2018 IEEE Wireless Communications and Networking Conference (WCNC) Ultra-Low Latency Cloud-Fog Computing for Industrial Internet of Things Chenhua Shi∗ , Zhiyuan Ren∗ , Kun Yang†∗ , Chen Chen∗ , Hailin Zhang∗ , Yao Xiao∗ and Xiangwang Hou∗ ∗ School of Telecommunications Engineering, Xidian University, Xi’an, China of Computer Sciences and Electrical Engineering, University of Essex, CO3 4HG, Colchester, UK E-mail: {chshi@s-an.org, zyren@xidian.edu.cn, kunyang@essex.ac.uk, cc2000@mail.xidian.edu.cn, hlzhang@xidian.edu.cn, yxiao@s-an.org, xwhou@s-an.org} † School Abstract—Recently, the industrial Internet of Things (IIoT) has drawn high attention in academia and industry in the context of industry 4.0. In the IIoT, smart IoT devices are adopted to improve production efficiency. But, these devices will generate huge amounts of production data, which need to be processed effectively. To support IIoT services efficiently, cloud computing is usually considered as one of the possible solutions. However, the IIoT services still suffer from the high-latency and unreliable links problem between cloud and IIoT terminals. To combat these issues, fog computing is a promising solution which extends computing and storage to the network edge. In this paper, we are motivated to integrate the fog computing to the cloud-based IIoT to build a cloud-fog integrated IIoT (CF-IIoT) network. To achieve the ultra-low service response latency, we introduce the distributed computing to the CF-IIoT network and propose leveraging the real-coded genetic algorithm for constrained optimization problem(RCGA-CO) algorithm to optimize the load balancing problem of the distributed cloud-fog network. Most importantly, considering the unreliable situation in the CF-IIoT (e.g., fog nodes damage, wireless links outage), we propose a task reallocation and retransmission mechanism to reduce the average service latency of the CF-IIoT network architecture. The performance evaluation results validate that the RCGA-CO-based CF-IIoT and our proposed mechanism can provide ultra-low latency service in IIoT scenario. Index Terms—Industry 4.0, industry Internet of Things, cloudfog computing, ultra-low latency. I. I NTRODUCTION In the industrial Internet of Things (IIoT), massive intelligent devices generate explosive information requiring processing. And in the context of industry 4.0, more and more IIoT applications, such as smart manufacturing and industrial automation, require real-time data processing. Therefore, a powerful data center will be important in the IIoT. In fact, cloud computing has been considered as the key enabler for meeting the requirements of IIoT applications [1]. However, the cloud-based IIoT network is still facing some unsolved challenges. The cloud datacenters are always deployed remotely, which leads to unbearable transmission latency. Besides, the surging information generated by intelligent services makes the burden of the cloud server heavier, and any fault in the network may results in large-scale communication network error. We notice that fog computing [2] is a promising solution to address above challenges in the cloud-based IIoT. Fog computing processes the workload locally on the fog nodes near the terminals to reduce latency, which supports a new breed of IoT applications and services that require low latency, 978-1-5386-1734-2/18/$31.00 ©2018 IEEE mobility support and geo-distribution [2]. Therefore, we propose the cloud-fog based IIoT (CF-IIoT) network architecture in the context of Industry 4.0, which leverages the current edge devices (e.g. gateway, relay, or router) to build a fog computing layer in the CF-IIoT architecture to meet the requirements of the latency-sensitive IIoT applications. Recently, there are many works about the fog computing and improved cloud computing. In [3], the low latency and low energy consumption performance of the fog computing is validated by comparing with the traditional cloud computing; In [4], authors focused on the service allocation problem in the Combined Fog-Cloud architecture to minimize the latency experienced; In [5], [6], authors proposed a novel C-RAN architecture with the mobile cloud computing. And in the IIoT