Nokia D211 Developer's Manual
Also see for D211: User guideUser guide
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Contents
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5/1132. CHARACTERISTICS2.1 WLANWireless LANs (WLAN) were originally designed to be used as an extension or as analternative to fixed LANs in order to avoid cabling costs, for example. Today, WLANs arerapidly gaining significance, as mobile devices, such as laptop computers, are replacingdesktop personal computers. At the same time, major advances in the WLAN technologyhave taken place. The overall trend is towards higher bit rates, interoperable networking withother systems, compatible standards, and interoperable products between different WLANproduct manufacturers.2.1.1 FrequenciesTraditional cellular systems operate on licensed frequency bands. Operators are given acertain piece of spectrum from the total spectrum allocation to a particular system for theirsole use. This scheme works nicely with systems covering large areas, such as GSM.However, given the limited amount of spectrum this means that only a few operators canhave licenses per a given geographical area. For WLANs licensed frequencies clearly arenot a feasible solution since by their nature these systems are operated, for example, bycompanies or individuals in a limited geographical area, such as a building. Therefore, picocell networks and personal area networks operate on unlicensed spectrum. This means thata certain piece of spectrum is available for a set of systems/technologies, provided that theyfulfil certain agreed, very basic requirements, like maximum allowed transmit power. Thismeans that the operating environment, and the related problems, are very differentcompared to traditional cellular networks.2.1.2 StandardsIEEE initiated the 802.11 /24/ working group in 1990 to develop a WLAN standard for the 2.4GHz unlicensed ISM (Industrial, Scientific, and Medical) frequency band that is availableglobally (with some national and regional limitations). The IEEE 802.11 standard wasapproved in 1997, and the first products appeared roughly at the same time.Like IEEE 802.3 (Ethernet) or 802.5 (Token Ring) also 802.11 only covers thetwo lowest layers of the OSI reference model. 802.11 reference model consistsof two main parts, MAC (Medium Access Control) and PHY (Physical) layers.MAC functions on the Data Link layer and PHY on the Physical layer of the OSImodel.Currently, the IEEE 802.11 family of standards includes specifications for five different typesof physical layers. Four of these are based on radio technologies and one on diffuse infraredtechnology. All physical layer alternatives utilise the same MAC protocol. Three of the radioPHY standards utilise the 2.4 GHz frequency band and one is designed for the 5 GHzfrequency band.IEEE 802.11b /25/, the current industry standard that the vast majority of the current WLANproducts -- including the Nokia D211 ñ support, specifies a 2.4 GHz radio that utilises Direct |
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