Wireless Sensor Networks : Technology, Protocols, and Applications

IEEE 802.15.4 LR-WPANs STANDARD CASE STUDY 181
by network beacons and divided into 16 equally sized slots. The first time slot
of each superframe is used to transmit the beacon. The main purpose of the beacon
is to synchronize the attached devices, identify the PAN, and describe the superframe
structure. The remaining time slots are used by competing devices for communications
during the contention access period (CAP). The devices use a slotted
CSMA-CA-based protocol to gain access to compete for the time slots. All communications
between devices must complete by the end of the current CAP and the
beginning of the next network beacon.
To satisfy the latency and bandwidth requirements of the supported applications,
the PAN coordinator may dedicate groups of contiguous time slots of the active
superframe to these applications. These slots are labeled as guaranteed time slots
(GTSs). The number of GTSs cannot exceed seven. A single GTS allocation, however,
may occupy more then one time slot. Together the GTSs form the contentionfree
period (CFP). As shown in Figure 5.16(b), the CFP always appears at the end
of the active superframe and starts at a slot boundary immediately following the
CAP. The CAP time slots remain for contention-based access between networked
devices and new devices wishing to join the network. All communication transactions
using contention-based access and GTS-based access must complete before
the end their associated CAP and CFP, respectively.
Network devices, which need GTS allocation, can send requests during the CAP
period to reserve a desired number of contiguous time slots. The requested slots can
be of either the ‘‘receive’’ or the ‘‘transmit’’ type. The receive slots are used by the
device to fetch data from the coordinator, while the transmit slots are used to send
data to the coordinator. Devices that have no data to exchange with the coordinator
can switch off their power and go into a sleep mode. Devices are expected to remain
active, however, during their allocated GTSs. Devices are allowed to go into a sleep
mode during the rest of the GTSs.
To reduce energy consumption, the coordinator may also issue a superframe containing
both an active period and an idle period, as shown in Figure 5.16(c). The
active period, composed of 16 time slots, contains the frame beacon, the CAP time
slots, and if applicable, the CAP slots. The inactive period defines a time period
during which all network nodes, including the coordinator, can go into a sleep
mode. In this mode, the network devices switch off their power and set a timer
to wake up immediately before the announcement of the next beacon frame.
It is worth noting that to accommodate a wide range of application requirements
and network deployment, the length of the active and inactive periods, the time slot
duration, and the number and usage of the slots designated as GTSs are configurable
network parameters. Consequently, depending on the network activity, the
types of devices connected to the network, and the nature of the application supported
by the network, the length of the inactive period varies and may be set to
zero.
Frame Types The general MAC frame format of the IEEE 802.15.4 MAC-layer
standard is depicted in Figure 5.17(a). It is composed of three basic components:
the MAC header, the MAC payload, and the MAC footer. The MAC header