Wireless Local Area Network

1
Lecture 8

• Wireless Local Area Network

2
Why Wireless

• Mobility Wireless LAN systems can provide LAN
  users with access to real-time information
  anywhere in their organization. This mobility
  supports productivity and service opportunities
  not possible with wired networks.
• Installation Speed and Simplicity Installing a
  wireless LAN system can be fast and easy and can
  eliminate the need to pull cable through walls
  and ceilings.
• Installation Flexibility Wireless technology
  allows the network to go where wire cannot go.
• Reduced Cost-of-Ownership While the initial
  investment required for wireless LAN hardware can
  be higher than the cost of wired LAN hardware,
  overall installation expenses and life-cycle
  costs can be significantly lower. Long-term cost
  benefits are greatest in dynamic environments
  requiring frequent moves and changes.
• Scalability Wireless LAN systems can be
  configured in a variety of topologies to meet the
  needs of specific applications and installations.
  Configurations are easily changed and range from
  peer-to-peer networks suitable for a small number
  of users to full infrastructure networks of
  thousands of users that enable roaming over a
  broad area.

3
Wireless LAN Technologies (I)

• Considerations for choosing IR technology
• Advantages
• No government regulations controlling use
• Immunity to electro-magnetic (EMI) and RF
  interference
• Disadvantages
• Generally a short-range technology (30 50 ft.
  radius under ideal conditions
• Signal cannot penetrate solid objects
• Signal affected by ice, snow, light, fog
• Dirt can interfere with infrared

4
Wireless LAN Technologies (II)

• Considerations for choosing Narrowband (UHF)
  technology
• Advantages
• Longest range
• Low cost solution for large sites with low medium
  data throughput requirements
• Disadvantages
• Low throughput
• No multi-vendor interoperability
• Interference potential
• RF site licence required for protected bands
• Large radios and antennas increase wireless
  client size

5
Wireless LAN Technologies (III)

• Wireless range for UHF compared to other RF
• technologies in a typical warehouse environment
• and outdoors in open area

6
Wireless LAN Technologies (IV)

• Wireless Technology Data Rates
• 400 MHz UHF 4.8 - 19.2 Kbps
• 900 MHz Spread Spectrum 100 - 400 Kbps
• 2.4 GHz Spread Spectrum 1 - 2 Mbps
• 2.4 GHz More than10 Mbps
• 5.7 GHz Future More than 20 Mbps

7
Wireless LAN Technologies (V)

• Considerations for choosing 900 MHz technology

Considerations for choosing 2.4 GHz technology
8
Comparison of Wireless LAN Technologies
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Wireless LAN Considerations (I)

• Business factors for selecting a WLAN vendor

10
Wireless LAN Considerations (II)

• Technical factors for selecting a WLAN solution

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Multiplexing

• Capacity of transmission medium usually exceeds
  capacity required for transmission of a single
  signal
• Multiplexing - carrying multiple signals on a
  single medium
• More efficient use of transmission medium

12
Multiplexing
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Reasons for Widespread Use of Multiplexing

• Cost per kbps of transmission facility declines
  with an increase in the data rate
• Cost of transmission and receiving equipment
  declines with increased data rate
• Most individual data communicating devices
  require relatively modest data rate support

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Multiple Access

• Three basic methods for multiplexing data from
  mobile devices into a given frequency spectrum
• Frequency Division Multiple Access (FDMA)
• Time Division Multiple Access (TDMA)
• Code Division Multiple Access (CDMA)

15
Frequency Division Multiple Access (FDMA)

• Takes advantage of the fact that the useful
  bandwidth of the medium exceeds the required
  bandwidth of a given signal
• Each source is given its own frequency band and
  can use it permanently.
• Multiple sources send on different frequency
  bands at the same time.

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Time Division Multiple Access (TDMA) (I)

• Takes advantage of the fact that the achievable
  bit rate of the medium exceeds the required data
  rate of a digital signal
• Each source is given certain time intervals
  during which it can use all the bandwidth.
• Multiple sources send at different points in time
  on the same frequency bandwidth.

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Time Division Multiple Access (TDMA) (II)

• Synchronous TDMA
• Fixed bandwidth allocation. Good for constant
  bit-rate streams. Inefficient for variable
  bit-rate, bursty streams. Simple, cost effective.

• Asynchronous TDMA
• On demand bandwidth allocation. Much more
  efficient for variable bit-rate, bursty streams.
  Various solutions.

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Code Division Multiple Access (CDMA)

• Multiple sources send with different data
  encoding, at the same time, on the same
  bandwidth. Each source encodes its data using a
  different code, from a set of orthogonal codes.
• Data divided into small packets and distributed
  into a predetermined pattern across the frequency
  spectrum.
• Each pattern designed by a code known as
  Pseudo-random Noise (PN) code.
• PN corresponds to a time slot number in TDMA or a
  carrier frequency in FDMA.

