Allowing for Interference

Allowing for Interference
Wireless means a radio link—and radio links are subject to interference.
Interference can impact both the quality of an audio (Synchronous Connection
Oriented [SCO]) connection or the throughput of a data (Asynchronous
Connectionless [ACL]) connection. High levels of interference can interrupt
communications for long enough to cause the protocol stack to timeout and
abandon the link altogether. Although this is addressed in the Bluetooth

Figure 1.4 A Wireless HAN for AV Control and Distribution
8 Chapter 1 • Introducing Bluetooth Applications
Specification with a frequency-hopping scheme which does provide robustness, it
is still a serious consideration for some applications.
Bluetooth technology should not be used for safety-critical applications
where data absolutely must get through, because there is always a possibility of a
burst of interference stopping the link. Interference can come from a variety of
sources: microwave ovens, thunderstorms, other communications systems (such as
IEEE 802.11b), even other Bluetooth devices in the area (although these will not
have a great effect as they are designed to cope with interference from one
another in normal use).
It is possible to overcome the problem of link failure. For example, if you are
relying on a Bluetooth link to monitor your baby and you know the environment
is such that the link will only fail approximately once a week, then you
might be happy to have the receiver alert you when the link fails. Once a week
you may be out of touch, but an alert will let you know that the link has failed,
so you have the option of returning within earshot of the infant. Since the
Bluetooth links only operate up to around 100 meters, it shouldn’t take you too
long to get there!
There are other safety-critical applications where an unreliable link may be
acceptable. An example is a system developed for Nokian tires, which allows tire
pressure to be automatically monitored and sent to the car dashboard display.A
wireless link will be subject to frequent failures in the harsh automotive environment,
but the link can be re-established. Even if it only works a tenth of the time,
it is still checking tire pressures far more often than will the average motorist!
Here again, the system could be set to alert the driver if the tire pressures have not
been reported recently.This way the driver knows that a manual check is needed.
So far, we have looked at effects of the Bluetooth link receiving interference,
but, of course, it can also interfere with other devices. Bluetooth devices are obviously
completely unsuitable for use in an environment where the Bluetooth link
would interfere with sensitive control equipment—an aircraft being the primary
example. Interference issues are explained in more depth later in this chapter.
Considering Connection Times
With a radio link, although the connections can be unconscious, connection
times can be lengthy as transmitters and receivers all need to synchronize before
communication can commence.These limitations could have serious consequences
if the wireless link was of a critical nature—for example, a “panic
button,” a life-dependant medical monitor, or an engine management system.
There are two delays in setting up a Bluetooth link. First, it takes time to discover
devices in the neighborhood. In device discovery, a device sends out
inquiry packets, and receives responses from devices in the area, then reports these
to the user. It can take ten seconds to find all the devices in an area, and even
then you will only find those devices which are willing to report their presence.
Some devices may not be set to scan for inquiries, in which case you will never
find them!
A second delay occurs when you set up the connection itself.Again, this can
take up to ten seconds.This lengthy connection time means that Bluetooth
devices are unsuitable for systems where a fast response is needed, such as automatic
toll collection on busy roads.
Coping with Limited Bandwidth
Wireless can also mean “slower.” An Internet connection via a Bluetooth LAN is
limited to the maximum data rate (723.2 Kbps) over the air interface. After
allowing for management traffic and the capacity taken up by headers for the various
protocol layers, even less is available to applications at the top of the stack.
This will not compete with a high-speed wired link.Thus, for sending or downloading
vast amounts of data, a Bluetooth wireless connection would not be the
optimum method.
This also impacts on audio quality: Bluetooth technology simply does not
have the bandwidth for raw CD quality sound (1411.2 Kbps). However, if a suitable
compression technique is employed (using MP3 to compress an audio stream
down to 128 Kbps, for example), it is feasible to use an ACL link for high-quality
audio.The quality of a Bluetooth SCO link is certainly not high quality—it is
approximately equivalent to a GSM telephone audio link (64 Kbps).
Compression can be useful for data devices. If large amounts of data are to be
sent, using a compressed format will obviously speed up transfer time.
Considering Power and Range
Power is a critical consideration for wireless devices. If a product is to be made
wireless, unleashed from its wired connection, where will its power come from?
Often the communication cable also acts as a power cable.With the cable gone,
the subject of batteries is brought into focus, and the inevitable questions arise
concerning battery life, standby time, and physical dimensions.
Some devices, such as headsets, have no need for power when they are connected
with wires. Audio signals come down a wire and drive speakers directly; a

