Biomedical Engineering Reference
In-Depth Information
Fig. 6 a
Link establishment (LE) frame format used for sensor initialization.
b
Data frame format
this MAC protocol assumes the use of a UWB receiver at the sensor node end in
order to receive LC messages from the coordinator, a considerable energy saving
can be achieved by avoiding the CCA. Reduced energy consumption makes it more
suitable for WBAN applications.
Drawbacks:
The suggested MAC protocol does not provide a method to re-initialize
the data transmission in the case of a lost LC frame. In the case of a lost LC frame,
the data transmission from the sensor nodes can be inhibited permanently. Also,
collisions can occur while using the common TH-code for control messages. The
MAC protocol does not suggest a method to avoid or minimize such collisions.
U-MAC
The U-MAC protocol described in [
33
] suggests the use of an adaptable pro-active
approach for UWB MAC design instead of a re-active approach. It suggests the
dynamic allocation of transmit power and data rate at the UWB sensor nodes using
hello
messages that are used by the sensor nodes in order to advertise their local state.
These messages are sent at a fixed power level that is known to all the sensor nodes. At
the reception of a
hello
message, a sensor node can determine the ranging information
of the neighbouring sensor nodes. This information can be used to dynamically
adjust the transmit power levels of the senor nodes. This MAC protocol suggests a
more sensor centric network organization approach compared with the coordinator
centric approach used in other UWB-based MAC protocols. This MAC protocol
also supports a prioritized delivery mechanism depending on the quality of sensor
(QoS) requirement of the sensor data. Similar to the UWB
2
protocol, U-MAC also
uses unique TH-codes in order to provide multiple access to the shared medium,
while control messages are sent using a common TH-code. Figure
7
demonstrates
the sensor initialization procedure suggested in the MAC protocol.
On the reception of a ready-to- send (RTS) message, the neighbouring sensor
nodes will determine whether the new sensor node is transmitting at an admissible
data rate and transmit power criteria that are determined by the interference level and
SNR at the neighbouring node. A not-clear-to-send (NCTS) message will be sent if
a neighbouring sensor node or a coordinator node disagrees with the parameters
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