Transfer time analysis of file transfer framework with AL-FEC in SOTM networks
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In this paper, we propose a fully reliable file-transfer framework with application layer forward error correction (AL-FEC) in satellite communications on the move (SOTM) systems. In particular, the proposed framework uses a two-way acknowledgement (ACK) exchange mechanism. The proposed framework is implemented into a performance-enhancing proxy to minimize the system change. Furthermore, we analyze the file-transfer time of the proposed framework by Markov chain. The analysis results show that the proposed framework outperforms TCP in terms of the file-transfer time and the goodput. Furthermore, in the transfer of a large-sized file, the overhead of the proposed AL-FEC mechanism is small in terms of the resource efficiency.
KeywordsSatellite communications SOTM network AL-FEC File transfer time Queuing analysis
Satellite communications on the move (SOTM) systems have become an essential part of commercial and military communications because they offer the high-speed wireless link to moving vehicles such as maritime vessels, trains, and land vehicles through a satellite [1, 2, 3]. In the SOTM system, the antenna of the SOTM terminal, which is equipped with an active control system and an inertial navigation system is pointed to the satellite. Thus, the wireless link between the SOTM terminal and the satellite can be continuously maintained. However, the link between the SOTM terminal and the satellite can experience a temporary outage owing to channel blockage due to pointing errors of the antenna on account of the rugged terrain and obstructions such as high buildings and trees. It can cause packet loss in the satellite link [2, 3].
There are two main error recovery techniques: retransmission and forward error correction (FEC). In satellite communications, both retransmission and FEC are used. However, in satellite communications using retransmission of TCP, even though optimized TCP can be applied to satellite link by a performance enhancing proxy (PEP) [4, 5], the throughput can be sharply reduced within a lossy environment because of the coupling of the congestion control and losses in TCP. Because TCP was designed for wired environment, a loss in TCP is always ascribed to congestion. As a result, the TCP congestion window is decreased even though the loss is actually not due to congestion but to channel blockage. This problem is worsened on satellite communications since it takes time to re-open the TCP congestion window and retransmit data due to the long propagation delay. Furthermore, an FEC technique such as channel coding that is applied in the physical layer of the satellite communications only corrects data corruption due to the noise and the interference. It cannot recover packets lost owing to the channel blockage in the SOTM network because of its fixed time diversity and fixed amount of protection .
Recently, an application-layer FEC (AL-FEC) is applied to satellite communications for SOTM terminals [6, 7]. The AL-FEC system can not only correct for packets lost owing to the channel blockage with its flexible time diversity and flexible amount of protection but also eliminate retransmission delay by FEC. Furthermore, the AL-FEC system can potentially achieve better performance as compared with an acknowledgement (ACK)-based system as TCP because the AL-FEC system is a rate-based system with a constant transmission rate through a user datagram protocol (UDP) . In this system, losses do not cause any impact on the transmission rate, resulting in enhancing the throughput. However, an additional feedback mechanism that notify the file-decoding completion is needed to provide fully reliable file transfer because AL-FEC system is based on the unreliable transfer method such as UDP . Some studies have been researched to provide the full reliability of file transfer based on AL-FEC by the additional feedback [9, 10, 11]. To deploy these systems, file-transfer systems of all servers and end-hosts should be changed.
A fully reliable file transfer framework with AL-FEC in SOTM networks
A performance analysis model evaluating the performance of the reliable AL-FEC mechanism with Markov chain model
2 Related works
Generally, TCP is used in the file-transfer system because TCP can provide the reliable, in-order delivery, and the congestion control thanks to its sliding window and ACK-based systems . However, TCP performance can be degraded in the wireless environment because of the coupling of the congestion control and losses in TCP. In the satellite communication with the long propagation delay, this problem is worsened. Thus, in the satellite communication, the optimized TCP such as TCP Hybla is used . In the optimized TCP, the TCP congestion window is aggressively increased to enhance the TCP throughput. However, the problem of the ACK-based system still remains. Therefore, we propose the file transfer framework with AL-FEC that is rate-based system with a constant transmission rate aided by the cross-layer design.
2.2 Benefit of AL-FEC
3 Proposed file transfer framework
3.1 System model
A two-state Markov chain model is used as the channel model . The channel model has channel open (o) and blockage (b) states.
The processing time for AL-FEC is insignificant .
