Image Processing Reference
In-Depth Information
TABLE .
Assumed Realizations of a TDMA Real-Time System, Based on Different Standard
Physical Layers and Resulting TDMA Frame in Which All Nodes Could Transmit
Actual Nodes
Byte Data
Resulting TDMA
ms Application Cylce (PLC)
Bandwidth (MHz)
Slot Time (ms)
Data Rate (Mbit/s)
Frame Time (ms)
(
a
)
IEEE . PHY
.
.
IEEE . PHY-WISA
.
.
IEEE g PHY:
.
a
Longer than necessary application cycle: not realizable.
TABLE .
Characteristic Figures for hree Different Physical Layers in an Assumed TDMA Real-Time
Implementation (Empirical, Indicative Only)
Characteristic Figures
Performance
Coexistence
E
Spectral
efficiency
=
P
=
Performance
F
Frequency usage (at
max. nodes)
=
R
=
Robustness (empirical)
Bit Rate/
bandwidth
Nodes/cycle time
and per Bandwidth
Bandwidth
∗
(nodes
∗
Min. latency/max. latency
slot time/application
cycle time)
Nodes/(ms
∗
MHz)
(MHz)
Ideal/worst case
a
IEEE . PHY-WISA . . . .
b
IEEE .g PHY . . . .
c
a
The worst case latency for a . system is very large, as fadings with a large bandwidth can frequently or even statically
occur, that is why normally a mesh layer should be used to find alternative routes then.
b
WISA has been measured, e.g., in Ref. [].
c
A WLAN latency as, e.g., measured in Ref. [] in a very quiet environment can be as fast as . ms, but in a realistic
environment, nodes connected with parallel other WLANs and users, up to ms can be frequently measured.
IEEE . PHY
.
.
.
∼
a number of wireless nodes. Table . calculates the characteristic figures defined in Chapter for
the three different physical layers.
The IEEE.. physical layer-based WISA implementation has, although it uses five .
frequencies simultaneously, a lower frequency usage
F
compared to the much higher data rate
WLAN,which,ifusedlikethat,sufersfromitsoverheadattheassumedsmalldatapacketsizes
of byte (e.g., a typical simple binary sensor).
hevaluesofTable.areplottedtovisualizeagainthediferentproilesofthetechnologiesfor
short packages (Figure .).
28.7 Summary
A truly wireless sensor/actuator interface with wireless power and wireless communication for
real-time factory has been presented. WISA uses IEEE .. radio transceivers, but adds an opti-
mized TDMA protocol to support a high number () of SAs per base station as well as short cycle
times
.
It is important to emphasize that WISA parameters differ significantly from the design goals of
existing short-range wireless office systems such as Bluetooth or ZigBee. This difference has been
veriiedgraphicallyandbyaigureofmerit,whichcomparestheperformancetothemostimportant
requirements in discrete factory automation applications.
The WISA FH sequences are constructed to guarantee adequate frequency separation between
consecutive hops and low correlation (low interference) in the case of multicell operation. In combi-
nation with an appropriate ARQ scheme, this provides reliable and low delay data transmission. All
cells use the entire available ISM frequency band of MHz at . GHz, without any requirement for
frequency planning.
(
µs
)
