ICOM 6505: Wireless Networks Medium Access Control

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ICOM 6505 Wireless Networks- Medium Access
Control -

• By Dr. Kejie Lu
• Department of Electronic and Computer Engineering
• Spring 2008

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Outline

• Basics
• Duplexing
• Coordinated MAC Schemes
• Random MAC Schemes

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Random MAC Schemes

• Aloha
• CSMA
• Reservation based
• Polling based
• Case study

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Aloha

• History
• Designed at the University of Hawaii to solve the
  channel allocation problem
• For ground-based radio broadcasting
• Categories
• Pure Aloha
• Slotted Aloha
• Time is divided into fixed-size slots
• Global time synchronization

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Pure Aloha

• Main idea
• Let users transmit whenever they have data to be
  sent
• Collision detection
• Directly
• Detection delay
• LAN negligible
• Satellite 270 msec
• Indirectly
• Via ACK frame
• Retransmission
• The sender wait a random amount of time to send
  again

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Illustration

• In pure ALOHA, frames are transmitted at
  completely arbitrary times
• The throughput of ALOHA systems is maximized if
  the frame size is fixed (the same) for all frames

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Vulnerable Period of A Frame
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Performance

• Throughput
• t/T
• T the total amount of time
• t the total amount of time that are used to
  transmit frame without collision
• Assumption
• The number of stations is infinite
• The frame size is fixed
• The overall traffic (frame) incoming rate follows
  a Poisson distribution with rate G (frames per
  frame time)
• The probability that k frames are generated
  during a frame time is given by the Poisson
  distribution

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Performance

• The probability that no frame is generated during
  the vulnerable period (two frames) is
• The throughput of pure Aloha is
• The maximum throughput is S1/(2e) or about 0.184

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Slotted Aloha

• Time is divided into fixed-size slots
• Global time synchronization is required
• Throughput
• The maximum throughput is
• S 1/e or about 0.368

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Throughput of Aloha Systems
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Carrier Sense Multiple Access (CSMA)

• Problem of the Aloha system
• A station does not consider what the others are
  doing
• Key idea of CSMA
• A station can detect what other stations are
  doing and can then adapt its behavior accordingly

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Categories

• 1-Persistent
• Nonpersistent
• p-persistent
• CSMA/CD
• CSMA/CA
• Performance

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1-Persistent

• A station that has a frame to send shall keep
  listening to (sensing) the channel
• If the channel is busy, the station will be
  waiting
• If the channel is idle, the station will transmit
  the frame

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Nonpersistent

• A station shall listen to (sense) the channel at
  the moment that it has a frame to send
• If the channel is busy, then the station will
• Stop sensing
• Wait for a random amount of time to sense again
• If the channel is idle, then the station will
  send the frame

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p-Persistence

• Assumption slotted channel
• A station shall listen to (sense) the channel at
  the moment that it has a frame to send
• If the channel is busy, then the station will
• Stop sensing
• Wait for a random amount of time (slots) to sense
  again
• If the channel is idle, then the station will
  send the frame with a probability p, and defer a
  slot with a probability (1-p)

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CSMA with Collision Detection

• Main idea
• If two stations detect the collision after
  transmitting, they shall stop the transmission
• CSMA/CD is used in Ethernet

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Illustration of CSMA/CD

• CSMA/CD can be in one of three states
  contention, transmission, or idle.

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Design Issues

• The maximum propagation delay
• E.g. about 5us to transmit signal through 1 km
  cable
• The capability to distinguish two signals from
  different stations
• This is an analogue process
• CSMA/CD system is half-duplex

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CSMA/CA

• Main idea
• To avoid collision in a network
• Example
• MACA
• IEEE 802.11 MAC
• To be discussed shortly later

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Medium Access with Collision Avoidance (MACA)

• Motivation
• CSMA protocols sense the carrier, but sensing the
  carrier does not always release true information
  about the status of the wireless channel
• There are two problems that are unique to
  wireless channels (different than wireline
  channels), that makes CSMA useless in some cases
• Hidden terminal problem
• Exposed terminal problem

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The Hidden Terminal Problem

• A is transmitting to B
• C is sensing the carrier and detects that it is
  idle (It can not hear As transmission)
• C also transmits and collision occurs at B
• A is hidden from C

Cs cell
As cell
B
C
A
Hidden terminal
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The Exposed Terminal Problem

• B is transmitting to A. C is hearing this
  transmission
• C now wants to transmit to D. It senses the
  existence of carrier signal and defers
  transmission to D
• However, C can actually start transmitting to D
  while B is transmitting to A,
• Since A is out of range of C and Cs signals can
  not be heard at A.
• C is exposed to Bs transmission.

