1. What is the difference between ZigBee & Bluetooth?
ZigBee is based on the IEEE 802.15.4
standard. Bluetooth was adopted as the IEEE 802.15.1 standard.
ZigBee is a multi-year battery life, low complexity, and low
data rate (20kbps-250kbps) operating at 868MHz, 915MHZ, and
2.4GHz with up to 100 meter ranges. ZigBee's target market
is large monitoring and control networks (up to 65,000
nodes). Potential applications include sensors, lighting
controls, meter reading, HVAC control, home security/automation,
medical sensing/monitoring, remote controls and industrial/building automation.
Bluetooth is a short-range standard operating at 2.45 GHz
with up to 10 meter ranges. Bluetooth's target market is cable
replacement in small networks of up 7 devices. These include
portable personal devices such as printers/mice/keyboards,
mobile phones and PDAs, digital cameras, etc.
2. What issues
affect the range?
As
with any radio technology range is affected by the following:
· Line of sight
(what obstacles are between the receiver and transmitter and
the nature of those obstacles)
· Transmit power
· Radio type (there
are three bands: 868 MHz Europe & Japan, 915 MHz USA, and
2.4 GHz international)
· Design of the
module antenna
Ranges vary from a minimum of 20
meters to a maximum of 100 meters or more depending on the
factors outlined above. One of the great things about
this standard is that the equivalent of a router (in ZigBee
parlance this would be called a full function device or FFD)
can be used to extend the range between nodes. In terms
of obstacles wood and concrete are friendlier than water and
steel. Devices operated by and held by humans are subject
to interference by the human body itself. External antennas
can easily double the range at the expense of size.
The
2.4 GHz radio offers the highest data rate but lowest range,
approximately 30 to 50 % less than the sub gigahertz radios. In
all cases ranges can be extended using full function devices.
3. What is the
biggest factor effecting node battery life?
The
biggest factor effecting battery life is how often and for
how long a node turns on to communicate with the network. The
system should be designed such that nodes that require battery
power are mostly idle. This is easily accomplished as
the standard is designed to support low power using the idle
mechanism. Systems requiring either low latency or very
controlled latency need
careful application design to avoid having on times which are
lengthy. These nodes, known as reduced function devices in
ZigBee parlance, cannot be used to relay messages from one
node to another in order to extend the range.
4. What would
be a good example in terms of applying this technology?
Any
low frequency (i.e time between events is measured in seconds
or more) low data content (switch closure as opposed to a JPEG
image) application that can tolerate variable delay between
the “event” and the “action” is suitable
for ZigBee technology. Examples would include heating and ventilation (remote
thermostats & sensors), security and fire alarms as well
as industrial controls.
5. What is the
difference between ZigBee and 802.15.4?
The
IEEE 802.15.4 specifies the lowest layers in the standard OSI
networking model: the physical and medium access layers. The
ZigBee standard builds on that foundation by specifying the
network, application, and security frameworks necessary to
build working applications that can interoperate with other
vendor products.
6. Can latency
be deterministic?
Like any shared medium, contention
and collisions are possible. When this occurs, message transmission
is retried to insure robustness, but message latency is affected.
Given a robust implementation of the ZigBee protocol stack, IEEE
802.15.4-compliant radios, and good application architecture,
these retries can be minimized to provide deterministic behavior
for the vast majority of cases. However unlike Bluetooth, latency
guarantees are not the primary focus of ZigBee - long battery
life is.
7. What are
reasonable throughput rates?
Like most network architectures,
a ZigBee based network is a layered architecture including
the top level, custom application software developed to
solve a specific problem. These software layers all add overhead
that can reduce the effective throughput primarily due to non-payload
data (addressing, checksum, etc.). The number of ZigBee
devices can also affect the aggregate throughput due to contention.
Finally, the application design can dramatically affect throughput,
for example, by the amount of concurrency in its communications.
In modest network of a few dozen nodes and proper application
design, a throughput in the 100Kbps range can be achieved.
8. How can I
have independent systems operate in the same physical space?
The
standard allows for up to 64K+ groups operating at the same
time and the same physical space. This means a member
of one group won’t interfere with a member of another. Additionally,
the IEEE 802.15.4 standard specifies a network identifier called
a PAN identifier that can be used to segregate networks.
9. What about
data security?
Security and data integrity are key
benefits of the ZigBee technology. It uses a security toolbox
approach to ensure reliable and secure networks without constraining
application development. Access control lists, packet freshness
timers, and 128-bit encryption capabilities can be leveraged
to help protect data transmission in security sensitive applications.
10. Can nodes
enter a network automatically?
ZigBee includes features in its core
architecture to enable dynamic and self-organizing networks.
Since in many applications ZigBee devices are mobile, battery
operated devices, these features, when properly used application
software, allow ZigBee networks to respond the dynamics of
the environment.
11. Does ZigBee
guarantee end-to-end delivery of my application messages?
ZigBee includes backoff and retry
mechanisms in an effort to insure message transmission. ZigBee
also includes acknowledgements that, when not received within
a certain time, cause a retransmission up to some retry limit.
Application design will normal require additional measures
to guarantee end-to-end control is achieved, for example, to
deal with duplicate transmissions.
12. How does
my application program get loaded into a ZigBee device?
Depending of the ZigBee device manufacturers
tools, applications will normally be downloaded over a serial
line, a (wired) network connection such as Ethernet, parallel
port or USB interface, or "over-the-air" bootstrapping.
13. How do I
debug my application program in a ZigBee device?
This depends on the tools that are
used to create and/or download the application into the ZigBee
device. Modern development tools often provide full source
level debugging, not unlike what is provided by PC development
tools. Other environments rely on routing debug print statements
embedded in the application to a host PC for display. Other
sophisticated approaches so as storing log files on a PC in
a flat file or a database can assist in debugging more subtle
issues.
14. Will ZigBee
radio signals interfere with or be inferred by my cell phone,
my wireless LAN, or my wireless mouse and keyboard?
Recent testing has confirmed that
there is no interference problem with these other devices even
when placed physically close (less than 2 feet) to ZigBee node
members.
15. Do ZigBee
radio signals need line of sight to operate or can they penetrate
walls and other objects?
ZigBee signals go through standard
commercial and industrial concrete walls and typical home construction. The
signals are somewhat attenuated which means that the range
is decreased depending on the number of obstacles between the
receiver and transmitter.
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