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					CWNA Guide to Wireless LANs,
      Second Edition



           Chapter Five
IEEE 802.11 Media Access Control and
     Network Layer Standards

                                       1
                        Objectives
• List and define the three types of WLAN
  configurations
• Tell the function of the MAC frame formats
• Explain the MAC procedures for joining, transmitting,
  and remaining connected to a WLAN
• Describe the functions of mobile IP




                                                      2
IEEE Wireless LAN Configurations:
       Basic Service Set
               •   Basic Service Set (BSS): Group
                   of wireless devices served by
                   single AP
                    – infrastructure mode
               •   BSS must be assigned unique
                   identifier
                    – Service Set Identifier
                       (SSID)
                         • Serves as “network name”
                           for BSS
               •   Basic Service Area (BSA):
                   Geographical area of a BSS
                    – Max BSA for a WLAN
                       depends on many factors
               •   Dynamic rate shifting: As mobile
                   devices move away from AP,
                   transmission speed decreases
                                               3
           IEEE Wireless LAN Configurations:
                Extended Service Set

•   Extended Service Set
    (ESS): Comprised of two or
    more BSS networks
    connected via a common
    distribution system
•   APs can be positioned so
    that cells overlap to
    facilitate roaming
     – Wireless devices choose
        AP based on signal
        strength
     – Handoff



                                           4
           IEEE Wireless LAN Configurations:
             Independent Basic Service Set
•   Independent Basic Service Set
    (IBSS): Wireless network that
    does not use an AP
     – Wireless devices communicate
        between themselves
     – Peer-to-peer or ad hoc
        mode
•   BSS more flexible than IBSS in
    being able to connect to other
    wired or wireless networks
•   IBSS useful for quickly and easily
    setting up wireless network
     – When no connection to
        Internet or external network
        needed

                                           5
            IEEE 802.11 Media Access Control
                (MAC) Layer Standards
• Media Access Control (MAC) layer performs several
  vital functions in a WLAN
   –   Discovering WLAN signal
   –   Joining WLAN
   –   Transmitting on WLAN
   –   Remaining connected to WLAN
• Mechanics of how functions performed center around
  frames sent and received in WLANs




                                                      6
                       MAC Frame Formats
• Packet: Smaller segments of a digital data
  transmission
   – Strictly speaking, other terms used to describe these smaller
     segments
• Frames: Packet at MAC layer
   – Or Data Link layer in OSI model
   – IEEE MAC frames different from 802.3 Ethernet frames in format
     and function
   – Used by wireless NICs and APs for communications and
     managing/controlling wireless network




                                                                     7
     MAC Frame Formats -Management

• Management Frames: Initialize communications
  between device and AP (infrastructure mode) or
  between devices (ad hoc mode)
    – Maintain connection




Figure 5-4: Structure of a management frame
                                                   8
                  MAC Frame Formats - Types
•   Types of management frames:
    1.    Authentication frame
    2.    Association request frame
    3.    Association response frame
    4.    Beacon frame
    5.    Deauthentication frame
    6.    Disassociation frame
    7.    Probe request frame
    8.    Probe response frame
    9.    Reassociation request frame
    10.   Reassociation response frame




                                              9
        MAC Frame Formats - Control

• Control frames: Provide assistance in delivering
  frames that contain data




Figure 5-5: Control frame
                                                     10
           MAC Frame Formats - Data

• Data frame: Carries information to be transmitted
  to destination device




Figure 5-6: Data frame
                                                      11
           Discovering the WLAN: Beaconing

•   At regular intervals, AP
    (infrastructure network) or
    wireless device (ad hoc
    network) sends beacon frame
     – Announce presence
     – Provide info for other
        devices to join network
•   Beacon frame format follows
    standard structure of a
    management frame
     – Destination address
        always set to all ones




                                             12
           Discovering the WLAN: Beaconing
                         Fields
•   Beacon frame body contains following fields:
      1.   Beacon interval
      2.   Timestamp
      3.   Service Set Identifier (SSID)
      4.   Supported rates
      5.   Parameter sets
      6.   Capability information
•   In ad hoc networks, each wireless device assumes
    responsibility for beaconing
•   In infrastructure networks beacon interval normally
    100 ms, but can be modified




                                                      13
            Discovering the WLAN: Scanning
• Receiving wireless device must be looking for beacon
  frames
• Passive scanning: Wireless device simply listens for
  beacon frame
   – Typically, on each available channel for set period
• Active scanning: Wireless device first sends out a
  management probe request frame on each available
  channel
   – Then waits for probe response frame from all available APs




