Wireless LANs and Bluetooth Lecture 5

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Wireless LANs and Bluetooth Lecture 5

• A wireless LAN uses wireless transmission medium.
• Used to have high prices, low data rates,
  occupational safety concerns, and licensing
  requirements.
• Problems have been addressed.
• Popularity of wireless LANs has grown rapidly.

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Applications of Wireless LANs
Wired LAN Extension

• Saves installation of LAN cabling
• Eases relocation and other modifications to
  network structure
• However, increasing reliance on twisted pair
  cabling for LANs
• Most older buildings already wired with Cat 3
  cable
• Newer buildings are prewired with Cat 5
• Wireless LAN to replace wired LANs has not
  happened
• In some environments, role for the wireless LAN
• Buildings with large open areas
• Manufacturing plants, stock exchange trading
  floors, warehouses
• Historical buildings
• Small offices where wired LANs not economical

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Applications of Wireless LANs
Single Cell Wireless LAN
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Applications of Wireless LANs
Multi-Cell Wireless LAN
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Applications of Wireless LANs
Ad Hoc Networking

• Peer-to-peer network,
• No centralised server,
• No infrastructure,
• Temporary nature.

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Wireless LAN Requirements

• Same as any LAN
• High capacity, short distances, full
  connectivity, broadcast capability
• Throughput efficient use of wireless medium
• Number of nodesHundreds of nodes across multiple
  cells
• Connection to backbone LAN Use control modules
  to connect to both types of LANs
• Service area 100 to 300 m
• Low power consumptionNeed long battery life on
  mobile stations
• Mustnt require nodes to monitor access points or
  frequent handshakes
• Transmission robustness and securityInterference
  prone and easily eavesdropped
• Collocated network operationTwo or more wireless
  LANs in same area
• License-free operation
• Handoff/roaming Move from one cell to another
• Dynamic configuration Addition, deletion, and
  relocation of end systems without disruption to
  users

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IEEE 802.11 Wireless LANs

• One of the major problems in wireless LANs today
  is the abundance of different wireless
  technologies and the lack of compatibility among
  them.
• IEEE 802.11 (also known as Wi-Fi) defines a
  standard for the physical and the data link
  layers of wireless LANs.
• The standard is defined for the license-free
  Industrial, Scientific, Medical (ISM) bands.

ISM Bands
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IEEE 802.11 Architecture
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IEEE 802.11 Protocol Stack
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IEEE 802.11 MAC Sublayer
CSMA/CD cannot be used in wireless LANs.
Because not all stations are within the radio
range of each other. It is also not possible to
detect collision while transmitting because most
stations are half-duplex.
(a) The hidden station problem. (b) The exposed
station problem
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IEEE 802.11 MAC Sublayer

• IEEE 802.11 MAC algorithm is called Distributed
  Foundation Wireless MAC (DFWMAC).
• DFWMAC defines two sublayers. Distributed
  Coordination Function (DCF) and Point
  Coordination Function (PCF).
• DCF is distributed with an optional centralised
  access control that works on top of that (i.e.,
  PCF).
• DCF is based on CSMA/CA (CSMA with Collision
  Avoidance) or MACAW.
• All frame transmissions are acknowledged with ACK
  packets. This is the way collision is avoided.

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IEEE 802.11 MAC Sublayer
Distributed Coordination Function (DCF)
A
B
C
D
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IEEE 802.11 MAC Sublayer
Point Coordination Function (PCF)

• The access point periodically broadcasts beacon
  frames that contains system parameters and
  invites stations to request bandwidth.
• The access point can provide guaranteed bandwidth
  to stations that are working in PCF mode.
• PCF allows the transport of real-time traffic
  over the wireless LAN.
• PCF is not very well defined in the standard, and
  not commonly implemented in most commercial
  access points. However, it exists in the standard
  specification.

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IEEE 802.11 MAC Sublayer
More on Distributed Coordination Function (DCF)
IFS InterFrame Space
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IEEE 802.11 MAC Sublayer
SIFS (Short IFS) Shortest IFS. Defined for ACKs,
CTSs, and poll responses PIFS (PCF IFS)
Middle-length IFS. Used by the access point when
issuing polls. DIFS (DCF IFS) Longest IFS. Any
station may attempt to acquire channel to send a
new frame.
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Bluetooth

• A piconet comprised by up to seven active slaves
  and a master.
• IEEE 802.15 (Personal Area Networks) is based on
  Bluetooth.
• Bluetooth is an end-to-end specification, whereas
  IEEE 802.15 only covers physical and data link
  layers.

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Bluetooth

• Bluetooth physical layer uses FHSS at 1600
  hops/sec.
• 2.4 GHz ISM Band is used with 79 channels of 1
  MHz each.
• Gross transmission rate is 1 Mbps.
• Dwell time (time spent in one carrier) is 625
  µsec.
• It takes 250-260 µsec to settle in one carrier
  frequency.
• The radio designated as the master makes the
  determination of the channel (frequency hopping
  sequence) and phase (timing offset, i.e., when to
  transmit) that shall be used by all devices on
  this piconet.
• A slave may only communicate with the master and
  may only communicate when granted permission by
  the master.

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Bluetooth

• Time is divided in 625 bits, 3x625 bits, or 5x625
  bits slots.
• Regardless of the slot type there is one settling
  time (250-260 µsec).
• Master uses even numbered slots.
• Slaves use odd numbered slots that are designated
  by the master.

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Bluetooth
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Bluetooth

• Two types of logical links are defined between
  master and slave.
• Asynchronous ConnectionLess (ACL) links are used
  bursty data traffic with no QoS guarantee. Frames
  can be lost and retransmitted on ACL links.
• Synchronous Connection Oriented (SCO) links are
  used for real-time traffic (typically telephony).
  This type of link is allocated a fixed slot in
  each direction. There are no retransmissions.
  Instead there is forward error correction on SCO.

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Bluetooth

• Access code defines the master node (in case
  there are several masters within the radio
  range).
• Address defines destination node.
• Type defines link type (e.g., SCO, ACL)
• F Flow control bit
• A Acknowledgement bit
• S Sequence number

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Bluetooth

• SCO Links always operate on single-slot (i.e.,
  240 bits data field) configuration. ACL links may
  operate on three or five slot configuration
  (i.e., 2744 bits data field).
• The most reliable configuration is single-slot
  with 80 bits of data repeated three times in a
  240-bit data field. 64000 bps can be achieved
  this way.

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• William Stallings, Data and Computer
  Communications, chapter 13, 14.
• A. S. Tanenbaum, Computer Networks, chapter 4.
• http//iamwww.unibe.ch/rvs/lectures/SS98/cn/apple
  ts/Ethernet/ethernet.htm (CSMA/CD applet)
• http//www.wi-fiplanet.com/
• http//www.vicomsoft.com/knowledge/reference/wirel
  ess1.html

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Tutorial

• 1- Explain the Hidden Node and Exposed Node
  problems in wireless LANs?
• 2- What is the solution for medium access control
  that is proposed by IEEE 802.11 standard?
• 3- How are PCF and DCF different in DFWMAC?
• 4-Explain the relationship between master and
  slave stations in Bluetooth.
• 5-What kind of logical links are supported by
  Bluetooth? What kind of a link would be required
  by an email application?
• 6-Explain why CSMA/CD is not used in wireless
  LANs?