How Bluetooth Works

[batishv]

Bluetooth networking transmits data via low-power radio waves. It communicates on a frequency of 2.45 gigahertz (actually between 2.402 GHz and 2.480 GHz, to be exact). This frequency band has been set aside by international agreement for the use of industrial, scientific and medical devices (ISM).

A number of devices that you may already use take advantage of this same radio-frequency band.Baby monitors, garage-door openers and the newest generation of cordless phones all make use of frequencies in the ISM band. Making sure that Bluetooth and these other devices don't interfere with one another has been a crucial part of the design process.
One of the ways Bluetooth devices avoid interfering with other systems is by sending out very weak signals of about 1 milliwatt. By comparison, the most powerful cell phones can transmit a signal of 3 watts. The low power limits the range of a Bluetooth device to about 10 meters (32 feet), cutting the chances of interference between your computer system and your portable telephone or television. Even with the low power, Bluetooth doesn't require line of sight between communicating devices. The walls in your house won't stop a Bluetooth signal, making the standard useful for controlling several devices in different rooms.
Bluetooth can connect up to eight devices simultaneously. With all of those devices in the same 10-meter (32-foot) radius, you might think they'd interfere with one another, but it's unlikely. Bluetooth uses a technique called spread-spectrum frequency hopping that makes it rare for more than one device to be transmitting on the same frequency at the same time. In this technique, a device will use 79 individual, randomly chosen frequencies within a designated range, changing from one to another on a regular basis. In the case of Bluetooth, the transmitters change frequencies 1,600 times every second, meaning that more devices can make full use of a limited slice of the radio spectrum. Since every Bluetooth transmitter uses spread-spectrum transmitting automatically, it’s unlikely that two transmitters will be on the same frequency at the same time. This same technique minimizes the risk that portable phones or baby monitors will disrupt Bluetooth devices, since any interference on a particular frequency will last only a tiny fraction of a second.

When Bluetooth-capable devices come within range of one another, an electronic conversation takes place to determine whether they have data to share or whether one needs to control the other. The user doesn't have to press a button or give a command -- the electronic conversation happens automatically. Once the conversation has occurred, the devices -- whether they're part of a computer system or a stereo -- form a network. Bluetooth systems create a personal-area network (PAN), or piconet, that may fill a room or may encompass no more distance than that between the cell phone on a belt-clip and the headset on your head. Once a piconet is established, the members randomly hop frequencies in unison so they stay in touch with one another and avoid other piconets that may be operating in the same room. Let's check out an example of a Bluetooth-connected system.

 

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[Android]

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[Sourabh]

Bluetooth A2Dp


even, at least for a while, the power cord. A telephone with a cord now seems almost antique.
Unwired audio has evolved from the transistor radio, to the Walkman, to the iPod, but until recently these snazzy gadgets still had one pesky cord--the one leading to the listener's eager ears. A new technology dubbed A2DP promises to ditch that last tether, finally allowing unplugged audiophiles to completely free their heads.

The Need


Headphones are great for privacy. Sleepy spouses and neighbors certainly appreciate them, and listening to your tunes on a bus without them is a serious faux pas. But try moving around with them, and you quickly discover their limitations. Headphone cords seem to seek out things to snag--a branch, a fellow pedestrian, a car door handle, even your own feet if the cord's long enough. A2DP headphones contain a wireless receiver and their own batteries, so they need no physical connection to the music player.

Technology


Advanced Audio Distribution Profile (A2DP) is a set of standards for devices using Bluetooth, the same radio technology used in cell phone headsets and wireless computer keyboards, among other things. A2DP devices offer a one-way stream of high-quality mono or stereo sound, unlike the more familiar Bluetooth headsets, designed for phone calls, that use a two-way connection.
Unlike older technology for wireless headphones, A2DB is digital; the headset is "paired" with the device sending, so you can have multiple headsets in range of each other without cross-talk.

Products


Headphones for portable players are by far the most popular A2DP devices, but the technology does not end there. Many laptop and desktop computers support A2DP, and adapters can be fitted to car stereos to create new opportunities for back-seat DJs. A2DP speakers allow your music to follow you around the house without blaring in the rooms where you aren't.
Potential


A2DP is fairly new technology, with potential uses far beyond untethered music. A2DP's high-quality audio stream is one-way, but it works in either direction; a camcorder, for example, could use A2DP to connect to wireless microphones. Because A2DP allows multiple simultaneous streams in the same place, headsets in movie theaters, stage shows or museums could provide translations in several languages at once, or assistance for the blind.
Warning


Many jurisdictions prohibit drivers from wearing headphones, but allow (or even require) hands-free phone headsets. Where A2DP headsets will fall remains unclear, and will likely vary from place to place. Be sure you know the law where you are. As with any headset, remain attentive to and aware of your surroundings, whether as a driver, cyclist or pedestrian.

 

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