Where Forth Art Thou, Bluetooth?

Bluetooth technology encompasses more than just hardware—it also covers software and interoperability factors.

The specification is relatively low speed—effectively under 1 Mbps—and operates in the 2.4 GHz Industrial - Scientific - Medical (ISM) band. Bluetooth transmission uses a frequency hopping, spread-spectrum technique, whereby the signal moves or 'hops' among 79 1-MHz-interval frequencies 1600 times per second (625-microsecond time slots). This results in a signal that is relatively immune from interference. The output signal is low power at 0dBm (1 mW), and has a specified effective range of 10 meters.

Bluetooth technology supports both voice and data channels. For voice, the technology handles up to three simultaneous synchronous voice channels, with each channel supporting a 64 kbit/s rate in each direction. Alternatively, you can have a single channel that handles asynchronous voice and synchronous data. The asynchronous data channel can support an asynchronous signal up to 723 kbits/s or a synchronous signal up to 434 kbits/s.

Bluetooth devices operate in a short-range local wireless network, a piconet or personal area network. The minimum piconet has two Bluetooth devices, a master and a slave, with point-to-point communication between the devices. A piconet can only have one master. A master can communicate with up to seven active slaves in one piconet, all sharing the same channel with the master establishing the hopping sequence for its own piconet (Figure 1). You can also place slaves in a Park mode, with a master supporting up to 255 slaves in this mode (point-to-multi-point communication). While in Park, Slaves do not participate in any activities but a master can synchronize these devices on a periodic basis.

Figure 1:  Bluetooth piconet with one master and three slaves

A scatternet consists of overlapping piconets (Figure 2). You can have a Bluetooth device that participates in several piconets simultaneously, but can only be active in one at a time. A single device can be a slave in several piconets, but can only act as a master in one piconet.

Figure 2:  Bluetooth scatternet comprising two piconets

The wider view of Bluetooth includes using the technology for establishing instant PANs. Two Bluetooth-enabled devices can start to talk to each other as soon as they are within range. Placing a number of such devices within the 10-meter boundary meets the establishment of an ad hoc PAN. Bluetooth allows the user to work within the PAN, sending and receiving data and voice, and also use devices such as cell phones and PDAs as communication gateways to other wireless networks, which use complementary technologies such as CDMA for a WAN or 802.11 for a LAN.

Another problem has been the evolving of target markets for Bluetooth. From an original PC-to-peripheral cable-replacement technology, Bluetooth has grown to encompass usage in PANs and network access points, the latter for support of tasks such as data acquisition, electronic commerce, and control of a home network comprising information, entertainment, comfort, and security applications. Although the widening of Bluetooth markets offers a richer future for the technology, the short-term impact has been detrimental due to uncertainty of where to use it.

Ray Burgess, Motorola's Corporate Vice President and Director, Strategy, Marketing and Communications, suggests several reasons why Bluetooth has not become as pervasive (up to now) as predicted a couple of years ago. First, Burgess thinks that the Bluetooth specification suffers from a "too many cooks" syndrome. The Bluetooth Special Interest Group, who formulates the Bluetooth specification and compliance criteria, comprises almost 2000 members. Burgess believes that so many organizations with, often, diverse opinions regarding the technology specification, have significantly slowed down Bluetooth development and implementation. This reasoning is, oddly enough, also applicable to 802.11x development by the IEEE.

Another reason for slower than expected Bluetooth adoption, according to Burgess, is the complexity of Bluetooth-enabled devices. The combination of difficult technology integration coupled with a very low desired Bluetooth chip cost ($5 or under) has made Bluetooth implementation more difficult than originally envisioned by the Bluetooth-chip vendors. Chip cost is still in the $10-15 range (higher than infrared links) and software adds even more to this figure. The problem will decrease somewhat as Bluetooth chip vendors move from a two-chip to a one-chip product and advancements continue in integrating the Bluetooth software protocol stack with the hardware.

Perhaps the most interesting of Burgess' reasons for slower Bluetooth growth is his comment that Bluetooth enablers worked on "mass-market solutions before components for early adopters." What this statement means is that developers of Bluetooth-enabled chips, with a low target price, concentrated on getting products into the hands of a broad consumer user-base before developing products targeted at initial users. The relatively tolerant initial users of a product serve the purpose of "shaking out" the technology, supplying valuable feedback to the product's developers, allowing them to implement changes and introduce products that will have more appeal to the mass market.

Chip cost has not dropped as rapidly as the technology's proponents had hoped. A contributing factor to these higher prices is that there are 20 different profiles (controlling Bluetooth device behavior) and three link types (which assign transmission modes between communicating devices). For two devices to communicate, each needs a modem layer and a voice/data link. Without these capabilities the devices, even if both are Bluetooth-certified, will not recognize each other. The more profiles on a device, the more expense associated with that device. Luckily, some profiles are not needed by certain types of devices—for example, a sound communication profile makes sense for a cell phone, but unnecessary for a keyboard.

Martin Reynolds, a Gartner Fellow concentrating on computer technology issues, blames the less than initially expected Bluetooth growth on two factors—security and profiles. The initial Bluetooth specification provided security features that were OK for cell phones and similar devices, but not for more security-sensitive equipment, such as PCs. As for the problem with Bluetooth profiles, they just make interoperability between two Bluetooth-enabled devices too difficult. For example, using an MP3 player and wireless headset from the same manufacturer might be fine, but they might not communicate properly if from different manufacturers.

Reynolds thinks that chip cost is progressing satisfactorily towards the $5 price point, but that system cost is still too high. As for Bluetooth 1.2, he doesn't think this version solves the security and interoperability problems but that 2.0 probably will—if it gets out in a timely fashion (meaning, before something comes along to usurp Bluetooth, perhaps a system which better integrates "small" wireless with LANs and WANs).

	The variety of articles you can access on the Web that compare Bluetooth to WiFi and other wireless technologies complicates the Bluetooth message for the consumer—for example, see Bluetooth and Wi-Fi: Understanding These Two Technologies and How They Can Benefit You.

Bluetooth 2.0 will probably handle data rates of 4, 8, and 12 Mbps over the same 10-meter distance at about double the power of devices supporting 1.2. 2.0 will also have a distributed protocol, to allow a piconet to remain in place after a master has left the net. With this protocol, any device in a piconet can be a designated as a supervisor, allowing intra-piconet communication between devices without identification of a master.

As for chip implementation, look for Bluetooth systems to migrate from two chips to one, as RF and digital-control combine on a single piece of silicon. Ultimately, this will allow price migration towards the $5 price point, still considered necessary for widespread adoption of the technology in wireless equipment. New applications will continue to appear, as WLAN technology becomes more pervasive, requiring a companion technology, such as Bluetooth, to handle the "last meter" of a network.

Jim Lipman is a consultant providing marketing, writing, and other electronics industry services, specializing in EDA tools and ASIC/SoC design methodologies. His job experience includes chip-design R&D, marketing, marcom, technical editing, and on-line publishing of technical content for engineers.