Wireless Networking for BE by changcheng2


Pradip Paudyal
 In 1997, the IEEE adopted the first standard for WLANs
  and revised in 1999.
 IEEE defines a MAC sublayer, MAC management
  protocols and services, and three physical (PHY) layers.
 PHY Layers: IR, FHSS, DSSS with 1-2 Mbps.
 IEEE 802.11a ; PHY Layer - OFDM at Unlicensed
  National Information Infrastructure (UNII) bands with 54
 IEEE 802.11b ; PHY Layer - DSSS at 2.4 GHz with
      Introduction cont…
 Supports both Asynchronous data transfer and time bound
   Asynchronous: traffic insensitive to time - email, FTP
   Time bound services: sensitive to time - voice traffic
   Different MAC strategies to support these classes of traffic
   Asynchronous traffic is supported through Distributed Co-
    ordination Function (DCF)
   Time bound traffic is supported through Point Co-
    ordination Function (PCF)
      DCF is mandatory, while PCF is optional
Characteristics of wireless LANs
 Advantages
   very flexible within the reception area
   Ad-hoc networks without previous planning possible
   (almost) no wiring difficulties (e.g. historic buildings,
   more robust against disasters like, e.g., earthquakes,
    fire - or users pulling a plug...
 Disadvantages
   typically very low bandwidth compared to wired
   products have to follow many national restrictions if
    working wireless, it takes a vary long time to establish
    global solutions like, e.g., IMT-2000
   Interference
Design goals for wireless LANs
   global, seamless operation
   low power for battery use
   no special permissions or licenses needed to use
      the LAN {ISM band, 2.4 GHz}
     robust transmission technology
     easy to use for everyone, simple management
     protection of investment in wired networks
     security (no one should be able to read my data),
      privacy (no one should be able to collect user
      profiles), safety (low radiation)
     transparency concerning applications and higher
      layer protocols.
 Architecture is designed to support a network where
  mobile station is responsible for the decision making.
 Components:
Station (STA)
  terminal with access mechanisms to the wireless medium
   and radio contact to the access point
  Supported services are authentication, privacy, and
   delivery of the data.
   Architecture cont..
 Access Point (AP):
    Similar to the base station in cellular network
    Supports range extension by providing network connectivity
     between multiple BSSs
    Multiple BSSs are connected together through a Distribution
     System (DS)
    DS is similar to a backbone network
        e.g., ethernet-based LAN
    BSSs connected by a DS form an Extended Service Set (ESS)
 BSS: Basic service set
    Group of stations under the direct control of a single co-ordination
   All stations in a BSS can directly communicate with each other,
     without any infrastructure
  bridge to other (wired) networks
Distribution System
  interconnection network to form one logical network (EES: Extended
   Service Set) based on several BSS
 Comparison: infrastructure vs. ad-hoc networks
                                   AP: Access Point

             AP    wired network

ad-hoc network
       802.11 - Architecture of an infrastructure network
          802.11 LAN
                                      802.x LAN

                Access                Portal
                   Distribution System
ESS                        Point


         STA2            802.11 LAN      STA3
   802.11 - Architecture of an ad-hoc network
        802.11 LAN
                                          Direct communication within a
                                          limited range
STA1                                          Station (STA):
       BSS1                  STA3              terminal with access mechanisms
                                               to the wireless medium
                                              Basic Service Set (BSS):
                                               group of stations using the same
          STA2                                 radio frequency



        STA4          802.11 LAN
IEEE standard 802.11
                                                              fixed terminal
  mobile terminal


                                         infrastructure network

                                     access point

  application                                           application
     TCP                                                   TCP
       IP                                                   IP
      LLC                    LLC                           LLC
  802.11 MAC        802.11 MAC   802.3 MAC             802.3 MAC
  802.11 PHY        802.11 PHY   802.3 PHY              802.3 PHY
802.11 - Layers and functions
      MAC                                           PLCP Physical Layer Convergence Protocol
             Medium access mechanisms,                                    clear channel assessment signal
              fragmentation (Segmentation),                                 (carrier sense)
              encryption                             PMD Physical Medium Dependent
      MAC Management                                                      modulation, coding
             synchronization, roaming, MAC          PHY Management
              Information Base (MIB), power
              management                                                   channel selection
                                                     Station Management
                                                                           coordination of all management

                                                       Station Management
       LLC (Logical Link Control)

      MAC (Medium Access Control) MAC Management
        PLCP (Physical Layer
         Convergence Protocol)

                                    PHY Management
        PMD (Physical Medium
Radio Transmission
 Orthogonal Frequency Division Multiplex (OFDM)
    a frequency-division multiplexing (FDM) scheme utilized as a
     digital multi-carrier modulation method
 FHSS (Frequency Hopping Spread Spectrum)
    spreading, despreading
    Operating at 1Mbps/2Mbps
 DSSS (Direct Sequence Spread Spectrum)
    chipping sequence: +1, -1, +1, +1, -1, +1, +1, +1, -1, -1, -1 (Barker
    max. radiated power 1 W (USA), 100 mW (EU), min. 1mW
 Infrared
    850-950 nm, diffuse light, typ. 10 m range
  Medium Access Control
 MAC protocol supplies the functionality required
  to provide a reliable delivery mechanism for user
  data over noisy, unreliable wireless media.
 MAC Functionality
   reliable data delivery
   fairly control access to the shared wireless medium.
   protect the data that it delivers.
Challenges 1: Hidden Node and Exposed Node
 Hidden terminals
     A sends to B, C cannot receive A
     C wants to send to B, C senses a “free” medium (CS fails)
     collision at B, A cannot receive the collision (CD fails)
     A is “hidden” for C

