- Gain—The energy an antenna adds to the RF signal.
- Directionality—How the radio coverage is distributed.
- Polarization—The physical orientation the RF element. Cisco Aironet antennas use vertical polarization.
- Multipath Distortion—Receiving both direct and reflected signals arriving from different directions.
- Effective Isotropic Radiated Power (EIRP)—The AP radio’s effective transmission power. Includes gain from the antenna and loss from the antenna’s cable.
Gain
Cisco measures gain in dBi, which stands for decibel isotropic and is a measure of decibels relative to an isotropic source in free space. A decibel is the ratio between two signal levels. An isotropic antenna is a theoretical one in which the signal spreads out evenly in all directions from one point. Thus, dBi is the ratio of an antenna’s signal to that of an isotropic antenna.
Directionality
Omnidirectional antennas have signals that theoretically extend in all directions, both vertically and horizontally. When gain in increased, the signal expands horizontally, but decreases vertically. One omnidirectional example is the dipole “Rubber Duck” antenna.
Directional antennas aim their signal in a specific direction. Signals can spread fairly wide in one direction or can be narrowly focused. Some examples include the Diversity Patch Wall Mount Antenna, Yagi, and dish antennas.
Multipath Distortion
Because radio waves are transmitted in many directions, not all go in a straight line to every client’s antenna. Some bounce off walls or other objects and arrive at the client in varying intervals. Thus, the client receives several copies of the same RF signal, which can cause degraded data quality. This is multipath distortion, or multipath interference. Diversity systems try to minimize this by using two antennas; you might try moving antennas or changing the frequency if this is a problem in your facility. OFDM uses multiple frequencies operating together to increase performance in multipath situations.
EIRP
EIRP is the actual power of the signal that comes from the antenna, measured in Decibel Milliwatts (dBm). (0 dBm equals 1 milliwatt of power.) EIRP is calculated by taking the transmitter power, subtracting the amount of signal lost traversing the cable between the transmitter and antenna, and adding the antenna’s gain. This can be expressed:
EIRP = (power – cable loss) + antenna gain.
Different countries have different rules about the amount of EIRP allowed. For instance, the maximum in the United States is 36 dBm. To minimize signal loss, use the shortest low-loss cable possible. Wider cables conserve more signal but are also more expensive.
Power over Ethernet (PoE) Switches
Access points can receive their power over Ethernet cables from Power over Ethernet (PoE) switches, routers with PoE switch modules, or midspan power injectors, thus alleviating the need for electrical outlets near them. APs require up to 15W of power, so plan your power budget accordingly. Two power standards are the Cisco Prestandard PoE and the IEEE’s 802.3af standard. Both have a method for sensing that a powered device is connected to the port. 802.3af specifies a method for determining the amount of power needed by the device. Cisco devices, when connected to Cisco switches, can additionally use CDP to send that information. Power can be supplied over the data pairs—1, 2, 3, and 6—or over the unused pairs of 4, 5, 7, and 8.
Cisco PoE switches are configured by default to automatically detect and provide power. To disable this function, or to re-enable it, use the interface command power inline {never | auto}. To view interfaces and the power allotted to each, use show power inline [interface].
Configuring Wireless LAN Devices
Autonomous APs must be configured individually, while the WLC provides configuration to lightweight APs. WLAN clients must also be configured; this process varies depending on the client software used.
Configuring Autonomous Access Points
Autonomous APs can be configured in one of three ways:
- IOS Command Line—Either via Telnet or the console port.
- Web browser—This is the Cisco preferred way.
- CiscoWorks WLSE—For centralized configuration control.
The AP must already have an IP address to use any of these except the console port. It attempts to obtain one via DHCP by default. This link has directions and screen shots for both the command line and web browser configuration:
http://www.cisco.com/en/US/products/ps6087/products_installation_ and_configuration_guides_list.html.
Aironet 1100, 1200, and 1300 series APs perform various functions:
- Wireless AP
- Root bridge
- Nonroot bridge
- Repeater
- Scanner
- Workgroup bridge
Configuring a WLAN Controller
Cisco lightweight APs receive their configuration from the Wireless LAN Controller, which must be configured first. Initial configuration of the lightweight WLC can be done via command line using the console port or via web browser using the service port. Subsequent configuration can be done via:
- IOS Command Line—Either by Telnet, SSH, or the console port.
- Web browser—Using the WLC’s IP address and Internet Explorer.
- Cisco Wireless Control System—For centralized configuration control.
You need to configure the WLC with information such as VLANs, SSIDs, and security policies. It downloads a configuration to its associated APs, and you can also configure, monitor, or reset individual APs through the web browser of the WLC. Review the material at this link
for screen shots and WLC configuration information:
http://www.cisco.com/en/US/products/ps6366/products_configuration_
guide_book09186a00806b0077.html.
WLCs use several different types of physical and logical interfaces that are described in Table 6-2.
