Image Processing Reference
In-Depth Information
TABLE .
IEEE .. Parameters for the Different Frequency Bands
Frequency band
MHz
MHz
. GHz
Location
Europe
North America
Worldwide
Number of center frequencies
Carrier spacing
MHz
MHz
Gross bit rate
kbps
kbps
kbps
Bit modulation
BPSK
-ary orthogonal
Symbol rate
kbps
kbps
. kbps
Spreading
-chips M-sequence
chips PN code
Chip modulation
BPSK
O-QPSK
Chip rate
kchips/s
kchips/s
Mchips/s
Beacons
Inactive
period
t
Contention-
free period
Contention period
Super frame15.36 ms
·2
n
, 0
n
14.
n
n
0
Beacon interval 15.36 ms·2
n
0
,
0
n
0
14
FIGURE .
Optional IEEE ../ZigBee beacon interval and super frame.
For medium access, there are in principle three possibilities: CSMA/CA in the unbeaconed mode,
slotted CSMA/CA in the contention period of a beacon system, and guaranteed time slots (GTS)
in the contention-free period of a beacon system. In the unbeaconed mode the star topology makes
most sense. he PAN coordinator then needs to be continuously awake (and attached to a permanent
power mode), while the other units have to be awake only when wanting to transmit something. Such
a network design is most power effective for the other nodes, but allows for some applications only. In
contrast, the mesh network with the beacon mode provides extended range and increases reliability
as traffic can be rerouted around hindrances. his though reduces battery life and introduces latencies
at the same time.
The beacon systems are defined by the beacon intervals and “super frames,” which can be between
. ms and . s (!) long (see also Figure .). The length of the super frame determines the
active period, which is divided into equal time slots. If desired, the super frame is divided into a
contention and a contention-free period (with a maximum length of seven slots), in which GTS can be
allocated. he beacons are used for synchronization, for identification, and to describe the structure
of the super frame. he used slotted CSMA/CA is somewhat different from the one in IEEE ..
First, the super frame time slots are further divided into back-off slots. he CSMA/CA is aligned to
this slot grid (which helps to save power). Second, a packet which found the channel busy too often
(the default value is five times) after the back-off timer expired, will be discarded.
For security IEEE ../ZigBee provides authentication, encryption, and integrity service. he
developer can choose between: no security, an access control list, and a to bit Advanced
Encryption Standard encryption with authentication.
25.5.2 Performance
An IEEE .. network can comprise a significant number of nodes, so the optimization of one
IEEE .. network is already a task not to underestimate.
