Biomedical Engineering Reference
In-Depth Information
Fig. 3.13 Variation of
average packet loss ratio with
the increase of number of
patients for each network
topology
0.20
Topology 1
Topology 2
0.18
0.16
0.14
0.12
0.10
0.08
0.06
0.04
1
2
3
4
5
6
7
Number of Patients
3.5.1 Packet Loss Ratio
Data transmitted over a WBAN is of utmost importance for a patient's health.
Hence keeping the data loss at a minimum level is an important characteristic of a
reliable WBAN. Packet loss ratio, which is determined by (
3.14
), is a good indi-
cator of WBAN's susceptibility to data loss. Packet losses in a network occur due to
various reasons, such as bad channel conditions and collisions. When the network is
divided into sub networks, comparatively small number of sensor nodes contends
for data transmission in a shared medium. Hence the number of collisions is
reduced. In Fig.
3.13
, it can be seen that the packet loss ratio for the network
topology that uses a router as the intermediate node is lesser than that of Topology 2
where the sensor nodes directly coordinate with the coordinator without an inter-
mediate node. In Topology 1, router nodes create a sub network with their child
sensor nodes; hence, control of the system is more distributed than in Topology 2.
3.5.2 Average Packet Acknowledgement Delay
In a real life hospital scenario, it is expected that patient information be delivered
as fast as possible to his life support systems. Hence packet delay of a WBAN
should be kept in the minimum value possible. Average packet acknowledgement
delay determined by (
3.15
) is an indicator of the time taken for a packet to be
successfully transmitted and acknowledged. In the system described in this
chapter, only periodic packets are acknowledged. Continuously transmitting sen-
sor nodes are provided with guaranteed time slots; hence offering reliable trans-
mission of packets. Data packets of continuously transmitting sensor nodes are not
acknowledged. Instead, BER of continuous data is monitored in real time by the
coordinator and a feedback is sent to sensor nodes only under degraded BER
conditions. This technique helps to keep the packet delay at a minimum level for
continuous sensor nodes.
