Wireless Local Area network and 802.11 Architecture
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Wireless Local Area network and 802.11 Architecture

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    Wireless Local Area network and 802.11 Architecture Wireless Local Area network and 802.11 Architecture Presentation Transcript

    • WLAN and 802.11 Architecture Ashish Babbar EE668
    • 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
    • What are WLAN’s
      • 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)
    • Why WLAN’s
      • LANs were originally created as short distance data communication networks.
      • Ease of Installation.
      • Mobility
      • Timesaving / Convenient / Logistically easy
      • Allows users to work where it’s most convenient
      • Cost advantages
        • Hard-to-wire auditoriums and classrooms
        • Impossible-to-wire outdoor spaces
        • Expandable at any time
    • 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
    • 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.
    • 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
    • Reference Model
    • 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.
    • 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.
    • 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 (>20 Mbps) at 2.4 GHz
      • 802.11h Dynamic Channel Selection and Transmit Power Control mechanisms
      • 802.11i Authentication and Security
    • 802.11 Frequency Bands
    • 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.
    • Basic Service Set STA1 STA2 STA3 STA4 BSS1 BSS2
    • 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
    • Ad-Hoc mode STA 1 STA 2 STA 4 STA 3
    • 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.
    • Infrastructure mode WLAN Distributed System STA 1 STA 2 STA 3 Access Point
    • 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.
    • Distribution Systems
    • 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.
    • Extended Service Set
    • 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.
    • Complete Architecture STA 1 STA 2 AP STA 4 AP Portal Wired LAN BSS 1 BSS 2 Distribution System STA 3
    • Channel Reuse
      • The 802.11b operates in the 2.4GHz band.
      • The transmitted signal uses approximately 30MHz which is approximately 1/3 rd 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.
    • Channel Allocation
    • Non Overlapping channels
    • Channel Reuse 802.11b
    • 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
    • 1 2 3 4 RTS2 RTS2 CTS1 CTS1 MEDIUM BUSY RTS2 DIFS SIFS CTS1 SIFS DATA SIFS ACK NAV(RTS) NAV(CTS)
      • Node 1 has to communicate with Node 2
      • 3 and 4 have to remain silent till 1 finishes transmission to 2
      Channel Access in 802.11
    • 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
    •  
    • 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
    • References
      • http://www.ifi.uio.no/infpri/Presentasjoner/stud_routingSwitching_13.02.04.pdf
      • www.lowrycomputer.com/webinars/files/rf01.pdf
      • http://wireless.ictp.trieste.it/school_2004/lectures/ermanno/Wi-Fi_overview.pdf
      • http://www.uic.edu/depts/accc/seminars/wireless/wireless.pdf
      • http://www.uncg.edu/apl/WirelessLANs.pdf
      • http://ieee1451.nist.gov/Workshop04June01/Gilsinn.pdf
      • http://www.sss-mag.com/pdf/802_11tut.pdf