Quản trị mạng - Chapter 7: Basic wireless concepts and configuration
Ensure that access points are not mounted closer than 7.9 inches (20
cm) from the body of all persons.
• Do not mount the access point within 3 feet (91.4 cm) of metal
obstructions.
• Install the access point away from microwave ovens. Microwave ovens
operate on the same frequency as the access point and can cause
signal interference.
• Always mount the access point vertically (standing up or hanging
down).
• Do not mount the access point outside of buildings.
• Do not mount the access point on building perimeter walls, unless
outside coverage is desired.
• When mounting an access point in the corner of a right-angle hallway
intersection, mount it at a 45-degree angle to the two hallways. The
access point internal antennas are not omnidirectional and cover a
larger area when mounted this way
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1Chapter 7: Basic Wireless
Concepts and Configuration
CCNA Exploration 4.0
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Objectives
• Describe the components and operations of basic
wireless LAN topologies.
• Describe the components and operations of basic
wireless LAN security.
• Configure and verify basic wireless LAN access.
• Configure and troubleshoot wireless client access.
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The Wireless LAN
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Why Use Wireless?
• Business networks today are evolving to support people who are on
the move.
• Mobility environment: where people can take their connection to the
network along with them on the road.
• There are many different infrastructures (wired LAN, service provider
networks) that allow mobility like this to happen, but in a business
environment, the most important is the WLAN.
• People now expect to be connected at any time and place
What is WLAN RF Technology?
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Benefits of WLANs
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Wireless Technologies
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Wireless LAN
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Comparing a WLAN to a LAN
• In an 802.3 Ethernet LAN, each client has a cable that connects the client NIC
to a switch. The switch is the point where the client gains access to the network.
• In a wireless LAN, each client uses a wireless adapter to gain access to the
network through a wireless device such as a wireless router or access point.
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Comparing a WLAN to a LAN
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Wireless standards
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Wireless standards
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Wi-Fi Certification
Extra: Modulation
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Extra: Modulation
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Extra: Modulation
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Extra: Modulation
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Extra: Modulation
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Extra: Modulation
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802.11a Uses OFDM Modulation
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802.11a 5-GHz Frequency Bands
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802.11b Access Point Coverage
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802.11b Scalability
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802.11a Access Point Coverage
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802.11a Scalability (Indoor UNII-1 and UNII-2)
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Wireless Infrastructure Components
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Extra: Wireless LAN Frame
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Wireless Access Points
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Carrier Sense Multiple Access with Collision Avoidance
(CSMA/CA)
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RTS/CTS
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Extra: RTS/CTS
• The optional request-to-send and clear-to-send (RTS/CTS) function
allows the access point to control use of the medium for stations
activating RTS/CTS.
• With most radio NICs, users can set a maximum frame-length
threshold for when the radio NIC activates RTS/CTS.
– For example, a frame length of 1,000 bytes triggers RTS/CTS for all
frames larger than 1,000 bytes.
• If the radio NIC activates RTS/CTS, it first sends an RTS frame to an
access point before sending a data frame. The access point then
responds with a CTS frame, indicating that the radio NIC can send the
data frame.
• With the CTS frame, the access point provides a value in the duration
field of the frame header that holds off other stations from transmitting
until after the radio NIC initiating the RTS can send its data frame. This
avoids collisions between hidden nodes.
• The RTS/CTS handshake continues for each frame, as long as the
frame size exceeds the threshold set in the corresponding radio NIC.
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Extra: RTS/CTS
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802.11 Frame Format
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802.11 Frames Type (cont)
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Configurable Parameters for Wireless Endpoints
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Configurable Parameters for Wireless Endpoints
2.4-GHz Channel Sets
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Học viện mạng Bach Khoa - Website: www.bkacad.com 38
802.11 Topologies: Ad hoc Network
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802.11 Topologies: Infrastructure
• Basic Service Sets
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802.11 Topologies: Infrastructure
• Extended Service Sets
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Extra: Roaming
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Extra: Roaming
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Extra: Scanning
• The 802.11 standard defines both passive and active scanning,
whereby a radio NIC searches for access points.
• Passive scanning is mandatory where each NIC scans individual
channels to find the best access-point signal. Periodically, access
points broadcast a beacon, and the radio NIC receives these beacons
while scanning and takes note of the corresponding signal strengths.
The beacons contain information about the access point, including
SSID and supported data rates. The radio NIC can use this information
along with the signal strength to compare access points and decide on
which one to use.
• Active scanning is similar, except the radio NIC initiates the process
by broadcasting a probe frame, and all access points within range
respond with a probe response. Active scanning enables a radio NIC to
receive immediate response from access points, without waiting for a
beacon transmission. The issue, however, is that active scanning
imposes additional overhead on the network because of the
transmission of probe and corresponding response frames.
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Client and Access Point Association
• Beacon
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Client and Access Point Association
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Client and Access Point Association
Step 3 - 802.11 Association
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Extra: Authentication and Association
• Open Authentication and Shared Key Authentication are the two methods
that the 802.11 standard defines for clients to connect to an access point.
• The association process can be broken down into three elements:
1. Probe
2. Authentication
3. Association.
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Extra: Open Authentication
• The Open Authentication method performs the entire authentication
process in clear text.
• Open Authentication is basically a null authentication, which means
there is no verification of the user or machine.
• Open Authentication is usually tied to a WEP key. A client can
associate to the access point with an incorrect WEP key or even no
WEP key. A client with the wrong WEP key will be unable to send or
receive data, since the packet payload will be encrypted.
• Keep in mind that the header is not encrypted by WEP. Only the
payload or data is encrypted.
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Extra: Shared Key Authentication
• Shared Key Authentication works similarly to Open Authentication,
except that it uses WEP encryption for one step.