area, authors in [7], [8] discuss the power consumption and the real life application about IIoT. In [9],an opportunistic spectrum access for Indoor Wireless Coverage is researched. In [10], an effective management scheme for heterogeneous sensor networks is proposed. However, these researches didn’t consider how to joint multiple fog nodes and the cloud server to perform tasks for further reducing the service latency. In this paper, considering the relatively weak computing and storage capacity of the fog nodes, we propose to joint multiple fog nodes and the cloud server to perform computationally intensive real-time tasks in a distributed manner for further reducing the latency. In the distributed computing, load balancing is a key technology to reduce latency. Therefore, we investigate the load balancing strategy of multiple fog nodes and the cloud server. And proposing leveraging the real-coded genetic algorithm for constrained optimization problem(RCGA-CO) load balancing algorithm to make the optimal load balancing strategy. Most importantly, the resource-poor fog node is easy to damage. Besides, the unstable wireless transmission links may lead to network faults. One single fog node cannot guarantee to complete the workload on it. Therefore, we propose a task reallocation and retransmission mechanism to reallocate the uncompleted subtasks on the failure nodes according to the RCGA-CO algorithm and retransmit the new subtasks to normal nodes to guarantee the task could be completed timely. The simulation results show that the reallocation and retransmission mechanism could reduce the average service latency in the case of fog nodes failure. The reminder of this paper proceeds as follows. Section II illustrates the CF-IIoT architecture. In section III, we 2018 IEEE Wireless Communications and Networking Conference (WCNC) establish the latency mathematical model of the CF-IIoT, and propose applying RCGA-CO algorithm to solve the problem. The performance evaluations are shown in section IV. Finally, we conclude this paper in section V. II. CF-II OT A RCHITECTURE To satisfy the applications’ requirements of low latency, we introduce the fog computing into the cloud-based architecture to build CF-IIoT architecture, which is shown in Fig. 1. Broadband communication Wifi 3G/4G Cloud Internet Gateway Fog III. T HEORETICAL M ODEL AND THE RCGA-CO- BASED L OAD BALANCING S TRATEGY IN CF-II OT In this section, we establish the theoretical latency model in the CF-IIoT and propose leveraging RCGA-CO algorithm to solve the problem. A. The Latency Mathematical Model in CF-IIoT Taking the real-time path planning of the industrial robot with laser navigation as an example, there exists a multitude of data processing during the path planning [11]. We analyze the data processing in the CF-IIoT architecture. Firstly, the robot collects the path data information in real time. And then the collected real-time path data is transferred to the local fog devices and cloud to perform fast distributed computing. Finally, the path planning results will be sent to the robot to conduct the behavior of the robot. Refer to the above task, the network architecture illustrated in Fig. 1 is abstract as a weighted undirected graph G = (V, E) as shown in Fig. 2. C Wireless sensor nodes Manufacturing equipments Users and smart terminals Wv3 ,vc Wv3 ,vk v3 Wv2 ,v3 Fig. 1. The CF-IIoT architecture. The architecture is divided into three layers: the cloud service layer, fog computing layer and infrastructure layer. The infrastructure layer is the basic component of the architecture, which is mainly composed of sensor nodes, manufacturing equipment, conveyor systems, intelligent industrial robots, manipulators, smart terminals and so on. The function of the infrastructure is performing specific production activities, such as collecting data, manufacturing, and logistics. The fog computing layer consists of multiple edge network devices (e.g., gateways, routers, switches) with relatively poor computing and storage capacity, which play an important role in the CF-IIoT architecture. In this architecture, we choose the gateways as the fog nodes. In order to facilitate the deployment and