基于NS2的无线传感器网络仿真平台设计
Mohsin+Raza1+Sajjad+Hussain2+Hoa+Le??Minh1+and+Nauman+Aslam1
Ultra?reliable and low?latency communications （URLLC） has become a fundamental focus of future industrial wireless sensor networks （IWSNs）. With the evolution of automation and process control in industrial environments， the need for increased reliability and reduced latencies in wireless communications is even pronounced. Furthermore， the 5G systems specifically target the URLLC in selected areas and industrial automation might turn into a suitable venue for future IWSNs， running 5G as a high speed inter?process linking technology. In this paper， a hybrid multi?channel scheme for performance and throughput enhancement of IWSNs is proposed. The scheme utilizes the multiple frequency channels to increase the overall throughput of the system along with the increase in reliability. A special purpose frequency channel is defined， which facilitates the failed communications by retransmissions where the retransmission slots are allocated according to the priority level of failed communications of different nodes. A scheduler is used to formulate priority based scheduling for retransmission in TDMA based communication slots of this channel. Furthermore， in carrier?sense multiple access with collision avoidance （CSMA/CA） based slots， a frequency polling is introduced to limit the collisions. Mathematical modelling for performance metrics is also presented. The performance of the proposed scheme is compared with that of IEEE802.15.4e， where the performance is evaluated on the basis of throughput， reliability and the number of nodes accommodated in a cluster. The proposed scheme offers a notable increase in the reliability and throughput over the existing IEEE802.15.4e Low Latency Deterministic Networks （LLDN） standard.
industrial wireless sensor network （IWSN）； IEEE802.15.4e； Low Latency Deterministic Network （LLDN）； low latency communications （LLC）； ultra?reliable low latency communication （URLLC）
1 Introduction
he past couple of decades have witnessed a relatively perpetual rise in industrialization and automation of the processes [1]. In the competitive industrial world， automation is the key to cost reduction whether it is a production， nuclear power， oil refinement or chemical plant [2]. These industrial plants can greatly benefit from technological advancements and can implement successful process control with efficient and effective formation of a close loop control system. However， to introduce a suitable process control， a reliable communication infrastructure is needed which should also offer scalable architecture for future enhancements and permits infrastructural extension in the ever?changing industrial plants [3]?[6]. To cope with the first objective （reliability）， wired networks offer suitable solutions but some intrinsic properties of these networks do not sit very well with the present?day industries. The lack of scalable architecture and flexibility of industrial wired networks poses serious limitations for dynamic industrial environments. On top of that， the high price tag in the wired networks also comes as a setback. As an alternate to the wired feedback systems in the industrial infrastructure， industrial wireless sensor networks （IWSNs） are also sometimes considered as a more cost?effective approach. However， the less predictable wireless communication links in IWSNs appear to be a major challenge [7]. Therefore， in order to establish a wireless feedback network， the reliability and real?time data delivery must be ensured in such networks.
The IWSNs offer a suitable reduction in the deployment cost as well as the maintenance cost of the feedback communication loop and in some cases these wireless networks offer a cost reduction by a factor of ten or even more [5]， [8]. The IWSNs also offer other significant benefits over traditional wired networks. These benefits include scalability， cost efficiency， self?healing ability， reduced planning overload for network formation， and relatively less time for new installations of IWSNs [7]. All these benefits have encouraged the use of IWSNs in industries， eventually leading to an extensive increase in research and development activities to ensure suitable IWSN solutions for enabling it for wider variety of applications in the industries.
In the past couple of decades， the use of IWSNs has exponentially increased and can primarily be credited to the improvements in Micro Electro Mechanical Systems （MEMS） technology， which enabled the cost and size reduction of the sensor nodes and an increase in processing capabilities and memory capacity as well. The present IWSNs are capable of taking in account the channel conditions， sensor readings， network specifications and suitable responses to the sampled sensor data. These abilities if properly utilized can also serve as a tool to overcome uncertainty in wireless links and timely delivery of the information. All these improvements in IWSNs have encouraged their use in industrial environments. In past few years many industrial protocols and IEEE standards surfaced， which include Zigbee， WirelessHart， 6LoWPAN， WiaPA， ISA100.11a， IEEE802.15.4， and IEEE802.15.4e [9]-[16]. The time?division multiple access （TDMA） based channel access was introduced in IWSNs over traditional carrier?sense multiple access with collision avoidance （CSMA/CA） based access for guaranteed channel access and improved reliability. The research community also contributed in many research oriented solutions targeting improved reliability， network lifetime enhancement and real?time data delivery. A few priority based schemes were also defined to offer hierarchical access to the wireless channel resources. In some cases， the use of multiple channels for enhanced data rates was also considered as well.
