First International Conference on Emerging Trends in Engineering and Technology
Simulation based comparison of different Modulation schemes for Mobile WiMAX using TCP and its Variants L.D.MALVIYA S.G.S.I.T.S., Indore(M.P.)
[email protected]
A. GAIWAK M.I.T.M., Indore(M.P.)
acknowledgements are exchanged between the two end hosts of the connection.
Abstract Adaptive modulation enables a WiMAX system to optimize the throughput based on propagation conditions. IEEE802.16 (WiMAX) system support BPSK, QPSK, 16-QAM and 64-QAM and the access scheme is OFDM. This paper presents the performance of different variants of transmission control protocols with different modulation schemes when density of mobile nodes changes.
2. Adaptive Modulation and WiMAX technology Different type of modulation scheme allows to send more bits per symbol and thus achieves higher throughputs or better spectral efficiency. The use of adaptive modulation allows a wireless system to choose the highest order modulation depending on the channel conditions. As the range increase IEEE802.16 step down to lower modulations (in other words, BPSK), but when the distance between transmitter and receiver is less it uses higher order modulations like QAM for increased throughput. In addition, adaptive modulation allows the system to overcome fading and other interference. WiMAX system adjust the signal modulation and coding scheme on the basis of Signal to Noise ratio (SINR) condition of the radio link. When the radio link quality is high the highest modulation scheme and light coding is used giving the system more capacity. During a signal fade or long signal path the WiMAX system can shift to a lower order modulation scheme with heavier coding to maintain the connection quality and link stability. Newer modulation techniques such as Orthogonal Frequency Division Multiplexing (OFDM), support high data rates/spectral efficiency and better multi-path immunity at the price of more complex signal processing at the transceiver [3].
Key Words - WiMAX, Adaptive modulation, TCP, Reno, NewReno, Vegas, Sack1, Fack, BPSK, QPSK, 16-QAM, 64-QAM, OFDM, Throughput, Packet loss ratio.
1. Introduction The demand for broadband services is growing exponentially. Traditional solutions which provide high-speed broadband access use wired access technologies. It is extremely difficult and expensive for carriers to build and maintain wired networks, especially in rural and remote areas. WiMAX technology may provide solutions to above problem of providing broadband communication and it has a potential to bridge the digital divide between developed and developing countries. The driving force behind the development of the WiMAX system has been the desire to satisfy the emerging need for high data rate applications such as voice over IP, video conferencing, interactive gaming, and multimedia streaming. WiMAX will resolve the “last-mile’’ problem in conjunction with IEEE 802.11. The IEEE 802.16-2004 standard is designed for stationary transmission, and the 802.16e amendment deals with both stationary and mobile transmissions [1]. Modulation is the process by which a carrier wave is able to carry the message or digital signal [2]. The TCP and UDP are the two widely used transport layer protocols in internet. To achieve both reliability and congestion control TCP uses a closed feedback loop, data packets and
978-0-7695-3267-7/08 $25.00 © 2008 IEEE DOI 10.1109/ICETET.2008.191
Dr. P.D.VYAVHARE S.G.S.I.T.S., Indore(M.P.)
3. Overview of TCP Variants used in paper The used TCP and its variants are explained below:3.1. TCP: Here reliability is achieved by assigning a sequence number to each packet and acknowledging every packet received. If this acknowledgment is not received within a certain timer duration, the segment is assumed to be lost and it is retransmitted. TCP’s acknowledgments
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are cumulative; this means that every acknowledgment sent indicates the highest inorder sequence number received so far. Therefore the newest acknowledgment received makes all previous ones redundant. The TCP receiver always acknowledges the highest in-order sequence number received, even if new packets with a higher sequence number arrive [4]. 3.2. TCP Reno: TCP-Reno is the most widely used TCP variant with three functions: Slow Start, Congestion Avoidance and Fast Recovery. Slow Start is activated at the start of TCP connection or when a timeout event occurs. During Slow Start phase, the congestion window size (called CWND) increases exponentially until a Slow Start threshold from which CWND increases linearly. The period during which CWND increases linearly is called Congestion Avoidance phase. In TCP-Reno, a lost packet is detected and retransmitted when triple duplicate ACKs are received (called Fast Retransmit) or a timeout event occurs at the sender. For a timeout event, TCP considers a serious congestion and enters Slow Start phase. [4]. 