expansion of the fog network, fog nodes communicate with each other via the wireless channels. The fog nodes interact with the cloud to obtain the related information and service according to the requirements of the infrastructure layer, store the pivotal product information transferred through them, and upload the valuable production status information to the cloud to achieve the global data sharing. Most importantly, all kinds of the data (e.g., product data, users’ requirement data and measurement data) could be processed in the fog computing layer to reduce the latency. And we propose executing distributed computing in the cluster composed of multiple fog nodes and cloud servers and balance the load of each fog node and cloud to achieve the ultra-low latency. In addition, due to the unreliable situation of the fog node and cloud, the distributed computing paradigm adopting task reallocation and retransmission mechanism in the cloud-fog network is a good method to further improve the real time performance of the architecture. The cloud service layer consists of high performance cloud servers, which is responsible for storing IIoT data,sharing the global information and simple data mining operations. In our architecture, the cloud is also seen as a computing node to improve the computing capacity of the architecture. vk v2 Wv1,vk v1 Wv1 ,v2 Fig. 2. The weighted undirected graph. In Fig. 2, the vertex set V = {v1 , v2 , · · · , vk , C}, where vertex vi and C represent the fog nodes and cloud server respectively. In order to enhance the computing capability of the CF-IIoT architecture, we introduce the cloud server as a computing node, and the computing capability is denoted by Cc . Meanwhile, the computing capacity of each fog node is represented by Cvi . The edge set E = {ev1 ,v2 , · · · , evi ,vj , · · · , evk−1 ,vk , ev3 ,c }, where each edge evi ,vj represents a wireless communication link between nodes vi and vj . And the weight of each edge, i.e., Wvi ,vj denotes the communication latency between vi and vj . In the CF-IIoT network, industrial terminals send their ultra-low latency service requests to the nearest fog node, i.e., vj , which is considered as the master node firstly. Then, these terminals transfer their application task i.e., D, to the master node. Next, the computing task could be divided into several small subtasks, i.e., Di . These subtasks are performed by all the fog nodes including master fog node vj and slave fog nodes, i.e, vi , and cloud server C. Finally, the computing results would be converged by the master node and sent back to the industrial terminals. Thus, the service latency t in CFIIoT can be expressed as: { } Di Dc t = max + Wvi ,vj lvi ,vj , + Wvj ,c lvj ,c Cvi Cc i, j = 1, 2, · · · , k (1) where CDvi is the computation latency of the subtask Di on i the fog node vi , Wvi ,vj lvi ,vj is the communication latency between fog nodes vi and vj . And lvi ,vj denotes whether there exists a subtask allocation relationship between the two fog nodes vi and vj . lvi ,vj = 1 means the relationship exists; 2018 IEEE Wireless Communications and Networking Conference (WCNC) lvi ,vj = 0 denotes the relationship doesn’t exist. Similarly, Dc Cc is the computation latency of the subtask Dc on the cloud server C, and Wvj ,c lvj ,c represents the communication latency between master fog node vj and the cloud C. Moreover, referring to [12], the wireless communication latency between the nodes in the CF-IIoT, i.e., Wvi ,vj , Wvj ,c , under the stop-and-wait ARQ protocol could be described as: 1 + P ei Di × (2) ri 1 − P ei Dc 1 + P ec Wvj ,c = × (3) rc 1 − P ec Where Di and Dc are the subtasks that are transmitted to the node vi and the cloud server C. ri and rc denote the data transfer rate of the link evi ,vj and evj ,c . P ei and P ec represent the packet error rate of the link evi ,vj and evj ,c . To obtain the ultra-low latency, we must find an optimal task allocation strategy, namely find a group of optimal subtasks {D1 , D2 , · · · , Di , · · · , Dk , Dc }. In summary, the load balancing problem in the CF-IIoT can be formulated as the following optimization problem: { } Di Di (1 + P ei ) Dc Dc (1 + P ec ) min max + lvi ,vj , + lvj ,c Cvi ri (1 − P ei ) Cc rc (1 − P ec ) i, j = 1, 2, · · · , k (4) Wvi ,vj = { s.t. lvi ,vj = 1, Di ̸= 0 0, Di = 0, 0 ≤ D i , Dc ≤ D k ∑ Di + Dc = D { lvj ,c = 1, Dc ̸= 0 0, Dc = 0 (5) i=1 B. The Average Service Latency Mathematical Model in the Case of Fog Nodes Failure in IIoT Considering the unreliable situation in the CF-IIoT, e.g., fog nodes damage or wireless links outage. In this paper, we are collectively called the fog node damages probability and the wireless link outage probability as the failure probability of one single fog node. Assuming the failure probability of each fog node is pi . When some fog nodes fail, the task can’t be completed if we do not adopt any mechanism. In this paper, when some fog nodes fail, we propose reallocating the subtasks on the failure fog nodes and retransmitting the corresponding subtasks to the normal fog nodes and cloud server to perform distributed computing for reducing the latency. The reallocation and retransmission mechanism could guarantee that the tasks could be completed timely and correctly. Based on the proposed mechanism, the mathematical model of the average service latency in the case of the fog nodes failure in CF-IIoT could be expressed as Eq. (6). Where V = {v1 , v2 , · · · , vk } is the collection of fog nodes, V ′ is the collection of normal fog nodes, and V − V ′ is the collection of failure fog nodes. pn or pi is the failure probability of the fog node. We set the subtask timeout period to be tout . When the master node can’t obtain the subtask processing results from the corresponding slave fog nodes within tout time period, the system think that the slave fog nodes fails, which means vn ∈ V − V ′ . Then, the master fog node, i.e., vj , reallocates the subtasks on the failure fog nodes, and the result of the reallocation is Di′ , Dc′ . Next, the master fog node retransmits the corresponding subtasks Di′ , Dc′ to the normal fog nodes and cloud server to execute distributed computing. Finally, the subtask processing results are transferred to the master node to converge and the final result is returned to the industrial terminals timely. In Eq. (6), Di , Dc , Di′ , Dc′ , lv′ i ,vj and lv′ j ,c meet the following constraints: 0 ≤ Di , Dc , Di′ , Dc′ ≤ D ∑ vi ∈V ′ { lv′ i ,vj = ∑ Di + Dc + vi ∈V Di′ + Dc′ = D 0, Di′ = 0, (8) ′ { 1, Di′ ̸= 0 (7) lv′ j ,c = 1, Dc′ ̸= 0 0, Dc′ = 0 (9) C. GA-based Load Balancing Algorithm To resolve the load balancing problem in Eq. (4) and optimize the latency in Eq. (6), we introduces a real-coded genetic algorithm [13] for constrained optimization problem(RCGACO). In the process of the real-coded GA, each individual Xi = {xi1 , xi2 , · · · , xi(k+1) } in the population denotes a possible solution of the optimization problem, which would be initialized with real number randomly. And then, an optimal individual is found through the constant evolution of selecting, crossing over and mutating the initial population. In order to solve the constrained optimization problems including inequality and equality constraints effectively, we levarage the RCGA-CO to solve the optimization problems with constraints, which transform the constrained optimization problems into the unconstrained optimization problems. The following gives a description of the RCGA-CO. Different from the traditional real-coded GA, the fitness function in the RCGA-CO calculated as follows [14]: t(X) X∈F f (X) = k+2 ∑ tj (X) + ξ(X, g) X ∈ S − F t(X) + h j=1 (10) Where F is the feasible region in the search space S, and S − F represents the infeasible region. h denotes the penalty factor, tj (X) is the constraint violation value of the infeasible individuals for the jth constraint, and ξ(X, g) indicates an additional heuristic value for infeasible individuals in the gth generation. tj (X) and ξ(X, g) could be expressed as follows: max(0, −X(j)) 1≤j ≤k+1 tj (X) = (11) k+1 ∑ | X(i) − D | j =k+2 i=1 ξ(X, g) = W orst(g) − min X∈S−F { } k+2 ∑ t(X) + h tj (X) j=1 { } W orst(g) = max W orst(g − 1), max {t(X)} X∈F (12) (13) 2018 IEEE Wireless Communications and Networking Conference (WCNC) ta = ∑ ∏ vn ∈V −V ′ pn ( { } Di (1 + P ei ) Dc Dc (1 + P ec ) Di + lvi ,vj , + lvj ,c , tout (1 − pi ) min max ′ vi ,vj ∈V Cvi ri (1 − P ei ) Cc rc (1 − P ec ) vi ∈V ′ { ′ }) Di D′ (1 + P ei ) ′ D′ D′ (1 + P ec ) ′ + min max ′ + i lvi ,vj , c + c lvj ,c vi ,vj ∈V Cvi ri (1 − P ei ) Cc rc (1 − P ec ) ∏ In Eq. (13), t(X) represents the fitness value of the gth generation feasible individuals. W orst(g) records the feasible individual with the best fitness through g generation evolution. In the RCGA-CO, each chromosome, namely each individual Xi in the population is designed as a one-dimensional real array with k + 1 genes, which should be randomly initialized with real number in the searching space S firstly. Then, the fitness value of each individual would be calculated according to Eq. (10) to evaluate the population. Next, the genetic operators are performed to update the initial population. And the specific genetic operators are given as follows: Selection: Selection is the process of preserving high fitness individuals from the current population. In this paper, 2tournament selection strategy is adopted. Crossover: Crossover is an important method of genetic algorithm to passed the original good genes onto the offspring. The arithmetic crossover is used in the RCGA-CO, in which two new children individuals , i.e., X1′ ,X2′ are generated by a linear combination of the two parent individuals, i.e., X1 , X2 . The relationship between offspring and parents could be described as follows: X1′ = λX1 + (1 − λ)X2 (14) X2′ = λX2 + (1 − λ)X1 (15) Where λ is a random number on interval (0, 1). Mutation: Mutation operation determines the local search capability of the RCGA-CO and improves the diversity of individuals in the population. In our paper, the non-uniform mutation operator is applied. Taking the load balancing problem in Eq. (4) as an example, the basic steps of RCGA-CO are shown in Algorithm 1. IV. P ERFORMANCE EVALUATION In this section, we present performance evaluations about the RCGA-CO based CF-IIoT and our proposed retransmission and redistribution mechanism. Referring to [15], the number of fog nodes in the CF-IIoT is set to 4 to achieve low latency. The Wireless transmission parameter settings used in the simulation were based on 802.11ac protocol. For simulating the real network environment,the computing capacity of each fog device is different to construct the heterogeneous fog network. Some related parameters are given in TABLE I. The parameter settings of the RCGA-CO are as follows: The population size is 100, the maximum number of generations is 200, the crossover and mutation probability are 0.9 and 0.05 respectively, and the value of W orst(0) is 106 . In our simulation, the task loads are the total service request tasks data from the smart industrial equipment. These task loads are simulation settings, which vary from 0Mb to 500Mb. To reduce the service latency, we can get the task allocation (6) Algorithm 1 RCGA-CO algorithm Input: D,Cvi ,Cc ,ri ,P ei ,P ec ,W orst(0) M axG: max iterations pc: Crossover probability; pm: Mutation probability Output: t(X): Best solution 1: Randomly initialize each individual Xi in the Population 2: for Generation:1 to M axG do 3: for each individual Xi ∈ Population do 4: Calculate the value of tj (X) using equation (11) 5: end for 6: Calculate the value of ξ(X, g) and W orst(g) using equation (12) and (13) 7: for each individual Xi ∈ Population do 8: Calculate fitness value f (Xi ) using equation (10) 9: if f (Xi )>localBestFitness then 10: localBestFitness = f (Xi ) 11: store Xi 12: end if 13: end for 14: if localBestFitness>globalBestFitness then 15: globalBestFitness =localBestFitness 16: store the corresponding Xi whose fitness is the globalBestFitness 17: end if 18: Select individuals from the Population; 19: if rand < pc then 20: crosspop = cossover(Population,pc) 21: end if 22: if rand < pm then 23: mutatepop = mutate(crosspop,pm) 24: end if 25: Update the Population: Population = mutatepop 26: end for 27: Calculate the optimal t(X) using equation (4) and (5) TABLE I T HE RELATED PARAMETERS OF CF-II OT Parameter type v1 v2 v3 v4 C Cvi /Cc (Gbps) Transfer rate(Mbps) Packet error rate 0.3 200 0.0132 0.25 200 0.0241 0.2 150 0.0204 0.15 250 0.0173 10 20 0.0091 ratio according to the RCGA-CO algorithm in advance. When the task arrives, the task allocation results could be obtained quickly according to the task allocation ratio. Therefore, the program running time for RCGA-CO could be ignored. All the simulation results are derived by MATLAB and the values are the means of many repeated experiments. 