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Cabling is a nightmare that gets worse the more systems that have to be connected. From the home office with a computer connected to a modem, a printer, a scanner and other peripherals, to the large office with thousands of cables strung in the floors or ceilings, the necessity to physically connect every device to at least one other device, if not a network, is a fact of modern high-tech life. The Bluetooth initiate is hoping to change all that by providing a low cost, medium speed radio interface that can be built into a variety of devices, with protocols that can establish connections easily and reliably.
The main competition to Bluetooth (apart from physical cabling) is infrared technology (e.g. IRDA). However, it has achieved only niche success because of the strict need for line-of-sight communications, and the short distances (1-2 meters) that can be covered. Bluetooth should provide a flexible alternative, although data speeds will not be any greater.
Bluetooth is named after Harald Bluetooth, a famous Scandinavian king who united Norway and Denmark. While he had many fine qualities, he was also a remarkably flawed man. His son so hated him, that he led a revolt which proved to be the end of Harald Bluetooth, who got an arrow in the eye (the same fate as befell another King Harold in England at the hands of the Normans in 1066). Perhaps the name is really just a subtle reference to the Scandinavian origins of the technology, within the laboratories of Ericsson. Perhaps one can even see the Ericsson logo as three stylized, blue teeth.
The main aim in referring to Harald Bluetooth, however, is to aim the technology at global dominance. Unlike cellular/PCS technology which is still fragmented into three main branches (TDMA, CDMA and GSM) and has several other smaller, but still living, technologies (e.g. AMPS, TACS and NMT), proponents of Bluetooth realize that the only chance for a low cost technology to survive is a single, global standard.
Bluetooth uses the unlicensed ISM (Industrial, Scientific, Medical) band at 2.400-2.4835 GHz – that is available in most countries. There is some spectrum in Japan, France and Spain that currently cannot be used, but the Bluetooth SIG (Special Interest Group) is attempting to resolve these issues. By using low power (1 mW in client devices and up to 100mW in network access devices) Bluetooth can transmit up to 10 meters (30 feet) between clients and up to 100 meters between a client and an access point operating at higher power. This provides a data link of about 1 Mbps, although this must be shared between up to 8 different devices. Support for asynchronous channels allows more bandwidth to be allocated in one direction (e.g. toward the client for web surfing) than in the other. Bluetooth uses frequency hopping over the entire range of 79 channels (with up to 1600 hops per second, making even the Easter Bunny look positively turtle-like).
Bluetooth was obviously designed with simplicity in mind, to produce the lowest possible cost. The modulation produces one bit per symbol. Although more efficient schemes are possible, they come at the cost of higher complexity. The same frequencies are used for transmit and receive, unlike cellular and PCS systems that use two separate frequencies.
Above the radio layer is a simple protocol known as the Baseband Specification, designed for data exchange and synchronization of the communicating devices. Although heavily customized, it has similarities to the lower layers of protocols like SS7 and X.25.
Bluetooth is based on a Master-Slave model, with one master device surrounded by up to 7 active sycophants. There may be any number of other slaves that are synchronized with the master, but not active. The use of a master-slave relationship obviates the need for collision avoidance schemes, unlike protocols used by Ethernet and cellular/PCS systems where multiple devices can autonomously attempt to access a shared medium at the same time. The model also simplifies the job of clock synchronization because, well ... the master is always right, and the slaves just derive their link clocking from it. One limitation of a master-slave model is that the slaves have no right to initiate a conversation, meaning that some bandwidth will be wasted by the master polling slaves that often have nothing to communicate.
A group consisting of a master and one or more slaves is known as a Piconet, and is completely autonomous with regard to frequency usage, timing and addressing.
Bluetooth links are based on time division duplexing, with a fundamental time-slot of 625 microseconds. A master may initiate a transmission of up to five time-slots, beginning in an even-numbered time-slot, and a slave can initiate a transmission (after being given permission) only in an odd-numbered timeslot.
Not only are Bluetooth links divided into frames by timing, each consecutive frame is transmitted on a different frequency. Each frame has a frequency determined based on the link clocking, although a multi-time-slot frame is always transmitted on one frequency. Frequency hopping is disabled for the duration of such a frame, and resumes with the next frame.
A second sub-layer in the Baseband Specification is the physical link, which divides the raw 1 Mbps bandwidth into multiple channels of two different types – synchronous and asynchronous.
Synchronous channels are most likely used for voice communications and consequently run at 64 kbps (the same speed as used in the PSTN for voice communications over a T1, and much higher than cellular or PCS protocols that generally use 8 kbps or 13 kbps data rates). The term synchronous means that data is continually transmitted. Because synchronous channels need data transmitted at regular intervals, the time-slots for these channels are reserved in advance. All remaining time-slots are available for asynchronous channels.
Asynchronous channels are more applicable for data, and transmit frames only when necessary (hence the name asynchronous). The asynchronous channels in Bluetooth may also be asymmetrical, meaning that the bandwidth in one direction could be as high as 732 kbps, with the other direction running at 58 kbps. Alternatively, both directions could run at 434 kbps. Asymmetric connections would be ideal for web surfing, where much more data is being received from a web server than is being sent to it.
Not only can synchronous channels and asynchronous channels be supported (e.g. a synchronous channel for voice between a wireless phone and a headset, and an asynchronous channel for data from a PC to the wireless phone), but it is possible to set up a mixed synchronous channel that contains some data along with every packet of voice. This type of channel could be very useful for applications involving a wireless phone.
