Troubleshooting Cisco Wireless LAN Solutions Chapter Summary

The following section is a summary of the major points you should be aware of in this chapter.

• Wireless networks use radio waves to transmit data and connect devices
• WLANs are meant to augment, not replace, wired LAN infrastructure

IEEE 802.11 Components

• IEEE 802.11 components include the following:

1. Client or Station (STA)
2. Access Point (AP)
3. Independent Basic Service Set (IBSS)
4. Basic Service Set (BSS)
5. Extended Service Set (ESS)
6. Distribution System (DS)

• The client is any appliance that interfaces with the wireless medium as an end user device
• The wireless AP functions as a bridge between the STAs and the network backbone
• An IBSS is a wireless network, consisting of at least two STAs
• The BSS is a cellular architecture which divides the wireless network (system) into cells
• The ESS is comprised of overlapping BSS sets (cells), usually connected by the DS
• The Distribution System (DS) allows for the interconnection of the APs of multiple cells

IEEE 802.11 Frames

• The 802.11 standard uses three types of frames:

1. Control Frames
2. Management Frames
3. Data Frames

• The 802.11 standard uses control frames to control device access to the wireless medium
• Management frames enable stations to establish and maintain communications
• Data frames are sent by any STA and contain higher layer protocol information or data

IEEE 802.11 Standards

At the physical (PHY) layer, IEEE 802.11 defines a series of encoding and transmission schemes for wireless communications the most common of which are the Frequency Hopping Spread Spectrum (FHSS), Direct Sequence Spread Spectrum (DSSS), and Orthogonal Frequency Division Multiplexing (OFDM) transmission schemes. Although Infra Red (IR) also exists at this layer, very little development of this standard has occurred due to line-of-sight limitations. The 802.11 standards described in this section are:

1. IEEE 802.11 (original)
2. IEEE 802.11b
3. IEEE 802.11a
4. IEEE 802.11g
5. IEEE 802.11n

• The original IEEE 802.11 standard defined WLANs that provided up to 2 Mbps throughput
• The original 802.11 standard is now considered obsolete because the throughput is too slow
• 802.11b is an extension to 802.11 that operates in the same unregulated 2.4 GHz band
• Devices operating on 802.11b use DSSS modulation for higher speeds
• The 2.4-GHz band consists of 14 channels, each 22 MHz wide
• In North America, the FCC allows channels 1 through 11
• Most of Europe can use channels 1 through 13. In Japan only channel 14 is used
• IEEE 802.11a is an extension to 802.11 that provides up to 54 Mbps throughput
• 802.11a uses OFDM and does away with spread spectrum
• 802.11a is not compatible with 802.11b or 802.11g and therefore is seldom used
• 802.11a equipment operates at 5GHz
• The 802.11g standard also works in the same 2.4GHz range as 802.11b
• IEEE 802.11g operates at a bit rate as high as 54 Mbps, but uses the S-Band ISM and OFDM
• 802.11g is compatible with 802.11b can operate at the 802.11b bit rates and use DSSS
• IEEE 802.11n improves on 802.11a and 802.11g maximum data rates
• The 802.11n standard includes several enhancements to the other 802.11 standards

The Cisco WLAN Solution

• The Cisco Wireless LAN solution of WLCs and their associated LAPs
• WLCs are responsible for system wide wireless LAN functions, such as:

1. Integrated Intrusion Prevention System (IPS)
2. Zero-Touch Deployment of Lightweight Access Points (LAPs)
3. Real-time Radio Frequency (RF) management
4. Wireless LAN Redundancy
5. Dynamic Channel Assignment for each LAP
6. Dynamic Client Load Balancing across LAPs
7. Dynamic LAP Transmit Power Optimization
8. Wireless LAN Security Management

• WLCs communicate with Controller-based APs over any Layer 2 or Layer 3 infrastructure
• WLCs communicate with LAPs using LWAPP or CAPWAP
• LWAPP is an IETF draft protocol
• CAPWAP, which is based on LWAPP, is a standard, interoperable protocol
• The following sequence of events must occur in order for an LAP to register to a WLC:

9. The LAPs issue a DHCP Discovery Request to get an IP address (if no static address is used)
10. The LAP sends a Layer 2 LWAPP discovery request message to the WLC
11. If Layer 2 discovery fails or is not supported, then the Layer 3 discovery method is used
12. Any available WLC responds with an LWAPP Discovery Response
13. The LAP selects the WLC to join, which is typically the first WLC to respond to the LAP
14. The LAP then sends an LWAPP Join Request to the WLC
15. The WLC validates the LAP and then sends an LWAPP Join Response to the LAP
16. The LAP validates the WLC, which then completes the Discovery and Join process
17. The LAP registers with the WLC and can begin accepting client associations

• The Cisco WLAN supports three types of roaming:

1. Intra-controller Roaming
2. Inter-controller Roaming
3. Inter-Subnet Roaming

Troubleshooting Cisco WLAN Solutions

• Generally speaking, Wireless LAN issues fall into one of the following problem areas:

1. Wireless Station (STA) or Client Issues
2. WLC Configuration Issues
3. AP Configuration Issues
4. AP and WLC Registration Issues
5. Infrastructure Issues
6. Antenna and Radio Frequency Issues

• Wireless client troubleshooting should include the following tasks:

1. Checking the client wireless NIC state
2. Checking client settings
3. Checking the state of the wireless client

• Some common WLC device misconfigurations include the following:

1. Mismatched Service Set Identifiers
2. Security Mismatches
3. WLANs are disabled on the WLC
4. Data Rate Mismatches
5. Incorrect Client or Station Filtering
6. Unsupported Features
7. IP Address Assignment Issues
8. SSID Broadcast is Disabled

• For APs operating in autonomous mode, ensure that the following parameters are the same:

1. The same Service Set Identifier
2. The same IP subnet
3. The same Layer 2 native VLAN

• When using DHCP for AP address allocation, ensure that DHCP Option 43 is configured
• When using Cisco IOS DHCP relay agent, forward UDP Broadcasts for port 12223
• When using PoE, ensure that there is enough available power to power up all the APs
• Excessive CRC and PLCP errors may point to one or more of the following issues:

1. Packet collisions due to densely populated clients
2. Overlapping channels
3. High multipath conditions due to bounced signals
4. Other signals in the 2.4GHz band

• When troubleshooting Layer 1 and Layer 2 issues, check cabling for both APs and antennas
• Common sources of antenna and RF problems include, but are not limited to, the following:

1. Radio Power Optimization
2. Radio Interference
3. Electromagnetic Interference
4. AP Channel Interference
5. Multipath
6. Antenna Power Issues