Transcript Linux+ Guide to Linux Certification

CWNA Guide to Wireless LANs,
Second Edition
Chapter Five
IEEE 802.11 Media Access Control and
Network Layer Standards
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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
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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:
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:
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 802.11 Media Access Control
(MAC) Layer Standards
• Media Access Control (MAC) layer performs several
vital functions in a WLAN
–
–
–
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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 -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
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MAC Frame Formats - Types
•
Types of management frames:
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
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
• Control frames: Provide assistance in delivering
frames that contain data
Figure 5-5: Control frame
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MAC Frame Formats - Data
• 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
Fields
•
Beacon frame body contains following fields:
•
In ad hoc networks, each wireless device assumes
responsibility for beaconing
In infrastructure networks beacon interval normally
100 ms, but can be modified
•
1.
2.
3.
4.
5.
6.
Beacon interval
Timestamp
Service Set Identifier (SSID)
Supported rates
Parameter sets
Capability information
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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: Active
Scanning
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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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Open System Authentication
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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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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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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
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Hidden node problem
Laptop A ,Laptop B and Laptop C can not see the transmission of
each other (more later on this topic)
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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
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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
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CSMA/CA and ACK
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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
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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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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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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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DIFS and DCF frames
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•
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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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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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 (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
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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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Power Management
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•
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
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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 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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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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Mobile IP-Computer relocated
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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 (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
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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
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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
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