3.3. TCP Vegas: TCP Vegas utilizes the congestion avoidance mechanism to avoid packet loss by decreasing its CWND as soon as it detects an incipient congestion. In TCP-Vegas, CWND is determined by difference between expected throughput and actual throughput. TCP-Vegas defines two thresholds, namely α and β. If difference is less than α, it considers the absence of congestion and increases its CWND by 1. If diff is greater than β, it expects an incipient congestion and decreases its CWND by 1. Otherwise, it keeps the current CWND. For the purpose of retransmitting lost packets, TCPVegas maintains fine-grained RTO (Retransmission Time-Out) value for each transmitted packet, which is used to determine the occurrence of a timeout event when a duplicate ACK for a corresponding packet is received [4]. 3.4. TCP NewReno: NewReno modifies the Fast Retransmit and Fast Recovery. These modifications are intended to fix the Reno problems above and are wholly implemented in the sender side. A modification in TCP Reno lead to NewReno which shows that Reno can be improved without the addition of Sack but still suffers without it. Here, the wait for a retransmit timer is eliminated when multiple packets are lost from a window. New Reno is the same as Reno but with more intelligence during fast recovery. It utilizes the idea of partial acks: when there are multiple packet drops, the acks for the retransmitted packet
will acknowledge some, but not all the segments sent before the Fast Retransmit [4]. 3.5. TCP Sack1: Traditional implementations of TCP use an ack number field that contains cumulative acknowledgement, indicating that the TCP receiver has received all of the data up to the indicated byte. A selective ACK option allows receivers to additionally report non-sequential data they have received. The Sack option is used within an ack packet to indicate which packets were received precisely and thus allows the sender to deduce which packets had been lost. This option aims to speed up the retransmission of lost packets and avoids retransmitting the whole window of data. The TCP Sack1 is modified Sack [5]. 3.6. TCP Fack: Fack is short for Forward Acknowledgment and is based on TCP Reno with Sack. TCP FACK is using the information provided by Sack to compute a better estimate of the amount of data currently in transit (outstanding data). This information is essential for any congestion control algorithm. To estimate the amount of outstanding data, Fack introduces a new variable, fack, denoting the highest sequence number known to have been received plus 1. The variables next and una represent the first byte of data yet to be sent and the first unacknowledged byte, respectively [6].
4. Simulation results The evaluation is done by varying the traffic by varying the number of mobile stations. The topology used here is point to point. The type of traffic used here is FTP. The performance matrices used for evaluations in this paper are throughput and packet loss ratio. The performance of each TCP variant can be seen with the used modulation technique in different scenarios shown below:-
4.1 Scenario-1
The modulation scheme used is OFDM_BPSK _1_2 with the TCP, TCP Reno, TCP NewReno, TCP Sack1, TCP Fack, and TCP Vegas, and the mobile station traffic is varied from 1 to 30. The performance of each used transport protocol is evaluated for OFDM_BPSK_1_2 modulation scheme by using throughput. The figure 1 shows the throughput comparison of TCP, Reno, NewReno, Sack1, Fack, and Vegas transport layer protocols. For OFDM_BPSK_1_2 TCP Fack performs better as compared to other TCP variants. The maximum throughput achieved here
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in Fack is 23.15% and Reno shows 23.06% and NewReno shows 23.06%. Similarly, TCP Vegas shows highest packet loss ratio for OFDM_BPS K_1_2 modulation scheme.
in this modulation scheme Vegas achieves highest throughput 48.78% as compared to the other TCP variants. And the Reno and NewReno achieve second best throughput of 48.15%. Similarly, TCP Vegas again achieves lowest packet loss ratio as compared to other used protocols.
25.00%
49.20%
24.00%
49.00%
23.50%
Throughput(%)
Throughput(%)
24.50%
23.00% 22.50% 22.00% 21.50% 1
10
30
48.60% 48.40% 48.20% 48.00% 47.80%
No. of m obile node s Tcp Tcp/Vegas Tcp/Fack
48.80%
47.60% 1
Tcp/Reno Tcp/Sack1 Tcp/New Reno
10
30
No. of mobile nodes
Tcp Tcp/Vegas Tcp/Fack
Figure1. Throughput comparison of TCP variants for OFDM_BPSK_1_2 modulation technique.
Tcp/Reno Tcp/Sack1 Tcp/NewReno
Figure 3. Throughput comparison of TCP variants for OFDM_QPSK_3_4 modulation technique.
38.80%
59.00%
38.40%
58.50%
38.20%
58.00% Throughput(%)
Throughput(%)
38.60%
38.00% 37.80% 37.60% 37.40% 1
10
57.00% 56.50% 56.00% 55.50%
30
55.00%
No. of mobile nodes
Tcp Tcp/Vegas Tcp/Fack
57.50%
54.50%
Tcp/Reno Tcp/Sack1 Tcp/NewReno
1
10
30
No. of m obile node s Tcp Tcp/Vegas Tcp/Fack
Figure 2. Throughput comparison of TCP variants for OFDM_QPSK_1_2 modulation technique.
Tcp/Reno Tcp/Sack1 Tcp/New Reno
Figure 4. Throughput comparison of TCP variants for OFDM_16QAM_1_2 modulation technique.
