A reliable data transfer protocol based on twin paths and network coding for underwater acoustic sensor network
© Cai et al.; licensee Springer. 2015
Received: 3 July 2014
Accepted: 19 January 2015
Published: 14 February 2015
MPNC (multiple path and network coding) is proposed as a reliable transport protocol for UWASN (underwater acoustic sensor network). In MPNC, after three disjoint paths being established, two groups of packets (A and B), coded by network coding, are transmitted over the two side paths respectively, and another group of packets C = A ⊕ B are transmitted over the middle path. That is, any two paths can work together as a redundant path for another path, and the reliable transmission of one packet can be guaranteed by 1.5 packets. In order to reduce the number of redundant packets without affecting the transmission reliability, the packets of group C are coded further at special ratio by network coding as shareable redundant packets according to the link error, and a reliable transport protocol TPNC (twin path and network coding) based on two paths and network coding is proposed in this paper. In TPNC, after two disjoint paths, called twin paths, being established, two groups of packets (A and B), coded by network coding, are transmitted with their own shareable redundant packets over the two paths respectively to guarantee the data packet transmission reliability. The results of simulations show that, compared with MPNC, TPNC can acquire similar data delivery ratio with lower energy consumption.
UWASN (underwater acoustic sensor network) is a special kind of WSN (wireless sensor network), which is consisted of underwater acoustic sensor nodes. The UWASN can be deployed for real-time warship monitoring, oceanographic data collection, environmental monitoring, and disaster prevention, etc. Hence, lots of researches have been done on it [1-3].
The design of a reliable data transfer protocol for UWASN is challenging due to the specific characteristics of acoustic channels: high bit error rates, high energy consumption, limited available bandwidth, low transmission speed, and long unstable packet delivery delay.
Traditionally, the data transmission reliability is guaranteed by acknowledgments and/or FEC (forward error correction). However, the data transmission protocols based on traditional acknowledgments, for example, ARQ, are not suitable for UWASN [4-6] because their acknowledgment packets not only prolong the transmission delay but also waste lots of limited bandwidth. Therefore, to reduce the bandwidth waste and transmission delay, caused by acknowledgment packets, without reducing data transmission reliability, some improved ARQ-based protocols [6-9] use data packets as implicit acknowledgments; some protocols [10-14], such as segmented data reliable transport (SDRT) [12,13], NCRF (network coding  in rateless fashion) , and NCIA (network coding with implicit acknowledgment) , send coded packets in burst, which only needs a acknowledgment packet; some protocols, such as ADELIN (adaptive reliable transport) [16,17], VBF-NC (vector-based forwarding-network coding)  and MPNC (multiple path and network coding) , guarantee transmission reliability only by sending enough coed packets.
In order to reduce data transmission overhead, the packets of group C are coded further at special ratio by network coding as shareable redundant packet according to the link error, and TPNC (twin path and network coding) is proposed in this paper. In TPNC, two disjoint paths are established firstly, and then, one group of packets A(B), coded by network coding, are transmitted with its respective shareable redundant packets on one path.
The rest of the paper is arranged as the following: firstly, TPNC is proposed in chapter 2; secondly, the performances of TPNC are analyzed in chapter 3; finally, a conclusion is drawn in chapter 4.
O A and O B are two groups of K original packets. They are coded by network coding individually to form two coded packet groups (A and B), each of which has K + K 1 coded packets.
According to the distance between node S and A 1(B 1), node S calculates the link error ratio e a1(e b1); node S coded every ⌈1/e⌉(e = max(e a1, e b1) packets of group C (C = A ⊕ B) to form a packet of group D by network coding.
Node S codes packets of group D by network coding to acquire the D(1 + ⌊e⌋) packets and forms packet group E.
At time slice 0, node S sends out the packets of group A with the packets of group E 1 (the first half of group E) to guarantee the packet transmission reliability.
At time slice 1, node S, it sends out the packets of group B with the packets of group E 2 (the latter half of packet group E) to guarantee the packet transmission reliability.