Two different types of channels mean that Bluetooth has to support two different, but integrated channels. For example, synchronous (e.g. voice) packets are never retransmitted and there is no error checking; it is just assumed that the amount of noise introduced by errors in these packets is either small enough that the user will not notice or large enough that the link is unusable. Asynchronous packets (data) will, on the other hand, be retransmitted if in error. This requires the inclusion of error checking data (e.g., a 16-bit CRC). Some Bluetooth frames can contain a mixture of voice and data – but both have to be handled independently. If there are errors in the packets, the transmission of voice continues uninterrupted, while the data must be retransmitted.
One of the major applications planned for Bluetooth, and likely one of the easiest to sell, is the ability to connect personal electronic devices together. For example, Bluetooth could allow a traveler to connect a PC to a cellphone to dial in to the office, or to a printer in the business center or even to eliminate the awkward tail that connects their PC and mouse. Eventually, some public areas may provide Bluetooth access points (e.g. airport lounges) allowing the business traveler faster access to the internet than most wireless phones provide, and even access to business services, such as printers and copiers. Mobile office users who need to work together could use Bluetooth to exchange files, working collaboratively to make last minute changes to a presentation.
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Another area where wireless connectivity could be extremely useful is in home office applications. Currently, cabling a network in a house can be dirty, time-consuming and even dangerous work. The alternative is to leave cabling lying along the floor, which is easy, but presents its own dangers. A wireless network can make the job of creating a hintranet (home intra-net) much easier, safer and quite possibly cheaper. Bluetooth will service the low end of this market need, as its 1 Mbps speed cannot compete with wireless Ethernet systems that operate in the 10-100 Mbps range. However, for people that have relatively simple networking requirements, a shared 1 Mbps link will be more than adequate. A major benefit to Bluetooth will be that it is much more likely to be built into consumer devices, so those who are interested in centralized control of their house may prefer it for that reason (turn up the fireplace, turn down the lights, move a bottle of bubbly from the fridge into the pneumatic drink delivery system ... and honey, sit a little closer).
Bluetooth is unlikely to be able to out-compete other wireless LAN technologies, because a shared 1 Mbps bandwidth simply is inadequate, and a limitation of 8 communicating devices is too small. However, Bluetooth may be combined with other networking technologies, for an optimal networking environment. Printers may be connected to Ethernet running at 10 Mbps or 100 Mbps, but may also have a Bluetooth connection for occasional users, to minimize the laying of cable, or to accommodate visitors to the office. It could be used within a single office or cubicle to connect devices that are used by a single person, such as a scanner, printer, or even a telephone that could be used in a cordless mode, to which the aggregate data requirements will be quite small.
If Bluetooth is a big success, its biggest advantage will be that it will be built into many devices. If a PC is purchased with Bluetooth, the issue will become whether a connection to a higher speed network is worth the cost. For someone who prints only a few pages every day, Bluetooth will not be noticeably slower than more expensive alternatives, but for someone who commonly sends hundreds of pages of complex text or photographic images to a printer, it would clearly be inadequate.
One of the major challenges of a wireless technology used in public places will be security. It would be very convenient to be able to connect a PC wirelessly to a cellphone and then to the internet, but what if the communications can be overheard? Even relatively innocuous-sounding features can act like a Trojan Horse. Consider a business card exchange service. Here, two people get into a conversation while waiting for a train, and decide to exchange virtual business cards using their PDAs. The secure way to do this is to first exchange encryption keys. If done manually, the convenience of the service is totally destroyed, but if done automatically, it would require highly complex cryptologic operations, such as Diffie-Hellman key exchange, which threaten the cost and convenience of the service. Alternatively, it could be considered an innocuous service that does not require security. But, that would mean that someone with a PDA in their pocket could walk through an airport exchanging business cards with every Bluetooth user they pass by. At the end of the day, our villain would have contact information for hundreds of people, and all the victims would have received a fake business card in return.
Bluetooth does provide a security layer, but it is probable that it will not get implemented, because of the cost and complexity of implementing the algorithms, and the time and bandwidth required for managing all necessary keys. Consequently, it may be some time before the use of Bluetooth devices in public areas can be considered secure for people who are concerned about their privacy or about some of the data or conversations they may be transmitting wirelessly.
Bluetooth is intended to be used for a wide variety of applications, so physical connectivity does not mean that two devices share compatible applications. A refrigerator is unlikely to support file transfer, printing or business card exchange, for example. Bluetooth incorporates Service Discovery to allow two devices to find compatible applications.
Even this process could be awkward and time consuming in an environment with many wireless devices, so a limited number of Bluetooth devices may act as Salutation Managers. These devices accept registrations from others and store their service capabilities in a database. Clients can later issue a search request to the Salutation Manager, obtaining in return a list of other clients that support the required function. For example, asking the Salutation Manager for a postscript printer may return two entries, but asking for a color postscript printer may return only one.
The salutation capability was not invented by Bluetooth, but is specified by the Salutation Consortium (www.salutation.org) for use in a variety of networking environments.
Bluetooth is one of a bewildering number of wireless initiatives. While some people are trying to promote it as a wireless elixir, it has distinct limitations. But, it also has unique advantages. If its promise of ubiquitous, cheap, cable replacement can be fulfilled, many people other than Scandinavian history students may learn about the uniting abilities of Bluetooth, whether the technology or the king.
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