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  3. Bluetooth When you use computers, entertainment systems or telephones, the various pieces and parts of the systems make up a

Bluetooth When you use computers, entertainment systems or telephones, the various pieces and parts of the systems make up a

admin2010-04-30  67
问题                                          Bluetooth
    When you use computers, entertainment systems or telephones, the various pieces and parts of the systems make up a community of electronic devices. These devices communicate with each other using a variety of wires, cables, radio signals and infrared light (红外线) beams, and an even greater variety of connectors and plugs.
    The art of connecting things is becoming more and more complex every day. And Bluetooth has now become a new popular method of connecting devices that can streamline the process. A Bluetooth connection is wireless and automatic, and it takes small-area networking to the next level by removing the need for user intervention and keeping transmission power extremely low to save battery power.
Bluetooth Operation
    Bluetooth networking transmits data via low-power radio waves. It communicates on a frequency of 2.45 gigahertz (千兆赫) (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 (微型监视器), garage-door openers and the newest generation of cordless phones (无绳电话) all make use of frequencies in the ISM band.
    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 wall 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 (扩展频谱) frequency 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 (传输设备) 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.
Piconets (个人域网络)
    Let’s say you have a typical modern living room with the typical modern stuff inside. There’s an entertainment system with a stereo, a DVD player, a satellite TV receiver and a television; there’s also a cordless telephone and a personal computer. Each of these systems uses Bluetooth, and each forms its own piconet to talk between the main unit and peripheral (外围设备).
    The cordless telephone has one Bluetooth transmitter in the base and another in the handset.  The manufacturer has programmed each unit with an address that falls into a range of addresses it has established for a particular type of device. When the base is first turned on, it sends radio signals asking for a response from any units with an address in a particular range. Since the handset has an address in the range, it responds, and a tiny network is formed. Now, even if one of these devices should receive a signal from another system, it will ignore it since it’s not from within the network. The computer and entertainment system go through similar routines, establishing networks among addresses in ranges established by manufacturers. Once the networks are established, the systems begin talking among themselves. Each piconet hops randomly through the available frequencies, so all of the piconets are completely separated from one another.
    Now the living room has three separate networks established, each one made up of devices that know the address of transmitters it should listen to and the address of receivers it should talk to. Since each network is changing the frequency of its operation thousands of times a second, it’s unlikely that any two networks will be on the same frequency at the same time. If it turns out that they are, then the resulting confusion will only cover a tiny fraction of a second, and software designed to correct for such errors weeds out the confusing information and gets on with the network’s business.
Bluetooth Security
    In any wireless networking setup, security is a concern. Devices can easily grab radio waves out of the air, so people who send sensitive information over a wireless connection need to take precautions to make sure those signals aren’t intercepted. Bluetooth technology is no different — it’s wireless and therefore susceptible to spying and remote access, just like WiFi is susceptible if the network isn’t secure. With Bluetooth, though, the automatic nature of the connection, which is a huge benefit in terms of time and effort, is also a benefit to people looking to send you data without your permission.
    Bluetooth  offers  several  security  modes,  and  device  manufacturers determine with mode to include in a Bluetooth-enabled gadget (配件). In almost all cases, Bluetooth users can establish trusted devices that can exchange data without asking permission. When any other device tries to establish a connection to the user’s gadget, the user has to decide to allow it. Service-level security and device-level security work together to protect Bluetooth devices from unauthorized data transmission. Security methods include authorization and identification procedures that limit the use of Bluetooth services to the registered user and require that users make a conscious decision to open a file or accept a data transfer. As long as these measures are enabled on the user’s phone or other device, unauthorized access is unlikely. A user can also simply switch his Bluetooth mode to "non-discoverable" and avoid connecting with other Bluetooth devices entirely. If a user makes use of the Bluetooth network primarily for synching devices at home, this might be a good way to avoid any chance of a security breach while in public.
    Still, early cell-phone virus writers have taken advantage of Bluetooth’s automated connection process to send out infected files.  However, since most cell phones use a secure Bluetooth connection that requires authorization and’ authentication before accepting data from an unknown device, the infected file typically doesn’t get very far. Other problems like "bluejacking", "bluebugging" and "Car Whisperer" have turned up as Bluetooth-specific security issues. Like a computer security hole, these vulnerabilities are an inevitable result of technological innovation, and device manufacturers are releasing firmware upgrades that address new problems as they arise.
Any two networks in piconets is unlikely be on the same frequency at the same time because each network ______ thousands of times a second.
选项
答案is changing the frequency
解析
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