10 Chapter 1 • Introducing Bluetooth Applications
very simple system with no need of extra power connections.When the wires are
replaced with a Bluetooth link, suddenly we need power to drive the link, power
to drive the microprocessor that runs the Bluetooth protocol stack, and power to
amplify the audio signal to a level the user can hear.With small mobile devices
you obviously do not want to install huge batteries, so keeping the power consumption
low is an important consideration.
Deciding on Acceptable Range
The Bluetooth specification defines three power classes for radio transmitters
with an output power of 1 mW, 2.5 mW and 100 mW.The output power defines
the range that the device is able to cover and thus the functionality of your
product must be considered when deciding which power class to use.The user
would not want to have to get up from his desk to connect to the LAN and
therefore requires a higher power radio. Conversely, a cellular phone headset is
likely to be kept close to the phone, making a lower range acceptable, which
allows smaller batteries and a more compact design.Table 1.1 details the respective
maximum output power versus range.
Table 1.1 Bluetooth Radio Power Classes
Power Class Max Output Power Range
Class 1 100 mW 100 meters+
Class 2 2.5 mW 10 meters
Class 3 1 mW 1 meter
It is important to realize that the range figures are for typical use. In the
middle of the Cambridgeshire fens, where the land is flat and there is not much
interference, a Class 1 device has been successfully tested at over a mile. But in a
crowded office with many metal desks and a lot of people, the Bluetooth signal
will be blocked and absorbed, so propagation conditions are far worse and ranges
will be reduced.
Recognizing Candidate Bluetooth Products
Taking into account the preceding sections, we can see that for a product to be a
candidate for Bluetooth technology, it needs to adhere to the six loosely defined
There are two delays in setting up a Bluetooth link. First, it takes time to discover
devices in the neighborhood. In device discovery, a device sends out
inquiry packets, and receives responses from devices in the area, then reports these
to the user. It can take ten seconds to find all the devices in an area, and even
then you will only find those devices which are willing to report their presence.
Some devices may not be set to scan for inquiries, in which case you will never
find them!
A second delay occurs when you set up the connection itself.Again, this can
take up to ten seconds.This lengthy connection time means that Bluetooth
devices are unsuitable for systems where a fast response is needed, such as automatic
toll collection on busy roads.
Coping with Limited Bandwidth
Wireless can also mean “slower.” An Internet connection via a Bluetooth LAN is
limited to the maximum data rate (723.2 Kbps) over the air interface. After
allowing for management traffic and the capacity taken up by headers for the various
protocol layers, even less is available to applications at the top of the stack.
This will not compete with a high-speed wired link.Thus, for sending or downloading
vast amounts of data, a Bluetooth wireless connection would not be the
optimum method.
This also impacts on audio quality: Bluetooth technology simply does not
have the bandwidth for raw CD quality sound (1411.2 Kbps). However, if a suitable
compression technique is employed (using MP3 to compress an audio stream
down to 128 Kbps, for example), it is feasible to use an ACL link for high-quality
audio.The quality of a Bluetooth SCO link is certainly not high quality—it is
approximately equivalent to a GSM telephone audio link (64 Kbps).
Compression can be useful for data devices. If large amounts of data are to be
sent, using a compressed format will obviously speed up transfer time.
Considering Power and Range
Power is a critical consideration for wireless devices. If a product is to be made
wireless, unleashed from its wired connection, where will its power come from?
Often the communication cable also acts as a power cable.With the cable gone,
the subject of batteries is brought into focus, and the inevitable questions arise
concerning battery life, standby time, and physical dimensions.
Some devices, such as headsets, have no need for power when they are connected
with wires. Audio signals come down a wire and drive speakers directly; a

