In recent years, with the rapid progress of the WSN (wireless sensor network), wireless devices have been widely deployed in industrial environments to improve the efficiency of the industrial process. The corresponding wireless networking standards and protocols have also emerged one after the other.
The most widely applied standard in WSN is the WirelessHART (highway addressable remote transducer) which is a variant of HART with a simple, reliable, and secure communication between wireless devices for an industrial automated process.
WirelessHART is a sensor mesh communication system which operates in the 2.4 GHz ISM (industrial scientific medical) band at the physical layer and uses the TDMA (time division multiple access) at the data link layer. In the WirelessHART data link layer, time is separated into equal time slots and a certain number of slots constitute a super frame. In order to avoid conflict, each connection is arranged into a certain time slot with the help of TDMA technology.
However, WirelessHART does not provide a clear strategy in relation to dynamic routing, which is important for the reliability and robustness of the industrial wireless network applications.
In the graph routing protocol applied in WirelessHART, each node only has two parents. If these two parents are both blocked, the node will be totally separated from the network which has been proved to be an embarrassment.
In this paper, a dynamic routing strategy is proposed by evaluating and improving the collection tree protocol to make it work as a solution to the dynamic routing mechanism for IWSN (industrial wireless sensor network).
CTP is a tree-based protocol which is designed for relatively low traffic rates in WSN. Some nodes in a network can advertise themselves as the tree roots of this network. Other nodes generate routes to roots using a routing gradient.
Two possible mechanisms are proposed as optional solutions for dynamic routing in IWSN. The main method of the dynamic routing mechanism involves adding a CSMA (carrier sense multiple access) period into the TDMA by occupying one or several time slots in the TDMA.
The reason why a CSMA period has been added into the TDMA is that a distributed period is required in order to complete some jobs, which prove not to be possible within the TDMA period. In the TDMA, everything has already been fixed. Every node knows its parent and which time slot is assigned to which link.
However, the TDMA is unable to determine whether there are an unknown number of devices and thus the requirement for a distributed compet- ing period to complete, for example, the synchronization and topology generation.
In the second mechanism, if the CTP cannot generate a sufficiently good topology in a short period of time, then a setup period is added which is based on the initial CSMA so as to generate the topology and disseminate the link scheduling information. In the next super frames, several time slots are assigned to the CSMA period in order to maintain the topology.
This mechanism is based on an assumption that the topology generated in the setup period is relatively stable which means that the nodes will not change. their parents frequently.
The dynamic routing mechanism was evaluated with regard to three aspects: time for generating the topology, link quality, and stability of network through several simulation experiments. The simulation and evaluation results show that this mechanism can act as a solution in relation to the dynamic routing mechanism for the TDMA-based wireless sensor network.
According to the results of the simulation experiments, the time of the setup period of the dynamic routing mechanism has been determined but how much time should be assigned to the CSMA period in each super frame has not. Thus some experiments should be designed and conducted to determine the time of CSMA period in each super frame in the future.
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