2018 IEEE Wireless Communications and Networking Conference (WCNC) A. Latency Performance Comparison among the Three Architectures in IIoT In the simulation, we compare the service latency of the CF-IIoT based on RCGA-CO load balancing algorithm with the cloud-based architecture and fog-based architecture in IIoT to validate the low latency performance of CF-IIoT. 12 10 Cloud Fog CF−IIOT C. The Average Service Latency Performance in the Case of Fog Nodes Failure Latency/s 8 6 4 2 0 0 100 200 300 Task/Mb 400 500 Fig. 3. Latency performance comparison among the three architectures. Fig. 3 shows the latency comparison results. With the increasing of the task, the latency in the cloud-based architecture is significantly higher than that in CF-IIoT and fogbased architecture due to the increasing transmission latency. Compared with the cloud-based architecture, the reason why fog-based architecture has lower latency is that the cloud is far from the smart industrial terminals and there is limited bandwidth. Furthermore, in Fig. 3, it’s seen that the service latency of CF-IIoT is lower than the fog-based architecture when the workload is large. This is because the computing capacity of the cloud server enhance the computation power of the CF-IIoT. When the task load is 500Mb,we can observe that the latency performance of CF-IIoT improved by 92.9% and 35.7% compared with the cloud-based architecture and fog-based architecture respectively. Therefore, the CF-IIoT is very suitable for IIoT scenario to provide low latency service. B. Latency Performance Comparison among Multiple Load Balancing algorithms In this section,we analyze the high efficiency of the RCGACO load balancing algorithm in reducing latency in CF-IIoT by comparing it with the PSO-CO [16], Weighted Round Robin (WRR) [17] and Greedy load balancing algorithm (GreedyLB) [18]. The simulation result is shown in Fig. 4. 2.5 Latency/s 2 WRR GreedyLB PSO−CO RCGA−CO 1.5 1 0.5 0 0 the service latency. Moreover, the PSO-CO may fall into the local optimum, the WRR and GreedyLB algorithm don’t consider the transmission latency when balancing the load, which would lead to higher latency. When the task quantity is 500Mb, the latency performance of RCGA-CO algorithm improved by 69.2%, 63.9%, 38.9% compared with WRR, GreedyLB and PSO-CO, respectively. Hence, the RCGACO load balancing algorithm is observed to be an efficient solution in CF-IIoT architecture to reduce the service latency. 100 200 300 Task/Mb 400 500 Fig. 4. Latency performance comparison among multiple load balancing algorithms. As the task quantity increases, we notice that the RCGACO algorithm obtained lower latency than other three algorithms. In RCGA-CO, the crossover operation guarantees the global searching ability and the mutation operation determines the local search capability. Therefore, the RCGA-CO algorithm can achieve a better global load balancing strategy to reduce In the simulation, we assume each fog node has a failure probability. To evaluate the performance of the CF-IIoT adopting task reallocation and retransmitting mechanism, we compare the average service latency in three cases: normal case where all the fog nodes are normal, a failure case where the CF-IIoT adopting task reallocation and retransmitting mechanism, and another failure case where the CF-IIoT only adopting retransmitting mechanism. Moreover, to investigate the influence of the fog nodes failure probabilities on the service latency in CF-IIoT, we compare the latency in four kinds of production environments where the failure probabilities of the fogs nodes are different. The failure probabilities of the fog nodes in four environments are given in TABLE II. The simulation results are shown in Fig. 5 and Fig. 6. TABLE II T HE FAILURE PROBABILITIES OF THE FOG NODES IN FOUR ENVIRONMENTS Environment v1 v2 v3 v4 C Best Environment Good Environment General