10 Chapter 1 • Introducing Bluetooth Applications
very simple system with no need of extra power connections.When the wires are
replaced with a Bluetooth link, suddenly we need power to drive the link, power
to drive the microprocessor that runs the Bluetooth protocol stack, and power to
amplify the audio signal to a level the user can hear.With small mobile devices
you obviously do not want to install huge batteries, so keeping the power consumption
low is an important consideration.
Deciding on Acceptable Range
The Bluetooth specification defines three power classes for radio transmitters
with an output power of 1 mW, 2.5 mW and 100 mW.The output power defines
the range that the device is able to cover and thus the functionality of your
product must be considered when deciding which power class to use.The user
would not want to have to get up from his desk to connect to the LAN and
therefore requires a higher power radio. Conversely, a cellular phone headset is
likely to be kept close to the phone, making a lower range acceptable, which
allows smaller batteries and a more compact design.Table 1.1 details the respective
maximum output power versus range.
Table 1.1 Bluetooth Radio Power Classes
Power Class Max Output Power Range
Class 1 100 mW 100 meters+
Class 2 2.5 mW 10 meters
Class 3 1 mW 1 meter
It is important to realize that the range figures are for typical use. In the
middle of the Cambridgeshire fens, where the land is flat and there is not much
interference, a Class 1 device has been successfully tested at over a mile. But in a
crowded office with many metal desks and a lot of people, the Bluetooth signal
will be blocked and absorbed, so propagation conditions are far worse and ranges
will be reduced.
Recognizing Candidate Bluetooth Products
Taking into account the preceding sections, we can see that for a product to be a
candidate for Bluetooth technology, it needs to adhere to the six loosely defined
Let’s examine the traditional headset and mobile phone and decide if
Bluetooth technology makes this more convenient for the user.With current
hands-free technology, you have to decide in advance if you require the handsfree
option.This involves fitting your car with a hands-free kit—a microphone or
headset plugged in, with the wire trailing from it to your phone which is either
in your pocket, clipped to your jacket/belt, in a cradle on your dashboard, or like
most of us, fallen down between the seat and the handbrake!
When you receive a call, you answer by pressing a button on the cable;
volume control is available via a button on the cable.The limitation is that you
always have to have your telephone with you; it can only be as far away as the
cable is long.Thus, it is always a conscious decision to use the headset, and to
decide to plug it in! With a Bluetooth headset and phone, the phone can be
inside your briefcase, in the boot of the car, in your jacket on the hook in the
office, in fact, absolutely anywhere—as long as it’s within the range of the
headset. In much the same way as the conventional technology, you press a
button on the headset to receive a call or to adjust the volume.The connection
between the two devices is extremely different, however, and although virtually
invisible to the user, it will incur a connection time overhead. First, the headset
must “pair” with the Audio Gateway (AG), the Bluetooth part of the phone.This
allows Bluetooth addresses to be swapped, and link keys to be established.The
headset will then be able to make a connection to the AG or the AG will be able
to connect to the headset—the exact operation is a software application issue. If
the headset connects to the phone, then the phone needs to know why, either to
set up voice dialing, action voice dialing, or some other function. If the phone
connects to the headset, it patches a SCO link across and the headset can be used
to take the incoming call.
The connection time could be a problem if you must connect every time a
call comes in. After ten seconds of trying to make a connection, the caller has
probably decided you are not going to answer and given up! A low power park
mode allows headset and phone to stay constantly connected without draining
their batteries; this overcomes the slow connection problem. So you must
beware—if connection time is an issue for your product, make absolutely sure
your system supports park mode—although it’s becoming increasingly common,
it’s still possible to buy devices that do not support it.
My conclusion would be that Bluetooth technology would make answering
my phone far more convenient, although extremely expensive at the moment! I
do not have to worry where my phone is, per-equip my car, or have to endure a

• Introducing Bluetooth Applications
cable running from my ear. If the complex connection issues are invisible to me
and I look as cool as Lara Croft (she wore the original Ericsson Bluetooth
headset in the Tomb Raider movie), who really cares! However if it turns into a
software setup nightmare and I have to read through vast user guides, I would not
be so sure.
The medical sector offers many opportunities for Bluetooth technology to
add convenience. In hospitals, patient medical data could be stored on PDA
type devices that would update a central database when brought within range
of an access point (small scale trials for this application in the neurology department
at the University Hospital in Mainz, Germany, have already begun).
Wireless foot controls for medical equipment, respiratory monitors that transmit
data to a PDA rather than a body-worn data collection system, ambulatory
monitoring equipment for easier patient access in emergency situations… the
list goes on.The questions of interference and security will need to be
addressed in some of these applications, but if they are not “life-dependant”
these issues could be overcome.
Regarding the LAN access points, we need to consider the issue of range. If
the consumer has to get up and walk to be within range, there is no added convenience—
in fact, it would become very inconvenient. A Class 1 Bluetooth
device has a range of approximately 100 meters. In reality, this could be much
further, which would be viable in an office, home, or a hotel/airport lounge scenario,
thus making possible the unconscious convenience of the airport check-in
and car rental confirmation detailed at the beginning of this chapter.
With our own personal “toys” the added convenience is unequivocal. Our
laptops will be able to play multiuser Quake with our colleagues in the airport or
the office! Our PDAs and phones will synchronise with our laptops—gone are
the days of database management. Our presentations can be shown at meetings
directly on the laptops of the attendees without the need for a projector or any
worries about forgetting your laptop’s I/O expander.
Against this optimistic picture there are a few inconveniences envisaged that
will affect the consumer. I wouldn’t be happy if my new wireless product spends
longer attached to a battery charger than it can be used without one, if the poor
placement of an antenna within a handheld product means I had be a contortionist
to be able to hold it and have it function, or if calls get dropped while
waiting for my headset to connect to my phone. But the BIG one is inevitably the
man-machine interface (MMI)—it must be simple to use, it must be simple to set
up, it simply must be simple:“connect to Adam’s PDA, Petra’s phone, or the