Biomedical Engineering Reference
In-Depth Information
Fig. 3.14 Variation of the
average packet
acknowledgement delay
(seconds) for periodic traffic
with increasing number of
patients for two topologies
with and without narrow band
feed back
0.024
Topology 1-With NB Feedback
Topology 2-With NB Feedback
Topology 1-With UWB Feedback
Topology 2-With UWB Feedback
0.022
0.020
0.018
0.016
0.014
0.012
0.010
0.008
0.006
0.004
0.002
0.000
0
1
2
3
4
5
6
7
Number of Patients
Figure
3.14
shows that the packet acknowledgement delay of periodic sensor
nodes in Topology 1 is lower than that of Topology 2 after the total number of
active periodic sensor nodes increases more than 20 (five patients). Packet
acknowledgment delay consists of the total time taken for queuing, medium
access, inter frame space, packet transmission and acknowledgment reception.
Medium access delay consists of time taken for allocating a time slot in the next
frame during random access of periodic sensor nodes and back-off time taken in
case of an unsuccessful time slot allocation. In Topology 1, only four periodic
sensor nodes per router have to contend for sending data, while in Topology 2, all
the periodic sensor nodes contend for a single shared medium. As the number of
sensor nodes increases above a certain limit, the time delay that a sensor node is
required to wait in order to send data through the shared medium will increase. It is
observed from the simulations that both the allocation time and the back-off delay
increase considerably in Topology 2 after total periodic sensor nodes increase
above 20. It is also observed that contention in the links between the routers and
the coordinator in Topology 1 is much lower than the contention level in the links
between sensor nodes and coordinator in Topology 2. From the results it can be
concluded that Topology 1 is more scalable than Topology 2.
With the introduction of narrow band feedback receiver, it is possible to do
simultaneous transmission and reception at sensor nodes. Hence the time taken to
switch from transmit state to a receive state when using a UWB transmitter and a
UWB receiver in the sensor node can be eradicated by using a narrowband
receiver. This is shown in the simulation results since the packet acknowledgement
delay for the system with a narrow band feedback is lower than that of the system
using UWB for both the transmitter and receiver.
3.5.3 Percentage Throughput
A WBAN should be capable of adjusting according to the varying load conditions
in a hospital environment. Although most of the physiological signals are of
