Bluetooth and IEEE 802.15.1: Wireless Personal Area Networks (WPANs)

Bluetooth^® and IEEE 802.15.1: Wireless Personal Area Networks (WPANs)

Bluetooth^® uses Frequency Hopping Spread Spectrum techniques and can coexist with IEEE 802.11b devices, and it serves a completely different application. The interference with a DSSS-based IEEE 802.11b WLAN is going to be minimal, but it's possible that two FHSS-based technologies operating in the same proximity would cause each other grief. In an effort to mitigate the effects of proximate operation, the IEEE LMSC has created the IEEE 802.19 TAG to study "coexistence" issues.

Bluetooth^® was conceived as a very low power, and thus short-range (i.e., 3 to 10 meters), wireless "cable replacement" to enable devices to communicate in very small ad hoc networks (termed "piconets"). A Bluetooth^® piconet can have up to a total of eight devices, with one being a master and the others known as slaves.^[2] A set of proximate piconets can merge into what's known as a "scatternet" by the fact that each Bluetooth^® station can be a slave in one piconet, and be a master in another.

^[2] The author would have preferred if the Bluetooth^® SIG had chosen less socially charged naming, perhaps "primary" and "secondaries" instead of "master" and "slaves." However, it is a fact that the standard does use these terms.

The "race" between IEEE 802.11b and Bluetooth^® that was covered in the trade press would have had readers believe that only one could survive. This type of overly simplistic "winner takes all" analysis is not limited to the trade press…. It is especially telling that few people look at 2.4 GHz cordless phones and imagine that they compete with WLAN devices^[3] (except in the sense that they can both use the same band of RF spectrum). In fact, microwave ovens and other devices, such as new forms of lighting based on using microwaves to excite glowing plasma, all share this 2.4 GHz ISM band.^[4]

^[3] In the near future, ultra-portable WLAN devices that look a lot like cellular phones will support VoIP over IEEE 802.11 WLANs. Once such products exist, then a comparison against cordless phones will be meaningful.

^[4] For more information on this new type of lighting, do a Web search on "RF lighting."

The standardization of Bluetooth^® began in an ad hoc fashion with an industry group known as the Bluetooth^® Special Interest Group (Bluetooth^® SIG), and the specification grew quite complex because certain members of the SIG wanted to endow Bluetooth^® with LAN-like capabilities. As it was originally conceived, Bluetooth^® had no "broadcast" capability, which was not a limitation in its target application as a cable replacement technology. Because of its intended application in the cell phone world, it supported synchronous data transfer, so that voice communications between a cell phone and a wireless headset would have acceptable quality, theoretically on par with what a wire could deliver.

Had the scope of the specification been limited to the original vision, of a short-range cable replacement supporting wireless point-to-point "links," interoperable Bluetooth^®-enabled devices may have entered the market much sooner. As it happened, the first Bluetooth^® "interoperability" event was attempted in late March 2001 at the giant CeBIT^[5] trade show in Hannover, Germany. At this trade show, a very spectacular public relations disaster happened when the high-profile new technology, Bluetooth^®, did not work as advertised. A Bluetooth^® network that was meant to span an entire exhibit hall did not work as advertised, primarily because of the fact that each vendor was using a slightly different version of the standard, and interoperability was just not possible at the time.^[6]

^[5] CeBIT is a German acronym that stands for "Centrum für Büro, Information und Telekommunikation" (which may be loosely translated into English as [World] "Center for Office, Information, and Communication" [Technology]. CeBIT is one of the largest trade shows in the world, perhaps the largest.

^[6] For more information on this event, one can do a Web search on "CeBIT Bluetooth demo 2001."

Rather than leave the reader with the impression that Bluetooth^® is a failed technology, if we fast-forward to 2003, we find many late-model cell phones with Bluetooth^® support, such as for "connecting" to a wireless headset, or to exchange data with another phone, or a PC. The standard for Bluetooth^® has now stabilized to the point where multiple interoperable implementations are the norm, not the exception. It would appear that Bluetooth^® has found its niche in the technology marketplace, but getting to this point was not easy!

While the Bluetooth^® SIG may have created the initial Bluetooth^® specification, the Bluetooth^® SIG is not a legally recognized standards body, and so its specification(s) would be effectively de facto standards. To make them official de jure standards, the IEEE LMSC created a WG IEEE 802.15 to formally standardize Bluetooth^® and related WPAN technologies. The first of the standards from this WG has already been published: IEEE 802.15.1-2002, which was "derived from the Bluetooth^® core, profiles, and test specifications" and contains "unaltered or minimally altered text of the Bluetooth specifications." To summarize the features of Bluetooth^®, here is an excerpt from IEEE 802.15.1-2002, Clause 8.1:^[7]

^[7] Excerpted from IEEE Std. IEEE 802.15.1-2002, copyright 2002. All rights reserved.

Bluetooth^® is a short-range radio link intended to replace the cable(s) connecting portable and/or fixed electronic devices. Key features are robustness, low complexity, low power, and low cost.

Bluetooth^® operates in the unlicensed ISM band at 2.4 GHz. A frequency hop transceiver is applied to combat interference and fading. A shaped, binary FM modulation is applied to minimize transceiver complexity. The symbol rate is 1 Msymbol/s. A slotted channel is applied with a nominal slot length of 625 µs. To emulate full duplex transmission, a Time-Division Duplex (TDD) scheme is used. On the channel, information is exchanged through packets. Each packet is transmitted on a different hop frequency. A packet nominally covers a single slot, but can be extended to cover up to five slots.

The Bluetooth^® protocol uses a combination of circuit and packet switching. Slots can be reserved for synchronous packets. Bluetooth can support an asynchronous data channel, up to three simultaneous synchronous voice channels, or a channel that simultaneously supports asynchronous data and synchronous voice. Each voice channel supports a 64 kb/s synchronous (voice) channel in each direction. The asynchronous channel can support maximal 723.2 kb/s asymmetric (and still up to 57.6 kb/s in the return direction), or 433.9 kb/s symmetric.

The IEEE 802.15.2 draft standard is entitled "Telecommunications and Information exchange between systems Local and metropolitan area networks Specific Requirements - Part 15.2: Coexistence of Wireless Personal Area Networks with Other Wireless Devices Operating in Unlicensed Frequency Bands," and it is intended to help devices based on IEEE 802.15.1, and other forthcoming IEEE 802.15 standards, coexist with other devices in this band.