3.2 Architecture of proposed file-transfer framework
The proposed FRFTF is shown in Fig. 2. To apply the AL-FEC to the file-transfer system, the AL-FEC system should be implemented in all file servers and end-host nodes. However, it is not easy to change the file system in all servers. Thus, in FRFTF, the PEP solution with the splitting connection is exploited [4, 5]. FRFTF is only implemented in the PEP of the ground station and SOTM nodes as shown in Fig. 2. In FRFTF, the TCP connection is used between the ground station and the file server. The UDP connection is used between the ground station and the SOTM node. As a result, FRFTF can be compliant with conventional a file-transfer system. In FRFTF, the file download should be completed between the ground station and the server to generate repair packets. However, the additional delay by the splitting connection is not incurred thanks to the high capacity of the wired link between the ground station and the file server. Its capacity is much greater than that of the satellite link. Therefore, the file download from the server to the ground station can be finished while transmitting native packets from the ground station to the SOTM node.
where R D and R A are the demanded rate for a file-transfer service and the available rate in FRFTF, respectively. μ is the number of connections for FRFTF. We assume that R A can be calculated by the information on available resource in link layer and header overheads of link layer and UDP/IP [21, 22]. If the quality of the service should be guaranteed in the system, a call admission control can be applied to limit the number of connections in FRFTF [23, 24]. When the SOTM node is file server, the information on available resource in the link layer is used to select R T X by the cross-layer design [25, 26].
Sender: Upon receiving the data of a file from the file server, FRFTF of the sender segments it into native packets by the size of L S . FRFTF then inserts native packets into both transmission and encoding queues. Packets in the transmission queue are transferred to UDP layer by constant TX rate of R T X . Next, when FRFTF receives a file completely, it generates repair packets from the k native packets in the encoding queue with RaptorQ code. After the repair packet generation, all the packets are inserted into the transmission queue. When receiving the receiver-ACK (R-ACK) message that indicates the completion of the file reception from the receiver, the FEC framework removes all packets from the transmission queue and terminates the file transfer to the receiver. The sender then sends the sender-ACK (S-ACK) that indicates the reception of R-ACK to the receiver. If the sender receives duplicated R-ACK, the sender retransmits the S-ACK.
Receiver: Upon receiving packets from the sender, FRFTF of the receiver inserts the packets into the reception queue. FRFTF checks whether the packets in the reception queue can be decoded to a file by RaptorQ code. If the decoding is completed, the receiver sends an R-ACK message to the sender (ACK-based scheme) and the file is forwarded to the application layer. If the receiver does not receive S-ACK message within round trip time (RTT), it retransmits the R-ACK message to the sender.
In FRFTF, the two-way ACK exchange is used to provide the reliability of data transfer because the S-ACK and R-ACK messages can be lost due to the channel blockage in the satellite communication environment. At the receiver, there is no extra delay caused by the ACK exchange because a file is forwarded to the application layer immediately after the completed decoding. However, the ACK exchange can incur the additional usage of satellite resource at the sender because the sender continuously transmits the packets to the receiver until receiving the R-ACK. The detailed analysis of this overhead is discussed in Section 4.
4 Performance evaluation
In this section, we theoretically derive the average file-transfer time and resource efficiency for transmitting a file by the simple Markov chain model. We also evaluate the performance of the proposed FRFTF in SOTM networks through the theoretical analysis and the simulation.
4.1 File transfer time
where π o and π b are the steady state probabilities in the channel model. They can be calculated from the transition probabilities of the channel model . T P is the propagation delay of the satellite link.
4.2 Resource efficiency
where ε is the processing time of the ACK message. However, it may be negligible.
4.3 Performance analysis
Environment in the performance analysis
Velocity of SOTM node
R T X (Prop. framework)
62.5 kbytes – 112.5 Mbytes
500 – 5000 bytes
Maximum window size (TCP)
Segment size (TCP)
K of RTO (TCP)
α of RTO (TCP)
β of RTO (TCP)
Maximum RTO (TCP)
R T T 0 (TCP Hybla in PEPsal)
In Figs. 5, 6, and 7, it is observed that FRFTF outperforms conventional AL-FEC mechanisms in terms of the average file-transfer time, the average goodput, and the average resource efficiency because fixed PP and CR reduce the efficiency of the AL-FEC mechanism in the SOTM system.
In this paper, we proposed a reliable file-transfer framework with AL-FEC in SOTM networks to reduce the file transfer time. We also analyze the file transfer time of the proposed FRFTF with the Markov chain model. Simulation results indicated that FRFTF can reduce the file-transfer time owing to the rate-based approach and elimination of retransmission delay. As a result, FRFTF outperformed conventional TCP in terms of the goodput. In the file transfer, the goodput of FRFTF is 6.6–10 times more than that of TCP. Furthermore, the overhead of FRFTF is small for large-sized file transfers in terms of the resource efficiency. Therefore, FRFTF can be applied to various services such as the transfer of urgent messages, mail service, web browsing, and file-transfer protocol in SOTM environments of military and commercial communications.
This research was supported by National GNSS Research Center program of Defense Acquisition Program Administration and Agency for Defense Development and Basic Science Research Program through the National Research Foundation of Korea(NRF) funded by the Ministry of Science, ICT and future Planning(NRF-2014R1A2A2A01002321).
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