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CWNA guide to wireless LANs 2nd ch11

CWNA Guide to Wireless
LANs, Second Edition
Chapter Eleven
Network Settings and Wireless LAN
Troubleshooting


Objectives
• Explain the wired network settings that can be
modified in a wireless network
• List troubleshooting techniques for solving RF
transmission problems
• Describe how to solve access point problems
• Describe the types of wireless device problems and
explain how to solve them

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Wired Network Settings for Wireless
Networks
• All APs have RJ-45 connector that allows them to
connect to an Ethernet hub or switch
– Provide wired network resources to wireless devices
– Settings for connecting to Ethernet network
occasionally need to be adjusted
• To improve wireless performance or provide additional
capabilities

• Mobile IP parameters can be set on APs

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Ethernet Parameters: Basic Settings

Figure 11-1: Basic Ethernet settings

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Ethernet Parameters: Basic Settings
(continued)
• Allow wireless network administrators to designate
Ethernet port as primary port of the AP
– Select whether port “adopts” identity of primary port

Table 11-1: Ethernet identification

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Ethernet Parameters: Advanced


Settings

Figure 11-2: Advanced DNS settings

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Ethernet Parameters: Advanced
Settings (continued)
• Setting on Figure 11-2:
– Default Domain: Name of network’s IP domain
– Current Domain: Domain that is serving the AP
– IP addresses of up to three DNS servers can be
entered under Domain Name Servers
– Domain Suffix: Last portion of domain name of
current network domain

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Ethernet Parameters: Advanced
Settings (continued)

Figure 11-3: Advanced Ethernet settings

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Ethernet Parameters: Filtering
• Allows control of types of network traffic that pass
from wired Ethernet network to WLAN devices
– Configure AP to act as type of firewall

• Different types of filtering:
– Some devices filter at high level and can block an
application from being requested
– Other filtering can reject request for specific IP port
– At the lowest level, filtering can look at received
frames and block based on type of frame

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Ethernet Parameters: Filtering
(continued)
• Frames can be filtered by protocol used
– e.g., TCP, UDP, IPX

• Frames can be filtered by frame format
– Four-character hexadecimal number found in each
frame that indicates protocol and frame format

Table 11-2: Frame formats

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Mobile IP Settings
• Most WLAN implementations follow standard IP
address plan
– Single subnet for entire WLAN
– Subnet: Portion of network that shares a common
address component

• Subnetting WLANs creates problems for users who
roam between WLAN subnets
– Cannot roam into new subnet without changing IP
address
– Need mechanism to ensure a device configured with
specific IP address can continue to communicate
when entering new subnet
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Mobile IP Settings (continued)

Figure 11-4: Roaming between wireless subnets

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Mobile IP Settings (continued)
• Mobile IP: Provides freedom to roam beyond home
subnet while maintaining home IP address
– AP forwards packets through Mobile IP enabled
router to router on client’s home network

• Five required devices:






Visiting device
Access point with Mobile IP enabled
Home agent
Authoritative access point
Foreign agent

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Mobile IP Settings (continued)
• Mobile IP begins with home agents and foreign
agents advertising their services
– APs with Mobile IP enabled listen to advertisements

• When visiting client associates to AP, AP compares
client’s IP address with own IP network
– Detects that client is a visitor
– Begins registration
– Gets home agent’s IP address by looking it up on a
subnet map table

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Mobile IP Settings (continued)

Table 11-3: Subnet map table

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Mobile IP Settings (continued)
• Authoritative access point (AAP) responsible for
maintaining/distributing master subnet map table of
APs and home agent information
• On some WLANs, may have multiple AAPs
• When client roams to another network, foreign
agent provides routing services
– Assigns mobile client new temporary IP number
• Care-of address

– Registers care-of address with home agent
– Home agent redirects frames to client via care-ofaddress
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Troubleshooting Wireless Networks
• Troubleshooting can be difficult with WLANs
– Many factors can impact wireless transmissions
• Many of them are “non-technical”

– Technology is relatively new
– Problems can be result of anything from overlooking
check box on a dialog box to metal objects in path of
RF signal

• Categorized into identifying and solving problems
with RF transmissions, APs, and wireless devices

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Solving RF Transmission Problems:
Near/Far

Figure 11-6: Near/far transmission problem

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Solving RF Transmission Problems:
Near/Far (continued)
• Two steps to identify device that is a victim of
near/far transmission problem
– Wireless protocol analyzer running on a device
displays signal strength
• Low signal strength may indicate problem
– May not indicate near/far problem
– Also determine signal strength on nearby devices

– Review placement of wireless devices

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Solving RF Transmission Problems:
Near/Far (continued)

Figure 11-7: Signal strength

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Solving RF Transmission Problems:
Near/Far (continued)
• Several solutions to near/far transmission
problems:
– Move device with stronger transmission power
farther away from AP
– Reduce transmission power of devices closer to AP
– Increase transmission power of devices farther away
from AP

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Solving RF Transmission Problems:
All-Band Interference
• FHSS uses range of frequencies that change
during transmission
– Bluetooth, for example, is a close-range, frequency
hopping technology that operates in same 2.4 GHz
ISM band as IEEE 802.11b/gWLANs
• Can create all-band interference

– IEEE 802.11b/g and Bluetooth devices do not
“peacefully coexist” in same environment

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Solving RF Transmission Problems:
All-Band Interference (continued)
• Several options have been proposed for 802.11b/g
and Bluetooth to work together:






Change the RF spectrum used
Modify power levels
Add switching software
Change the MAC layer
Change PHY layer

• Best solutions is to not use the two devices
together or migrate to 802.11a wireless technology
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Solving RF Transmission Problems:
System Throughput
• Many factors influence WLAN transmission speed:









AP processor speed
Distance from AP
Implementing security solutions
Number of users associated with an AP
Packet size
RTS/CTS protocol
Types of RF interference
Using PCF protocol

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Solving RF Transmission Problems:
System Throughput (continued)
• To troubleshoot:
– Determine if all devices experiencing problem or only
a single device
– Identify potential causes that may have least impact
on system if changed

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