This first chapter from the book Linux Unwired by Roger Weeks et al. introduces radio waves, antennas, connections without wires, Bluetooth, cellular data and infrared. (O'Reilly Media, ISBN: 0596005830, 2004.)
Bluetooth is a low-power radio technology aimed at replacing cables for connecting devices. It was originally developed by the Swedish telecommunications manufacturer Ericsson and then formalized by an industry consortium. The name is taken from a Danish king, Harald Bluetooth, who ruled Denmark and Norway in A.D. 936.
The standards for Bluetooth define a low-power radio with a maximum range of 300 feet. The radios are actually on a transceiver microchip to keep size and power consumption to a minimum. Bluetooth uses the 2.45 GHz band of the ISM radio spectrum and divides the band into 79 channels. To further reduce any crosstalk into other ISM bands, Bluetooth devices can change channels up to 1,600 times per second.
Bluetooth is becoming widely available on mobile phones and PDAs, and one of its “killer” applications is hands-free wireless headsets for mobile phones. Bluetooth is also a popular way to “tether” a notebook computer to a cellular phone, which allows you to connect to the Internet even when an 802.11 network is not available (because current cellular data speeds are much slower than Bluetooth, Bluetooth’s relatively slow speeds are not the limiting factor). Bluetooth adapters are available for PDAs, desktops, and notebooks. There are some printers and keyboards available that use Blue-tooth to communicate with the host device as well.
Compared to Wi-Fi, Bluetooth speeds are not impressive, but they are quite useful for transferring small amounts of data. Download speeds can max out at 720 kbps with a simultaneous upload speed of 56 kbps. Every Bluetooth device can simultaneously maintain up to seven connections, making a personal Bluetooth LAN a real possibility.
With the rise of digital cellular phone networks, it became possible to use these networks to transfer data rather than just voice. There are several differing and competing technologies available.
Cellular Digital Packet Data (CDPD) was one of the first data networking technologies available for mobile phones. CDPD utilizes unused bandwidth in the 800–900 MHz range normally used by mobile phones. Data transfer rates max out at a theoretical 19.2 kbps. Today, CDPD is obsolete, and cellular carriers are actively trying to phase it out.
General Packet Radio Service (GPRS) is an add-on technology to existing Time Division Multiple Access (TDMA)–based GSM mobile phone networks. Timeslots in the GSM network are normally allocated to create a circuit-switched voice connection. With a GPRS-enabled network, the timeslots are used for packet data as needed. This by design creates a very slow data network with high latency and, theoretically, the speed of a 56 kbps modem. AT&T Wireless, T-Mobile, and Cingular Wireless use this technology. In 2003, an enhancement to GPRS, Enhanced Data Rates for Global Evolution (EDGE), was partially rolled out in the United States by AT&T Wireless and Cingular. In theory, EDGE can triple the data rate of GPRS, but you need an EDGE-capable handset, such as the Nokia 6200, to use it.
1xRTT stands for Single Carrier Radio Transmission Technology and is part of the CDMA2000 family of protocols, which includes successors to 1xRTT such as Single Carrier Evolution Data Only (1xEV-DO). It is built on top of the CDMA-based mobile phone networks and allows for ISDN-like data transfer speeds up to 144 kbps (1xEV-DO is capable of much higher speeds). Sprint’s PCS Vision and Verizon’s Express Network use this technology. As of this writing, Verizon Wireless is experimenting with 1xEV-DO in two U.S. markets, with testers obtaining data rates between 500 and 800 kbps.
The electromagnetic (EM) spectrum contains many different wavelengths of which the RF spectrum is a small part. Another part of the EM spectrum is infrared light. This light has a longer wavelength than visible light, but a much shorter wavelength than radio or microwave radiation. Infrared is usually linked to body or mechanical heat, as many objects above room temperature emit infrared radiation. These emissions can be seen by night vision equipment.
Infrared is used in television remote controls, because the signal does not interfere with the TV transmission. Remote controls and Infrared Data (IrDA) equipment utilize light-emitting diodes to emit infrared radiation that is then focused by a lens into a narrow beam. The beam is modulated on and off to encode the data transmission.
The IrDA Association publishes specifications that are used by PDA, notebook, and mobile phone device manufacturers for the infrared ports on their devices. IrDA devices typically have a maximum throughput of 4 Mbps. While most mobile devices still have IrDA, many manufacturers are replacing these with Bluetooth.
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