The article gives a short overview about the wireless local area network (WLAN) and the IEEE 802.11 standards.

Content:

• 1. Abstract
• 2. IEEE 802.11/802.11b Wireless Ethernet Standard
• 3. IEEE 802.11 PHY-Layer
• 4. IEEE 802.11 MAC-Schicht

The possibilities of WLANs seems to be an infinite and important step forward to ubiquitous computing. Over a connection between two computers to a LAN-coupling of several miles everything is practicable. The simplest solution comprises an ad-hoc network as afore mentioned with a wireless network interface cards for each computer. The data transmission within WLANs is realised by the radio technology. The range of a wireless network can be increased with the implementation of a access point (AP). A AP within a WLAN is comparable with a bridge in a LAN and supports the connection to several clients. In many topologies APs operate as a data gateway which receives, buffers and exchanges data between several mobile clients and a WLAN. In general a WLAN based on the IEEE 802.11 standard and specifies the communication ”between a wireless client and a base station or access point, as well as among clients” [IEEE01]. A WLAN supports a data transfer rate up to 11 Mbit/s. But a new WLAN product generation offers 54 Mbit/s and it is called HiperLAN2 (High Performance LAN) [tecchan03]. WLAN operates in the unlicensed 2.4-GHz ISM-band as well as Bluetooth. The range of WLANs comprise up to 100 meters. In the following there is a short overview about this standard.

In 1997 IEEE published the 802.11 Wireless Ethernet Standard with a data transfer rate of 1-2Mbit/s. But two years later IEEE adopted the 802.11b standard which based on the 802.11 standard and extended the data transfer rate of 5.5 Mbit/s and 11 Mbit/s. The IEEE 802.11 standard describes a detailed software, hardware and protocol specification for the PHY- and MAC-layer (”physical” and ”media access control”) of the OSI Reference Model (Open System Interconnection).

In general is the PHY-Layer responsible for the data transmission between two nodes. Therefore it was necessary to define a uniform interface and protocols for communication among wireless nodes. It also includes access points (APs) in a network as well. The PHY-Layer comprises the FHSS (Frequency Hopping Spread Spectrum) technology, DSSS (Direct Sequence Spread Spectrum) technology and the infrared technology. Whereas the infrared technology is rarely used in the practice today.

FHSS is a modulation procedure in WLANs. It is switching of frequencies multiple per second during a radio transmission to minimize any interferences. In contrast to FHSS DSSS generates for every bit that have to be transmitted a redundant bit (also known as a chipping code). The chipping code supports to recover the original data if data information are damaged during transmission [WLAN03].

The MAC-Layer is one of two sublayers that make up the Data Link Layer in the OSI Reference Model. It consists of a number of MAC protocols which are responsible for a controlled data transmission between two stations. The 802.11 standard defines for example the Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) protocol. In general the CSMA/CA protocol tries to realise a collision-free data transmission. Such collision avoidance is achieved by a carrier sense scheme. At first the station that intends to transmit any data packages sends a test signal. Such test signal scans the transmission channel for any jams. After the waiting for a sufficient time for all stations to receive the test signal the intended station transmits its data frame. If there is a jam in the channel the station waits sufficient time. While transmitting there is a jam signal from another station it stops data transmitting for a random time and tries again.