Like wired LAN solutions, WLANs are comprised of many different elements. Therefore, in order to troubleshoot the WLAN solution, it is important to have a solid understanding of WLAN components and how they interact with each other in the overall solution. This understanding simplifies the overall WLAN solution troubleshooting process. Generally speaking, the majority of Wireless LAN issues fall into one of the following problem areas:
- Wireless Station (STA) or Client Issues
- WLC Configuration Issues
- AP Configuration Issues
- AP and WLC Registration Issues
- Infrastructure Issues
- Antenna and Radio Frequency Issues
The sections that follow describe some common problems that may occur in these three areas. Additionally, suggested solutions to correct (or avoid) such issues are also described.
Wireless Station (STA) Issues
Troubleshooting wireless clients (STAs) is an integral component of the overall WLAN solution troubleshooting process. This process can be used to narrow down the root of the wireless problem. For example, if a single client is unable to connect to the WLAN but every other client is able to, you can eliminate the WLAN solution devices, e.g. Access Points and WLCs, and troubleshoot the client itself. However, if multiple or all clients are unable to access the WLAN, you can eliminate the clients and focus your efforts on WLAN solution devices instead. Basic Wireless client troubleshooting should include the following tasks:
- Checking the client wireless NIC state
- Checking client settings
- Checking the state of the wireless client
Checking the client wireless network connection includes tasks such as validating that the client wireless network card is enabled and is functioning properly. Some wireless devices have a radio button that can be toggled back and forth to enable and disable the wireless network connection. It is not uncommon for this to be accidentally disabled; for example, such as when one is removing a laptop out of a carrying case. In most cases, the Operating System will indicate that the radio button is disabled by displaying a warning or other error message or using some kind of visual indicator, such as a red cross for example. Figure 11-3 shows the warning message displayed on a Windows-based machine after the wireless radio is disabled:
Fig. 11-3. Client Wireless Radio Warning Indicator and Message
In addition to verifying whether the wireless network connection is enabled, it is also important to check whether the wireless NIC has been correctly installed and is operating as expected, i.e. there are no error or warning messages printed by the Operating System. Additionally, it is also prudent to validate that the TCP/IP stack has been properly installed, is working and that the client is correctly configured to dynamically receive IP addressing information using the DHCP service. Keep in mind that these checks will vary depending on platform.
After validating that the wireless NIC is functioning as it should be, the next logical step would be to check the wireless settings. These include the SSID, security or authentication configuration (if applicable) and station configuration. Once the AP has been discovered, the client must establish an association. The AP may have some specific requirements that must be satisfied before allowing the STA to join the cell. For example, the AP may request a matching Service Set Identifier (SSID), a supported 802.11 standard, or some form of authentication. It is important to ensure that these parameters match on both the client and the AP; otherwise, the client will not be able to establish an association with the AP.
Checking the state of the client involves verifying whether or not the client is able to detect any wireless networks, based on the assumption that the wireless NIC is working as it should be. Additional client checks should include checking for signal interference, and verifying whether or not the client is associated. Different Operating Systems and vendors have tools that can be used to troubleshoot the client state. Check the vendor or manufacturer documentation for additional information on using their utilities. As an example, Figure 11-4 shows the Dell Wireless WLAN Card Utility that can be used to troubleshoot client state issues on a machine installed with a Dell WLAN Card:
Fig. 11-4. Client Wireless State Troubleshooting Tools
NOTE: When troubleshooting client association, you should also check the station’s status on the Access Point. Access Point troubleshooting is described later in this section.
WLC Configuration Issues
As is the case with all of the other technologies and protocols described in this guide, device misconfigurations are a common cause of problems. Some common WLC device misconfigurations include the following:
- Mismatched Service Set Identifiers
- Security Mismatches
- WLANs are disabled on the WLC
- Data Rate Mismatches
- Incorrect Client or Station Filtering
- Unsupported Features
- IP Address Assignment Issues
- SSID Broadcast is Disabled
NOTE: It should be noted that while the section that follows does include common WLC and AP misconfigurations as well as recommendations for resolving such problems, you are not expected to perform any WLC troubleshooting in the current TSHOOT exam. Emphasis should instead simply be placed on understanding the potential root of the problem.
