Low-PowerWireless Sensor Network Platforms - Signal Processing Systems - page 391

status information, such as the duration of the active period and the time between
active periods. After a beacon, nodes exchange frames during the active period. The
active period is followed by a sleep period to save energy. Together, the active period
and the sleep period are referred to as an access cycle . The access cycle is repeated
periodically.
For establishing the synchronized operation, neighboring nodes are typically
discovered by a network scan. The network scan means a long-term reception of
frequency channels for receiving beacons from neighbors, since their schedules
and frequency channels are unknown. Clearly, this is energy-hungry. However, the
synchronized operation after the network scan is very energy-efficient [ 58 ] .
While the channel access in the unsynchronized protocols is usually contention-
based, the synchronized MAC protocols use either contention-based, contention-
free, or hybrid channel access mechanism.
Sensor-MAC (S-MAC) [ 57 ] utilizes purely contention-based channel access by
using CSMA/CA with RTS/CTS mechanism. The protocol utilizes a fixed active
period length and an adjustable, network specific wake up period. Neighboring
nodes may coordinate their active periods to occur simultaneously to form virtual
clusters. At the beginning of an active period nodes wake up and exchange
synchronization (SYNC) frames for synchronizing their operation. The fixed access
cycle length causes idle listening, which decreases energy efficiency. T-MAC [ 52 ]
is a variation of the S-MAC that improves the energy-efficiency by adjusting the
active period according to traffic. It utilizes a short listening window after the CTS
phase and each frame exchange. If no activity occurs during the listening window,
node returns to sleep mode.
IEEE 802.15.4 [ 19 ] standard defines a MAC layer that can use both contention-
based and contention-free channel access. It operates on beacon-enabled and non-
beacon modes. In the non-beacon mode, a protocol is based on a simple CSMA/CA.
Energy-efficient synchronized low duty-cycle operation is provided by the beacon-
enabled mode, where all communications are performed in a superframe structure.
The superframe is divided into three parts: the beacon, Contention Access Period
(CAP) and Contention Free Period (CFP). CAP is a mandatory part of a superframe
during which channel is accessed using a slotted CSMA/CA scheme. CFP is an
optional feature of IEEE 802.15.4 MAC, in which a channel access is performed
in dedicated time slots. CFP can be utilized only for a direct communication
with a PAN coordinator. Thus, its applicability and benefits are very limited in
multi-hop networks. The cluster-tree type IEEE 802.15.4 network can provide
comparably good energy-efficiency in static and sparse networks. The hidden node
problem reduces performance in dense networks, since any handshaking prior to
transmissions is not used.
TUTWSN MAC [ 25 , 27 ] is another example of protocol that uses both contention-
based and contention-free channel access. The superframe structure is similar to the
IEEE 802.15.4. However, instead of using carrier sensing, CAP and CFP are divided
into fixed time-slots. To allow implementation on the simplest radios without carrier
sensing capabilities, TUTWSN MAC uses slotted ALOHA on CAP. Each time slot