Image Processing Reference
In-Depth Information
.a a mechanism has been standardized in IEEE .h. he most relevant parameter for WLAN
frequency planning is the number of mobile terminals that have to be served (assuming that this cor-
relates with the demand for bandwidth). From it the optimum number of APs and their location
(i.e., the distance between APs) can be determined. If it is desired that MS can seamlessly change
between APs (due to mobile deployment) hand-over algorithms have to be added. [HF] addition-
ally recommends a centralized approach of load sharing between APs, for being able to double or
triple the throughput.
IEEE . provides reliable best-effort traffic. For Ethernet traffic, the (theoretical) maximum
transmissionratesforIEEE.a,g,andbare.,.,and.Mbps,respectively,andthetech-
nology is suitable for applications that require respective data rates. To achieve network capacity
values three times these values (three parallel systems are possible for IEEE . b and g, for IEEE
.a networks capacity is less critical, owing to the larger frequency band at GHz), appropriate
receivers and sophisticated frequency planning should be used. IEEE . systems are not efficient
for applications, in which small packet sizes prevail or when power consumption is a critical issue.
25.5 IEEE 802.15.4/ZigBee
25.5.1 Technical Background
The idea behind IEEE ../ZigBee
∗
was to create a very low cost, very low power, two-way wire-
less communication solution that meets the unique requirements of sensors and control devices
needed in consumer electronics, home and building automation, industrial controls, PC periph-
erals, medical sensor applications, and toys and games. To allow long battery lives of years and
more (and thus minimizing the efforts in maintenance), the system supports low data rates at low
duty cycles. Owing to its nominal range of m, IEEE ../ZigBee is generally considered a
WPAN Technology [Bah,Con,Kin]. Nevertheless, this is not as obvious as with Bluetooth.
Owing to the large number of units an IEEE .. network can support, one network can cover
a very large area. Furthermore, IEEE ../ZigBee is also often discussed in the context of WSN
[KANS,KAT,BGSV].
To encourage deployment also IEEE ../ZigBee is placed in unlicensed frequency bands. Like
Bluetooth and the .b/g systems, IEEE ../ZigBee can be used almost globally in the . GHz
band. Additionally IEEE ../ZigBee has been specified for the ISM-bands at MHz in Europe
and MHz in North America. To comply with the respective sharing rules and to allow simple
analogue circuitry, the system uses DSSS. Table . gives an overview on the respective physical
layer parameters. Note that the maximum
user
-data rate is about kbps, though smaller values are
likely in case unsuitable parameters are used (see also Section ..).
The IEEE .. standard allows for two different types of devices: The full function device (FFD)
and the reduced function device (RFD). FFDs are unlimited in their communication, while RFDs can
communicate with FFDs only. In an IEEE ../ZigBee network one of the FFDs serves as a “PAN
coordinator,” who organizes the network, independent of its topology (which can be “star,” “cluster
tree,” or “mesh”). Each network can handle up to devices in case of a bit address pad and even
more in case of a bit addressing. In contrast to Bluetooth ACL or IEEE . systems, ACKs are
not mandatory for normal IEEE .. data traffic, but are sent only upon request.
∗
Note, the technology comprises two specifications he specifications published under IEEE .. for the physical and
MAC layer as well as the ZigBee specification, which covers the upper layers from network to application profiles [Cal].
For the investigation topic of this chapter mainly the IEEE .. part is relevant.
