Bluetooth has offered the promise of real connectivity. But what exactly is it?
Over the past few years, most people with an interest in technology will have stumbled across the term 'Bluetooth'. This article provides some examples of Bluetooth technology and describes what happens not only when it works, but what might be going on when it does not as well.
What does 'Bluetooth' mean?
'Bluetooth' is referred to as a cable replacement technology, but the profiles make it much more than that. It is a combination of radio hardware and different sorts of software. Defining Bluetooth continues to be a substantial task; it would have been gargantuan if it did not build on existing specifications including DECT, Infrared, GSM and many telephony standards.
The cable replacement part of Bluetooth is initially being manufactured as a module, which contains a selection of hardware and software. To a user, it may not matter what the module looks like unless it has to be replaced. Of a module's two interfaces, the radio looks less complex because there are only two choices. It can be an RF connector or an antenna. The experts in the Bluetooth special interest group (SIG) have established how the radio should work.
A module's digital interface has many more variants. There is a SIG-defined host controller interface, but designs are often broken into somewhat different blocks for cost or technology reasons. This is especially true if the host device already has signal processing hardware.
In the same way new software is added to a PC to make it do something new, Bluetooth solutions may need software upgrades to get them running. It will take some care to get device configuration right.
Example one: The ultimate headset
One way to go hands-free in the car is to use a Bluetooth headset and Bluetooth-enabled phone. It is referred to as 'K6', a good example of one of the dozen or so Bluetooth profiles or use cases, and a simple way to realise that a little thought about performance is required when using a Bluetooth link. A phone may have a Bluetooth battery pack, but if it originally came as a data transfer package with a compact flash card, it may not support audio yet. So it would not qualify as an audio gateway in Bluetooth terminology, and would not work with a headset.
In the headset, the module, antenna, audio and digital circuits and all necessary software are fully integrated. The user interface includes a button or two, the microphone and an earpiece along with, perhaps, an LED.
With batteries in the headset, attention can be turned to the phone. Bluetooth can be added to a cellphone in a few different physical and electrical ways. A new battery pack is an easy way to upgrade an existing phone. Because it helps isolate certain functions, it will serve as the next example. Did I say not to worry about modules? Perhaps I fibbed. Features depend on the module inside the battery pack. The power class is an obvious and important differentiator. There are three power classes, with a Class 1 device being the highest and most flexible. It provides a maximum of 100 mW of RF power, which is enough to light a small torch bulb. This gives it greater distance coverage, and would be needed if a cellphone were also desired to act as a cordless phone or 'walkie-talkie'. These are two more profiles that Bluetooth can support, but do not necessarily come with a purchase. When the battery pack is replaced, the phone does not yet know it has a new way of talking to the outside world. The software inside the phone needs to be upgraded. If the user does not need to take it to a supplier, it could be the last time the 'butt' plug is needed for PC hook-up.
It works
When the phone is turned on, it knows it can speak in a new way, but still does not know if there is anyone to talk to. Either when told to or automatically, it will start acting as a Bluetooth master transmitting short messages, inquiring if any other devices are within range. Like modern, nonCDMA, cellphones and cordless phones, the radio transmits all information as bursts of digital data. Figure 1 shows what these bursts look like over time. They are short and would make a fast (800 Hz) buzzing sound if they could be heard as a GSM phone sometimes can when it interferes with an audio system.
Inquiry is a critical step, rather like the process needed to get a cordless phone to recognise its base station. Since it turned on, the headset has been periodically listening, or scanning. A button may have been pressed to activate the scanning. Within a minute, the phone should receive a message from the headset. The phone display will show what it has found. If away from home, it may well be more than just the headset. Every Bluetooth device should be programmed with its own unique address, and a few button pushes will let the phone know whom it should consider friends. Once the phone knows, and has stored, the headset address, it is ready to use the audio link. At this stage, it is more like a cabled earpiece and microphone, but the radio gets busier.
Unlike computers, humans cannot work with random breaks in speech. The headset and phone need to use a synchronous connection. If the link gets noisy, some speech is simply lost or the receiver has to be smart enough to fill in the gaps. There are three ways to transmit the audio signal, with the delta modulated CVSD being the least prone to continuous noise and the one the headset must use. It is not as fancy as the tricks played by cellphones because that would take battery current and add costs that Bluetooth tries to avoid. Standard AT modem commands are used to make and break the phone call placed over the Bluetooth link. Bluetooth interoperability testing should ensure that a user is unaffected by most of this complexity. However, some profile features are optional, so one may still want to check what the device is supposed to do.
If the headset is left on after phoning, both it and the Bluetooth part of the phone can do some clever things to save battery current. The sleep modes involve listening and talking less frequently. If there were any way to measure battery current, one could see how it went up and down over several seconds. Next time the phone is switched on, it will automatically look for the same headset, and use it as told.