                                                                  14
Discovering the WLAN: Active
          Scanning




                               15
           Joining the WLAN: Authentication
• Unlike standard wired LANS, authentication
  performed before user connected to network
   – Authentication of the wireless device, not the user
• IEEE 802.11 authentication: Process in which AP
  accepts or rejects a wireless device
• Open system authentication: Most basic, and default,
  authentication method
• Shared key authentication: Optional authentication
  method
   – Utilizes challenge text




                                                           16
Open System Authentication




                             17
Shared Key Authentication




                            18
           Joining the WLAN: Authentication
• Open system and Shared key authentication
  techniques are weak
   – Open System: Only need SSID to connect
   – Shared Key: Key installed manually on devices
       • Can be discovered by examining the devices

• Digital certificates: Digital documents that associate
  an individual with key value
   – Digitally “signed” by trusted third party
   – Cannot change any part of digital certificate without being detected




                                                                       19
              Joining the WLAN: Association
• Association: Accepting a wireless device into a
  wireless network
   – Final step to join WLAN
• After authentication, AP responds with association
  response frame
   – Contains acceptance or rejection notice
• If AP accepts wireless device, reserves memory space
  in AP and establishes association ID
• Association response frame includes association ID
  and supported data rates



                                                       20
          Distributed Coordination Function
                       (DCF)
• MAC layer responsible for controlling access to
  wireless medium
• Channel access methods: Rules for cooperation among
  wireless devices
  – Contention: Computers compete to use medium
     • If two devices send frames simultaneously, collision results and frames
       become unintelligible
     • Must take steps to avoid collisions




                                                                             21
    Carrier Sense Multiple Access with Collision Detection
             (CSMA/CD)Used on Ethernet LANs
•   Carrier Sense Multiple Access with Collision Detection (CSMA/CD):
    Before networked device sends a frame, listens to see if another
    device currently transmitting
     – If traffic exists, wait; otherwise send
     – Devices continue listening while sending frame
         • If collision occurs, stops and broadcasts a “jam” signal
•   CSMA/CD cannot be used on wireless networks:
     – Difficult to detect collisions
     – Hidden node problem




                                                                    22
                 Hidden node problem
Laptop A ,Laptop B and Laptop C can not see the transmission of
each other (more later on this topic)




                                                                  23
              Transmitting on the WLAN:
           Distributed Coordination Function
                    and CSMA/CA
• Distributed Coordination Function (DCF): Specifies
  modified version of CSMA/CD
   – Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA)
   – Attempts to avoid collisions altogether
   – Time when most collisions occur is immediately after a station
     completes transmission
   – All stations must wait random amount of time after medium clear
      • Slot time




                                                                  24
                               CSMA/CA
• CSMA/CA also reduces collisions via explicit frame
  acknowledgment
   – Acknowledgment frame (ACK): Sent by receiving device to sending
     device to confirm data frame arrived intact
   – If ACK not returned, transmission error assumed
• CSMA/CA does not eliminate collisions
   – Does not solve hidden node problem




                                                                   25
CSMA/CA and ACK




                  26
             Request to Send/Clear to Send

•   Request to Send/Clear to Send (RTS/CTS) protocol: Option
    used to solve hidden node problem
     – Significant overhead upon the WLAN with transmission of
       RTS and CTS frames
         • Especially with short data packets
     – RTS threshold: Only packets that longer than RTS threshold
       transmitted using RTS/CTS




                                                                    27
                        Interframe Spacing
•   Interframe spaces (IFS): Intervals between transmissions of data
    frames
     – Short IFS (SIFS): For immediate response actions such as ACK
     – Point Coordination Function IFS (PIFS): Time used by a device to
       access medium after it has been asked and then given approval to
       transmit
     – Distributed Coordination Function IFS (DIFS): Standard
       interval between transmission of data frames




                                                                      28
CSMA/CA with two stations
     transmitting




                            29
                 Transmitting on the WLAN:
                       Fragmentation
• Fragmentation: Divide data to be transmitted from
  one large frame into several smaller ones
   – Reduces probability of collisions
   – Reduces amount of time medium is in use
• If data frame length exceeds specific value, MAC
  layer fragments it
   – Receiving station reassembles fragments
• Alternative to RTS/CTS
   – High overhead
       • ACKs and additional SIFS time gaps




                                                      30
            Point Coordination Function (PCF)
• Polling: Channel access method in which each device
  asked in sequence if it wants to transmit
   – Effectively prevents collisions
• Point Coordination Function (PCF): AP serves as
  polling device or “point coordinator”
• Point coordinator has to wait only through point
  coordination function IFS (PIFS) time gap
   – Shorter than DFIS time gap




                                                        31
                       DIFS and DCF frames
•   If point coordinator hears no traffic after PIFS time gap, sends out
    beacon frame
     – Field to indicate length of time that PCF (polling) will be used
        instead of DCF (contention)
          • Receiving stations must stop transmission for that amount of
            time
     – Point coordinator then sends frame to specific station, granting
        permission to transmit one frame
•   802.11 standard allows WLAN to alternate between PCF (polling) and
    DCF (contention)