• Shared key requires the client and the access point to have the same
WEP key.
• An access point using Shared Key Authentication sends a challenge
text packet to the client. If the client has the wrong key or no key, it will
fail this portion of the authentication process. The client will not be
allowed to associate to the AP.
• Shared key is vulnerable to a man-in-the-middle attack, so it is not
recommended.
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Extra: ARS
• When a source node sends a frame, the receiving node returns a
positive acknowledgment (ACK).
– This can cause consumption of 50% of the available bandwidth.
• This overhead when combined with the collision avoidance protocol
overhead reduces the actual data throughput to a maximum of 5.0 to
5.5 Mbps on an 802.11b wireless LAN rated at 11 Mbps.
• Performance of the network will also be affected by signal strength and
degradation in signal quality due to distance or interference.
• As the signal becomes weaker, Adaptive Rate Selection (ARS) may
be invoked and the transmitting unit will drop the data rate from 11
Mbps to 5.5 Mbps, from 5.5 Mbps to 2 Mbps or 2 Mbps to 1 Mbps.
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Planning the Wireless LAN
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Planning the Wireless LAN
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Planning the Wireless LAN
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Planning the Wireless LAN
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Activity 7.1.5.2
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Activity 7.1.5.2
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Wireless LAN Security
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Wireless LAN Security Threats
Unauthorized Access
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Wireless LAN Security Threats
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Wireless LAN Security Threats
• Denial of Service
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Extra: Securing a WLAN
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Extra: SSID
• Most access points have options like ‘SSID broadcast’ and ‘allow any
SSID.’ These features are usually enabled by default and make it easy
to set up a wireless network.
– Using the ‘allow any SSID’ option lets the access point allow
access to a client with a blank SSID.
– The ‘SSID broadcast’ sends beacon packets, which advertise the
SSID.
• Disabling these two options do not secure the network, since a
wireless sniffer can easily capture a valid SSID from normal WLAN
traffic.
• SSIDs should not be considered a security feature.
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Wireless Security Protocols
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Extra: Wireless Security Protocols
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Extra: Encryption Methods
• Many encryption methods, such as the 802.11 Wired Equivalent Privacy
(WEP), are symmetric—that is, the same key that does the encryption is also
the one that performs the decryption.
• If a user activates WEP, the NIC encrypts the payload (frame body and cyclic
redundancy check [CRC]) of each 802.11 frame before transmission using an
RC4 stream cipher provided by RSA security. The receiving station, such as an
access point or another radio NIC, performs decryption upon arrival of the
frame. As a result, 802.11 WEP only encrypts data between 802.11 stations.
Once the frame enters the wired side of the network, such as between access
points, WEP no longer applies.
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Extra: Encryption Methods
• Wi-Fi Protected Access
– The Wi-Fi Protocol Access (WPA) standard provided by the Wi-Fi
Alliance provides an upgrade to WEP that offers dynamic key
encryption and mutual authentication.
– Most wireless vendors now support WPA. WPA clients utilize
different encryption keys that change periodically. This makes it
more difficult to crack the encryption.
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Wireless Security Protocols
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Wireless Security Protocols
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Wireless Security Protocols
• Encryption
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Securing a Wireless LAN
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Configure Wireless LAN Access
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Configuring the Wireless Access Point
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Setup: Basic Setup
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Administration: Management
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Configuring Basic Wireless Settings
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Security Mode
• Select the mode you want to use: PSK-Personal, PSK2-
Personal, PSK-Enterprise, PSK2-Enterprise, RADIUS, or
WEP.
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Mode Parameters
• Enterprise modes are not configured in this chapter
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Configure Encryption and Key
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Configure a wireless NIC: Scan SSID
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Configure a wireless NIC: Scan SSID
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Select the Wireless Security Protocol
• Practice: 7.3.2.4
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Troubleshooting
Simple WLAN Problems
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Systematic Approach to WLAN Troubleshooting
• Step 1 - Eliminate the client device as the source of the
problem.
• Step 2 - Confirm the physical status of WLAN devices.
• Step 3 - Inspect wired links.
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Updating the Access Point Firmware
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Incorrect Channel Settings
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Incorrect Channel Settings: Solution
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Solving RF Interference
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Solving RF Interference
• Site Surveys
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Site Survey
• Two categories: Manual and utility assisted.
• Manual site surveys can include a site evaluation to be followed by a more thorough
utility-assisted site survey. A site evaluation involves inspecting the area with the goal of
identifying potential issues that could impact the network. Specifically, look for the
presence of multiple WLANs, unique building structures, such as open floors and
atriums, and high client usage variances, such as those caused by differences in day or
night shift staffing levels.
• Note: you do not conduct site surveys as part of this course
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Access Point Misplacement
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Access Point Misplacement: Solution
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Access Point Misplacement: Solution
• Ensure that access points are not mounted closer than 7.9 inches (20
cm) from the body of all persons.
• Do not mount the access point within 3 feet (91.4 cm) of metal
obstructions.
• Install the access point away from microwave ovens. Microwave ovens
operate on the same frequency as the access point and can cause
signal interference.
• Always mount the access point vertically (standing up or hanging
down).
• Do not mount the access point outside of buildings.
• Do not mount the access point on building perimeter walls, unless
outside coverage is desired.
• When mounting an access point in the corner of a right-angle hallway
intersection, mount it at a 45-degree angle to the two hallways. The
access point internal antennas are not omnidirectional and cover a
larger area when mounted this way.
Học viện mạng Bach Khoa - Website: www.bkacad.com 93
Problems with Authentication and Encrytion
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Problems with Authentication and Encrytion
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Problems with Authentication and Encrytion
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Summary
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