Biomedical Engineering Reference
In-Depth Information
Fig. 8
Super frame structure used in multiband MAC
By sensing those UWB pulses within the corresponding time slots of the
availability
frame
, other sensor nodes can determine the availability or occupancy of a particular
band for data transmission. This super frame structure is shown in Fig.
8
.
Drawbacks:
This MAC protocol requires the sensor nodes to operate in multiple
frequency modes in order to access different frequency bands. This method increases
the hardware complexity of the sensor nodes, which is disadvantageous in WBAN
applications. Also, it adds the complexity of modulating the data signals into TH-
codes in order to access the common control channel. The sensor nodes have to
sense the narrow UWB pulses during the
availability frame.
This implies the use of a
UWB receiver at the sensor node and of energy consuming pulse sensing procedures.
The authors also have not specified how the node synchronization is achieved in this
MAC protocol.
PULSERS
The PULSERS project [
38
] uses an altered format of the IEEE802.15.4a standard
super frame structure in order to provide high guarantee of delivery for sensor nodes
with high QoS requirements. It uses an extended CFP that facilitates GTS for priority
traffic in the IEEE802.15.4a standard. The CAP is limited to two time slots, where
initialization messages and control messages are exchanged. When a sensor node
wants to transmit data, it requests the allocation of a GTS in the CFP in the next
super frame using the one of the time slots available in CAP. Hence, this MAC pro-
tocol follows a time division multiple access (TDMA)-based approach in providing
multiple access to the shared UWB channel.
This MAC protocol is well suited for WBAN applications with high data rate
requirements. It also proposes a peer-to-peer relay mechanism using a static routing
table, which is known to all the sensor nodes during the initialization. This mechanism
helps in decentralizing the control of the network, hence reduces the latency. The
sensor nodes can be put into inactive mode in between the data transmission slots
used in the MAC protocol; hence the power consumption can be reduced.
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