Performance Evaluation and Comparison 131

In this section, we compare the performance of our proposed DDH-MAC protocol with another CR MAC protocol. For our simulation experiments we compare the DDH-MAC protocol with the CREAM-MAC [38] protocol. This protocol is highly cited and is the latest in the literature. A performance comparison of the DDH-MAC protocol for the pre-transmission time and the analytical aggregated throughput has already been provided in Chapter 3 and Chapter 4 respectively. In this section, we present some simulation experiments to compare the performance of the DDH-MAC protocol for throughput, the number of frames exchanged as control information, and the number of frames exchanged when a PU claim is detected.

We evaluate our proposed DDH-MAC protocol in the saturated network case. Figure 5.11 shows the simulation results for DDH-MAC and CREAM-MAC protocols, given that the number of available data channels is 6 and that the total number of SUs is 10. Figure 5.12 shows the average throughput in bits/sec of a secondary user as the mean of the results of 20 simulations each running for 50 seconds. We observe that the throughput of DDH-MAC is higher and thus better when compared with other CR MAC protocols.

Figure 5.12. Performance comparison between DDH-MAC and CREAM-MAC on the average throughput in bits/sec of an SU pair on a data channel.

0 500000 1000000 1500000 2000000 2500000 3000000 0 5 10 15 20 25 30 35 40 45 50 DDH_MAC CREAM_MAC A ve ra ge thr oughp ut in bi ts /s ec sec .

- 132 - The very first reason for a higher throughput value of DDH-MAC is the pre- transmission time which heavily affects the performance of a CR MAC protocol. As discussed in detail in Chapter 3, DDH-MAC has the least amount of the pre- transmission time which is advantageous for DDH-MAC. The pre-transmission time itself depends on several elements. The most important is how easy it is to find and access a control channel.

The second reason for the higher throughput in DDH-MAC is re-dialoguing on control information if the primary control channel becomes unavailable. Nodes deploying the DDH-MAC protocol always have access to a backup control channel and can easily converge on the backup control channel by performing channel-switching. Switching from the primary control channel to the backup control channel consumes a time less than 5µs. However, the other CR protocols have to first find a common control channel, and then disseminate the information about the newly found control channel in the CR network, and last exchange control information.

The third reason for the DDH-MAC to outperform other CR MAC protocols is the number of control frames. Each data transaction amongst any pair of SUs is subject to successful exchange of control frames over the control channel. This control information is an overhead in the CR network and should be minimized in all possible ways. Since, CREAM-MAC protocol exchange 4 control frames for each data transmission, while DDH-MAC only exchanges 3 control frames, the overall throughput in DDH-MAC is greater.

In order to observe the behaviour of the CR MAC protocols while exchanging the control information, we ran another simulation experiment. Figure 5.13 presents the number of frames exchanged as control information on the control channel by DDH- MAC and CREAM-MAC protocols. We strongly believe that a smaller number of frames exchanged as control information can be beneficial in several aspects, e.g., i) it reduces the MAC layer overheads and thus CR nodes can quickly start data transmission; ii) nodes holding delay-sensitive data have to wait less, which will ultimately contribute towards better QoS; and iii) fewer number of control frames exchanged will make the CR network more secure and more energy efficient4_.

4

- 133 - 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 0 3 6 9 12 15 time (ms) DDH-MAC CREAM-MAC

Figure 5.13. Performance comparison of DDH-MAC with CREAM-MAC on the number of control frames re-exchanged when the control channel becomes unavailable.

We further extend our experiment on performance evaluation and comparison by investigating the response of SUs when a PU claim is sensed on a control channel and thus the control channel becomes unavailable. Also, the CR node that first detects the PU claim on the control channel is unable to propagate the information about unavailability of control channel by any means. In this case, all other CR MAC protocols have to re-exchange the entire configuration dialogue and all frames need to be retransmitted. However, DDH-MAC is specially designed to handle this situation, and any node which first detects the PU occupancy on a control channel will launch a single frame which lets all CR nodes in the vicinity start using the backup control channel. Hence, there is no need to find the control channel and then re-dialogue the whole control information.

Clearly, DDH-MAC has to broadcast only one management frame to let other CR nodes know to switch onto the BCCH which has already been established and all CR nodes are aware of, while the protocol design of CREAM-MAC has to broadcast 4 control frames. Nu mber of c on tr ol fr ame s e xc hang ed

- 134 - We also examine the performance of DDH-MAC in terms of queuing delay with another protocol and simulate an experiment when a PU claim has been sensed. By comparison, the re-exchange of the entire configuration dialogue forces the SUs deploying CREAM-MAC protocol to wait for longer. We capture this scenario in our simulation experiment and plot the results obtained in Figure 5.14. It can be clearly observed that more time spent re-exchanging the control information results in high values of queuing delay.

0 1 2 3 4 0 2 5 7 10 12 14 17 19 22 24 sec

DDH-MAC Queuing Delay in ms CREAM-MAC Queuing Delay in ms

PU claim

Figure 5.14. Performance comparison between DDH-MAC and CREAM-MAC on the queuing delay in milliseconds when a control channel becomes unavailable

Que uing de la y in millisec on ds

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