Network setup

In order to evaluate the performance of communication between the sensor nodes and between the controller and the sensor nodes, a network has to be setup. Different scenarios as explained in the Chapter 5, is used to evaluate the network. The network is setup in a star-topology with a central coordinating node and four client nodes. As shown in Fig. 6.5, the implementation is done with a central coordinator and four clients.

Figure 6.5: The network topology

6.3.2

Results and discussion

The data obtained are analyzed in MATLAB. The results are presented in the following subsec- tions.

Duty cycleDuty cycle is the proportional duration for which the main radio is turned on in use- ful data communication. Variation in inter-packet arrival time (IPAT) will cause the duty cycle to be higher, which means radio has to be turned on for much longer time, if the time between the arrival of packets at the receiver is longer. In the CSMA/CA-based access mechanism the node have to sense the medium randomly after every back-off time, which causes the radio to be turned on unnecessarily. In case of the WuR mac, the duty cycle is lesser because the node will have the information about the data at which it has to send. Ensuring that the node will turn

CHAPTER 6. PERFORMANCE EVALUATION OF WAKE-UP FEATURE BASED CSMA/CA PROTOCOL

on the radio only when the node sends a data to the controller.

Power consumption The power consumption can be predicted from duty cycle and the values

of current and voltage consumption from the data sheet of the radio. The CC430 has a CC1101 radio chip embedded on it. The current consumption for sleep mode is 200nA and 15 mA in active mode [30]. The radio is operated with a battery power supply of 3.3 V. Hence if the radio is active for 1 second 49.5 mW (power(W) =current(A)∗voltage(V)) is consumed and while sleeping 0.06 mW is consumed. This power calculation is to show that the longer the sleep mode, shorter the duty cycle and hence lesser the power consumption1_{. From the Fig. 6.6, the duty}

cycle of WUR based CSMA/CA is much lower than the CSMA/CA, which means that in WUR based CSMA/CA the radio is in longer sleep duration than the CSMA/CA itself. Hence the power consumption of the sensor node with WUR based CSMA/CA access mechanism will be lesser than CSMA/CA based access mechanism.

0 5 10 15 20 25 30 35 40 45 50 0 10 20 30 40 50 60 70 80 90 100

Packet Inter−arrival Time (seconds)

Duty cycle (%)

CSMA/CA without WuR CSMA/CA with WuR

Figure 6.6: Inter packet arrival time vs Duty cycle. Comparison between CSMA/CA with and without Wake up radio

Packet delivery ratio The packet delivery ratio (PDR) is the ratio of the total number of packets received at the receiver to the total number of packets generated at the sender. A 100%

1_{In reality, the power consumption of the sensor node also depends on the processor power consumption which}

will be added to the power consumption of the embedded radio.

5 10 15 20 25 30 35 40 45 50 0 10 20 30 40 50 60 70 80 90 100

Packet Inter−arrival Time (seconds)

Packet delivery ratio (%)

CSMA/CA without WuR CSMA/CA with WuR

Figure 6.7: Inter packet arrival time vs Packet delivery ratio. Comparison between CSMA/CA with and without Wake up radio

PDR means there is no loss of packets. PDR is an important metric to analyze the network performance with different physical layer parameters with a specific topology. In case of the in-body sensor network, inter-packet arrival time can vary due to the highly varying physical properties of the human body. The PDR is evaluated for different IPAT to find the reliability of the different CSMA/CA protocols with wake-up radio. As a result, it is shown that all the two implementations perform with high PDR at lower IPAT. However, as the IPAT increases the PDR of the protocols decreases. Out of the two implementations chosen, CSMA/CA with WuR has a better delivery ratio, which is due to the reduced overheads and hence packets can be transferred in less time similar the case of TDMA-based approach. The wake-up feature implemented is similar to that of the TDMA based approach except for the fact that there is no real hardware based time synchronization in the network.