When multiple clients are unable to connect to the WLAN, a good point to start is by checking the configured SSID on the WLC. Just as the SSID could be misconfigured on the client, it is possible for it to also be misconfigured on the WLC. When verifying the configured SSID, it is important to remember that the SSID is case sensitive.
Security mismatches are another common cause of WLAN problems. These parameters must match on the client and WLC. If the authentication type is Static WEP, verify that the appropriate encryption key and key index on the WLC matches that of the client. Alternatively, if the authentication type is 802.1x or WPA, ensure that the authentication type and the encryption key size match on both the client and the WLC. In the event that these parameters are indeed mismatched, they should be corrected on both the WLC and the client.
When a WLAN is configured on the WLC, it is important to ensure that it is enabled. By default, the status of the WLAN is not enabled on the WLC. Instead, it must be manually enabled following its configuration. If the WLAN is disabled, clients will not be able to associate. Verify the configuration of the WLC to ensure that all configured WLANs are enabled.
The Cisco Unified WLAN Solution allows administrators to specify data rates for the AP radio. Data rates can be specified as either mandatory or supported. If a data rate is specified as mandatory, the client must support it, otherwise association will fail. If a certain data rate has been configured as mandatory, verify that it is supported by the wireless client. To avoid situations such as these it is recommended that you set the lowest data rate to mandatory on the WLC and then specify any other data rates as supported.
On the WLC, there is an option to manually disable the clients which helps to prevent rogue clients from trying to access the network. While such policies enhance WLAN security, misconfigurations can result in legitimate clients being denied access to the WLAN. If your organizations policy requires such security, check the WLC configuration to ensure that the client that cannot connect to the WLAN is not included in the list of filtered client MACs.
Cisco WLCs include some proprietary features and functions that may not be supported by non-Cisco clients. Common proprietary features that are enabled by default include radio preambles and Management Frame Protection (MFP). The radio preamble, which is sometimes called a header, is a section of data at the head of a packet, which contains information that wireless devices need when they send and receive packets.
MFP ensures the integrity of the 802.11 management frames by allowing the Access Point to add a Message Integrity Check Information Element (MIC IE) to each frame. Any attempt made by the intruders to copy, alter, or replay the frame invalidates the MIC, which causes any receiving access point, which is configured to detect MFP frames, to report the discrepancy.
Although enabled by default, these features are not supported by all clients. When non-Cisco clients exist, it is recommended that such features be disabled if not supported.
IP addressing issues are one of the most commonly experience WLAN issues. The WLC can be configured as a DHCP relay agent or as a DHCP server itself. When the WLC operates as a DHCP relay agent, it forwards DHCP messages from clients to the specified DHCP server(s). When the DHCPOFFER comes back to the controller, it changes the DHCP server IP address to its virtual IP address, which is typically set to 1.1.1.1. When clients roam, the first thing they attempt to do it contact the DHCP server to renew their IP address. By using its own virtual IP as that of the DHCP server, the WLC is able to intercept the client DHCPREQUEST packets. Given this behavior, it is important to ensure that all WLCs (if more than one exists) are configured with the same virtual IP address as it prevents clients from beginning the entire DHCP process each time they roam between Access Points because they believe that they are communicating with the same DHCP server.
With Cisco 4400 series WLCs, by default, the Broadcast SSID parameter is disabled by default. This is a problem for non-Cisco clients or other devices that only perform passive scans, i.e. those that do not transmit probe requests to locate an Access Point. In hybrid or in non-Cisco client environments, you should enable the Broadcast SSID parameter so that any passive clients will be able to associate. This action also allows for any clients that do not have an SSID explicitly configured to associate.
