The industrial wireless sensor network (IWSN) has gathered more and more notice in recent years. Superior reliability, determinism, timeliness, and security are emphasized. Since 2007, three standards, the WirelessHART, ISA100.11a, and WIA-PA, have been released based on the IEEE 802.15.4.
One of the strong similarities of them is the TDMA (time division multiple access) -based media access mechanism. In the TDMA mechanism, all communications among nodes are allocated and limited within corresponding timeslots.
For example, WirelessHART uses 10ms fixed timeslots which allows for maximum packet sizes while still allowing time drift to be technical feasible. ISA100.11a needs flexible time slots to allow for duocast. WIA-PA uses 802.15-4-2006 super-frame with configurable timeslot too.
This is essential to reduce the possibility of collision (and thus increase the communication reliability), and to meet the critical requirement of timing determinism of industrial applications. To do this, all the nodes must be synchronized precisely, i.e. the jitter of synchronization should be much smaller than the length of time slot. Also, the stack designer must guarantee that the node can finish everything within the time slot.
Such timing critical requirement has become one of the primary challenges to design the protocol stacks. Firstly, it is challenging to finish the complicated tasks (packet parsing, encryption, decryption, authentication, etc.) within such a short timeslot by the processor with limited resource (clock frequency, memory, energy supply, etc.). Secondly, the IWSN stacks are often only a part of the timing critical tasks that the device should execute. It is much more difficult to ensure the timing integrity in a complicated multi-task system.
At the same time, the rapidly increasing complexity and other specific requirements of industrial systems have made it necessary to adopt the real time operation system (RTOS) in the IWSN stacks. These requirements include safety, security, availability, support for actuators, system integration, network size, product life cycle, etc.. Furthermore, the use of multiprocessor (and/or multi-core) architecture has become another trend in the industrial system design because it is an effective way to improve the system scalability and manage the complexity and cost.
The multi-processor support of IWSN stacks is, therefore, needed to follow this trend. Additionally, dedicated high performance chips for IWSN are rare, but low cost (also low performance) IEEE802.15.4 system-on-chips (SoCs) are very common in commercial markets.
It is attractive if we can use these low cost commercial chips combining with high performance industrial processors, so- called “low-high combination”. So, the multi-processor support of IWSN stacks is a realistic requirement for the best trade-off between performance and cost.
In this paper, we propose an RTOS-based architecture for IWSN stacks with multi-processor support. This architecture offers significant benefits in terms of platform independency, product life cycle, safety and security, system integration, and performance scalability.
The stack comprises two major parts: the stack core and the platform abstraction layer (PAL). The stack core is the body of the protocol which is platform independent. It is composed of a Data Engine for zero-copy data management, a Protocol Engine to handle inter-layer communication, and all the layers specified by the standard.
The PAL is an abstraction of the implementation platform including the memory, radio frequency transceiver, and RTOS. So an instance of a PAL is platform dependent. By separating the PAL from the stack core, the stack core is isolated from the implementation platform.
In most industrial applications this is important since the communication stack is normally only a small part of the total device application and the design cannot be designed according to the communication stacks. In order to port the stack from one platform to another (e.g. to use new hardware or RTOS or reuse the implementation in new products), we only need to modify the PAL instead of the stack core.
This could significantly simplify the porting work which is important to prolong the product lifecycle of the industrial systems as aforementioned. An implemented WirelessHART stack has proven the feasibility of the proposed architecture in practice and challenges are discussed as well.
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