Monday, March 07, 2005

How Bluetooth Cuts The Cord

Bluetooth is a quaint sounding wireless networking standard that solves the problem of last-inch connectivity. Let's face it, there are lots of standards for last-mile connectivity including DSL, Cable Modem Internet, PON (Passive Optical Networks), T1, BPL (Broadband over Power Line), and even WiFi. Well, WiFi is really more of a last-foot connectivity solution designed to eliminate Ethernet wiring within a home or office. Bluetooth is more likely to eliminate USB and parallel printer cables within a room. It also eliminates other short wires, such as the tiny but annoying cable that links a headset to a cellular phone.

But why call it Bluetooth? It's not a joke and it's not just because engineers got tired of those arcane acronyms like USB and IrDA. No, the serious reason is that it was named after a Danish king who united Denmark and Norway just before the turn of the first millennium. His name translated into English sounds like "blue tooth." Symbolically, the Bluetooth communications standard unites disparate devices like cell phones and computers.

If that's not quaint enough for you, consider that one of the primary technologies behind the Bluetooth transmission standard is FHSS or Frequency Hopping Spread Spectrum, based on a patent issued to a stunning 1930's film actress and a noted music composer who were trying to invent a secret military radio control system for torpedoes. I'm serious. I relate that story in "Thank Hedy Lamarr for Bluetooth."

Moving right along to the Bluetooth protocol itself... This is a very low power wireless networking scheme that draws as little as a milliwatt and has a maximum range of about 32 feet. That's more than enough range for a keyboard, mouse, desktop printer, PDA, laptop computer or cell phone. All of these devices could theoretically link together via Bluetooth. By keeping the power low, the component size is small and the battery consumption is low. That's just what you want in a cord replacement.

Bluetooth really is a networking standard. Instead of a LAN or Local Area Network, it's a PAN or Personal Area Network. Bluetooth devices establish what are known as piconets. A piconet contains a minimum of two devices and a maximum of 8. No manual intervention is needed. The process of setting up the network is completely automatic when Bluetooth enabled devices are within range of each other. One device assumes the role of the master and invites other nearby Bluetooth enabled devices to join the net as slaves. Once all 8 available slots are filled, no other device can join. The master and the slaves take turns communicating in a round-robin scheme. Communications between slaves must be sent via the master and not directly.

All of this is going on at a data rate of 1 Mbps for the standard Bluetooth and up to 3 Mbps for Bluetooth version 2.0. They are compatible standards and run at a speed that the slowest device in the piconet can keep up with. Deducting overhead in the transmission protocol, the basic communications rate is around 720 Kbps. There are options including half-duplex, full duplex, asynchronous connectionless and synchronous connection oriented links. The data bits can be information, digital control words or even two-way audio at 64Kbps. That's perfect for telephone applications, as 64Kbps is the legacy standard for toll quality digitized voice.

Bluetooth operates smack in the middle of the unlicensed 2.4 GHz ISM (Industrial, Scientific and Medical) band. If that has a familiar ring, it's because WiFi uses the same frequencies. What keeps them from clashing is different modulation schemes. Bluetooth deliberately picked a frequency hopping scheme to avoid interfering or being interfered with. It switches randomly among 79 channels at a rate of 1,600 times per second. Only devices on a particular piconet are synchronized to hop to the same frequencies at the same time. This greatly reduces the chances of noise or other transmitters blocking out the entire data stream. If bits are lost on once channel, they can be resent on another.

What are typical uses for Bluetooth? A popular application is wireless headsets for cell phones. If your phone has Internet capability, a Bluetooth piconet can be established between your phone and nearby laptop computer to give the computer Internet access as well. Bluetooth enabled printers can print pictures from a cell phone or camera that has Bluetooth without needing any wires. Likewise, a Bluetooth enabled PDA can synchronize with a Bluetooth enabled cell phone, laptop or desktop computer. As devices that meet the 2.0 standard become more commonly available, the higher throughput will be used for wireless audio components and appliances as well. It seems likely that Bluetooth will replace infrared links that need direct line of site and perhaps the bulk of interface cables we're so accustomed to.

Find Bluetooth enabled cell phones and cellular service plans at Cell Phone Plans Finder.

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