AP Configuration Issues
While the Cisco Unified WLAN Solution includes both WLCs and APs, it is also possible to implement a Cisco WLAN solution with just APs running in autonomous mode. In such implementations, all APs must be configured individually and it is important to ensure that configuration parameters are consistent between the APs.
The most common issues with Access Points that are running in autonomous mode are service interruptions when clients are roaming. When implementing APs in autonomous mode it is important to ensure that all of the Access Points are configured with the following parameters:
- The same Service Set Identifier
- The same IP subnet
- The same Layer 2 native VLAN
When a client roams from one AP to another, the client will discard 802.11 probe responses and beacons received from access points unless they have matching SSID and encryption settings, which results in connectivity issues as the client moves from one Access Point to another. When roaming, WLAN clients first perform a Layer 2 roam as they move from one Access Point to another within the same subnet. However, if the Access Points are in a different subnet, clients perform Layer 3 roaming, which entails the client to acquire a new IP address, interrupting previously connected sessions. This behavior will adversely impact wireless VoIP phones and similar services, as well as any other applications using the original IP address.
And finally, the Access Points use the native VLAN to communicate information about clients that are roaming with other Access Points. For this reason, it is important to ensure that a consistent native VLAN is used when implementing multiple autonomous Access Points as management traffic is sent and received across this VLAN.
AP and WLC Registration Issues
In order to effectively troubleshoot LAP and WLC integration issues, it is important to have a solid understanding of the interaction between the LAP and WLC as was described earlier in this chapter. One of the most common issues that prevent LAP registration with the WLC is forgetting to configure DHCP Option 43. DHCP Option 43 is required to provide the LAP with the IP address(es) of the WLC in the DHCPOFFER message. If this option is not specified, the LAP is unable to register with the WLC.
As stated earlier in this chapter, the LAP attempts to use the Layer 2 discovery method first and then reverts to the Layer 3 discovery method if the Layer 2 method is not supported or fails.
When the LAP and WLC reside on different subnets, the LAP uses Layer 3 discovery to locate a WLC. In this mode, the LAP will Broadcast a Layer 3 LWAPP discover message on the local subnet. If the WLC resides on a remote subnet, the DHCP relay agent is required to ensure that these messages are relayed to the WLC. When using the Cisco IOS DHCP relay agent, it is important to remember that LWAPP discovery messages are not forwarded by default when theip helper-address <address> interface configuration command is issued. Given this, it is important to ensure that LWAPP Broadcasts, which use UDP port 12223 are forwarded by the DHCP relay agent by adding the ip forward-protocol udp 12223 global configuration command to the Cisco IOS DHCP relay agent configuration file. Without this configuration command, the LAP will not be able to communicate with the remote WLC.
Infrastructure Issues
Infrastructure issues can also cause WLAN issues. As stated earlier in this chapter, the wireless LAN relies on the wired LAN for connectivity. For this reason, it is important to ensure that switches and other intermediate devices are adequately configured to support the WLAN extension. For example, when connecting a WLC to a switch, the port should be configured as a trunk link. Verify the switch port configuration by checking the configuration or by using theshow interfaces <name> switchport command. Unlike WLC ports, the switch ports connected to LAPs should not be configured as trunk links, but as access ports. These ports should then be assigned to an active VLAN, which is typically the management VLAN. Again, verify switch configuration by checking the configuration or using the show interfaces <name> switchport command. Additionally, ensure that PortFast is enabled on ports that are connected to LAPs so that these ports transition to the forwarding state immediately.
Infrastructure devices such as Multilayer switches can also be configured as DHCP servers or DHCP relay agents. When the device is configured as a DHCP server, it is important to verify that DHCP Option 43 is included in the Cisco IOS DHCP server configuration. This is implemented using the option 43 ascii “<address>” DHCP configuration command. Likewise, if the device is configured as a Cisco IOS DHCP relay agent, ensure that the ip forward-protocol udp 12223global configuration command is used if the LAP and WLC reside on different IP subnets.