DelayThe delay in the sensor network is defined as the difference in time taken for a set of data that is sent from the source to reach at the destination. Varying IPAT in different protocols will have influence on the delay due to the effect of preambles and control beacons sent alongside the data. In Fig. 4.5, two different implementations of CSMA/CA with and without WuR are compared in terms of delay. It is shown that the CSMA/CA without WuR has the highest delay with higher IPAT. The delay is larger in CSMA/CA without WuR because at higher

CHAPTER 6. PERFORMANCE EVALUATION OF WAKE-UP FEATURE BASED CSMA/CA PROTOCOL 0 1 2 3 4 5 6 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5

Packet Inter−arrival Time (seconds)

End−End Delay (seconds)

CSMA/CA without WuR CSMA/CA with WuR

Figure 6.8: Inter packet arrival time vs End to End delay. Comparison between CSMA/CA with and without Wake up radio

IPAT the carrier sense overheads such as CCA, and back-off time for the data communication to complete adds to the effective data communication. In the case of WuR CSMA, delay is almost constant for the data communication, because the only major delay is by wake-up timer data and transmission of packets. The synchronization bits are encoded in the packets which is also a reason for constant delay, however it does not reduce the effective throughput of the transmitted data. From the experimental analysis of the delay, it is known that WuR-based protocols has less delay even when the real hardware is not present. The slot allocation based on the predefined timer value for the channel access for each node is the crucial reason for delay in case of WuR. In case of higher IPAT, the delay increases linearly in both the cases of CSMA/CA implementation, which is due to the fact that the number of nodes and topology remains the same whereas the inter-packet arrival time is increased. However, similar results can be achieved with CSMA-based protocols without WuR using guaranteed time slot (GTS) mechanisms which requires time synchronization in the network, adding extra overheads.

6.4

Conclusion

Despite the fact that there is no dedicated for wake-up radio,we simulated the wake-up radio using the WOR feature of the chip. The WuR implementation is more close to the TDMA based approach but with no time synchronization required. The time is synchronized with information from the data packets from base station, no separate preamble is required for synchronisation. The analysis of CSMA/CA with WuR feature performs better than CSMA/CA without WuR due to unique reasons such as prevention of idle listening, and over hearing using WUR feature. The power measurement is not made since no dedicated wake-up radio is used. However, the power consumed can be predicted using the duty cycle analysis. From the results the CSMA/CA without wake-up feature has higher duty cycle than the CSMA/CA with the wake-up feature. Higher duty cycle shows that long period of time the radio is switched on, hence higher power consumption. As explained in section 6.3.2, CSMA/CA with WUR has lower power consumption than CSMA/CA without WUR.

As a conclusion, the use WuR feature increases the network performance in terms of low latency, shorter overheads, guaranteed packet delivery ratio. The results obtained in section 6.3.2 verifies the software simulation performed earlier except for the power consumption. The performance of the network increases in terms of packet delivery ratio and lower latency,by incorporating WuR to the CSMA/CA implementation. From the duty cycle measurement, it can also be claimed that the power consumed will also be lesser when real wake-up radio is implemented, as observed in the software simulation of Chapter 4 the power consumed by the external wake-up radio is much lower than that of the main radio.

Chapter 7

Conclusions

In this work, the communication mechanisms for the in-body sensor networks is characterized by evaluating the physical layer and MAC sub-layer of the IBSN. A brief introduction about the need for IBSN in health-care is presented in chapter 1. The main drawbacks of the state-of-the- art medical devices are listed and the possible solution with the closed loop medical devices is presented. In order to realize the closed loop architecture the need of IBSN is mandatory, hence the requirements of the IBSN are formed aiming at a network of closed loop medical devices. An introduction to IBSN and the current research topics of MICS band in physical layer and wake- up radio integration in the MAC sub-layer was given and their need to materialize the IBSN is discussed. Based on this discussion a research question is framed to justify the hypothesis which states that wake-up radio integration in the MAC layer will increase the network performance and reduce the power consumption while operating in the MICS band physical layer. In chapter 2, the requirements for the IBSN are presented with the description of appropriate technologies needed to fulfill the requirements of IBSN. Wake-up radio operation is explained and the need for MICS band in medical device communication is discussed. As a result, the requirements for characterizing the IBSN are listed.