Environment Poor Environment 0 0.001 0.31 0.6 0 0.1 0.36 0.63 0 0.01 0.4 0.65 0 0.016 0.33 0.69 0 0 0 0 In Fig. 5, we compare the average service latency in three cases: all the nodes in the CF-IIoT are normally running(Normal.Latency), some fog nodes fail in the CF-IIoT adopting task reallocation and retransmitting mechanism(FReaRet.Latency) and some fog nodes fail in the CF-IIoT only adopting retransmitting mechanism (F-Ret.latency) that the uncompleted subtasks on the failure fog nodes only retransmitted to one normal node to compute. The failure probabilities of the fog nodes in the failure case are set according to Good Environment in TABLE II. From Fig. 5, we can firstly observe that the task could be completed in a certain latency instead of can not being completed when fog nodes fail. With the increase of the task quantity, the latency in failure case (F-ReaRet.Latency and F-Ret.Latency) is higher than the latency in normal case(Normal.Latency). This is because the uncompleted tasks on the failure nodes need to be reprocessed on the normal nodes, which will lead to the increasing of the latency. Besides, we can see that the F-ReaRet.Latency is lower than the F-ret.latency and the difference of the latency between them is increasing gradually as the task increases. Because reallocating the uncompleted task on the failure fog nodes to all the normal nodes to perform distributed computing could efficiently reduce the 2018 IEEE Wireless Communications and Networking Conference (WCNC) ACKNOWLEDGMENT 1 Latency/s 0.8 F−Ret.Latency F−ReaRet.Latency Normal.Latency 0.6 0.4 0.2 0 0 100 200 300 Task/Mb 400 500 Fig. 5. The average service latency comparison in three cases. 1.4 1.2 Latency/s 1 Poor ENV General ENV Good ENV Best ENV R EFERENCES 0.8 0.6 0.4 0.2 0 0 100 200 300 Task/Mb This work was supported in part by the National Key Research and Development Program of China(2016YFE0123000),the National Natural Science Foundation of China (61201133, 61571338,61671347,61572389, 61620106011), the National Key Research and Development Program of China(SQ2016YFHZ021501), the key research and development plan of Shaanxi province(2017ZDCXL-GY05-01), UK EPSRC Project NIRVANA (EP/L026031/1), EU H2020 Project iCIRRUS (GA-644526), 111 Project in Xidian University of China (B08038). 400 500 Fig. 6. The average service latency comparison in four environments. processing latency of the uncompleted task, especially when the task is large.When the task load is 500Mb,we can observe that the F-ReaRet.Latency reduced by 8.1% compared with the F-Ret.latency. Therefore, the CF-IIoT adopting task reallocation and retransmitting mechanism could provide low latency service in the case of fog nodes failure in IIoT. Fig. 6 shows the influence of the fog nodes failure probabilities on the average service latency in CF-IIoT. We compare the average service latency among four kinds of different failure probabilities production environments. The failure probabilities of the fog nodes in four environments are shown in TABLE II. It’s obvious that the service latency increases with the growing number of the task loads in four kinds of environments. Moreover, we can observe that when the task load is fixed, the average service latency shows an upward trend with the increase of the failure probability. When the task load is 500Mb, the latency in the best environment reduced by 31.8%, 23.3% and 12.1% compared with the latency in the poor environment, general environment and good environment respectively, which means the large failure probability of the fog nodes would lead to large average service latency in the CF-IIoT. V. C ONCLUSION In this paper, we integrate fog computing to the cloudbased IIoT architecture to build CF-IIoT network architecture and introduce distributed computing to the network. We first establish the latency mathematical model of the CF-IIoT. Then, we introduced RCGA-CO algorithm to balance the load for reducing latency. Furthermore, considering the unreliable situation in the CF-IIoT, we proposed a task reallocation and retransmission mechanism to reduce the average service latency in the case of fog nodes failure. 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