Like Cisco IP phones, Cisco Access Points can use an external power source to draw their power or draw their power from the switch to which they are connected. This power is sent within the Ethernet cable connecting the switch and the Access Point using either the IEEE 802.3af-2003 standard or the Cisco Inline Power method. Because PoE is increasingly used in today’s converged networks, it is important to accurately calculate the amount of power that will be required by the devices that will draw power from the switch. Cisco provides an online power calculator tool (requires login) that can be used to make this determination.
If an AP is connected to a switch and is unable to draw sufficient power, perhaps because other devices connected to the switch are consuming all available power, or because the switch itself is unable to provide sufficient power to all devices due to incorrect power calculations by administrators, in some cases, a message similar to the following will be logged on the console:
| %CDP_PD-2-POWER_LOW: All radios disabled – LOW_POWER_CLASSIC inline |
This message means that the Access Point has detected that the switch port (PSE) is not able to provide sufficient power and therefore has transitioned to low-power mode. In low-power mode, the AP will disable all radios, effectively meaning that no stations or clients will be able to associate with that Access Point. An important fact to remember is that if the AP is connected to both an external power injector and a PoE switch port, if the switch is not able to provide the Access Point with sufficient power, it will still log this message and disable the radios, even though the external power injector can provide enough power. In essence, PoE information received via CDP takes precedence. In such cases, the AP must be configured to ignore the CDP information and use the external power injector.
NOTE: AP configuration is beyond the scope of the TSHOOT exam and will not be described in any additional detail in this chapter or the remainder of this guide.
On the switch side, you can use the show power suite of commands to verify available power for the entire chassis, on a per-module or per-interface command as shown below. As an example, the show power inline <interface> command shows how much power is drawn by the device connected to the switch port. The following example shows the power used by a connected IP phone:
| Cat-6500-1#show power inline gigabitethernet 2/1 Interface Admin Oper Power(Watts) Device Class From PS To Device ——— —— ———- ———- ———- ——————- —–Gi2/33 auto on 13.5 12.0 Cisco IP Phone 7945 3Interface AdminPowerMax (Watts) ———- —————Gi2/1 15.4 |
The following example shows the same output for a switch port connected to an Access Point. In this case, the AP is using an external power injector and is not drawing power from the switch, implying the AP has been appropriately configured to use the external power source:
| Cat-6500-1#show power inline gigabitethernet 2/2 Interface Admin Oper Power(Watts) Device Class From PS To Device ——— —— ———- ———- ———- ——————- —–Gi2/2 auto off 0 0 cisco AIR-LAP1252AG n/aInterface AdminPowerMax (Watts) ———- —————Gi2/2 15.4 |
In addition to the previously described infrastructure checks and verifications, it is important to perform basic additional infrastructure checks, such as verifying Layers 1 and 2. When troubleshooting Layer 1 and Layer 2 issues, it is important to understand that given that wireless networks operate in a shared media, more so than wired networks; it is not uncommon to see Cyclic Redundancy Check (CRC) or PLCP (Physical Layer Control Protocol) errors. However, having stated that, it is important to understand that while these errors are normal, an excessive amount of these errors may indicate wireless network issues, which include:
- Packet collisions due to densely populated clients
- Overlapping channels
- High multipath conditions due to bounced signals
- Other signals in the 2.4GHz band
Recall from the SWITCH guide that radio interfaces (WLANs in general) operate in half duplex mode because a single frequency is used to transmit and receive data. Therefore, in environments with a dense population of clients (STAs), keep in mind that while the 802.11 standard does have some mechanisms for avoiding collisions, i.e. CSMA/CA, it is still possible for collisions to occur in environments with a dense client population. This can adversely impact WLAN performance and result in intermittent connectivity issues.
