networking (SDN) [1] achieves easy re-policing by decoupling the control plane ... Besides, novel network control services, such as NC strategy, also can be ...
Network-Coding-based Multipath Transmission in Software Defined Fiber-Wireless Networks Xin Liu1+, Muriel Médard2, Wenzhu Li1 1
2
School of Information and Electric Engineering Hebei University of Engineering Handan 056038, China
Terminal (OLT) to the Internet. Network virtualization was adopted to set up multipath in FiWi networks and a frequentlyused scheduling strategy Weighted Round Robin (WRR) in the multipath for traffic distribution in the virtual resource manager (VRM) was proposed in [3]. Further, a Modified Weighted Round Robin (MWRR) algorithm based on the model of FiWi network virtualization was proposed in [4] to achieve smaller end-to-end delay and better load balancing. In [5], the authors provided an integer linear programming (ILP) solution for the disjoint multiple path computation problem in FiWi networks and proposed two approximation algorithms that have a constant factor approximation bound.
Abstract—A network coding based multipath transmission scheme in Software defined Fiber-Wireless (SD-FiWi) networks is proposed in this paper. By exploring the broadcast nature of the passive optical networks and by activating the ability of optimizing the globe resources of SD-FiWi networks via powerful control, encoded packets can be transmitted through multiple paths via different Optical Network Units (ONUs) efficiently. Simulation results showed that the proposed scheme can effectively improve the goodput of SD-FiWi networks by fairly considering the batch size and the number of redundancy packets. Keywords—Fiber-Wireless networks; multipath transmission; Network Coding; Software Defined Networking
I.
Department of Electrical Engineering and Computer Science, Massachusetts Institute of Technology Cambridge, MA 02139 USA
However, due to dynamic noise, interference, and dynamic channel effects, handling packet loss is a fundamental challenge in WMNs but none of the above researches have focused on this issue. More recently, the network coded SDN was investigated in [6], which showed that the inherent flexibility of both SDN and Network Coding (NC) can provide a fertile ground to envision more efficient, robust, and secure networking designs, that may also incorporate content caching and storage, as well as to influence the focus of network theory. In addition, NC has been proved to be one possible solution that can increase transmission performance over lossy networks, also in the multipath scenario [7].
INTRODUCTION
By integrating optical and wireless technology and providing high capacity and high flexibility, Fiber-Wireless (FiWi) access networks, as a promising solution for the future broadband access networks, have received considerable attention in the research community during recent years. In FiWi networks, the optical back-end provides the high bandwidth by adopting passive optical fiber infrastructures and the wireless front-end offers the flexibility by constructing a wireless mesh network (WMN). Since software defined networking (SDN) [1] achieves easy re-policing by decoupling the control plane from data plane, an SDN architecture makes it easy to define and enforce consistent policies across both wired and wireless connections, resulting in a great potential for seamless and fully integrated FiWi networks.
In this paper, a software defined FiWi (SD-FiWi) network architecture is designed to improve the network flexibility in the developing, control, and implementing. Based on the proposed network architecture, an NC based multipath transmission scheme is proposed in this paper. By exploring the broadcast nature of the passive optical networks and by activating the ability of optimizing the globe resources of SDFiWi networks via powerful control, encoded packets can be transmitted through multiple paths via different Optical Network Units (ONUs), achieving higher goodput in the lossy wireless domain. The remainder of the paper is organized as follows. The network architecture of SD-FiWi networks is presented in Section II. The NC-based multipath transmission scheme is described in Section Ⅲ. We present the simulation results in Section IV and conclude the paper in Section V.
Multipath transmission can effectively improve load balancing and fault tolerance as well as network throughput. Several recent efforts have been devoted to its development in FiWi networks, focusing on the packet reordering, path selection, and traffic splitting. In [2], the authors proposed an optimal flow assignment and fast packet resequencing algorithm in order to reduce the out-of-order probability of multipath upstream packets injected by the Optical Line This work was jointly supported by Wireless@MIT and by the State Scholarship Fund of China (201408130043), the National Natural Science Foundation of China (61440001), the Program for New Century Excellent Talents in University (NCET-13-0770), the Research Project of High-level Talents in University of Hebei Province (GCC2014062), as well as the scientific research projects of the Department of Education of Hebei Province (ZH2012020).
II.
SD-FIWI NETWOTKS
According to the definition of SDN in [1], the architecture of the SD-FiWi networks is illustrated in Fig. 1. The network
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Fig. 2. The basic idea of the NC-based multipath transmission in FiWi networks. (a) Multipath routing. (b) Encoding at the OLT. (c) Decoding at the destination. (d) Encoding at an ONU.
the implementation of NC in PONs [10-12], which aimed to construct a more robust network and to improve the performance of network throughput, end-to-end delays, as well as energy efficiency.
