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With the increasing requirements of the multicast services in the whole data traffic service, the optical multicast technology becomes a key technology supporting wide bandwidth and high speed multicasting communication. The transmission efficiency, capacity and robustness of optical multicast network can be further improved by introducing network coding technology into optical multicast networks. Meanwhile, facing to demand of emerging rate-variable multi-granularity multicast service, a multi-path transmission scheme based on network coding for routing and spectrum allocation (RSA) is proposed. It can not only allocate spectrum resources effectively and flexibly for various-rate multicast traffic, but also balance the network load, improve network throughput and reduce transmission blocking rate. In this paper, RSA problem is decomposed into two subproblems, namely routing allocation based on network coding and spectrum allocation based on maximum spectrum first (MSF) strategy, which are solved sequentially. Simulation experiments are carried out to analyze transmission performance with proposed RSA scheme. The simulation results show that the proposed RSA mechanism can allocate spectrum resources efficiently and flexibly for multi-granularity multicast traffic. Compared with RSA schemes based on shortest path tree (SPT) and minimal spanning tree (MST), the proposed RSA scheme is more efficient for spectrum resource utilization and load balancing, and spectrum resource is saved more than 20%.

With the rapid development of broadband network, the rate of backbone network communication has been into T bit/s recently [

The optical multicast technology overcomes the limitations on transmission rate caused by optical-electric-optical convention, so it becomes a key technology supporting wide bandwidth and high speed multicasting communication. And the transmission efficiency, capacity and robustness of optical multicast network can be further improved by introducing network coding technology [

Facing to rate-variable traffic requests, elastic optical network employ flexible routing and spectrum allocation algorithms to improve resource utilization efficiency [

Hence, in this paper, a novel elastic resource allocation scheme for multi-granularity multicast traffic is proposed. A multi-path transmission scheme based on network coding for routing allocation relieves the pressure of single link throughput efficiently, and the distributed bandwidth allocation is beneficial to balance the network load and avoid network congestion. Meanwhile, network coding technology supports the two traffic flows sharing the same transmitting links and spectrum resource, so the spectrum utilization is improved. The maximum spectrum first (MSF) strategy is adopted for spectrum allocation, which gives priority to the high bandwidth multicast traffic, and it is helpful for decreasing network blocking probability.

This paper is organized as follows: the elastic spectrum allocation scheme and multi-path multicast routing scheme based on network coding scheme are illustrated in Section 2. Section 3 shows the integer linear programming (ILP) formulation and heuristic algorithm based on GA for solving routing and spectrum allocation. Section 4 presents the simulations on proposed scheme, and relevant spectrum resource utilization efficiency and load balancing performance on multi-granularity multicast traffic are analyzed and discussed. Finally, our conclusion is summarized in Section 5.

Traditional WDM network assigns a fixed spectral grid (50 GHz or 100 GHz) for each traffic demand. When the multicast traffic is lower than the entire wavelength capacity, it results in inefficient resource utilization because of the rigid fixed-grid and coarse bandwidth allocation. However, elastic optical networks based on optical OFDM technology adopt flexible bandwidth allocation to match multi-granularity traffic by elastically accommodating multiple subcarriers [

Optical OFDM technology can satisfy multi-granularity traffic demand by adjusting the number of OFDM multiplexing subcarrier.

The routing allocation scheme for three multicast traffic streams is shown in

Network coding technique changed the traditional multicast scheme for setting up multicast tree. Traditional multicast tree set a single path between the source

node and destination node, while multicast tree scheme based on network coding technology will build at least two paths between the source node and destination node. The data will be transmit to the destination node via two different path. As shown in

In

Previous researches mainly focus on allocating single path for each traffic. However, if multicast traffic is beyond the bandwidth of path, it will increase the blocking probability. Meanwhile, single transmitting method is likely to result in network load imbalance. Multi-path transmission scheme contributes to improve the network throughput and resource utilization efficiency [

Based on the hardware structure of encoding node supporting multi-granularity multicast traffic transmission in optical network [

proposed a NC-based multi-path transmission RSA scheme. The different transmission rate of multicast service is unable to match the network coding operation, so the NC-based multi-path routing allocation operates every multicast traffic separately. As shown in

In this paper, we formulate integer liner programming (ILP) model. In order to reduce the computation complexity, we allocate the resource separately for every multicast traffic request.

