CWNA Guide to Wireless LAN's Second Edition - Chapter 11

CWNA Guide to Wireless LANs, Second Edition

Chapter ElevenNetwork Settings and Wireless LAN

Troubleshooting

CWNA Guide to Wireless LANs, Second Edition 2

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


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


Ethernet Parameters: Basic Settings

Figure 11-1: Basic Ethernet settings


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


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)

Figure 11-3: Advanced Ethernet settings


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

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


Mobile IP Settings (continued)

Figure 11-4: Roaming between wireless subnets



• 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



• 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



Table 11-3: Subnet map table



• 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

Figure 11-6: Near/far transmission problem


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



Figure 11-7: Signal strength



• 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: 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


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)

• 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


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)

Figure 11-8: Adjacent Channels 1 and 6



• Possible solutions:– Change channels to Channels 1 and 11– Migrate to 802.11a

• 8 non-overlapping channels



Figure 11-9: Separate channels


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)

Table 11-4: Cisco Aironet LED information



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



• If AP does not communicate with devices, check:– SSID– WEP keys– Security settings

• 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



(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



(continued)

• Possible causes if wireless device intermittently disconnects from AP (continued):– Faulty hardware

• Run diagnostic tests for AP or wireless network adapter

• Use current drivers

– Wireless attacks• May require special scanners to pinpoint attacker

– Device misconfiguration


Summary

• 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

Documents

CWNA Guide to Wireless LAN's Second Edition - Chapter 11