Bs cell
Cs cell
B
C
A
D
Exposed terminal
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The Main Idea of MACA
Jeff, lets talk! I am available.
Bob
Bob, I want to talk to you!
Bob, I want to talk to you!

Jeff
John
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The MACA Protocol

• When a station wants to transmit data
• It sends an RTS (Ready-to-Send) packet to the
  intended receiver
• The RTS packet contains the length of the data
  that needs to be transmitted
• Any station other than the intended recipient
  hearing RTS defers transmission for a time
  duration equal to the end of the corresponding
  CTS reception
• The receiver sends back CTS (Clear-to-Send)
  packet back to sender if it is available to
  receive.
• The CTS packet contains the length of the data
  that original sender wants to transmit
• Any station other than the original RTS sender,
  hearing CTS defers transmission until the data is
  sent.
• The original sender upon reception of the CTS,
  starts transmitting

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The MACA Protocol
A is transmitting to B.
Cs cell
As cell
CTS(n)
RTS(n)
RTS(n)
B
C
A
X
CTS(n)
C defers transmission for duration of n bytes of
data transmission. Node A is no longer hidden
from C effectively.
X defers transmission until expected CTS
reception time by RTS sender.
Data(n)
Waiting time of node X is much smaller than
waiting time of node C.
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The MACA Protocol
B is transmitting to A
Bs cell
Cs cell
RTS(n)
RTS(n)
B
C
A
D
RTS(m)
CTS(n)
CTS(m)
Data(n)
Data(m)

• C defers transmission upon hearing Bs RTS until
  B could get CTS from A.
• After that C can start transmission to D. For
  that it first sends an RTS.
• C is not longer exposed to the data transmission
  of B.

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Performance

• The metrics
• The assumptions
• Evaluation methods
• Throughput comparison
• The delay issue

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The Performance Metrics

• Throughput
• May be normalized by various figures
• Utilization
• Delay
• End-to-end
• Access
• Queuing

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The Assumptions

• System
• Timing
• Slotted
• Non-slotted
• Queue
• Finite
• Infinite
• Delay
• Propagation
• Detection/Sensing
• Traffic
• Saturated
• Unsaturated
• Arrival model
• Others
• Transmission errors
• Many more

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How to Evaluate the Performance?

• Simulation
• Simulator
• NS2, OPNET, QALNET, etc.
• Self-developed program
• Analytical model
• Assumptions
• Probability theory
• Queuing theory

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Throughputs Comparison
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Performance of CSMA

• Comparison of the channel utilization versus load
  for various random access protocols.

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The Delay Issue

• Detection delay and propagation delay are two
  important parameters for CSMA
• Detection delay time required to sense the
  carrier and to decide if it is idle or busy
• Propagation delay The time required for bit to
  travel from the transmitter to the receiver
• distance/speed_of_wave
• Example
• Ethernet
• WLAN with directional antenna

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Reservation Based MAC

• The reservation can increase the efficiency to
  more than 80
• A sender shall reserve some time in the future
  for transmission
• The reservation attempts can be collision free or
  with collision
• The reservation delay may be large
• Suitable for satellite communications
• Examples
• Packet reservation MA (PRMA)
• Reservation-TDMA

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PRMA
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Reservation-TDMA
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Polling Based MAC

• There is a controller/coordinator/master in a
  wireless network
• The coordinator will send polling message to ask
  whether a node has message to send
• The node been asked will reply with its demand
• Example
• IEEE 802.11 MAC PCF mode
• Bluetooth

