Access Point in wireless system

Access points are transmitter / receivers (transceivers) that connect to a network through an interface such as a bus or connector. They receive, store, and transmit data between the wireless LAN and the wired structure. End-users access the WLAN through wireless LAN adapters, similarly equipped with transmit-receive capabilities.

Access points are implemented as computer cards connected to the wired network through a bus, or as standalone modules connected to the wired network by mean of a suitable interface. Internal access points are computer cards that are attached to the computer through a slot. They are implemented, among others, as PCI, ISA, or PC cards (PCMCIA) in desktop computers, in notebook or palmtop computers, or integrated within hand-held computers. External access points are modules that connect to the system by means of a physical port.

Access points use a number of radio techniques to transfer data, while avoiding as much interference as possible. The most commonly used technique is referred to as spread spectrum. Spread spectrum is used to reduce the impact of localized frequency interferences. To achieve this, it uses more bandwidth than the system needs. There are two main spread spectrum modalities: direct sequence and frequency hopping. Direct sequence is a technique where the signal is spread on a large band by multiplexing it with a code (signature) to minimize localized interference and noise. The system works over a large band. To spread the signal, each bit is modulated by a code. Frequency hopping uses a technique where the signal walks through a set of narrow channels in sequence. The transmitted frequency band is divided in certain number of channels, and periodically the system hop to a new channel, following a predetermined cyclic hopping pattern. The system avoids interference by never staying in the same channel a long period of time.

When selecting between available access points, the most important specifications to consider (once the type of network and interface has been established), are the rate at which the access point transfers data, the maximum output power, the number of channels, and the sensitivity. Sensitivity is the measure of the weakest signal that may be reliably sensed by the receiver. Sensitivity is measured in dBm, and the lower the value (higher in absolute value) the better is the receiver.


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