                                                                           32
        Quality of Service (QoS) and 802.11e
• DCF does not work well for real-time, time-dependent
  traffic
• Quality of Service (QoS): Capability to prioritize
  different types of frames
• Wi-Fi Multimedia (WMM): Modeled after wired
  network QoS prioritization scheme
• 802.11e draft: defines superset of features intended
  to provide QoS over WLANs
   – Proposes two new mode of operation for 802.11 MAC Layer




                                                               33
            Quality of Service and 802.11e




Table 5-1: Wi-Fi Multimedia (WMM)
                                             34
        Transmitting on the WLAN: Quality
        of Service and 802.11e (continued)
• 802.11e draft (continued):
  – Enhanced Distributed Channel Access (EDCA): Contention-based
    but supports different types of traffic
     • Four access categories (AC)
     • Provides “relative” QoS but cannot guarantee service
  – Hybrid Coordination Function Controlled Channel Access (HCCA):
    New form of PCF based upon polling
     • Serves as a centralized scheduling mechanism




                                                                   35
          Remaining Connected to the WLAN:
                    Reassociation
• Reassociation: Device drops connection with one AP
  and establish connection with another
   – Several reason why reassociation may occur:
       • Roaming
       • Weakened signal
   – When device determines link to current AP is poor, begins scanning to
     find another AP
       • Can use information from previous scans




                                                                       36
                  Power Management
•   A WLAN laptop must remain “awake” in order to receive network
    transmissions
     – Original IEEE 802 standard assumes stations always ready to
        receive network messages
•   Power management: Allows mobile devices to conserve battery life
    without missing transmissions
     – Transparent to all protocols
     – Differs based on WLAN configuration
     – AP records which stations awake and sleeping
     – Buffering: If sleeping, AP temporarily stores frames




                                                                   37
                          Power Management
• At set times AP send out beacon to all stations
   – Contains traffic indication map (TIM)
   – At same time, all sleeping stations switch into active listening mode
• Power management in ad hoc mode:
   – Ad hoc traffic indication message (ATIM) window: Time at which
     all stations must be awake
       • Wireless device sends beacon to all other devices
           – Devices that previously attempted to send a frame to a
             sleeping device will send ATIM frame indicating that
             receiving device has data to receive and must remain
             awake




                                                                         38
                       WLAN IP Addressing
• In standard networking, IP protocol responsible for
  moving frames between computers
   – Network layer protocol
• TCP/IP works on principle that each network host has
  unique IP address
   – Used to locate path to specific host
   – Routers use IP address to forward packets
   – Prohibits mobile users from switching to another network and using
     same IP number
       • Users who want to roam need new IP address on every network




                                                                       39
                           Mobile IP
•   Provides mechanism within TCP/IP protocol to support mobile
    computing
     – Computers given home address,
         • Static IP number on home network
     – Home agent: Forwarding mechanism that keeps track of where
       mobile computer located
     – When computer moves to foreign network, a foreign agent
       provides routing services
         • Assigns computer a care-of address
         • Computer registers care-of address with home agent




                                                                    40
Mobile IP-Computer relocated




                               41
                          Summary
• A Basic Service Set (BSS) is defined as a group of
  wireless devices that is served by a single access point
  (AP)
• An Extended Service Set (ESS) is comprised of two or
  more BSS networks that are connected through a
  common distribution system
• An Independent Basic Service Set (IBSS) is a wireless
  network that does not use an access point
• Frames are used by both wireless NICs and access
  points for communication and for managing and
  controlling the wireless network



                                                       42
                  Summary (continued)
• The MAC layer provides four major functions in
  WLANs: discovering the WLAN signal, joining the
  WLAN, transmitting on the WLAN, and remaining
  connected to the WLAN
• Discovery is a twofold process: the AP or other
  wireless devices must transmit an appropriate frame
  (beaconing), and the wireless device must be looking
  for those frames (scanning)
• Once a wireless device has discovered the WLAN, it
  requests to join the network; This is a twofold process
  known as authentication and association



                                                       43
                 Summary (continued)
• The IEEE 802.11 standard specifies two procedures
  for transmitting on the WLAN, distributed
  coordination function (DCF) and an optional point
  coordination function (PCF)
• The 802.11 standard provides for an optional polling
  function known as Point Coordination Function (PCF)
• The 802.11e draft defines a superset of features that
  is intended to provide QoS over WLANs




                                                     44
                  Summary (continued)
• Power management allows mobile devices to be off as
  much as possible to conserve battery life but not miss
  data transmissions
• Mobile IP provides a mechanism within the TCP/IP
  protocol to support mobile computing




                                                      45

				
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