In: Root » Electronics and communication » Wireless and mobile computing
About the same time WECA approved the 802.11 standard, several other types of wireless technologies were being introduced. Although a few have made a rather impressive niche in the Personal Area Network (PAN) market, the only other WLAN technology that came close to competing with 802.11 was HomeRF. Using the Shared Wireless Access Protocol (SWAP), HomeRF merges the 802.11 FHSS standard with the six voice channels based on Digital Enhanced Cordless Telecommunications (DECT). In other words, the home network included both voice and data streams that could all work together at the same time. In addition, HomeRF devices do not require an access point to convert signals. The HomeRF devices do all the required conversion. Understanding FHSSHomeRF uses another frequency control standard called FHSS (Frequency Hopping Spread Spectrum). Used in combination with a 2.4GHz frequency, a signal can change channels 50 times per second. This helps provide reliable service, even with the existence of other HomeRF networks. By using the entire frequency range, multiple networks can operate in the same area without fear of collision. Ironically, FHSS was also used in the preliminary implementations of the 802.11 standard. However, HomeRF used an enhanced version and managed to achieve a data rate of 1.6Mbps, as compared to the 1Mbps 802.11 reached. This standard was short-lived because of the low bandwidth (1–2Mbps) and the relatively short effective distance. One advantage that helped keep HomeRF in the market was its low cost. However, after the wireless fad caught on, 802.11b devices quickly dropped in price, and numerous vendors started producing equipment for WLANs. Understanding IrDAIrDA stands for the Infrared Data Association, which is a standard controlled by another of the previously mentioned groups. This organization is responsible for standardizing the hardware and software protocols that make up this wireless technology designed for the PAN. IrDA is a wireless technology that facilitates the communication of devices that only need to transmit or receive small amounts of information. Because this technology is cheap, it is integrated into all sorts of personal devices, such as watches, PDAs, phones, laptops, and even wireless mice and keyboards. Although Bluetooth (discussed in the next section) threatens to take over the PAN market, IrDA will be around for some time to come. IrDA's strength is its versatility. It is a standard all to itself, which makes it simple and cheap to integrate into almost anything. However, it has multiple weaknesses that are closely tied to its functionality. IrDA uses timed pulses of light to transmit data using a simple light bulb wired into a circuit board — in other words, a computerized version of a flashlight. By turning a light on and off at modulated times, it transmits data bit by bit up to 4Mbps. Although this is sufficient for many uses, such as a mouse or watch programming, a file over 1MB will take several minutes to transmit, and then only if the IrDA devices are right next to each other. Because IrDA uses light as its data medium, it is subject to transmission problems in a lighted environment. For example, office room lights flicker at 60Hz because of the AC current modulation. You don't see this because your brain doesn't detect it, but electric lights and most other light emitting devices go on and off 60 times a second. If the light is too bright, it will scramble the data as it is being transmitted. NOTE For those of you who like to design circuits, adding a 60Hz "spike" filter to your receiving device can minimize this interference. In addition, distance becomes an issue for IrDA. Typically, an IrDA device will not work beyond one meter. Although slower pulse times can increase this distance, anything slower than a solid 4Mbps, at a realistic range of several meters is inefficient by modern standards. Still, IrDA will be around for a while. However, it will probably remain exclusively in devices that can only be used within a few feet of each other.
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