Despite these benefits and the recent improvements， the researchers are still struggling to offer substantial solutions for the improved reliability and real?time data delivery in IWSNs to match the strict deadlines as needed for the close loop process control and ultra?reliable and low?latency communications （URLLC） [17]， [18]. URLLC is mandatory for IWSNs， especially when dealing with emergency communications and regulatory and supervisory control feedback systems. To improve the overall acceptability of IWSNs， and to cope with fast paced improvements in industry and protocol stack developments， restructuring and procedural changes for URLLC in IWSNs are very important. Furthermore， the inclusion of Machine?to?Machine （M2M） communications in 5G offers potentially benefitting framework， targeting three main aspects of IWSNs： 1） supporting for large number of low?rate network devices， 2） sustaining a minimal data rate in all circumstances to satisfy the feedback control requirements， and 3） enabling very low?latency data transfer [19]. Further architectural improvements and procedure restructuring are necessary for 5G M2M infrastructure to address such requirements in IWSNs.
In this paper， a multi?channel TDMA based hybrid scheme is proposed， which benefits from the use of multiple?channels and short frame communication for the communication of time critical data. The proposed scheme targets real?time data delivery with improved data reliability. The effectiveness of the scheme is demonstrated with a test case with two frequency channels： one is used for the slotted access of the medium of communication for low latency networks， and the other channel used for the communication of the urgently required or critically needed information to be delivered to the control center within a specified time deadline. The second channel is also used for the retransmission of the failed communications to improve reliability of the communication taking place in the network. The time deadline enforced by the control society is taken into consideration to offer a reliable solution for close loop control systems in industrial plants.
The performance of the proposed work is also presented in this paper in comparison with the IEEE802.15.4e LLDN， specifically defined for industrial automation. Since the proposed scheme considers typical IWSNs， the work can further be extended to certain specific 5G network application areas. Communications for ubiquitous machine type devices， Moving Networks （MNs） and Ultra Reliable Communication （URC） [20] are some of the examples. Besides， as demonstrated in the results， the proposed protocol is able to use multi?channel diversity to incorporate a large number of nodes. This ability enables it to be considered for applications in 5G ultra dense networks （UDNs） [21].
The rest of the paper is organized as follows. Section 2 presents literature review. The proposed system model is presented in Section 3. Section 4 discusses the results and presents performance analysis. Finally， Section 5 gives conclusions and future directions.
2 Literature Review
The IWSNs are now widely used in various industrial processes. Different industrial wireless protocols are also defined to facilitate certain industrial applications and to encourage the use of IWSNs in these applications. Most of the industrial protocols presently used in the industry are CSMA/CA based and the core functionalities of physical and media access control （MAC） layers are inherited from IEEE802.15.4. Zigbee and 6LoWPAN are examples of such protocols. Although the specified protocols offer flexibility of operation and can be used to establish ad?hoc on?demand networks with the ability of active network formation and handling runtime changes， these protocols are more suitable for monitoring traditional wireless sensor networks （WSNs） applications. Since the suitability of WSNs in time insensitive applications is widely accepted， this paper is focused on time sensitive and reliable IWSNs.
In 2012， the IEEE 802.15.4e standard was launched， which mainly targets the critical applications of WSNs and primarily focuses on industrial environments where time?sensitive and information?critical data are to be routed [15]. It uses TDMA based channel access to ensure collision free access to the wireless resources on pre?specified and dedicated time slots. This standard also takes into consideration the low latency demands of the industrial processes and so a special framework， Low Latency Deterministic Network （LLDN） is introduced to meet the critical time deadlines for the emergency and close loop control applications. Some widely used industrial protocols （ISA100.11a， WirelessHART， etc.） also use TDMA based channel access.
Some recent researches have also targeted the priority based communications in IWSNs. In [22]， the authors established priority levels based on the critical nature of information. The entire traffic in the network is divided in four levels where the highest priority nodes get instant channel access and its communication is facilitated by allocating the channel bandwidth of low priority nodes to it. In other words， based on the assigned priority level， a high priority node can hijack the low priority traffic bandwidth. The protocol offers an improved QoS for high priority nodes at the price of the low priority nodes communication sacrifice. In [23]， the authors present an arbitration based protocol where each node is assigned with a unique frequency. The frequency assignment is linked to the priority of the node and hence， on the basis of these preassigned frequencies a priority?wise schedule of transmission is created. The protocol executes in two phases， an arbitration decision period and an arbitration execution period. In the arbitration decision period， each node that wants to communicate broadcasts its preassigned frequency to determine a deterministic channel access order. This allows each node participated in the arbitration decision period to know how many time slots it has to wait before its transmission can take place， thus， the node assigned with arbitration frequency of the highest priority communicates instantly， while the node with the lowest priority arbitration frequency waits until all the other nodes have communicated. The scheme allows multilevel priority system， however， it requires a special coordinator to identify the received arbitration frequencies and respond to them accordingly. The frequencies are also preassigned on the basis of the pre?specified priority system which implements static priority system. Furthermore， with the increase in number of nodes， the problem in defining orthogonal frequencies also surfaces.