In cases where you do observe a large amount of CRC errors, consider checking for possible radio interference, antennas and cabling, as well as the line of sight (LOS) between the transmitter and the receiver, to ensure that the LOS is clear from possible interfering objects.
NOTE: WLAN problems caused by overlapping channels, multipath conditions, and the presence of other signals in the 2.4GHz band are described in the following sections.
In addition to checking for errors, it is also important to remember basic Ethernet fundamentals. For example, if the WLAN is experiencing intermittent connectivity or connectivity with errors, there may be a possibility that the cable length is greater than the recommended Ethernet segment lengths. This is applicable not only to Access Points but also to antenna cabling. When implementing a WLAN solution, cable runs should be kept as short as possible to allow for optimum efficiency and prevents loss, which is likely if the cable runs are long. Instead of using standard cabling, such as traditional coaxial cable to connect antennas, for example, consider using Cisco antenna cables instead. While they may be slightly more expensive than other standard cables, they are recommended for optimum efficiency of the overall WLAN solution.
Finally, if the LAP and WLC are located on remote subnets, ensure that no ACLs are blocking communication between the two devices. In addition to these checks, ensure that configurations on the infrastructure devices are correct. For example, if using the Cisco IOS DHCP relay agent to forward LWAPP Broadcasts to the WLC ensure that the ip forward-protocol udp 12223global configuration command is included in the configuration in addition to the ip helper-address <address> interface configuration command. Similarly, when using Cisco IOS DHCP server, ensure that DHCP Option 43 is also included in the configuration.
Antenna and Radio Frequency Issues
Antennas are an integral component of wireless implementations. Antennas provide the WLAN system with three fundamental properties, which are gain, direction and polarization. Gain is a measure of increase in power and is used to describe the amount of increase in energy that an antenna adds to a radio frequency (RF) signal. Direction is the shape of the transmission pattern. Polarization is the physical orientation of the element on the antenna that actually emits the RF energy. Understanding basic antenna functionality and operation is fundamental to understanding the overall wireless solution. In addition to this, a solid understanding of these principles is necessary for supporting and troubleshooting wireless problems.
Cisco wireless equipment support different styles of antenna. Each of these types has different coverage capabilities. The Supported antenna types include omnidirectional and directional antenna types. Omnidirectional antennas are designed to provide a 360-degree radiation pattern and are commonly used when coverage in all directions from the antenna is required. Omnidirectional antenna operation is illustrated in Figure 11-5. The coverage provided by this type of antenna is shown in gray (U.S English) or grey (UK English):
Fig. 11-5. Omnidirectional Antenna Coverage
Directional antennas come in different styles and shapes. Directional antenna types include yagi antennas, patch antennas and parabolic dishes. Yagi antennas are simply antenna types that radiate in only a specific direction. Figure 11-6 (below) illustrates basic yagi antenna operation. The coverage is shown in gray (U.S English) or grey (UK English):
Fig. 11-6. Directional Yagi Antenna Coverage
Directional patch antennas are simply a type of flat antenna. Like directional yagi antennas, they provide coverage in a specific direction. Figure 11-7 shows a mounted patch antenna:
Fig. 11-7. Mounted Directional Patch Antenna
And finally, parabolic antennas are simply antennas that look like satellite dishes, which we all should be familiar with. These antennas are also commonly referred to simply as a dish antenna. Parabolic antennas have a very narrow RF energy path. These antennas are typically used only in outdoor wireless implementations.