It does not work
As illustrated, one must start with the correct expectations. Especially when using devices that did not arrive as a pair, it is important to check what functions they support.
The headset software may be able to run self-tests. Some Bluetooth radio designs are highly sophisticated and allow internal signal path loops; so the LED being lit may tell a user a lot more than that the battery is OK. Frequently, however, the engineering effort to make user troubleshooting easy gets eaten up when adding features. Therefore, the user needs some other tools, as well as a little knowledge of what should be happening.
A major part of the problem is one of not being able to see what is going on. Any kind of software diagnosis in the cellphone/headset combination is impracticable, but one can deduce whether the right things are happening when looking or listening to the RF. In some cases, unwanted interference can be put to good use.
The examples below use an RF detector {eg HP8782B) connected to an ordinary Bluetooth antenna and an oscilloscope. If these are unfamiliar, the simplest tool is probably an FM radio, but it is impossible to predict how useful the result will be. One could get some idea by manually tuning slightly offchannel to an FM station and listening to what happens when switching on and making a call with a nonCDMA cellphone. If the radio starts buzzing, it may be able to detect a Bluetooth radio when placed close to it. Portable cassette players are worth a try too.
In this example, when the phone is looking for new devices, it transmits the short inquiry bursts of Figure 1. Although the channel (frequency) hops randomly on every burst, the RF detector will pick up all of them.
Leave the headset turned off. If the Bluetooth function in the phone is working as expected, the inquiry bursts should keep going for about 10 s. If there is no RF transmission, software revisions should be checked, and beyond that, a trip back to the store is necessary. When the headset hears the phone inquiring, it transmits a few bursts of its own back to the phone. If the antenna is moved next to the headset, these can be captured with a triggered oscilloscope.
Example two: file transfer
This profile (K12) is different from the headset, and the differences may give rise to unexpected behaviour. Terms such as client-server and object push and pull come into play. Radio is subject to eavesdroppers. Bluetooth security can be increased with use of a PIN. This determines the link keys, which in turn are part of what is used to encrypt transmitted data. Sometimes a bundled solution may have link keys pre-determined, so the user has to find and enter them to make it work. The other big difference in file transfers is the way data are sent over the radio. It is now essential that data be transferred correctly. Time is of secondary importance. An asynchronous connection-less link is used. It is a packet-based method, like the Internet. Several mechanisms are used, including changing of the size of packets and re-transmitting of corrupted packets.
Transmit power levels may be continuously adjusted to reduce the effect of noise. All this gives useful performance indicators and can be seen using the RF detector. One can get a good idea why file transfers take longer, or battery life is shorter than expected.
It works
The initial inquiry process is the same as the headset uses, with the addition of link keys.
With two flexible user interfaces, it is likely a user will be able to get a reasonable amount of information about the status of the Bluetooth function in both organiser and PC. If one tries to transfer a multimegabyte file, the radio transmission should look like those in Figure 2. This is a multislot burst, which makes more efficient use of transmit time.
The direction of file transfer determines which transmitter is on the most. Note how for every burst of data that is sent, a much shorter null, or 'it arrived', burst is sent in response.
It does not work as well as it should
Once basic configuration problems are resolved, the most likely questions will revolve around time, distance and battery life. Subtle configuration problems may affect these, for example, if the link is not set up the way it should be. Noise or specific types of interference may be present. Class 1 devices may not always be instructed to reduce power. The server could be servicing another client at the same time.
Observing the waveform from the RF detector will give some clues. Figure 3 shows what happens as the link degrades, in this case because the author's hand was placed firmly over one of the antennas. First, the long bursts get changed to the shortest type, DH1. If a burst becomes corrupted, the link has not invested so much time in sending it. If the burst is not believed to be good, the return null burst is not sent - as shown at the bottom right-hand-side of Figure 3. This can be spotted on the oscilloscope, and is an indication of packet error rate.
Bad bursts are re-transmitted again and again, until a timeout expires. Depending on the nature of the interference, the transfer could become really slow.
One final idea - if it is possible, try moving the antenna for the (desktop) PC. The metal back of a PC tower is a pretty nasty RF environment. It means using a cable (just when you thought they were destined for the trash can), but it could make quite a difference in how a Bluetooth link performs.
More comprehensive testing
These simple techniques will allow basic troubleshooting. If a more thorough look into device performance is required, the first step is to consider if the interest is application- or radiobased. Equipment is available to explore most topics from radio parameters to profile evaluation. Details can be found on the Bluetooth website at www.bluetooth.org.
Conclusion
Bluetooth can be made simple for the user, even though it is a complex system in operation. It is a combination of software and hardware that needs to work together to make use of the central part of Bluetooth - the radio link. By making simple observations of the radio transmissions, it is possible to deduce where problems lie when it does not work at all or not as well as expected. More complex test techniques can provide the information to understand what needs to change to make it work.
For further information on test equipment for your needs contact Concilium Technologies, 011 678 9200.
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