wilamowski-b-m-irwin-j-d-industrial-communication-systems-2011

50-6 Industrial Communication Systems
• Data are sent from one participant to another (peer-to-peer-topology). As in the first scenario, the
receiver must always be capable of receiving.
• Data are sent from the PAN coordinator to one participant. The participant asks periodically
for data at the coordinator. The coordinator sends an ACK and the data if something is available,
otherwise it sends a null packet. The coordinator is always in receiving mode when not
transmitting.
50.5.2.2 Slotted Mode
In the slotted mode, there is a superframe structure coordinated by the PAN coordinator as the transmitter
of periodic beacons. The superframe is started by the beacon (without using CSMA/CA) and
followed by 16 equal time slots. The beacons are used to synchronize the attached devices to identify the
PAN and to describe the structure of the superframes.
For low-latency applications, the PAN coordinator may optionally dedicate portions of the active
superframe to GTSs. The GTSs form the contention-free period (CFP), which always appears at the end
of the active superframe. The other slots are right after the beacon and are the so-called contention access
slots. Any device wishing to communicate during the contention access period (CAP) between two beacons
has to compete with other devices using a slotted CSMA/CA mechanism. All transactions have to
be completed by the time of the next network beacon. The first nine slots shall always be CAP slots, which
can be used by any device, while the following optional up to seven slots may be allocated by the coordinator
for individual devices upon a request. Figure 50.5 shows the concept of the superframe structure.
With the superframe order (SO) parameter, the superframe can be divided in an active and inactive
phase. The active phase consists of 16 identical time slots as described above followed by the inactive
phase where the PAN coordinator as, of course, also the other participants can enter a low power sleep
mode to save batteries. This is an improvement compared to the unslotted mode; however, it comes with
a higher synchronization effort. Also here, there are three basic communication scenarios:
• Data are sent from participating nodes to the PAN coordinator. The participant tries to obtain a
CAP slot with CSMA/CA if no GTS for it has been defined in the superframe structure. If a GTS
has been reserved, it can send data immediately after synchronizing to its slot.
• When a peer-to-peer communication is initiated, participants synchronize themselves before
transmitting the data in one of the CAP slots.
• If data are sent from the PAN coordinator to one participant, it shows this information in the following
beacon. The participant listens periodically to the beacons and responds in the CAP slots
to pick up the data.
Contention
access period
(CAP)
Optional
guaranteed
time slots (GTS)
Beacon
0
1
8 9 10 15
Beacon
Inactive
Superframe duration
FIGURE 50.5
Superframe.
© 2011 by Taylor and Francis Group, LLC