Having discussed antenna basics, the remainder of this section will describe some of the more common sources of problems, which include, but are not limited to, the following:
- Radio Power Optimization
- Radio Interference
- Electromagnetic Interference
- AP Channel Interference
- Multipath
- Antenna Power Issues
In some instances, when the Access Point and the clients that are associated with the Access Point are within close proximity, the clients may be disconnected from the Access Point. Although rare, this can result in poor WLAN performance and intermittent connectivity for clients associated with the Access Point(s). The recommended solution for such issues is keeping clients away from the Access Point. This can be performed by installing the Access Point in locations that, while still accessible to clients, are not within close proximity to the clients. For example, instead of placing an Access Point on a conference room table, the AP should be placed or mounted on a wall or ceiling in the conference room instead.
In some cases it may not be possible to prevent clients from being within such close proximity to the Access Point. For example, it may not be possible to mount the AP in a factory for example. If the AP and clients will be in such close proximity, you can reduce the power of the Access Point to prevent clients that are too close to the Access Point from being disconnected.
Radio interference is a common cause of WLAN problems due to the shared media. Proactively, such issues can be avoided by performing site surveys prior to implementing the WLAN solution. Radio interference issues are a common phenomenon because a license is not required to operate radio equipment in the 2.4 GHz band, which is the same band Cisco Aironet WLAN equipment operates. It is therefore possible for other devices, such as microwave ovens or wireless phones, to be using the same band, resulting in interference. You can use a spectrum analyzer to determine the presence of any other activity on your frequency. In the event that there is too much interference, consider changing frequencies, if possible.
Electromagnetic Interference (EMI), also referred to as Radio Frequency Interference or RFI, is a disturbance that affects an electrical circuit due to either electromagnetic induction or electromagnetic radiation emitted from an external source. While EMI does not necessarily affect signal transmissions, per-se, it can affect the components of the transmitter, resulting in poor WLAN performance and intermittent connectivity issues for example. To avoid the potential problems caused by EMI, you should ensure that APs are placed away from any potential EMI sources such as fluorescent lights as well as high-voltage power lines for example. In some environments, e.g. factories, if you cannot completely remove the AP from sources of EMI, such as power lines, you could alternatively supply conditioned power to the WLAN equipment in order to lessen the effects of EMI generated on those power circuits. However, the recommended solution would still be to isolate the equipment from such sources.
Channel interference, which is a direct result of a poor implementation, is also a common cause of WLAN issues. As was stated earlier in this chapter, 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. Taking this into consideration, when installing APs, you should ensure that adjacent APs use non-overlapping channels.
Within the 2.4-GHz range, there are three channels that do not overlap. These channels are 1, 6, and 11. Therefore, use these channels alternately when deploying Access Points in an ESS. Figure 11-8 illustrates a recommended AP deployment using these non-overlapping channels:
Fig. 11-8. Implementing APs Using Non-Overlapping Channels
Referencing Figure 11-8, three APs deployed within close proximity or overlapping coverage areas are configured to use overlapping channels to avoid RFI issues, which may lead to connectivity issues and poor throughput. If an additional AP is added, say AP # 4, then the Access Point can be configured to use channel 1. If yet another Access Point is added, say AP # 5, this AP would be configured to use channel 6, and so forth.
Multipath is a common cause of WLAN problems due to the nature of the medium used. This situation occurs when RF signals take different paths from a source to a destination. When Radio Frequency signals are transmitted, they become wider as they are transmitted further. This increase in width increases the likelihood of the RF signals running into objects that reflect, refract, diffract or interfere with the signal, such as furniture, walls, or coated glass.
When the RF signal is reflected off an object, this causes multiple, duplicate, wavefronts to be created and propagated, resulting in multiple wavefronts being received by the receiver. The WLAN multipath concept is illustrated in Figure 11-9:
Fig. 11-9. Understanding WLAN Multipath
Referencing Figure 11-9, the AP transmits a signal. As the signal travels further, it widens. Part of the signal goes straight to the destination while other parts bounces off an obstruction, such as the ceiling or any other reflector object, such as a steel cabinet, and then goes on to the destination. As a result of this obstruction or interference, the obstructed signals will encounter some delay and travel a longer path to reach the same destination. This results in the client receiving multiple wavefronts.
