WLAN and 802'11 Architecture

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WLAN and 802.11 Architecture

• Ashish Babbar
• EE668

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Overview

• What are WLANs
• Why use WLANs
• Technology Overview
• 802.11 standard
• Reference model
• Various standards
• 802.11 Architecture
• Basic Service Set
• Ad hoc mode
• Infrastructure mode
• Distribution System
• Extended Service Set
• Integration with wired LANs
• Channel Reuse
• Channel Allocation
• Channel Access in 802.11
• Comparison between 802.11a and 802.11b

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What are WLANs

• A supplement to wired LANS!!
• LANs that use radio waves as a transport medium
• Transmission can occur using radio, microwaves or
  Infrared.
• Configured with transmitter/receiver (access
  point or AP) that is connected to a wired network
• Accessed by end users with wireless LAN adapters
  (usually cards in notebook computers)

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Why WLANs

• LANs were originally created as short distance
  data communication networks.
• Ease of Installation.
• Mobility
• Timesaving / Convenient / Logistically easy
• Allows users to work where its most convenient
• Cost advantages
• Hard-to-wire auditoriums and classrooms
• Impossible-to-wire outdoor spaces
• Expandable at any time

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Technology overview

• How does is work?
• Access Points stationary transceivers
• APs act as broadcast and receive base stations
  for wireless network traffic
• APs can offer encryption, security and other
  services, including access
• Once you have an Access Point in operation,
  wireless networking is enabled in its vicinity
• Radio frequency signal coverage typically ranges
  from 100 to 300 feet

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802.11 Standard

• 802.11 refers to specifications developed by the
  IEEE for wireless LAN technology.
• 802.11 specifies an over-the-air interface
  between a wireless client and a base station or
  between two wireless clients.
• 802.11 is based on the Open System
  interconnection (OSI) model.
• IEEE 802.11 framework defines the lowest two
  layers of the OSI model- the physical and the
  data link layers.

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Reference Model

• The 802.11 deals with only the data link layer
  and the physical layer.
• The Data link layer is divided into the MAC layer
  and LLC layer.
• The objective of the standard is to define a
  mechanism for delivery of MAC service data units
  (MSDUs) between the Logic Link controls
• The subdivision makes it possible to support
  different wireless media, such as radio frequency
  signaling and infrared transmission.
• For the radio frequency signaling the frequency
  hopping spread spectrum (FHSS) or direct sequence
  spread spectrum (DSSS) is used

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Reference Model
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Spread Spectrum methods

• Frequency Hoping Spread Spectrum
• A narrowband signal moves or hops from one
  frequency to another using pseudorandom sequence
  to control hopping.
• Signal lingers in the pre-defined frequency for a
  short period of time
• The possibility of interference from another
  signal source generating power at the specific
  hop frequency is reduced.

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Spread Spectrum methods

• Direct Sequence Spread Spectrum
• The data entering a transmitter is combined with
  a higher data rate bit sequence.
• This bit sequence is called chipping code and is
  known to both the transmitter and the receiver.
• Chipping code forms the basis of spreading data
  over a wide range of frequencies.
• At receiver the same chipping code is employed to
  recover the original data.

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Various 802.11 Standards

• 802.11 Specification of WLAN MAC and PHY layers
  (IR, FH and DSSS at 2.4 GHz), ratified in
  1997/1999
• 802.11a PHY layer at 5 GHz (54 Mbps OFDM),
  ratified in 1999
• 802.11b 11 Mbps DSSS at 2.4 GHz, ratified in
  1999
• 802.11c Improvements of the MAC layer (Internal
  Sub-Layer Service)
• 802.11d Update (frequency spectrum regulations)
• 802.11e Improvements of the MAC layer (Quality
  of Service)
• 802.11f Inter-Access Point Protocol (IAPP)
• 802.11g Higher Data rate (gt20 Mbps) at 2.4 GHz
• 802.11h Dynamic Channel Selection and Transmit
  Power Control mechanisms
• 802.11i Authentication and Security

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802.11 Frequency Bands
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802.11 Architecture

• The basic building block of an 802.11 network is
  the Basic Service Set (BSS), which is simply a
  group of stations communicate with each other.
• Communication take place within a somewhat fuzzy
  area, called the Basic Service Area.
• When a station is in the basic service area, it
  can communicate with the other member of the BSS
• When a station moves out of its BSS, it can no
  longer directly communicate with each other.

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Basic Service Set
STA1
STA2
STA3
BSS1
STA4
BSS2
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Basic Modes of BSS

• An ad-hoc or peer-to-peer mode
• - One Basic Service Set, BSS
• - A set of wireless stations communicate
  directly with one another.
• - No need of a central access point or wired
  network connection
• - Limited coverage area

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Ad-Hoc mode
STA 2
STA 1
STA 3
STA 4
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Basic Modes of BSS

• An infrastructure mode
• At least one central access point connected to a
  wired network.
• Access points are used for all communications in
  infrastructure networks
• Communications between mobile nodes in the same
  service area is also done through Access points.

