Cisco Wireless Network Components

You should review the following link for more information on Cisco wireless controllers and access points before you take the exam: http://www.cisco.com/en/US/products/hw/wireless/products_category_ buyers_guide.htmlWireless Clients.

Cisco has a wireless NIC that can be installed on Windows 2000 and Windows XP systems. It comes with some utilities: Aironet Desktop Utility (ADU), Aironet Client Monitor (ACM), and Aironet Client Administration Utility (ACAU). Cisco recommends using the ADU and ACM utilities to control your wireless card, rather than the built-in Windows controls to get the increased functionality Cisco provides. The Cisco ACAU allows loading and configuration of the Cisco client software over the network, using encrypted files. There is also an Aironet Site Survey Utility to scan for APs and get information about them.

Cisco wireless IP phones have the same features as Cisco wired IP phones and can use LEAP for authentication.

The Cisco Compatible Extensions Program tests other vendors' devices for compatibility with Cisco wireless products. Using products certified by this program ensures full functionality of Cisco enhancements and proprietary extensions. A list of these products can be found at www.cisco.com/go/ciscocompatible/wireless.

Autonomous APs

Autonomous APs run Cisco IOS, are programmed individually, and act independently. They can be centrally managed with the CiscoWorks Wireless LAN Solution Engine (WLSE) and can use Cisco Secure Access Control Server (ACS) for RADIUS and TACAS+ authentication. Redundancy consists of multiple APs.

Lightweight Access Points

Lightweight APs divide the 802.11 processing between the AP and a Cisco Wireless LAN Controller (WLC). This is sometimes called "split MAC," because they split the functions of the MAC layer—Layer 2. Their management components also include the Wireless Control System (WCS) and a location-tracking appliance. Redundancy consists of multiple WLCs. The AP handles real-time processes, and the WLC handles processes such as:

■ Authentication

■ Client association/mobility management

■ Security management

■ QoS policies

■ VLAN tagging

■ Forwarding of user traffic

The Lightweight Access Point Protocol (LWAP) supports the split MAC function in traffic between a lightweight AP and its controller. LWAP uses AES-encrypted control messages and encapsulates, but does not encrypt, data traffic. LWAP operates at Layer 2, and also at Layer 3 over UDP. (However, Layer 2 operation has been deprecated by Cisco.) The controller can be either in the same broadcast domain and IP subnet or in a different broadcast domain and IP subnets for Layer 3 operation. The AP follows this process to discover its controller:

Step 1. The AP requests a DHCP address. The DHCP response includes the management IP address of one or more WLCs.

Step 2. The AP sends an LWAPP Discovery Request message to each WLC.

Step 3. The WLCs respond with an LWAPP Discovery Response that includes the number of APs currently associated to it.

Step 4. The AP sends a Join Request to the WLC with the fewest APs associated to it.

Step 5. The WLC responds with a Join Response message, the AP and the controller mutually authenticate each other and derive encryption keys to be used with future control messages. The WLC then configures the AP with settings, such as SSIDs, channels, security settings, and 802.11 parameters.

The Cisco Aironet 2000 series WLC can handle up to six APs; thus, it is sized for small- to medium-sized operations.

The Cisco Aironet 4400 series WLC supports medium to large facilities with the 4402 handling up to 50 APs, and the 4404 handling up to 100 APs.

Wireless LAN Antennas

Several concepts are important in understanding wireless antennas:

■ 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:

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.

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].

Continue reading here: MAC Address Flooding

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