At time slice 2, node A 1 broadcasts its received packets of group A and group E 1 to improve the packet transmission reliability.
At time slice 3, node B 1 broadcasts its received packets of group B and group E 2 to improve the packet transmission reliability.
By their received packets of group A, group B, and group E from node S, node A 1, and node B 1, node A 1 and node B 1 can decode these packets to acquire the original packets of group O A and group O B.
At time slice 4, according to link error ratio, calculated from the distance between node A 1 and node A 2, node A 1 forms packet group E and sends out the packets of group A and group E 1 , which are not sent out at time slice 2.
At time slice 5, according to link error ratio, calculated from the distance between node B 1 and node B 2, node B 1 forms packet group E and sends out the packets of group B and group E 2 , which are not sent out at time slice 3.
At time slice 6, node A 2 broadcasts its received packets of group A and group E 1 to improve transmission reliability.
At time slice 7, node B 2 broadcasts its received packets of group B and group E 2 to improve transmission reliability.
By their received packets of group A, group B, and group E from node A 1 , node A 2 , node B 1 , and node B 2 , node A 2 and node B 2 can acquire the original packets of group O A and group O B.
Node A 2 and node B 2 do as node A 1 and node B 1 do.
By the way above, packets of group A, B, and E are transmitted over the twin path, and node E can acquire the original packets of group O A and group O B by decoding its received coded packets of group A, B, and E finally.
From the descriptions above, it can be known that the packet number of group E is much less than that of group C. So, the data transmission overhead of TPNC should be much less than that of MPNC.
3 Performance analysis
In this section, the performance of protocols is analyzed in a 9-hop UWASN. The simulation environment of MATLAB and Aqua-sim  is defined according to Zheng's work . In the experiments, the bit error of channel is a direct ratio to the distance between nodes when the optimal frequency f(d) is used for their communication . Furthermore, in order to acquire average results, every experiment is done 1000 times.
K′ = 0, when d = <10 km;
K′ = 5, when 10 km < d = <30 km;
K′ = 10, when 30 km < d = <35 km;
K′ = 15, when 35 km < d = <37.5 km;
K′ = 18, when 37.5 km < d < 41.5 km.
In the above formula, R presents the successful delivery ratio; T i presents the number of average received packets of a node.
3.1 MATLAB analysis
MATLAB is used to analyze the performance of the protocols firstly in this sub-section.
From Figures 3 and 4, a conclusion can be drawn that the data delivery ratios of TPNC and MPNC are similar, and the normalized energy consumption of TPNC is much lower than that of MPNC because TPNC can guarantee its data transmission reliability by less redundant packets.
3.2 Analysis by Aqua-sim
In order to approve the analysis results of MATLAB above, the performance of the protocols is analyzed by Aqua-sim , which is specially developed for UWASN on the basis of NS-2 by Underwater Wireless Sensor Network Lab of Connecticut University, in this sub-section.
On the basis of analyzing the existed reliable data transfer protocols for UWASN, TPNC is proposed to improve MPNC in this paper. In order to reduce the number of redundant packets, which belong to the group C in MPNC, without affecting the transmission reliability, the packets of the group C are coded further by network coding as shareable redundant packet according to the link error. And then, two groups of packets (A and B), coded by network coding, are transmitted with their respective shareable redundant packets over the two established disjoint paths to transfer the data packet reliably. The simulation results show that the redundant packet number of TPNC is reduced greatly without affecting the strength of TPNC protocol. So, compared with MPNC, TPNC can acquire similar data delivery ratio with lower energy consumption.
The paper is supported by National Science foundation of China (41176082, 61073082), supported by Program for New Century Excellent Talents in University (NCET-13-0753), Specialized Research Fund for the Doctoral Program of Higher Education of China (20132304110031), National Science foundation of Heilongjiang Province, China (42400621-1-14076), and Specialized Research Fund for the Innovation talents of science and technology (2014RFQXJ012).
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