There are numerous MAC protocols which already exists for different applications of WSN, and more specifically for the body sensor networks. Very few protocols have been proposed so far in literature for an application in IBSN. However, some of the existing MAC protocols for BSN may suit the requirements of IBSN as well. In order to understand the existing MAC protocols, a analytical survey was made in Chapter 3, with more than 30 MAC protocols which has the wake- up feature and that could fulfill the requirements of IBSN. The MAC protocols were surveyed on their design, and network performance based on the selected parameters such as reliability, latency, effective throughput, duty cycle. As a result it is found that the MAC protocols with wake-up radio are better than the MAC protocols without wake-up radio, in terms of energy efficiency, latency, less overhead and reliability. However, it is also found that the wake-up radio will increase the hardware overheads if a dedicated radio is embedded on the implant. Neverthe- less, practicality of wake-up radio in the current radio chips is studied and found that the added hardware overhead is not a big problem since differnet implants are packed with dual radio on chip and adds no extra hardware. The minor increase in hardware overhead will be negligible when compared to the benefits in increase performance and reduced power consumption. Out of the surveyed MAC protocols with and without wake-up radio, three protocols with wake- up radio is chosen for software evaluation. The reason is that in literature the performance of contention based (CSMA/CA) access mechanism with wake-up radio was better than perform- ance of contention-free (TDMA) access mechanism. In order to study the reasons behind the

performance increase, and to validate the design of the MAC protocols with wake-up radio a software simulation of the implantable sensor network is done in MATLAB following the IEEE 802.15.6 standard. The results matched with the survey results and main reason for the im- proved performance was the wake-up radio eliminated the conventional problems of CSMA/CA approach such as over-hearing and idle-listening.

The results from software simulation was as alongside to the hypothesis made. To verify the findings from software analysis, an analysis of physical layer and MAC layer with the implant- able sensor node was made in Chapter 5. The implants were first characterized for the physical parameters such as distance between transmitter and receptor, transmission power, orientation of antenna. Upon characterizing the optimum parameters, the values are used to characterize the physical layer of OSI network model such as transmission data rate, packet delivery ratio and packet length. Upon characterizing the optimum values for each parameter are found, which are then used for the evaluation of MAC sub-layer. The characterization was done in order to know the effect of flesh as the physical medium of radio communication. The values are found to be deviating largely for different values of physical layer parameters. This explains why a characterization of physical layer was necessary for a given implantable sensor node. Upon find- ing the physical layer characteristics, the MAC sub-layer is anaylsed with and without wake-up feature in chapter 6. The performance of CSMA/CA was found to be largely deviating from the CSMA/CA with wake-up radio. To verify this in real hardware, the CSMA/CA with wake-up feature was analyzed with the sensor node inside the animal flesh. The three main network parameters, interpacket arrival time, latency, packet delivery ratio, were analyzed, since these parameters ensure the reliability of a network and also influences the energy efficiency of the network. The CSMA/CA with wake-up radio was implemented using a wake-on radio timer of the chip simulating the effect of dedicated wake-up radio. In all the cases, the wake-up radio was performing well in terms of the analyzed network parameters. Lower latency due to reduced overheads and lesser collision, higher packet delivery ratio due to lesser collision of data pack- ets and the absence of unwanted back-off from the medium are the findings of the performance analysis CSMA/CA with wake-up feature.

7.1

Answer to the research question

The research question which was raised during the start of this thesis work was

Can a wake-up radio integrated with MAC protocol, meet the QoS requirements and power constraints of an IBSN while operating inside the human body ?

.

The chapters 3 -6 were focused on the research about wake-up radio integration int he MAC layer and find the effect of the same in software environment and in real hardware environment. We found that a MAC protocol integrated wake-up radio can meet the QoS requirements and power constraints of an IBSN more appropriate than a MAC protocol without wake-up radio. From our analysis it is known that the least performing access mechanism can be improved using the wake-up radio integration and careful designing of the MAC protocols. In all the expected medical scenarios of IBSN, the wake-up radio based MAC protocol can serve the purpose better than the non-WUR MAC protocols.

CHAPTER 7. CONCLUSIONS