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Infrastructure mode
WLAN Distributed System
Access Point
STA 3
STA 1
STA 2
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Distribution System Concept

• PHY limitations determine the direct station-to-
  station distance supported.
• Instead of existing independently, the BSS may
  interconnected.
• The Distribution system (DS) enables the
  interconnection of the BSS.
• Data moves between the BSS and the DS via an
  access point.
• The DS can represent an existing LAN
  infrastructure or a twisted pair wire that simply
  interconnects two access points.

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Distribution Systems
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Extended Service Set (ESS)

• The DS and BSSs allow IEEE 802.11 to create a
  wireless network of arbitrary size and
  complexity, called the ESS.
• An ESS is created by chaining BSSs together with
  a backbone
• The stations within the ESS may communicate and
  mobile stations may move from one BSS to another
  (within the same ESS).
• Within an ESS the BSSs may overlap or may be
  physically disjoint.

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Extended Service Set
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Integration With Wired LANs

• To integrate the IEEE 802.11 architecture with a
  traditional wired LAN, a final architectural
  component called the Portal is used.
• A portal is the logical point at which the MSDUs
  from a wired LAN enter the IEEE 802.11
  distribution system
• All data from non IEEE 802.11 LANs enter the IEEE
  802.11 architecture via a portal.
• It is possible for one device to provide the
  functions of both the AP and the portal.

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Complete Architecture
STA 1
STA 2
BSS 1
Distribution System
AP
BSS 2
AP
STA 4
Portal
STA 3
Wired LAN
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Channel Reuse

• The 802.11b operates in the 2.4GHz band.
• The transmitted signal uses approximately 30MHz
  which is approximately 1/3rd of the band.
• Thus 802.11b provides only 3 non overlapping
  channels, i.e. channel number 1, 6 and 11.
• These 3 channels can be reused and allocated to
  access points in different regions for effective
  channel re-use.

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Channel Allocation
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Non Overlapping channels
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Channel Reuse 802.11b
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Channel Access in 802.11

• A Request To Send (RTS) frame is sent by a
  potential transmitter to a receiver.
• In response the receiver sends a Clear To Send
  (CTS) frame back to the transmitter.
• When an RTS or CTS frame is sent out by a
  transmitter or receiver the requesting station
  notifies all stations within the radio range not
  to initiate any transmissions for a given time
• This is called the Net Allocation Vector (NAV)
• The NAV indicates the amount of time that must
  elapse before the current transmission session is
  complete and the channel can be sampled again for
  idle status

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Channel Access in 802.11
3
4

• Node 1 has to communicate with Node 2
• 3 and 4 have to remain silent till 1 finishes
  transmission to 2

RTS2
RTS2
CTS1
CTS1
1
2
SIFS
SIFS
SIFS
DIFS
MEDIUM BUSY
RTS2
DATA
CTS1
ACK
NAV(RTS)
NAV(CTS)
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Comparison between 802.11a 802.11b

• 802.11b operates in the 2.4 GHz band.
• Supports bit rates up to 11Mbps.
• Uses the Direct Sequence Spread Spectrum (DSSS).
  
• Only 3 non overlapping channels can be used
  without interference.
• Backward compatibility with 802.11 products
• Better range as compared to 802.11a.

• 802.11a operates in the 5 GHz band.
• Supports bit rates up to 54 Mbps.
• Uses Orthogonal frequency division multiplexing
  (OFDM)
• Up to 8 non overlapping channels can be used
  without interference.
• No backward compatibility with the 802.11 and
  802.11b products.
• Reduced range owing to higher operating frequency

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References

• Gil Held- Deploying Wireless LANs McGraw
  Hill
• Benny Bing-Wireless Local Area Networks Wiley
  Interscience
• Asuncion Santamaria Wireless LAN standards and
  applications Artech House
• James Larocca, Ruth larocca -802.11 Demystified
  McGraw Hill
• Neil Reid and Ron Seide - 802.11 Networking
  Handbook

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References

• http//www.ifi.uio.no/infpri/Presentasjoner/stud_r
  outingSwitching_13.02.04.pdf
• www.lowrycomputer.com/webinars/files/rf01.pdf
• http//wireless.ictp.trieste.it/school_2004/lectur
  es/ermanno/Wi-Fi_overview.pdf
• http//www.uic.edu/depts/accc/seminars/wireless/wi
  reless.pdf
• http//www.uncg.edu/apl/WirelessLANs.pdf
• http//ieee1451.nist.gov/Workshop04June01/Gilsinn.
  pdf
• http//www.sss-mag.com/pdf/802_11tut.pdf