer that is responsible
  for coordination of the interactions between MAC
  and PHY layers.
•  

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Carrier Sensing in 802.11

• PHY Sensing
• Clear Channel Assessment (CCA) signal
• Generate by the PLCP
• Sensing Detected data sensing or Carrier
  Sensing
• Decision using detected bits slow but reliable
• Decision using RSS of carrier against threshold
  fast but many false alarms due to interference.

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Carrier Sensing in 802.11Cont

• Virtual carrier sensing
• It is based on the reservation information found
  in the Duration field of all frames.
• MAC Coordination will monitor the Duration field
  in all MAC frames and place this information in
  station's Network Allocation Vector (NAV)
  indicates the medium is occupied for a given
  (length field) time duration.
• If the NAV0, then the channel is virtually free
• Used for RTS/CTS and PCF based schemes mainly

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IEEE802.11 MAC Layer

• MAC has two sublayers and all the
  responsibilities are divided between them.
• Major responsibilities of MAC sublayer
• Define access scheme
• Define packet formats
•  Major responsibilities of management sublayer
• Support ESS
• Power management
• Security

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Accessing the Wireless Medium

• To access the medium, IEEE 802.11 provides
  different access schemes
• CSMA/CA contention data
• RTS/CTS contention-free
• PCF contention-free, intended for time-bounded
  traffic
• First two schemes are also referred as
  DCF(Distribution Coordination Function)

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 IFS (Inter Frame Spacing)
DFIS DCF-IFS (DIFS) used for contention data
spacing that has the lowest priority and longest
duration SIFSShort IFS (SIFS), used for highest
priority packets such as ACK and CTS (clear to
send),has the lowest duration of time. PIFS The
PCF IFS (PIFS), designed for PCF operation, has
the second priority rate with duration between
DIPS and SIFS.
       
 
DIFS
   
PIFS
SIFS
Medium Busy
Time
IFS
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CSMA/CA access Scheme

• Procedure
• When a station ready for a new data frame
  transmission senses the channel status
• If the channel is busy, the station defers its
  transmission and continues to sense the channel
  until it is idle.
• If it is idle for DIFS period, the station choose
  a random number as a backoff timer.
•  

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CSMA/CA access Schemecont..

• The backoff timer is decreased by one for each
  idle slot
• Stopped if the channel is sensed busy, and then
  reactivated if the channel is idle again and
  remains idle for more than a DIFS time duration.
• When the backoff timer reaches zero, the data
  frame is transmitted.

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RTS/CTS

• RTS/CTS Operation
• When source is ready RTS (20 bytes) is sent
• Destination responds with CTS (16 bytes) after
  SIFS
• Source terminal received CTS and after SIFS
  sends data
• Destination terminal sends ACK after SIFS
• Other terminal listening to RTS/CTS will turn
  their NAVsignal on used for virtual carrier
  sensing
• NAV signal turned off when after the
  transmission and reception of the ACK frame

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PCF (Point Coordination Function)

• Optional MAC service Not implemented by all
  manufacturers
• Available only for infrastructure networks not
  Ad-hoc
• AP point coordinator organizes periodical
  contention-free periods
• (CFP) for delay-sensitive services
• PCF operation
• During PCF operation (part of CFP) NAV signal is
  on
• During the remainder of the CFP NAV signal is
  off and that can be used for contention

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MAC frame Format of 802.11
Frame Control This field carries control
information being sent from station to station.
Duration/ID In most frames, this field contains
a duration value, depending on the type of frame
sent. Address 1, 2, .3,and 4 The address fields
contain different types of addresses, depending
on the type of frame being sent. Sequence
Control The sequence control is used for
fragmentation numbering to control the
sequencing. Body Field This field has a
variable length payload and its range is 0-2312
bytes. Frame Check Sequence (FCS) The MAC layer
at the sending station calculates a 32 bits FCS
using CRC and place the result in this field.
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Physical Layer Architecture

• Physical Layer has the following three components
  for each station
• Physical Layer management
• Channel tuning to different options within PHY
• PLCP
• Carrier sensing
• Forming packets for different PHYs
• PMD
• Modulation, Coding

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Physical Layer Technologies

• Two main technologies are used for wireless
  communications Radio Frequency and InfraRed.
• For the Radio frequency there are two modulation
  schemes
• frequency hopping spread spectrum (FHSS) and
• direct sequence spread spectrum (DSSS)
• For InfraRed we use Diffused Infrared

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FHSS

• FHSS allows for a less complex radio design than
  DSSS. It has the following features
• FHSS PMD hops over 78 channels of 1 MHz each in
  the centre of the 2.44 GH ISM band
• Modulation is GFSK 1 bit/symbol 1 Mb/s or
  2bit/symbol 2 Mb/s
• Hopping rate 2.5 hop per second
• Therefore three APs can coexist in the same area
  which means maximum throughput of 6 Mb/s
• Lowest cost and Power consumption
• Most tolerant to signal interference

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DSSS

• The Direct Sequence Spread Spectrum (DSSS) has
  the following properties
• DSSS PMD uses 26 MHz chunks to transmit 11 Mc/s
  Modulation DBPSK for 1 Mb/s and DQPSK for 2 Mb/s
•  ISM band at 2.4 GHz 11 overlapping channels
  with 5 MHz spacing
• Max tx power 100 mW
• Wider range the FHSS
• Highest cost and Power consumption

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DFIR

• Diffused Infrared has the following features
• DFIR PMD utilizes 250 ns pulses
• Pulse Position Modulation (PPM)
• 16-PPM for the 1 Mb/s option
• 4-PPM for the 2 Mb/s option
• Low cost
• Higher tolerance to RF signal interference
• Lower range compare to spread spectrum

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IEEE 802.11 DCF simulation

• In this project we have simulated the IEEE 802.11
  MAC layers Distributed coordination function
  (DCF)
• The simulation is done using the standards
  defined by IEEE for 802.11

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MODEL

• Assumptions
• Perfect channel conditions have been assumed
• The packet propagation delays between the
  communicating stations are assumed to be zero
• Only basic access is employed, no RTS/CTS
  messages are exchanged

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• We have studied the performance of 802.11 by
  subjecting it to different conditions
• Parameters that were useful in the performance
  evaluation are
• Number of Communication stations
• Ranges of transmission
• Frame sizes

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Simulation Results

• Our simulation calculates the following
  parameters
• Total Transmissions
• Successful Transmissions
• Total Collisions
• Unreachable Packets
• Total Acknowledgments
• Successful Acknowledgments
• Acknowledgment Collisions
• Unreachable Acknowledgments

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GRAPHS
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Conclusion

• It is clear that increasing the load in a WLAN
  system drastically reduces the efficiency of the
  network.
• We propose to increase the transmission range for
  stations. Increasing the transmission range would
  improve the efficiency better for the heavy load

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Proof

• For the frame size 5, range 3 and No. Of Stations
  10, the efficiency of network is 0.4423. If we
  increase the transmission range from 3 to 10,
  keeping the frame size 5 and No. Of Stations 10,
  the network efficiency comes close to 0.6. That
  is 15.77 improvement in efficiency.