CWNA Guide to Wireless LANs, Second Edition

1
CWNA Guide to Wireless LANs, Second Edition

• Chapter Five
• IEEE 802.11 Media Access Control and Network
  Layer Standards

2
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

3
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

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IEEE Wireless LAN Configurations Basic Service
Set
Figure 5-1 Basic Service Set (BSS)
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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

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IEEE Wireless LAN Configurations Extended
Service Set
Figure 5-2 Extended Service Set (ESS)
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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

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IEEE Wireless LAN Configurations Independent
Basic Service Set
Figure 5-3 Independent Basic Service Set (IBSS)
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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

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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

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MAC Frame Formats

• Frame control field identifies
• Specific 802.11 protocol version
• Frame type
• Indicators that show WLAN configuration
• All frames contain
• MAC address of the source and destination device
• Frame sequence number
• Frame check sequence for error detection

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MAC Frame Formats

• 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
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MAC Frame Formats

• Types of management frames
• Authentication frame
• Association request frame
• Association response frame
• Beacon frame
• Deauthentication frame
• Disassociation frame
• Probe request frame
• Probe response frame
• Reassociation request frame
• Reassociation response frame

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MAC Frame Formats

• Control frames Provide assistance in delivering
  frames that contain data

Figure 5-5 Control frame
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MAC Frame Formats

• Data frame Carries information to be transmitted
  to destination device

Figure 5-6 Data frame
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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

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Discovering the WLAN Beaconing
Figure 5-7 Beaconing
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Discovering the WLAN Beaconing

• Beacon frame body contains following fields
• Beacon interval
• Timestamp
• Service Set Identifier (SSID)
• Supported rates
• Parameter sets
• 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

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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

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Discovering the WLAN Scanning
Figure 5-8 Active scanning
21
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

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Joining the WLAN Authentication
Figure 5-9 Open system authentication
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Joining the WLAN Authentication (continued)
Figure 5-10 Shared key authentication
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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

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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

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Transmitting on the WLAN 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

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Transmitting on the WLAN Distributed
Coordination Function

• 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

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Transmitting on the WLAN Distributed
Coordination Function
Figure 5-11 Carrier Sense Multiple Access with
Collision Detection (CSMA/CD)
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Transmitting on the WLAN Distributed
Coordination Function
Figure 5-11 (continued) Carrier Sense Multiple
Access with Collision Detection (CSMA/CD)
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Transmitting on the WLAN Distributed
Coordination Function
Figure 5-12 Hidden node problem
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Transmitting on the WLAN Distributed
Coordination Function

• 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

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Transmitting on the WLAN Distributed
Coordination Function

• 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

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Transmitting on the WLAN Distributed
Coordination Function
Figure 5-13 CSMA/CA and ACK
34
Transmitting on the WLAN Distributed
Coordination Function

• 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

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Transmitting on the WLAN Distributed
Coordination Function
Figure 5-14 Request to Send/Clear to Send
(RTS/CTS)
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Transmitting on the WLAN 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

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Transmitting on the WLAN Interframe Spacing
(continued)
Figure 5-15 CSMA/CA with one station transmitting
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Transmitting on the WLAN Interframe Spacing
Figure 5-16 CSMA/CA with two stations
transmitting
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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

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Transmitting on the WLAN 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

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Transmitting on the WLAN Point Coordination
Function (continued)

• 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)

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Transmitting on the WLAN Point Coordination
Function
Figure 5-18 DIFS and DCF frames
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Transmitting on the WLAN 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

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Transmitting on the WLAN Quality of Service and
802.11e
Table 5-1 Wi-Fi Multimedia (WMM)
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Transmitting on the WLAN Quality of Service and
802.11e

• 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

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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

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Remaining Connected to the WLAN Power Management

• When laptop is part of a WLAN, 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

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Remaining Connected to the WLAN Power Management
Figure 5-19 Power management in infrastructure
mode
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Remaining Connected to the WLAN 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

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WLAN Network Layer Standards 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

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WLAN Network Layer Standards 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

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WLAN Network Layer Standards Mobile IP
Figure 5-20 Mobile IP components
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WLAN Network Layer Standards Mobile IP
Figure 5-21 Computer relocated in Mobile IP
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WLAN Network Layer Standards Mobile IP
Figure 5-22 Encapsulated Mobile IP frame
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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

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Summary

• 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

57
Summary

• 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

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Summary

• 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