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IEEE 802.11 Wireless LAN Standard
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(No Transcript)
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Protocol Architecture

• Functions of physical layer
• Encoding/decoding of signals
• Preamble generation/removal (for synchronization)
• Bit transmission/reception
• Includes specification of the transmission medium

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Protocol Architecture

• Functions of medium access control (MAC) layer
• On transmission, assemble data into a frame with
  address and error detection fields
• On reception, disassemble frame and perform
  address recognition and error detection
• Govern access to the LAN transmission medium
• Functions of logical link control (LLC) Layer
• Provide an interface to higher layers and perform
  flow and error control

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Separation of LLC and MAC

• The logic required to manage access to a
  shared-access medium not found in traditional
  layer 2 data link control
• For the same LLC, several MAC options may be
  provided

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MAC LLC Frames Format

• Characteristics of LLC not shared by other
  control protocols
• Must support multi access, shared-medium nature
  of the link
• Relieved of some details of link access by MAC
  layer

• MAC control
• Contains Mac protocol information
• Destination MAC address
• Destination physical attachment point
• Source MAC address
• Source physical attachment point
• CRC
• Cyclic redundancy check

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LLC Services

• Unacknowledged connectionless service
• No flow- and error-control mechanisms
• Data delivery not guaranteed
• Connection-mode service
• Logical connection set up between two users
• Flow- and error-control provided
• Acknowledged connectionless service
• Cross between previous two
• Datagrams acknowledged
• No prior logical setup

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IEEE 802.11 Services
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IEEE 802.11 Architecture

• Distribution system (DS)
• Access point (AP)
• Basic service set (BSS)
• Stations competing for access to shared wireless
  medium
• Isolated or connected to backbone DS through AP
• Extended service set (ESS)
• Two or more basic service sets interconnected by
  DS

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Distribution of Messages Within a Distribution
Service (DS)

• Distribution service
• Used to exchange MAC frames from station in one
  BSS to station in another BSS
• Integration service
• Transfer of data between station on IEEE 802.11
  LAN and station on integrated IEEE 802.x LAN

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Transition Types Based On Mobility

• No transition
• Stationary or moves only within BSS
• Basic Service Set (BSS) transition
• Station moving from one BSS to another BSS in
  same ESS
• Extended Service Set (ESS) transition
• Station moving from BSS in one ESS to BSS within
  another ESS

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Access and Privacy Services

• Authentication
• Establishes identity of stations to each other
• Deauthentication
• Invoked when existing authentication is
  terminated
• Privacy
• Prevents message contents from being read by
  unintended recipient

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IEEE 802.11 Medium Access Control

• MAC layer covers three functional areas
• Reliable data delivery
• Access control
• Security WEP (Wired Equivalent Privacy), WPA
  (Wi-Fi Protected Access), WPA-PSK (Wi-Fi
  Protected Access with the PreShared Key)

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Reliable Data Delivery

• More efficient to deal with errors at the MAC
  level than higher layer (such as TCP)
• Frame exchange protocol
• Source station transmits data
• Destination responds with acknowledgment (ACK)
• If source doesnt receive ACK, it retransmits
  frame
• Four frame exchange
• Source issues request to send (RTS)
• Destination responds with clear to send (CTS)
• Source transmits data
• Destination responds with ACK

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Access Control
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MAC Frame Format Fields

• Frame Control frame type, control information
• Duration/connection ID channel allocation time
• Addresses context dependant, types include
  source and destination
• Sequence control numbering and reassembly
• Frame body MAC Service Data Unit (MSDU) or
  fragment of MSDU
• Frame check sequence 32-bit Cyclic Redundancy
  Check (CRC)

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Frame Control Fields

• Power management 1 if transmitting station is
  in sleep mode
• More data Indicates that station has more data
  to send
• WEP (Wired Equivalent Privacy) 1 if wired
  equivalent protocol is implemented
• Order 1 if any data frame is sent using the
  Strictly Ordered service

• Protocol version 802.11 version
• Type control, management, or data
• Subtype identifies function of frame
• To DS 1 if destined for DS
• From DS 1 if leaving DS
• More fragments 1 if fragments follow
• Retry 1 if retransmission of previous frame

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Physical Media Defined by Original 802.11 Standard

• Direct-sequence spread spectrum
• Operating in 2.4 GHz ISM band
• Data rates of 1 and 2 Mbps
• Frequency-hopping spread spectrum
• Operating in 2.4 GHz ISM band
• Data rates of 1 and 2 Mbps
• Infrared
• 1 and 2 Mbps
• Wavelength between 850 and 950 nm

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IEEE 802.11a and IEEE 802.11b

• IEEE 802.11a
• Makes use of 5-GHz band
• Provides rates of 6, 9 , 12, 18, 24, 36, 48, 54
  Mbps
• Uses orthogonal frequency division multiplexing
  (OFDM)
• Subcarrier modulated using BPSK, QPSK, 16-QAM or
  64-QAM
• IEEE 802.11b
• Provides data rates of 5.5 and 11 Mbps
• Complementary code keying (CCK) modulation scheme

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Sources

• From
• William Stallings - Wireless communications and
  networks /Second Edition, Prentice Hall 2005
• Chapter 2 2.5
• Chapter 14
• Whitepapers
• Wireless LAN Concepts available at
  http//www.utdallas.edu/ir/wlans/whitepapers/wlanc
  oncepts.pdf
• Guide to Wireless LAN Technologies available
  at http//www.utdallas.edu/ir/wlans/whitepapers/w
  lan_wp.pdf
• Wireless Networking available at
  http//www.utdallas.edu/ir/wlans/whitepapers/WLANA
  .pdf
• Wireless LAN Evaluation Guide available at
  http//www.utdallas.edu/ir/wlans/whitepapers/wlang
  de.pdf
• What is a Wireless LAN available at
  http//www.utdallas.edu/ir/wlans/whitepapers/whatw
  lan.pdf
• In-building Wireless LANs available at
  http//www.utdallas.edu/ir/wlans/whitepapers/1862.
  pdf