Fig. 1. Architecture of the SD-FiWi networks.
control of the SD-FiWi networks is decoupled from the infrastructure layer, forming a control layer that maintains a global view of the whole network and controls the network devices through the flow-based control interfaces. Hence, the network devices, including the Optical Line Terminal (OLT), ONU Mesh Portal Points (ONU MPPs), and Mesh Access Points (MAPs), in the infrastructure layer only need to accept instructions from the controllers, instead of understanding and processing various protocol standards.
The intra-flow NC can be implemented within the wireless domain [13] or across both the optical and wireless domains [10] in FiWi networks to alleviate wireless interference by reducing the number of transmissions and overcoming high loss rate in WMNs for both unicast [13] and multicast traffic [10]. With intra-flow NC, the source node sends random linear combinations for each batch of outgoing packets. To compensate packet losses in the wireless domain, generally more combinations than original packets, i.e. degrees of freedom, are sent, according to a constant or adaptive redundancy factor. Finally, the destination can recover all the original packets by performing Gauss-Jordan elimination [14], as long as it correctly receives a sufficient number of independent linear combinations.
The control layer is directly programmable and manages the entire network through intelligent orchestration and provisioning systems. All the essential network services, including topology management, node configuration, MAC & bandwidth management, network virtualization, flow control, network performance measurement, fault management, etc., are implemented in this layer to meet business objectives. Besides, novel network control services, such as NC strategy, also can be deployed in this layer. The control layer support open APIs to the application layer. With them, business applications can operate on an abstraction of the network, leveraging network services and capabilities without being tied to the details of their implementation. III. NC-BASED MULTIPATH TRANSMISSION
The inter-flow NC can be implemented in the optical or wireless domain of FiWi networks when two or more ONUs or MAPs exchange data flows via the OLT or a relay MAP, by using the broadcast nature of the wireless transmission as well as the optical downstream transmission in PONs. Due to the length of this paper, the interested readers are referred to [15] and [11] for further information on the implementation of the inter-flow NC in WMNs and PONs, respectively. B. NC-based Multipath Transmission in SD-FiWi networks The basic idea of the NC-based multipath transmission in FiWi networks is presented in Fig. 2. The data flow from the backbone network can be split into two or more subflows by PONs and transmitted to the destination through different paths in the WMN via different ONUs in FiWi networks.
To illustrate the high flexibility and efficiency of SD-FiWi networks when developing new network control services, the implementation of NC, as an example, is discussed in this section. We give a brief review on the implementation of NC in FiWi networks first and then propose an NC-based multipath transmission scheme based on the proposed architecture of SDFiWi networks.
Since the transmission in the optical domain is characterized as lossless, an intra-flow encoding area is formed in PONs. That is, with intra-flow NC, the random linear combinations for each batch of the packets from the backbone network can be generated at the OLT and then be transmitted to the destination through multiple paths via different ONUs, as shown in Fig. 2(b). After that, when the destination receives
A. NC in FiWi networks NC can be implemented under the scenarios of intra- and inter-session, also referred to as intra- and inter-flow. Both the two scenarios have been widely and deeply investigated in the wireless domain [8, 9] and several studies have also addressed
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TABLE I.
REDUNDANCY PACKETS (BATCH SIZE = 100 PKS)
PLR(%)
RT
Rp (pks)
PLR(%)
RT
Rp (pks)
1
1.0101
2
6
1.0638
7
2
1.0204
3
7
1.0753
8
3
1.0309
4
8
1.0870
9
4
1.0417
5
9
1.0989
10
5
1.0526
6
10
1.1111
12
In the infrastructure layer, taking encoding at the ONUs as an example, the OLT only needs to forward the data flow from the backbone network to the ONUs in a broadcast manner. When an ONU receives a batch size of the specific data flow indicated by the control layer, it generates a certain number of linear combinations, each of which is set with the Batch ID and different Coding Vector ID, according to the encoding instruction and the coding vectors table, as well as the redundancy factor and then forwards them along the paths defined by the flow table. After that, when a sufficient number of the combinations with the same Batch ID are received by the destination, the original data packets can be recovered by the decoding process, according to the decoding instruction and the coding vectors table. Overall, in SD-FiWi networks, the network devices in the infrastructure layer only need to accept instructions from the control layer, instead of processing the protocols by themselves.
Fig. 3. NC-based Multipath Transmission in SD-FiWi networks (Encoding at the ONUs).
enough independent linear combinations, all the original packets of the current batch can be recovered by the decoding process, as shown in Fig. 2(c).
Finally, since the NC strategy module is reprogrammable and the control layer maintains a real-time global view of the whole SD-FiWi network, the network devices can be simplified and the development of novel NC control schemes (e.g. dynamic redundancy policy according to the real-time condition of the network) will be reasonable and in a very quick manner.
