Introduction to WirelessHART

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Technical Background

A Wireless Sensor Network (WSN) consists of spatially distributed individual sensor nodes which work together to monitor physical and environment parameters, and transmit monitor result to base station. One of the common use structures of WSN is showed in figure 2.1. Sensor nodes are deployed into a monitoring region, and constitute a wireless ad-hoc network automatically. Sensor nodes support multi-hop algorithm, and together, they forward data packets to the base station. Some of the sensor nodes in network may be fixed at a certain position in order to monitor some unmoved parameters at its appropriate place.
The environment can be either the physical world or an information technology framework. That is why WSN can be used in many industrial and civilian applications, including industrial process monitoring and control, healthcare applications, habitat monitoring, home automation and many others [5] [6] [7].
Sensor node typically consists of one or more sensors, radio transceiver, a small microcontroller, and an energy source, usually a battery. Its size can be as small as a grain and its price can down to a penny.
The main features and challenges of WSN can be summarized as:

  • Low cost devices
  • Energy-efficient devices
  • End-to-end quality of service
  • Seamless operation under context changes
  • Secure operation
  • Large scale of deployment
  • Mobility of nodes

Here in this thesis, we use WSN, with self-development standard, for industrial automation applications

IEEE 802.15.4

IEEE 802.15.4 – 2006 is a standard which specifies the physical layer and media access control (MAC) for low-rate wireless personal area networks (LR-WPANs). It is revised version released in September 2006. It is maintained by the IEEE 802.15 working group.
The targeted application for IEEE 802.15.4 is focus on low-cost, low-speed areas like wireless sensor network, home network, industrial control, remote monitor, building automation, and so on. Beside, these applications usually has low bitrates (up to some few hundreds of kbps), flexible, ad-hoc self-organize, not too stringent delay guarantees, and sometime low power consumption requirement.
The physical layer, based on direct sequence spread spectrum (DSSS), offers bitrates of 20 kbps (a single channel in the frequency range 868-868.6 MHz), 40 kbps (ten channels in the range between 905 and 928 MHz) and 250 kbps (16 channels in the 2.4 GHz ISM band between 2.4 and 2.485 GHz with 5- MHz spacing between the center frequencies). Even there are 27 channels available, but the MAC protocol use only one of these channels at a time. This standard is not a multichannel protocol [4].
ZigBee, WirelessHART, and MiWi specification use the services offered by IEEE 802.15.4 and attempts to offer a complete networking solution by developing the upper layers which are not covered by the standard. Our designed specification also follows this standard (using 2.4 GHZ ISM band in physical layer and super frame structure with CSMA/CA protocol in MAC layer) and adds network construction (mesh networks), security, application services, and more

Introduction to WirelessHART

WirelessHART is a secure and Time Division Multiple Access (TDMA) based wireless mesh networking technology operating in the 2.4Ghz ISM band. WirelessHART is an optional HART Physical Layer that provides a low cost, relatively low speed(e.g., compared to IEEE 802.11g) wireless connection to HART-enabled devices. TDMA is used to schedule the communication of the various devices. All communication is performed within a designated time slot of 10ms. A series of time slots form a superframe. WirelessHART also enables channel hopping to avoid interference and reduce multi-path fading effects. One or more sources and one or more destination devices may be scheduled to communicate in a given slot. The slot may be dedicated to communication from a single source device or a slot may support shared communication.
HART is loosely organized around the ISO/OSI 7-layer model for communication protocols (see Figure 2.2) [3]. With the introduction of wireless technology to HART, two Data-Link layers are supported: the token-passing and TDMA. Both support the common HART Application layer.
All the communications of the WirelessHART network pass through the gateway. Consequently, the gateway must route packets to the specified destination (network device, host application or network manager). The gateway uses standard HART commands to communicate with network devices and host applications. The plant automation network could be a TCP based network, a remote IO system, or a bus such as PROFIBUS DP. The network manager creates an initial superframe and configures a Gateway. A detailed description of the components of a WirelessHART network is given in [2]. The structure of WirelessHART network is shown in the figure.

Physical Layer

TheWirelessHART physical layer is based mostly on the IEEE STD 802.15.4-2006 2.4GHz DSSS physical layer [5]. This layer defines radio characteristics, such as the signalling method, signal strength, and device sensitivity. Just as IEEE 802.15.4 [5], WirelessHART operates in the 2400-2483.5MHz license-free ISM band with a data rate of up to 250 kbits/s. Its channels are numbered from 11 to 26, with a 5MHz gap between two adjacent channels.

