CWNA guide to wireless LANs 2nd ch11

• 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 poin

CWNA Guide to Wireless LANs, Second Edition

Chapter Eleven

Network Settings and Wireless LAN

Troubleshooting

• 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

Wired Network Settings for Wireless

occasionally need to be adjusted

• To improve wireless performance or provide additional capabilities

• Mobile IP parameters can be set on APs

Ethernet Parameters: Basic Settings

Figure 11-1: Basic Ethernet settings

Ethernet Parameters: Basic Settings

Ethernet Parameters: Advanced

Settings

Figure 11-2: Advanced DNS settings

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

Ethernet Parameters: Advanced

Settings (continued)

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

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

Mobile IP Settings

• Most WLAN implementations follow standard IP

address plan

– Single subnet for entire WLAN

– Subnet: Portion of network that shares a common

Mobile IP Settings (continued)

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:

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

Mobile IP Settings (continued)

Table 11-3: Subnet map table

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-of-address

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

Solving RF Transmission Problems:

Near/Far

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

Solving RF Transmission Problems:

Near/Far (continued)

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

Solving RF Transmission Problems:

• Can create all-band interference

– IEEE 802.11b/g and Bluetooth devices do not

“peacefully coexist” in same environment

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

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

Solving RF Transmission Problems: System Throughput (continued)

Solving RF Transmission Problems:

Co-Location Throughput

• When a mobile wireless user enters into range of more than one AP, wireless device will choose an

AP based on signal strength or packet error rates

– For 802.11b and 802.11g wireless LANs only

channels 1, 6, and 11 can be used to co-locate

multiple APs in same area

• Small amount of overlap between channels 1 and

6, as well as between channels 6 and 11

– Co-location throughput

– Can affect wireless network performance

Solving RF Transmission Problems: Co-Location Throughput (continued)

Solving RF Transmission Problems: Co-Location Throughput (continued)

• Possible solutions:

– Change channels to Channels 1 and 11

– Migrate to 802.11a

• 8 non-overlapping channels

Solving RF Transmission Problems: Co-Location Throughput (continued)

Solving Access Point Problems

• All APs have external light emitting diodes

– Ethernet indicator: Signals traffic on wired LAN

• Green when Ethernet cable is connected

• Blinks green when packet received or transmitted

• Off when Ethernet cable not connected

– Status indicator: Signals operational status

• Green indicates AP associated with at least one wireless device,

• Blinking green means AP operating normally but is not associated with any wireless devices

– Radio indicator: Blinks green to indicate RF activity

Solving Access Point Problems

(continued)

Solving Access Point Problems

(continued)

Table 11-4 (continued): Cisco Aironet LED information

Solving Access Point Problems

• In extreme circumstances, may be necessary to

delete current AP configuration and return all

settings to factory defaults

Solving Wireless Device Problems: Wireless Device Connection Process

• Five-step connection process:

– Scan for access points

• Wireless network adapter sends series of Probe Request frames

• APs within range respond with Beacon frame that contains the capabilities of the wireless AP

– Choose an access point

• Decision based on:

– Wireless AP capabilities – SSID matches a preferred network – Signal strength

Solving Wireless Device Problems: Wireless Device Connection Process

(continued)

• Five-step connection process (continued):

– Authenticate with the access point

• Type of authentication depends on security capabilities of AP and how wireless device has configured to authenticate with AP

– Associate with the access point

– Obtain an IP address

• Manual addressing

• DHCP addressing

• APIPA addressing

Solving Wireless Device Problems: Troubleshooting Wireless Devices

• Possible causes if problems makings connection:

– Incompatible IEEE 802.11 standards

– Mismatched authentication methods

– Different WEP keys

• Will not prevent association, but will prevent successful communication

– Incorrect WEP key order

– Mismatched pre-shared key

– Conflict between Windows Wireless Auto

Configuration and a third-party configuration tool

– Incorrect MAC address

Solving Wireless Device Problems: Troubleshooting Wireless Devices

(continued)

• Possible causes if wireless device intermittently

disconnects from AP:

– Incompatible 802.1x authentication

• For computers running Windows XP with SP1, clear

Enable IEEE 802.1x authentication for this network

check box in Authentication tab

– Duplicate SSID

• Generally result of default SSID being used on APs

– Infection by computer virus

• Run antivirus software

Solving Wireless Device Problems: Troubleshooting Wireless Devices

(continued)

• Possible causes if wireless device intermittently

disconnects from AP (continued):

• Maintaining a wireless LAN involves adjusting the wired Ethernet settings to ensure peak

performance

• Most WLAN network implementations follow the

standard Internet protocol (IP) address plan, which calls for a single subnet to be used for the entire

WLAN

• Mobile IP provides users the freedom to roam

beyond their home subnets while still maintaining their home IP addresses

Summary (continued)

• A device that is transmitting at higher signal

strength and is located closer to the access point will drown out a weaker signal from a device that is farther away and is using less power

• Technologies such as Bluetooth which use the

entire 2.4 GHz spectrum can impact IEEE 802.11 WLANs that use the same frequency spectrum

• Co-location throughput can affect the performance

of access points that use 802.11b/g technology

because of channel overlap on channels 1, 6, 11

Summary (continued)

• Solving access point problems often starts with

observing the LED lights that provide information regarding Ethernet and RF activity along with

operational status

• Wireless device problems fall into two broad

categories: the inability to make a successful

wireless connection and intermittently

disconnecting from the access point