When these different waveforms combine, they cause a distortion of the waveform, resulting in poor signal quality, even though the actual signal strength itself may be strong. This delay causes the information symbols represented in 802.11 signals to overlap, which then affects the decoding capability of the client (receiver) and results in poor performance and connectivity issues. The recommended solution is to implement diversity.
Diversity is the use of two antennas for each radio. Not only does this increase the probability of receiving better signals from either one of the antennas, it also allows the radio to compensate for errors due to RFI and provides relief to a wireless network in a multipath scenario. With diversity, only a single antenna (the best antenna for transmitting to the receiver) is used. The antennas are not used at the same time to avoid introducing multipath issues themselves. By default, Cisco APs default to antenna diversity, i.e. to using dual antennas. Although not recommended, this default behavior can be modified because only a single antenna is required to provide radio operations.
NOTE: An alternative to implementing the dual antennas is to implement the 802.11a standard, which provides higher data rates than DSSS and minimizes the effects of multipath propagation on signal quality and throughput. However, keep in mind that 802.11a is not compatible with the other more commonly used 802.11b and 802.11g standards, and typically costs more, from a monetary standpoint, to implement.
The final issue discussed in this section pertains to antenna power issues. Antenna gain ratings are measured in decibels (dB) which is a ratio between two values. An antenna rating is typically to the gain of an isotropic (dBi) or dipole (dBd) antenna. The isotropic antenna is a theoretical antenna that transmits equal power density in all directions. These antennas are used only as theoretical (mathematical) references, and do not exist in the real world; however, because the U.S FCC uses dBi its calculations, this same standard is also used by most wireless equipment manufactures and vendors, such as Cisco, for example.
Dipole antennas are more real-world antennas. While some antennas are rated in dBd, most of the rating use dBi because all FCC calculations are based on the dBi measurement. Antenna power is a major factor that should be taken into consideration when designing the WLAN as incorrect calculations may result in poor WLAN performance, resulting in issues such as intermittent connectivity or even in an outright or complete loss of connectivity in some areas.
When you are determining which antenna to use or when you are troubleshooting potential RF power issues, keep in mind that as the gain of an antenna increases so does the signal strength and directivity; however, this comes with a tradeoff in that the antennas coverage area is diminished. Directivity measures the power density an antenna radiates in the direction of its strongest emission. To further clarify this point, consider Figure 11-10 which shows the coverage provided by a low gain antenna (LGA):
Fig. 11-10. Understanding Low Gain Antennas
Referencing Figure 11-10, the low gain antenna (LGA) is able to provide a broad coverage area which includes STA 2, STA 3, and STA 4. However, because it is low gain, the antennas coverage distance is limited and STA 1 is not included in this range. If the gain were increased, the antennas signal strength and directivity would be increased, allowing it to reach greater distances as is illustrated in Figure 11-11:
Fig. 11-11. Understanding High Gain Antennas
Referencing Figure 11-11, the LGA shown in Figure 11-10 has been replaced by an HGA, or the gain on the LGA was simply increased. Either way, the signal is stronger, more direct and traverses a greater (further) distance which means that STA 1 now resides within the coverage area. However, this comes at the expense of the area that is actually covered by the AP. Previously, STA 3 was comfortably within the coverage area while STA 1 was not; however, now STA 3 is no longer within the coverage area, while STA 1 is. The recommended solution in this case would be to reduce the adjusted gain on the AP and integrate another AP into the WLAN solution, keeping in mind standard recommended configuration fundamentals, such as using overlapping channels, and ensuring that both APs are configured the same, e.g. SSIDs.
In summation, you should be aware of this tradeoff when deciding on antennas or adjusting the gain values for antennas used in the WLAN solution. Careful consideration must be taken before increasing or decreasing gain because this can result in issues such as path loss, which is the distance the signal can be transmitted, as well as a reduced coverage area.