Notations:

G = ( V , E ) : the physical topology, where V is the node set, E is the link set.

M i = ( s i , D i , F S i ) : the multicast requests set, where S_{i} is the source node of M_{i}, D_{i} is the destination node set of M_{i}, and FS_{i} is the bandwidth of M_{i}.

d_{ij}: the jth destination node in D_{i}, d i j ∈ D i , j = 1 , 2 , ⋯ , | D i | .

C_{e}: the capacity of fiber links, e ∈ E .

R s , d : the path from the source node to the destination node.

Variables:

λ r : Boolean variable that equals 1 if path r( r ∈ R ) is allocated for M_{i}, and 0 otherwise.

ω e i : Boolean variable that equals 1 ifmulticast request M_{i} occupies the spectrum resource on link e, and 0 otherwise.

π e : the times of multicast traffic occupies link e.

G F S : the number of frequency slots for guard spectrum, GFS is equal to the space of subcarrier

ξ : the number of frequency slots for multicast traffic spectrum.

Objective:

Minimize ξ (1)

In this paper, optimization goal is minimizing the total number of all the multicast traffic requests.

Constraints:

1) Multicast tree constraints:

∑ r ∈ R λ r = 2 , ∀ d i j ∈ D i (2)

Equation (2) constrains that the multicast traffic is divided into two paths for transmitting from s i to d i j , and

π e ≤ 1 (3)

Equation (3) constrains that every link is only occupied once for each multicast traffic.

2) Spectrum allocation constraints:

F S i ≤ C e (4)

Equation (4) constrains that the spectrum resource allocated for multicast traffic is less than the capacity of fiber link. If cannot be satisfied, the traffic will be blocking.

ω e i ≤ 1 (5)

Equation (5) constrains that the same frequency slot only can be allocated for one traffic.

The proposed RSA algorithm is illustrated as follow, and the relevant flow diagram is shown in

When multicast tree based on network coding is established for one multicast traffic, it prefer to choose spare path for multicast routing, so as to balance the network load. If there is no spare path for multicast routing, the other available frequency slots is allocated for new multicast traffic on the same path.

In this paper, genetic algorithm (GA) is used for planning the multicast routing based on network coding. The multicast routing from a single source node to multi-destination nodes is optimized by GA, whose objective is minimizing the cost of encoding. As a result, the proposed optimized multicast tree is of minimum number of encoding node and encoding sides. The individuals population are denoted as binary symbol, and each bit is generated randomly from the set of [0,1]. The each bit of individuals is denoted a link for encoding node, and the each chromosome of populations is denoted a network topology. The fitness of chromosome R_{i} is set:

F ( R i ) = { L − L coding R i is feasible solution 0 R i is not feasible solution (6)

where L is the number of links in network topology, and L_{coding} is the number of encoding sides. In proposed scheme, individuals of higher fitness have a larger probability to survive. Thus, when R_{i} is feasible solution, if L_{coding} is smaller, the fitness is higher, and the corresponding individual has larger probability to survive in next generation. The tournament selection is adopted for selection operation, and the lowest fitness individual is replaced by the highest fitness individual in each generation, so that the highest fitness individuals is easier be chosen. Meanwhile, the crossover is single-point operation and the mutation is multi-point operation.

