3.3.3 This page explains the communication process of a WLAN.
After establishing connectivity to the WLAN, a node will pass frames in the same manner as on any other 802.x network. WLANs do not use a standard 802.3 frame. Therefore, using the term wireless Ethernet is misleading. There are three types of frames: control, management, and data. Only the data frame type is similar to 802.3 frames. The payload of wireless and 802.3 frames is 1500 bytes; however, an Ethernet frame may not exceed 1518 bytes whereas a wireless frame could be as large as 2346 bytes. Usually the WLAN frame size will be limited to 1518 bytes as it is most commonly connected to a wired Ethernet network.
Since radio frequency (RF) is a shared medium, collisions can occur just as they do on wired shared medium. The major difference is that there is no method by which the source node is able to detect that a collision occurred. For that reason WLANs use Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA). This is somewhat like Ethernet CSMA/CD.
When a source node sends a frame, the receiving node returns a positive acknowledgment (ACK). This can cause consumption of 50% of the available bandwidth. This overhead when combined with the collision avoidance protocol overhead reduces the actual data throughput to a maximum of 5.0 to 5.5 Mbps on an 802.11b wireless LAN rated at 11 Mbps.
Performance of the network will also be affected by signal strength and degradation in signal quality due to distance or interference. As the signal becomes weaker, Adaptive Rate Selection (ARS) may be invoked. The transmitting unit will drop the data rate from 11 Mbps to 5.5 Mbps, from 5.5 Mbps to 2 Mbps or 2 Mbps to 1 Mbps.
The next page explains authentication and association.
Authentication and association
3.3.4 This page describes WLAN authentication and association.
WLAN authentication occurs at Layer 2. It is the process of authenticating the device not the user. This is a critical point to remember when considering WLAN security, troubleshooting and overall management.
Authentication may be a null process, as in the case of a new AP and NIC with default configurations in place. The client will send an authentication request frame to the AP and the frame will be accepted or rejected by the AP. The client is notified of the response via an authentication response frame. The AP may also be configured to hand off the authentication task to an authentication server, which would perform a more thorough credentialing process.
Association, performed after authentication, is the state that permits a client to use the services of the AP to transfer data.
Authentication and Association types
• Unauthenticated and unassociated
• The node is disconnected from the network and not associated to an access point.
• Authenticated and unassociated
• The node has been authenticated on the network but has not yet associated with the access point.
• Authenticated and associated
• The node is connected to the network and able to transmit and receive data through the access point.
Methods of authentication
IEEE 802.11 lists two types of authentication processes.
The first authentication process is the open system. This is an open connectivity standard in which only the SSID must match. This may be used in a secure or non-secure environment although the ability of low level network ‘sniffers’ to discover the SSID of the WLAN is high.
The second process is the shared key. This process requires the use of Wireless Equivalency Protocol (WEP) encryption. WEP is a fairly simple algorithm using 64 and 128 bit keys. The AP is configured with an encrypted key and nodes attempting to access the network through the AP must have a matching key. Statically assigned WEP keys provide a higher level of security than the open system but are definitely not hack proof.
The problem of unauthorized entry into WLANs is being addressed by a number of new security solution technologies.
The next page explains radio waves and modulation.
3.3.5 This page describes radio waves and modulation.
Computers send data signals electronically. Radio transmitters convert these electrical signals to radio waves. Changing electric currents in the antenna of a transmitter generates the radio waves. These radio waves radiate out in straight lines from the antenna. However, radio waves attenuate as they move out from the transmitting antenna. In a WLAN, a radio signal measured at a distance of just 10 meters (30 feet) from the transmitting antenna would be only 1/100th of its original strength. Like light, radio waves can be absorbed by some materials and reflected by others. When passing from one material, like air, into another material, like a plaster wall, radio waves are refracted. Radio waves are also scattered and absorbed by water droplets in the air.
These qualities of radio waves are important to remember when a WLAN is being planned for a building or for a campus. The process of evaluating a location for the installation of a WLAN is called making a Site Survey.
Because radio signals weaken as they travel away from the transmitter, the receiver must also be equipped with an antenna. When radio waves hit the antenna of a receiver, weak electric currents are generated in that antenna. These electric currents, caused by the received radio waves, are equal to the currents that originally generated the radio waves in the antenna of the transmitter. The receiver amplifies the strength of these weak electrical signals.
In a transmitter, the electrical (data) signals from a computer or a LAN are not sent directly into the antenna of the transmitter. Rather, these data signals are used to alter a second, strong signal called the carrier signal.
The process of altering the carrier signal that will enter the antenna of the transmitter is called modulation. There are three basic ways in which a radio carrier signal can be modulated. For example, Amplitude Modulated (AM) radio stations modulate the height (amplitude) of the carrier signal. Frequency Modulated (FM) radio stations modulate the frequency of the carrier signal as determined by the electrical signal from the microphone. In WLANs, a third type of modulation called phase modulation is used to superimpose the data signal onto the carrier signal that is broadcast by the transmitter.
In this type of modulation, the data bits in the electrical signal change the phase of the carrier signal.
A receiver demodulates the carrier signal that arrives from its antenna. The receiver interprets the phase changes of the carrier signal and reconstructs from it the original electrical data signal.
The first Interactive Media Activity explains electromagnetic fields and polarization.
The second Interactive Media Activity shows the names, devices, frequencies, and wavelengths of the EM spectrum.
The next page describes problems caused by signals and noise.
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