More specifically, due to the broadcast nature of the downstream transmission in PONs, the encoding process can be implemented at the ONUs alternatively to alleviate the processing load at the OLT side. That is, the OLT forwards the original packets to the ONUs naturally in a broadcast manner by the optical splitter, instead of encoding them by itself. When the ONUs receive those packets, the intra-flow NC encoding process is implemented (as shown in Fig. 2(d)) and the encoded combinations of the original packets are transmitted to the destination through the corresponding wireless path from each ONU, following a decoding process at the destination.
IV.
SIMULATION EVALUATIONS
In this section, we evaluate the performance of the proposed NC-based multipath transmission scheme in SDFiWi networks by computer simulations. The network topology is the same as that shown in Fig. 2(a). The data transmission rate in the optical and wireless domains are 1 Gbit/s and 100 Mbit/s, respectively. The packet length is assumed to be 1000 bits. For simplicity, instead of considering the packet loss ratio (PLR) on each link, only the PLRs on each of the two paths are considered, which is reasonable due to the globe view of the whole network maintained by the control layer of SD-FiWi networks. The bandwidth allocated on each path is assumed to be 5 Mbit/s. The transmission delay from the OLT to the destination is set to be 5 ms. The ordered transmission and retransmission are considered. The backward transmission delay from the destination to the nearest ONU and the OLT are set to be 5 ms and 10 ms, respectively. This difference is due to the polling cycle in the upstream in PONs that increases the transmission delay.
When implementing the NC-based multipath transmission in SD-FiWi networks, both the control and infrastructure layers need to be further designed. As shown in Fig. 3, in the control layer, a multipath routing algorithm needs to be added into the flow control module to define the multiple routing paths. Meanwhile, an NC strategy module is also added, in which the batch size as well as the redundancy policy can be defined and the random coding vectors can be generated. With them, the control information, including the multipath routing-based flow control info with NC operations and the network coding info that includes the batch size, redundancy factor, as well as the coding vectors table, is sent to the corresponding network devices through the flow-based control interface. The coding vectors table contains the generated random coding vectors and their corresponding ID. Instead of generating random coding vectors at the source node and transmitting them with the corresponding combinations along the data paths, the source and destination maintain the coding vectors table received from the control layer and only the coding vector IDs need to be used to identify the combinations. Hence, the overhead of each combination can be reduced and the network efficiency can be improved.
First, we set the batch size to be 100 packets with a certain redundancy packets Rp computed by the theoretical redundancy factor, which is defined as RT=1/(1-PLR), where PLR is the packet loss ratio. The relationship among PLR, RT, and Rp is illustrated in Table I and the simulation results of the goodput
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Fig. 4. Results of the goodput with different packet loss ratio. The bandwidth allocated on each path is 5 Mbit/s.
Fig. 5. Results of the goodput with different batch size and different number of redundancy packets. The packet loss ratio is 5%. NC and retransmission are performed at the ONUs.
of the network with retransmission from the OLT or ONUs and with or without NC are plotted in Fig. 4.
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From Fig. 4, it can be observed that without NC, with the increase of the PLR, the goodput of the incoming data flow decreases accordingly. Moreover, due to the longer backward transmission time to the OLT for retransmissions, the goodput with retransmission from the OLT is lower than that from ONUs. On the other hand, when implementing NC-based multipath transmission, since the dropped packets are easily recovered by the redundancy packets, the goodput increases greatly. The maximum coding gain can reach to 2.6x when the packet loss ratio is 10% and retransmission is from the ONUs.
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[2]
[3]
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Then, we further investigate the influence of the batch size and redundancy packets on the goodput performance. We set the PLR to be 5% and NC as well as retransmission are performed at the ONUs. The simulation results of the goodput with different batch size and different number of redundancy packets are shown in Fig. 5. Using the same method of Table I, we can obtain the corresponding Rp with different batch size (i.e., 3, 4, 5, and 6 redundancy packets for the batch sizes with 40, 60, 80, and 100 packets, respectively). After that, we increase Rp by 1 packet each time. From Fig. 5, we can observe that with the further increase of Rp, the goodput increases accordingly. This is because that by increasing the redundancy packets, the retransmission times due to the packet loss are reduced. However, when the number of the redundancy packets increases to a certain number (e.g., 7 packets with a batch size of 40 packets or 9 packets with a batch size of 60 packets), the goodput reaches the maximum value. This is because that when too much redundancy packets are transmitted through the link, the effective bandwidth for the incoming data flow is reduced. It should also be noted that the batch size is directly related to the buffer size and processing complexity of the NC source and destination nodes. As a result, the batch size and the redundancy packets should be fairly considered. V.
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CONCLUSION
In this paper, an NC-based multipath transmission scheme in SD-FiWi networks was proposed and evaluated by computer simulations. Results showed that the proposed scheme can effectively improve the goodput of SD-FiWi networks by fairly considering the batch size and the number of redundancy packets.
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