Wi-Fi
The IEEE 802.11 working group published the 802.11 standard for wireless LANs in 1999. 802.11b is a supplement to 802.11 that specifies a higher data rate and is currently the industry-accepted standard for WLANs. 802.11b products that meet a base standard of interoperability are certified by the Wireless Ethernet Compatibility Alliance (WECA) with the Wi-Fiâ„¢ logo. Several IEEE 802.11 working groups are creating standards for improvements in data rates, quality of service, and security.
Wi-Fi is not a fully wireless LAN. It is an extension to the existing wired network. By using device called Access Point (AP) wireless nodes can be connected to the wired network. Otherwise wireless nodes has to be connected in AdHoc style. Its typical coverage area is 100 to 500 feet.
Wireless LANs give the enterprise more mobility and flexibility by allowing workers to stay connected to the Internet and to the network as they roam from one coverage area to another. This increases efficiency by allowing data to be entered and accessed on site.
In September of 1999, the Institute of Electrical and Electronic engineers (IEEE) ratified the specification for IEEE 802.11b, also known as Wi-Fi. IEEE 802.11b defines the physical layer and media access control (MAC) sublayer for communications across a shared, wireless local area network (WLAN).
At the physical layer, IEEE 802.11b operates at the radio frequency of 2.45-gigahertz (GHz) with a maximum bit rate of 11 Mbps. It uses the direct sequence spread spectrum (DSSS) transmission technique. At the MAC sublayer of the Data Link layer, 802.11b uses the carrier sense multiple access with collision avoidance (CSMA/CA) media access control (MAC) protocol.
A wireless station with a frame to transmit first listens on the wireless medium to determine if another station is currently transmitting (this is the carrier sense portion of CSMA/CA). If the medium is being used, the wireless station calculates a random backoff delay. Only after the random backoff delay elapses can the wireless station again listen for a transmitting station. By instituting a random backoff delay, multiple stations that are waiting to transmit do not end up trying to transmit at the same time (this is the collision avoidance portion of CSMA/CA).
Collisions can occur and, unlike with Ethernet, they might not be detected by the transmitting nodes. Therefore, 802.11b uses a Request to Send (RTS)/Clear to Send (CTS) protocol with an Acknowledgment (ACK) signal to ensure that a frame is successfully transmitted and received.
Wi-Fi wireless networking consists of the following components:
1. Stations
2. Wireless Access Points
3. Ports
STATIONS (STA)
A station (STA) is a network node that is equipped with a wireless network device. A personal computer with a wireless network adapter is known as a wireless client. Wireless clients can communicate directly with each other or through a wireless access point (AP). Wireless clients are mobile.
Wireless Access points (AP)
A wireless AP is a wireless network node that acts as a bridge between stations and a wired network. A wireless AP contains:
1. At least one interface that connects the wireless AP to an existing wired network (such as an Ethernet backbone).
2. A wireless network device with which it creates wireless connections with stations.
3. IEEE 802.1D bridging software, so that it can act as a transparent bridge between the wireless and wired networks.
The wireless AP is similar to a cellular phone network's base station. Wireless clients communicate with both the wired network and other wireless clients through the wireless AP. Wireless APs are not mobile and act as peripheral bridge devices that extend a wired network.
PORTS
A port is a channel of a device that can support a single point-to-point connection. For IEEE 802.11b, a port is an association, a logical entity over which a single wireless connection is made. A typical wireless client with a single wireless network adapter has one port and can support only one wireless connection. A typical wireless AP has multiple ports and can simultaneously support multiple wireless connections. The logical connection between a port on the wireless client and the port on a wireless AP is a point-to-point bridged LAN segment similar to an Ethernet-based network client that is connected to an Ethernet switch.
HOW IT WORKS?
When a wireless adapter is turned on, it begins to scan across the wireless frequencies for wireless APs and other wireless clients in AdHoc mode. Assuming that the wireless client is configured to operate in infrastructure mode, the wireless adapter chooses a wireless AP with which to connect. This selection is made automatically by using a SSID and signal strength and frame error rate information. Next, the wireless adapter switches to the assigned channel of the selected wireless AP and negotiates the use of a port. This is known as establishing an association.
If the signal strength of the wireless AP is too low, the error rate too high, or if instructed by the operating system (in the case of Windows XP), the wireless adapter scans for other wireless APs to determine whether a different wireless AP can provide a stronger signal or lower error rate. If such a wireless AP is located, the wireless adapter switches to the channel of that wireless AP and negotiates the use of a port. This is known as Reassociation.
Reassociation with a different wireless AP can occur for several reasons. The signal can weaken as either the wireless adapter moves away from the wireless AP or the wireless AP becomes congested with too much traffic or interference. By switching to another wireless AP, the wireless adapter can distribute the load to other wireless APs, increasing the performance for other wireless clients. You can achieve contiguous coverage over large areas by placing your wireless APs so that their signal areas overlap slightly. As a wireless client roams across different signal areas, it can associate and reassociate from one wireless AP to another, maintaining a continuous logical connection to the wired network. It communicates with the AP using Direct Sequence Spread Spectrum (DSSS) technique.
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