Performance comparison under different MCSs

In this section, we demonstrate the performance evaluation under different MCS configuration. Under the number of spatial streams is 2, the MCS for simulation are MCS(QPSK,1/2,60Mbps), MCS(16QAM,3/4,180Mbps), and MCS(16QAM,3/4,180Mbps)

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respectively. The rest configurations, J and RS-codec, are set by 10 and RS(255,239) respectively.

Fig. 28 Performance comparison among three architectures under MCS(QPSK,1/2,60Mbps)

Fig. 29 Performance comparison among three architectures under MCS(16QAM,3/4,180Mbps)

Fig. 30 Performance comparison among three architectures under MCS(64QAM,5/6,300Mbps)

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Fig. 28,

Fig. 29, and Fig. 30 show the performance comparison for both throughput and mean service time under different MCS consideration. As the result of these three figures, we notice that the throughput performance under MCS(QPSK,1/2,60Mbps), MCS(16QAM,3/4,180Mbps), and MCS(16QAM,3/4,180Mbps) declines as the SNR are lower than 5.5, 8, 12 and eventually reaches the retransmission threshold when SNR are 2.5, 6, 6.5 due to high B_e. The maximum throughput ratio of AH-ARQ to AH-FCCMP are 99.98%, 87.89%, and 57.73% respectively, and the ratio of AH-ARQ to AH-CCMP are 73.26%, 50.13%, and 39.74% respectively. In AH-FCCMP scheme, the mean service time ratio of AH-ARQ decreases from 1.749 to 1.0025, 3.0812 to 1.3153, and 4.225 to 2.5834 respectively in high SNR circumstance.

We notice that the mean service time increases as long as the SNR raises after the SNR is 10.5, and it is unusual from the other figures shown before. The reason of this rebound is the limitation of cipher chip's computational speed, and the detail is stated in Chapter 4.3.

The sender's strategy in simulation program is that transmitting a new packet as long as the previous packet is all received correctly within AH-ARQ but not take into account whether it is fully decrypted by CCMP or not. Therefore, higher input rate leads early initial time, but the ending time of each packet is bounded by AES. On the other hand, the difference increases as the AH-ARQ throughput raises.

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Fig. 31 Performance comparison under different MCS with AH-FCCMP scheme

Fig. 32 Performance comparison under different MCS with AH-ARQ scheme

Fig. 31, and Fig. 32 provide performance comparison within different MCS

configuration in AH-ARQ and AH-FCCMP scheme. The ratio of data rate to maximum throughput are 82.73%, 75.08% and 68.73% in three setting respectively in AH-ARQ scheme, and 82.72%, 65.99% and 39.67% in AH-FCCMP scheme. In AH-FCCMP scheme, the mean service time ratio of MCS(16QAM,3/4,180Mbps) are 2.12 and 1.266 in high SNR condition and 1.326 and 0.3413 in low SNR condition for MCS(QPSK,1/2,60Mbps) and

MCS(16QAM,3/4,180Mbps) respectively. In addition, the values shown in AH-ARQ scheme are 2.78 and 0.645 in high SNR condition and 1.38 and 0.355 in low SNR condition.

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Chapter 5.

Conclusion

In this thesis, we propose the efficient structure of 802.11n with WPA2 protocol, Aggregated Hybrid Automatic Repeat Request Mechanism with Fragmentation Counter Mode with CBC-MAC Protocol (AH-FCCMP), while we consider different parameters in 802.11n configuration so as to analyze the performance of the AH-FCCMP scheme in practice. The AH-FCCMP scheme is composed of two algorithms: AH-ARQ protocol and FCCMP protocol.

AH-ARQ is designed with the consideration of frame aggregation and block acknowledgement, which are proposed in 802.11n, for boosting the throughput under low SNR channel quality by using Reed-Solomon block code as the forward error correction code (FEC). Based on the feature of AH-ARQ, we modify the CCMP to FCCMP so that we can compute in parallel not only the AES decryption but the CBC-MAC calculation. The modification of CCMP may raise some flaws such as replay attack, but we demonstrate the solution for preventing replay attack in Chapter 3.2.2 and 3.2.3. As long as AES is not cracked, FCCMP should be as safe as CCMP.

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From the simulation results in Chapter 4, we can conclude that the throughput of AH-FCCMP is close to the one without security requirement. AH-FCCMP makes the cost of security operation decrease and provides the same security level. Moreover, we find that the total throughput is bounded by either data rate or cipher chip operation capability. So that high data rate does not necessarily lead to high system throughput since low level cipher chip.

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