4.2 Scenario-2
The modulation scheme used is OFDM_QPSK _1_2. The figure 2 shows the throughput comparison of TCP, Reno, NewReno, Sack1, Fack, and Vegas transport layer protocols. Here in OFDM_QPSK_1_2 modulation scheme Vegas performs better, and the throughput achieved here is 38.30%. Second best performance is shown by Reno (37.91%) and NewReno (37.91%). Similarly, Vegas achieve lowest packet loss ratio as compared to other used protocols.
4.4 Scenario-4
The modulation scheme used is OFDM_16QAM _1_2. The figure 4 shows the through-put comparison of TCP, Reno, NewReno, Sack1, Fack, and Vegas transport layer protocols. Here in this modulation scheme TCP Fack achieves highest throughput 56.79% and Reno and NewReno achieve the same 56.73% throughput as compared to other TCP variants. Therefore the packet loss ratio achievable in Fack is lowest as compared to the other TCP variants.
4.3 Scenario-3 The modulation scheme used is OFDM_QPSK _3_4. The figure 3 shows the throughput comparison of TCP, Reno, NewReno, Sack1, Fack, and Vegas transport layer protocols. Here
4.5 Scenario-5 The modulation scheme used is OFDM_16QAM _3_4. The figure 5 shows the throughput
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-son of TCP, Reno, NewReno, Sack1, Fack, and Vegas transport layer protocols. Here the TCP Reno achieves 100% throughput as compared to the other TCP variants. Therefore the highest throughput and the lowest packet loss ratio is shown by the TCP Reno transport protocol. Similarly the lowest packet loss ratio is shown by the TCP Reno among all the used TCP transport protocols.
66.60% 66.50% 66.40% 66.30% 66.20% 66.10% 66.00% 65.90% 65.80% 65.70% 65.60% 65.50%
120% 100%
Throughput(%)
Throughput(%)
comparison of TCP, Reno, NewReno, Sack1, Fack, and Vegas transport layer protocols. Here the Vegas achieves highest throughput as 66.42%, as compared to the other TCP variants. NewReno achieves 65.88% throughput and the Reno, Sack1 and Fack shows 65.87% throughput. Therefore the lowest packet loss ratio is shown by the TCP Vegas as compared to others.
10
30
40%
0%
No. of mobile nodes
Tcp Tcp/Vegas Tcp/Fack
1
Tcp/Reno Tcp/Sack1 Tcp/NewReno
10
30
No. of mobile nodes
Tcp Tcp/Vegas Tcp/Fack
Figure 5. Throughput comparison of TCP variants for OFDM_16QAM_3_4 modulation technique.
Tcp/Reno Tcp/Sack1 Tcp/NewReno
Figure 7. Throughput comparison of TCP variants for OFDM_64QAM_3_4 modulation technique.
73.00% 72.00% Throughput(%)
60%
20%
1
5. Conclusion
71.00% 70.00%
This paper presented the throughput and packet loss ratio comparison of TCP, TCP Reno, TCP NewReno, TCP Sack1, TCP Fack, and TCP Vegas transport protocols for digital modulation techniques used in adaptive modulation scheme in WiMAX. From the simulation and comparison it is clear that TCP Reno transport protocol is performing best in terms of throughput and packet loss ratio for all the used digital modulation techniques used for adaptive modulation in WiMAX.
69.00% 68.00% 67.00% 66.00% 65.00% 64.00% 63.00% 1
10
30
No. of m obile node s
Tcp Tcp/Vegas Tcp/Fack
80%
Tcp/Reno Tcp/Sack1 Tcp/NewReno
Figure 6. Throughput comparison of TCP variants for OFDM_64QAM_2_3 modulation technique.
6. References
4.6 Scenario-6
[1] www.wimaxforam.org [2] Qingwen Liu, Shengli Zhou and Georgios B. Giannakis, “TCP performance in wireless access with adaptive modulation and coding”, IEEE Communications Society, 0-7803-8533-0/04/$20.00 (c) 2004. [3] www.intel.com/netcomms/technologies/wimax/inde x. htm [4] Mahbub Hassan and Raj jain, “High performance TCP/IP networking, concepts, issues, and solutions”, pearson education publishing ltd.-2007. [5] RFC2581for SACK. [6] M. Mathis and J. Mahdavi. Forward acknowledgement: Refining TCP congestion control. Association for Computer Machinery (ACM), 26(4):11,1996.
The modulation scheme used is OFDM_64QAM _2_3. The figure 6 shows the throughput comparison of TCP, Reno, NewReno, Sack1, Fack, and Vegas transport layer protocols. TCP Reno, TCP NewReno, and TCP Fack achieve the same throughput as 71.66%. And the TCP Sack1 achieves the highest throughput as 71.68%. Therefore the lowest packet loss ratio is shown by the TCP Sack1 transport protocol.
4.7 Scenario-7 The modulation scheme used is OFDM_64QAM _3_4. The figure 7 shows the throughput compare
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