In this section, we compare the performance of proposed NC-MSF RSA scheme with RSA based on Shortest Path Tree (SPT) and Minimal Spanning Tree (MST). The multicast tree based on SPT has the shortest path from source node to multi-destination nodes, and multicast traffic will go through the least number of nodes. The multicast tree based on MST contains the source node and all the destination nodes, its topology has the least sum of all the path. The transmission performance of three RSA schemes is evaluated using topology 1 and topology 2, as shown in

In simulation, C band is deployed for elastic spectrum allocation, and flexible grid spectrum scheme based on OFDM is set 12.5 GHz to be the bandwidth of each frequency slot, so each fiber link can accommodate 358 frequency slots for multicast traffic requests. In simulation scenario, multi-granularity multicast traffic requests of diverse rate 10, 20, 40, 50, 80, 100 Gbit/s are chosen randomly, and the spectrum utilization is analyzed when the number of multicast traffic requests are set 3, 5, 10, 15, 20 respectively.

Topology 1 for simulation experiment has 25 nodes and 36 links, and topology 2 has 20 nodes and 35 links. In topology 1, the set of source nodes is {1, 2, 3, 4, 5, 6, 7}, and the set of destination nodes is {22, 23, 24, 25}. In topology 2, the

set of source nodes is {1, 2, 3, 4, 5, 6}, and the set of destination nodes is {14, 17, 18, 19, 20}. According to the number of multicast traffic requests, the nodes are chosen from the set of nodes separately, and the number of destination nodes is set as 2, 3, 4 and 5 in sequence. All the links is set normalization for simple.

In this paper, the NC-MSF algorithm employs the genetic algorithm based on network coding for multicast routing. The number of population is set 30, the crossover probability is set 0.8, and the mutation probability is set 0.1. When the maximum number of iterative reached 100, the algorithm is terminated, and corresponding multicast routing is established. Then, the utilization of spectrum

The number of the frequency slots on each link | 358 |
---|---|

The bandwidth of each frequency slot | 12.5 GHz |

The capacity of each frequency slot | 12.5 Gbit/s |

The number of multicast traffic requests | 3, 5, 10, 15, 20 |

The number of destination nodes | 2, 3, 4, 5 |

The rate of multicast traffic | 10, 20, 40, 50, 80, 100 Gbit/s |

resource is calculated for multicast traffic requests. In each simulation scenario, according to the bandwidth of multicast traffic requests and the number of multicast traffic requests, average maximum frequency slots occupied on topology links is calculated by simulating 10 times respectively. Afterwards, the utilization of routing and spectrum resource and the performance of network load balance are discussed.

In

As shown in

simulation is operated in topology 1 scenario. When the same number of multicast traffic requests is transmitted, the proposed multi-path NC-MSF scheme will improve the utilization of spectrum resources more than 20% comparing with SPT scheme. The simulation results illustrate that multi-path NC-MSF scheme is beneficial to reduce the maximum average usage of frequency slots on each links, and the performance of load balance is improved obviously.

A multi-path transmission scheme based on network coding for routing and spectrum allocation is proposed for multi-granularity multicast traffic transmitting in elastic optical network. The routing spectrum allocation is optimized by decomposing into multi-path routing based on network coding and spectrum allocation based on maximum spectrum first. In simulation experiments, the spectrum resource can be allocated dynamically and efficiently to satisfy diverse rate multicast traffic demand, and the maximum average usage of frequency slots on each links is analyzed. The simulation results verify that the multi-path transmission scheme based network coding can balance the network load, increase network throughput and reduce blocking probability effectively.

This research was jointly supported by National Youth Science Foundation of China (No. 61703297), Youth Foundation of Shanxi Province (No. 201601D021065, No. 201701D221109), and Doctoral Scientific Research Foundation of Taiyuan University of Science and Technology (No. 20152023).

The authors declare no conflicts of interest regarding the publication of this paper.

Li, L.J., Xing, H.L., Wang, Z.N. and Shi, H. (2018) Elastic Resource Allocation for Multi-Granularity Multicasting Traffic in OFDM-Based Optical Networks. Optics and Photonics Journal, 8, 323-336. https://doi.org/10.4236/opj.2018.811028