Chapter 3 - Proposed Algorithm
3.3 AH-ARQ with FCCMP
Typically, we obtain whole packet which is encrypted in plaintext within the following two phases: First, receive all the packet(s) and ensure that there is no errors after error correcting. Second, decrypt the ciphertext into plaintext and check if this packet is
authenticated or not. Therefore, the time that the receiver obtains a packet successfully is:
total AH dec
T T K T
(3.17)
where K is the number of RS block in a packet, Tdec is the decryption time of a RS block, and TAH is the expected time of AH-ARQ.
But we can reduce the total service time to almost TAH by applying AH-FCCMP. The main idea of this structure is that we want to decrypt the packet not until whole bytes are received correctly. And the group, Gi, defined in FCCMP is RSi here in the MPDUs.
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Fig. 15 AH-CCMP example
Fig. 16 AH-FCCMP example
Now we can calculate the service time, Ttotal*
, with the following formulation:
* *
25
where Lnum is the expected block number of the last retransmitted packet. Obviously,
Ttotal* is less than or equal to Ttotal and there is a high positive correlation between Tdec* and
Lnum.
Fig. 17 Lnum under different Be
As the result shows above, we notice that Tdec*
increases when the Be is low, but TAH
decreases in the same condition. On the contrary, TAH rises but Tdec* descends under high Be circumstance. Therefore, the growing rate of Ttatol*
decreases as Be declines, and Ttatol*
is close to TAH when SNR is small.
As the structure we illustrate above, we can decrypt some blocks earlier after the first successful block and make the service time shorten if all RS blocks satisfies those two features below. First, all the information payload in each RS block contains D encryption
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blocks at most and D must be an positive integer. Therefore, every RS block can be decrypted independently. Second, redundancy in RS block has better include FCS. Otherwise, we need to know if this block is cracked or not until it is been decrypted.
Fig. 18 RS block format in AH-FCCMP
The original RS block in AH-ARQ contains ni bytes information and (4+2θ) bytes redundancy, including CRC-32 and FEC. But because of the first feature stated above, D
must be
255 (4 2 ) 16
bytes. For RS(255,239) codec, there will be 11 bytes waste in each RS block. The solution of this situation is reduce θ from 8 bytes into 4 bytes, RS(255,247), enlarge the RS block length. These two cases will be simulated in next section.
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Chapter 4.
Simulation
4.1 System configurations
In this section, the performance of the original AH-ARQ, AH-ARQ with CCMP, and AH-ARQ with FCCMP schemes will be validated and compared via simulations. For simulating the performance, we apply this system with Multi-mode RS-codec chip[5] for RS-codec, Motorola PowerPC G4 7410, referenced by [15], for (F)CCMP, respectively, and other MAC-defined parameters, which are described in 802.11n standard, are showing in Table. 2.
Table. 2 Simulation System Parameters
Parameter Value
Min / Max window size ( Wmin / WMax ) 7/31
Maximum back-off stage ( M ) 5
Maximum Retransmission ( RT ) 25
# of RS blocks in one MPDU ( R ) 16
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4.2 Performance comparison under different numbers of MPDUs
In this section, we demonstrate the performance evaluation under different number of aggregated MPDUs within an A-MPDU, i.e., J = 1,10,20. The special case, J = 1, is shown for comparison purpose because it is also the same as the SR-ARQ, which transmits only one MPDU within each transmission. The rest configurations, RS-codec and MCS, are set by RS(255,239) and MCS(16QAM,3/4,180Mbps) respectively.
Fig. 19 Performance comparison among three architectures when J = 1
Fig. 20 Performance comparison among three architectures when J = 10
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Fig. 21 Performance comparison among three architectures when J = 20
Fig. 19, Fig. 20, and Fig. 21 show the performance comparison for both throughput and mean service time under different Js consideration. As the result of these three figures, we notice that the throughput performance declines as the SNR is lower than 8 and eventually reaches the retransmission threshold when SNR is 6 due to high Be. The maximum
throughput ratio of AH-ARQ to AH-FCCMP are 99.8%, 87.89%, and 86.096% respectively, and the ratio of AH-ARQ to AH-CCMP are 75.79%, 50.13%, and 47.99% respectively. The difference of output rate between AH-ARQ and AH-FCCMP are extremely close especially when the SNR is low and the reason is shown in Fig. 17 and Eq.(3.17) in Chapter 3. The mean service time of AH-CCMP is the highest one in these three figures due to the time wasting in the CCMP procedure. In AH-FCCMP scheme, the mean service time ratio of AH-ARQ decreases from 1.894 to 1.1105, 3.0812 to 1.3153, and 3.2195 to 1.3447 respectively.
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Fig. 22 Performance comparison under different value of J with AH-FCCMP scheme
Fig. 22 provide performance compared to the SR-ARQ scheme, whose number of MPDU per packet is one, since frame aggregation can improve channel utilization effectively.
More MPDUs in one packet reduces the time consumptions by shared contention phase and PHY header. The maximum throughput enhancement to SR-FCCMP are 97.55% and
107.9% for J = 10 and 20 respectively. However, the mean service time increments are not the multiple of the number of MPDUs. In AH-FCCMP scheme, the mean service time ratio of J=1 to J = 10 and 20 are 5.896 and 11.382 respectively in high SNR circumstance. Based on the simulation result, we notice that the performances are close in J=10 and 20's schemes, so the configuration of J in the next two cases is set with 10.