                                    A          B          C
 Exposed terminals
     B sends to A, C wants to send to another terminal (not A or B)
     C has to wait, CS signals a medium in use
     but A is outside the radio range of C, therefore waiting is not
     C is “exposed” to B
Challenges 2: Near and Far Terminals
   Terminals A and B send, C receives
       signal strength decreases proportional to the square of the

                             A              B        C

       the signal of terminal B therefore drowns out A’s signal
       C cannot receive A
   Also severe problem for CDMA-networks - precise power
    control needed!
802.11 - MAC layer I – DFWMAC (Distributed Foundation
Wireless Medium Access Control)
 Traffic services
    Asynchronous Data Service (mandatory) {ad hoc}
      exchange of data packets.
      support of broadcast and multicast.
   Time-Bounded Service (optional) {ad hoc / infrastructure}
 Access methods
  CSMA/CA (mandatory)
              avoidance via randomized „back-off“ mechanism
      collision
      minimum distance between consecutive packets
      ACK packet for acknowledgements (not for broadcasts)
  RTS/CTS (optional)
      Distributed Foundation Wireless MAC
      avoids hidden terminal problem
802.11 - CSMA/CA access method                          contention window
     DIFS                          DIFS                 (randomized back-off

                medium busy                              next frame

                   direct access if                                      t
                   medium is free  DIFS          slot time

     Distributed Inter-Frame Space (DIFS)
     station ready to send starts sensing the medium
     if the medium is free for the duration of an Inter-Frame Space
      (IFS), the station can start sending (IFS depends on service type)
     if the medium is busy, the station has to wait for a free IFS, then
      the station must additionally wait a random back-off time
      (collision avoidance, multiple of slot-time)
     if another station occupies the medium during the back-off time of
      the station, the back-off timer stops (fairness)
  802.11 - Frame format
         Types
            control frames, management frames, data frames
         Sequence numbers
            important against duplicated frames due to lost ACKs
         Addresses
            receiver, transmitter (physical), BSS identifier, sender (logical)
         Miscellaneous
            sending time, checksum, frame control, data

bytes  2          2        6       6       6        2       6                   0-2312   4
    Frame      Duration Address Address Address Sequence Address
                                                                                 Data    CRC
    Control      ID        1       2       3     Control    4

              version, type, fragmentation, security, DS (ditribution system){2 bits}
    802.11 MAC Frames
Management Frames: Used for
   Station association, dissociation, timing and synchronization,
Control Frames: Used for
   Handshaking during CP (Contention Period ) (RTS/CTS)
   ACK frames during CP (Contention Period )
Data Frames: Used for
   Sending data during CP and CFP (Contention Free Period )
802.11 - MAC management
  Synchronization
     try to find a LAN, try to stay within a LAN
     timer.
     Beacon.
  Power management
     sleep-mode without missing a message
     periodic sleep, frame buffering, traffic measurements
  Association/Re-association
     integration into a LAN
     roaming, i.e. change networks by changing access points
     scanning, i.e. active search for a network
  MIB - Management Information Base
     managing, read, write
Power management
Idea: switch the transceiver off if not needed
  States of a station: sleep and awake
 Timing Synchronization Function (TSF)
   stations wake up at the same time
 Infrastructure
    Traffic Indication Map (TIM)
      list of   unicast receivers transmitted by AP
   Delivery Traffic Indication Map (DTIM)
     list of broadcast/multicast receivers transmitted by AP

 Ad-hoc
   Ad-hoc Traffic Indication Map (ATIM)
      announcement of   receivers by stations buffering frames
      more complicated - no central AP
      collision of ATIMs possible (scalability?)
802.11 - Roaming
 No or bad connection? Then perform:
  Scanning
     scan the environment, i.e., listen into the medium for beacon
      signals or send probes into the medium and wait for an answer
  Re-association Request
     station sends a request to one or several AP(s)
  Re-association Response
     success: AP has answered, station can now participate
     failure: continue scanning
  AP accepts Re-association Request
     signal the new station to the distribution system
     the distribution system updates its data base (i.e., location
     typically, the distribution system now informs the old AP so it
      can release resources
    Fast roaming – 802.11r : e.g. for vehicle-to-roadside networks
Future developments
 IEEE 802.11a
    compatible MAC, but now 5 GHz band
    transmission rates up to 20 Mbit/s
    close cooperation with BRAN (Broadband Radio Access
     Network; European Standard)
 IEEE 802.11b
    higher data rates at 2.4 GHz
    proprietary solutions already offer 10 Mbit/s
 IEEE WPAN (Wireless Personal Area Networks)
    market potential
    compatibility
    low cost/power, small form factor
    technical/economic feasibility
WLAN Security Concerns
 • Anyone within the geographical network range of an
   open, unencrypted wireless network can 'sniff' the
   traffic, gain unauthorized access to internal network
 • If router security is not activated or if the owner
   deactivates it for convenience, it creates a free hotspot.

 • Modern operating systems such as Microsoft Windows
   make it fairly easy to set up a PC as a wireless LAN 'base
   station' using Internet Connection Sharing, thus
   allowing all the PCs in the home to access the Internet
   via the 'base' PC.
WLAN Security Options
 • For closed networks (like home users and
   organizations) the most common way is to configure
   access restrictions in the access points; Encryption, Checks
   on MAC address
 • For commercial providers and large organizations, the
   preferred solution is often to have an open and
   unencrypted, but completely isolated wireless network.
   -Captive portal which provides for payment and/or
   -Connect securely to a privileged network using VPN
 • Finally, a general solution may be end-to-end
   encryption, with independent authentication on all
   resources that shouldn't be available to the public.
Important Standards
 Thank You……???????????????

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