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Case Study IEEE 802.11b MAC

• IEEE 802.11b High Data-rate Wireless LAN
  standard
• Operates in 2.4-2.483 GHz ISM RF Band.
• 83 MHz spectrum width
• Max data-rate 11Mbps simplex
• Spectrum Usage FHSS or DSSS
• Modulation Technique CCK with QPSK
• For 11Mbps
• Symbol rate 1.375 MSps
• Bits/symbol 8
• Range a few hundred meters

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Network Mode

• Infrastructure Mode
• Ad-Hoc Mode

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Infrastructure Mode

• All traffic has to go through access points
• Access point provides connectivity to the wired
  backbone

Infrastructure Mode
Access Point
Access Point
Wireless Link
Wireless Link
Wireless Link
Mobile Station
Basic Service Set (BSS)
Extended Service Set (ESS) comprised of a
number of IEEE 802.11 BSS and enables limited
mobility within the WLAN.
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Ad Hoc Mode

• Mobile Stations can talk directly with each-other
• All stations in an IBSS
• Stations need to be in the range of each-other

Ad-Hoc Mode
Independent Basic Service Set (IBSS)
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MAC Mode

• DCF Distributed Coordination Function
• Based on CSMA/CA
• PCF Point Coordination Function
• Connection oriented
• Contention free service
• Polling based

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802.11b PHY Layer

• Can support data rates at 1,2,5.5,11 Mbps

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802.11b MAC Sublayer

• CRC is added to each MAC frame
• Packet fragmentation is supported to chop large
  higher layer packets into small pieces
• Advantage
• Probability a packet gets corrupted increases
  with the packet size
• In case of corruption, only a small fragment
  needs to be re-transmitted
• Disadvantage
• Introduce more overhead
• Preamble, access
• Not suitable for high-data rate physical layer

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CSMA/CA Protocol

• CSMA/CA
• Carrier Sense Multiple Access/Collision Avoidance
• CSMA/CA is the protocol to implement the DCF mode
  of the MAC sub-layer.
• RTS/CTS is optional
• To avoid collisions
• To improve the throughput performance
• A threshold will be applied
• Why?

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The Basic Scheme

• Transmission of MPDU (Data) without the use of
  RTS/CTS
• Procedure
• When a STA has data to send, it senses medium
• The STA may transmit a MAC Protocol Data Unit
  (MPDA) when medium idle time is greater or equal
  to DIFS
• If medium is busy, wait for a random backoff time
• Each node maintains a data structure called
  Network Allocation Vector (NAV)
• When neighboring nodes receive the header of a
  data frame, they know somebody is talking
• During the NAV period, these nodes will not try
  to send

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Backoff Procedure

• Backoff Procedure
• Backoff procedure is invoked for a STA to
  transfer a frame but the medium is busy
• Set Backoff Timer to be random backoff time
• Backoff Timer starts decreasing after an idle
  time of DIFS following the medium business
• Backoff Timer is suspended when medium is busy,
  and wont resume until the medium is idle for
  DIFS
• A frame may be transmitted immediately when
  Backoff Timer is 0

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Illustration of the Basic Scheme
DIFS
Data
Source
SIFS
ACK
Destination
Contention Window (Slot Times)
DIFS
Data
Others
Defer Access
Backoff afterDefer
A station backoffs a random number of slot times.
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The RTS/CTS Scheme

• Please refer to the discussion in MACA
• The RTS and CTS frame will carry duration
  information such that neighboring nodes can
  update their NAV

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Illustration of the RTS/CTS Scheme
DIFS
RTS
DATA
Source
SIFS
SIFS
SIFS
CTS
ACK
Destination
Others
DIFS
Defer Access for NAV(RTS)
Defer Access for NAV(CTS)
Backoff afterDefer
Defer Access for NAV(Data)
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The ACK Mechanism

• ACK is required to understand if the packet is
  correctly received (without any collisions ) at
  the receiver
• Positive ACK
• Ethernet does not require ACK to be sent, since
  the transmitter can detect the collision on the
  channel (cable)
• A wireless transmitter can not detect collision,
  because
• Transmit power is much larger than the received
  power received signal is regarded as noise (not
  collision)
• There could be a hidden terminal

Access Point
Mobile
RTS
CTS
DATA
ACK