The multi?channel schemes in IWSNs for MAC optimization offer improved medium utilization. Many schemes are presented in literature which use multi?channels to offer improvements in the existing scenarios. In [24]， the authors demonstrated the effectiveness of the multi?channel schemes in improving throughput over other schemes. In [25]， the authors took into account the benefit of the availability of multiple channels， defined a scalable media access and considered the limitation of the presently available sensor nodes. However， this scheme requires frequent channel hopping and has relatively high scheduling overhead. In [26]， the authors used TDMA based channel access in a multi?channel scenario. The scheme is relatively static and does not exploit the available resources. In [27]， a pseudo random scheduling was introduced where each node randomly decides two factors， the wakeup time and the channel sequence. The primary aim of the protocol is to distribute the traffic in the available communication resources. However， the scheme fails to offer an efficient traffic scheduling and resource sharing mechanism. In [28]， the authors used multiple channels to increase the network throughput. The proposed algorithm eliminates collisions by establishing coordinated transmissions. The scheme schedules both periodic and event based traffic using reinforcement learning to establish collision free transmissions on parallel data streams using multiple channels. However， the scheme fails to offer a differentiated treatment for different datasets of different priorities. It also fails to suggest a suitable alternate in case of communication failure. In [29]， the authors proposed a multi?channel scheme where the network is divided into sub?trees. Once divided， each sub?tree is allocated a unique channel. In [30]， the authors present a multi?channel scheme for the static networks. The scheme benefits from the TDMA based source aware scheduling. However， the scheme fails to give satisfactory assurance on reliability of the scheme. In [31]， the multichannel overhead reduction was achieved using the regret matching based algorithm. For the evaluation of the proposed scheme， both software and hardware based analyses were presented. In [32]， the authors proposed a hybrid scheme that uses both TDMA and CSMA/CA for communication purposes. The proposed work offers a mechanism to switch between the two access schemes based on the traffic density. The proposed protocol also considers multi?channel scenario. However， in this proposed scheme suitable reliability and QoS can only be achieved using much higher delays acceptable in critical industrial processes， making the scheme unsuitable for time critical and information sensitive industrial processes.endprint
The discussed schemes though offer suitable improvements in the existing scenarios， almost all of the encountered schemes fail to offer suitable plans for retransmission of failed communications. In most of the cases， the importance of retransmission is ignored， which results in extended delay and failure in deterministic behavior of the network. The proposed scheme in this paper focuses on how to ensure the retransmission of failed communications up to certain desirable extent within the superframe and time deadline， which helps in improving both the overall delay and communication reliability.
3 Proposed System Model
Feedback control systems play a very important role in automation and process control. In such applications， the IWSNs serve as the feedback path for the sensory information. For better control of the processes， the reliability of the feedback link is very important and can be termed as an integral ingredient for smooth running of control processes. The deadline in the discrete feedback systems also alters the performance of the implemented process control， hence， making the processes more time sensitive.
The proposed scheme uses TDMA based channel access to minimize interference and to ensure URLLC. Apart from this， the scheme also considers a multi?channel scenario where channels are effectively used to offer improved throughput， reliability and timely delivery. The proposed scheme focuses on short burst communications where a dedicated channel is used to facilitate the retransmissions and urgently required data. A detailed description of the network topology， superframe structure， channel specifications and system modelling is presented as follows. Before this， the frequently used parameters in the discussion are first listed in Table 1.
3.1 Superframe Structure
The proposed scheme targets improvement in MAC layer architecture by taking in account the availability of multiple channels. More specifically， the frequency and time division multiple access is utilized so as to improve the QoS and meet the time and reliability requirements of critical industrial processes. From the available frequency channels， a Special Purpose （SP） channel is specified which is dedicated for the short frame communications for highly time sensitive or erroneous packet communications. Any failures in transmissions from Regular Communication （RC） channels which require urgent retransmission due to sensitive nature of the data are facilitated by the SP channel. Superframe structure for the proposed system is presented in Fig. 1， where the superframe structure for RC and SP channels are presented. Except for the SP channel， all the channels use TDMA based access scheme for collision free communications. The SP channel， however， is implemented using CSMA/CA based as well as TDMA based access.