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Data – Link Layer

One distinct feature of WirelessHART is the time synchronized data link layer. WirelessHART defines a strict 10ms time slot and utilizes TDMA technology to provide collision free and deterministic communications. The concept of superframe is introduced to group a sequence of consecutive time slots. Note a superframe is periodical, with the total length of the member slots as the period. All superframes in a WirelessHART network start from the ASN (absolution slot number) 0, the time when the network is first created. Each superframe then repeats itself along the time based on its period

Network and Transport Layer

The network layer and transport layer cooperate to provide secure and reliable end-to-end communication for network devices. As shown in Figure 4, the basic elements of a typical WirelessHART network include: (1) Field Devices that are attached to the plant process, (2) Handheld which is a portable WirelessHART-enabled computer used to configure devices, run diagnostics, and perform calibrations, (3) A gateway that connects host applications with field devices, and (4) A network manager that is responsible for configuring the network, scheduling and managing communication between WirelessHART devices.
The WirelessHART standard does not give any specification concerning about a particular scheduling algorithm to be used in a WirelessHART network.

 Introduction to Collection Tree Protocol

The Collection Tree Protocol (CTP) is a simple tree based protocol proposed for collection of data from sensor nodes into the root node. All the communication is many to- one or one-to-many. The nodes form a set of routing trees, whereas multiple root nodes are allowed. The CTP is address-free protocol. The sensors send data packets to the next hop whereas routes are based on a routing gradient. The protocol has the following properties [9]:

  • CTP assumes that it has link quality estimates of some nearby neighbours. These provide an estimate of the number and quality of transmissions it takes for the node to send a unicast packet whose acknowledgment is successfully received.
  • CTP has several mechanisms to improve delivery reliability, but it does not promise 100 % reliable delivery. This has been evaluated on 12 test beds using 20 to 310 range sizes of nodes. It is best effort, but a best effort that tries very hard.
  • CTP is designed for relatively low traffic rates. Bandwidth-consuming systems might benefit from a different protocol, which can, for example, pack multiple small frames into a single data-link packet.
  • CTP uses expected transmissions (ETX) as its routing gradient. A root has an ETX of 0. The ETX of a node is the ETX of its parent plus the ETX of its link to its parent. If several routes are valid, CTP should choose the one with the lowest ETX value.
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This protocol was proposed for data gathering with low energy demands. Routes established during node deployment are not updated periodically, but only if inconsistency in the network topology is detected. A loop is detected when node chooses a route with gradient value significantly higher than its old one. This may be caused by losing connectivity with the current parent node. The CTP offers two solutions: (1) beacon frame and (2) ignoring routes with an ETX higher than a reasonable constant. The protocol provides also a handle for packet duplication. When a duplicate instance of a packet is detected during its forwarding, it is dropped.
The CTP is designed for minimum power consumed consumer products. It has no time synchronization mechanism for the industrial process automation. Also the reliability can be improved using channel hopping technique. Our aim is to propose such improvement of CTP that offers high reliability compare to original CTP and to be a time synchronized network

 Related Work

Time Synchronized Mesh Protocol (TSMP) [12] was designed to improve reliability. TSMP employs frequency hopping: different links use different frequency channels and the same link hops during its lifetime across different channels. This reduces the impact of narrow-band interference and persistent multipath fading. [13] presents experimental results in which 44 nodes run TSMP for 26 days in a printing facility. The authors shows how channel hopping combined with a retransmission policy. TSMP uses a central coordinator which retrieves the list of nodes, their neighbours and their traffic requirements. This allows it to construct a schedule which is then communicated back to the network.
Critical applications require reliable solutions, and channel hopping is one answer to this need. With the success of proprietary solutions such as TSMP, standardization bodies have been working on similar solutions.
TSMP and WirelessHART uses central controller to schedule communication. The reliability is increased by having each node maintain connectivity to at least two parent nodes in the routing graph, enabling the network to resist link failures. Another industrial wireless standardization body is the ISA100 Wireless Compliance Institute. Their latest standard ISA100a [14] is similar in essence to TSMP or WirelessHART, yet features interesting channel hopping mechanisms

1. Introduction
1.1. Background
1.2. Purpose and Aim
1.3. Delimits
2. Technical Background 
2.1. IEEE 802.15.4
2.2. Introduction to WirelessHART
2.3. Introduction to Collection Tree Protocol
2.4. Related Work
3. Methodology and Implementation process 
3.1. Research Method
3.2. Implementation process
3.3. TinyOS v2.0
4. WirelessHART
4.1. WirelessHART MAC protocol
4.2. Source Code – TSCH (Time Synchronized Channel Hoping)
5. Review of the Collection Tree Protocol (CTP)
5.1. CTP routing and Data Packets
5.2. Link Estimator module
5.3. Routing Engine module
5.4. Forwarding Engine
6. CTP over WirelessHART 
6.1. CTP – TSCH packet structure
6.2. CTP – TSCH wiring and working
7. Experiments and Results 
8. Conclusion and Future Work 
References
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