4.3 Performance comparison under different RS-codec schemes
In this section, we demonstrate the performance evaluation under different RS coding rate. While the number of AES encrypted payloads must be an integer and the total payload should be lower than RS's information data, the payloads in a MPDU with AH-ARQ scheme with RS(255,223), RS(255,239), and RS(255,247) are 3300, 3556, and 3812 bytes
respectively as the number of RS blocks in one MPDU, R, is 16. Note that the 3556-byte MPDU is computed from (D Block AES) R MAC_Header3556 bytes , where
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239 4 16 14
D , BlockAES = 16 bytes, and MAC_Header = 28 bytes. The rest configurations, J and MCS, are set by 10 and MCS(16QAM,3/4,180Mbps) respectively.
Fig. 23 Performance comparison among three architectures under RS(255,223)
Fig. 24 Performance comparison among three architectures under RS(255,239)
Fig. 25 Performance comparison among three architectures under RS(255,247)
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Fig. 23, Fig. 24, and Fig. 25 show the performance comparison for both throughput and mean service time under different RS-codec consideration. As the result of these three figures, we notice that the throughput performance under RS(255,223), RS(255,239), and RS(255,247) FEC code declines as the SNR are lower than 6, 8, 10 and eventually reaches the
retransmission threshold when SNR are 4, 6, 8 due to high Be. The maximum throughput ratio of AH-ARQ to AH-FCCMP are 93.89%, 87.89%, and 85.53% respectively, and the ratio of AH-ARQ to AH-CCMP are 52.061%, 50.13%, and 48.35% respectively. In AH-FCCMP scheme, the mean service time ratio of AH-ARQ decreases from 2.928 to 1.165, 3.0812 to 1.3153, and 3.233 to 1.467 respectively in high SNR circumstance.
Fig. 26 Performance comparison under different RS-codec with AH-FCCMP scheme
Fig. 27 Performance comparison under different RS-codec with AH-ARQ scheme
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Fig. 26, and Fig. 27 provide performance comparison within different RS-codec in AH-ARQ and AH-FCCMP scheme. The maximum throughput of AH-ARQ are 125.36, 135.148, and 144.958 Mbps and throughput of AH-FCCMP are 117.69, 118.78, and 119.64 Mbps in three schemes. In AH-FCCMP scheme, the mean service time ratio of RS(255,239) are 0.886 and 1.115 in high SNR condition and 0.47 and 1.498 in low SNR condition for RS(255,223) and RS(255,247) respectively. In addition, In addition, the values shown in AH-ARQ scheme are 1.0001 and 0.9994 in high SNR condition and 0.4889 and 1.5596 in low SNR condition for RS(255,223) and RS(255,247) respectively in AH-ARQ scheme.
The result shows that larger latency used for error correction leads to higher error tolerance under noisy channel quality but less efficiency when channel quality is good. But there is a special case showed in Fig. 26 when the SNR is high but the throughputs are all close to 118Mbps. It is because of the limitation of Motorola PowerPC G4 7410's
computational speed. Each AES received encrypted block needs two AES calculation, which are used for data confidentiality and authentication respectively, to recover the original information. This chip computational speed for AES and CCMP calculation are
approximated as 265Mbps and 120Mbps respectively. When the throughput of AH's is over 120Mbps, the system output rate will be saturated by cipher chip's speed. Upgrading the cipher chip is one of the solution, but the cost of each device will raise. It can be a consideration for trade-off between throughput and cost.
4.4 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)
34
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 Be. 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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References
[1] Cisco PSE, Inside 802.11n Technical details about the new WLAN standard, Mar. 2009.
[2] J.-C. C. e. al., "WIRELESS LAN SECURITY AND IEEE 802.11i," IEEE Wireless Communications, pp. 24 - 36, Feb. 2005.
[3] Committee, LAN/MAN Standards, "Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications," IEEE Computer Society, 2012.
[4] C.-X. W. e. al., "A Novel Generative Model for Burst Error Characterization in Rayleigh Fading Channels," IEEE PIMRC Proceedings Vol.1, pp. 960 - 964, Sept. 2003.
[5] H.-Y. Hsu, Reconfigurable Multi-mode Reed-Solomon Codec for High-Speed Communication Systems, National Central University,, 2001.
[6] J.-S. L. e. al., "Novel Design and Analysis of Aggregated ARQ Protocols for IEEE 802.11n Networks," IEEE Trans. Mobile Computing vol.12, no.3, pp. 556-570, Mar.
2013.
[7] Y. Wu, "Novel Burst Error Correction Algorithms for Reed-Solomon Codes,"
nformation Theory, IEEE Trans. on , vol.58, no.2, pp. 519 - 529, Feb. 2012.
[8] D. e. a. Skordoulis, "IEEE 802.11n MAC frame aggregation mechanisms for
next-generation high-throughput WLANs," Wireless Communications, IEEE , vol.15, no.1, pp. 40 - 47, Feb. 2008.
[9] Advanced Encryption Standard (AES), NIST, 2001.
[10] V. Technologies, Counter CBC-MAC Protocol (CCMP) Encryption Algorithm, 2003.
[11] L. C. T. Shi, "Combining techniques and segment selective repeat on turbo coded hybrid ARQ," WCNC. 2004 IEEE , vol.4, pp. 21-25, Mar. 2004.
[12] S. C. Tinnirello I., "Efficiency analysis of burst transmissions with block ACK in contention-based 802.11e WLANs," ICC 2005. on , vol.5, pp. 16 - 20, May 2005.
[13] D. J. Bernstein, "AES speed," Sept. 2008. [Online]. Available:
http://cr.yp.to/aes-speed.html.
[14] V. R. Joan Daemen, "AES Proposal: Rijndael," http://www.esat.
kuleuven.ac.be/~rijmen/rijndael, 2001.
[15] Y.-T. H. e.al, "Performance analysis for aggregated selective repeat ARQ scheme in IEEE 802.11n networks," IEEE PIMRC, pp. 37,41, 13-16, Sept. 2009.
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