In this first article on the reliable network communication protocol WirelessHART, we will focus our technical discussion on the OSI layers which are implemented in the protocol and their function within the protocol. Future articles will examine each of these layers in more detail.
The HART standard for networking smart field devices has been in around since the late 1980’s. WirelessHART was introduced with the release of HART 7 in 2007. The wireless protocol makes it easier and cheaper to use HART technologies, which already had the most field devices deployed of any field network.
Wireless opens the door to handheld devices for monitoring and controlling the operation of a plant. It also becomes easier to install a wireless device in previously inaccessible or cost prohibitive locations – such as the top of a reaction tank, inside a pipe, or at widely separated warehouses.
HART includes five layers of the OSI model: physical layer, data link layer, network layer, transport layer, and application layer. The main difference between the wired and unwired versions is in the physical layer, data link layer and network layer. In fact, wired HART does’t have a network layer. Let’s take a brief look at each layer.
The physical layer of the protocol is derived from the IEEE 802.15.4 standard. Basically, it is a subset of the IEEE standard with modifications. It only operates in one of the bands specified in the IEEE 802.15.4 standard – the 2450MHz ISM band. The physical layer employs 15 channels of the band which the data link layer exploits to increase reliability. Several transceivers developed to meet the IEEE standard have also been approved for use with WirelessHART, reducing development time and cost.
Data Link Layer
The data link layer introduces the use of superframes and time dimension multiple access (TDMA) technology to provide collision free, deterministic communication. Timeslots 10ms in length are grouped into superframes. These superframes are used to control the timing of transmissions to insure reliable communication and reduce collisions.
The data link layer employs channel hopping and channel blacklisting to increase security and reliability. In channel hopping, every time a transmission occurs, the channel is switched. Channel blacklisting identifies channels consistently affected by interference and removes them from use.
Network and Transport Layers
The network and transport layers cooperate to handle various types of traffic, routing, session creation, and security. WirelessHART establishes a mesh network, requiring each device be able to forward packets for other devices. Each device is provided with an updated network graph to handle routing duties. In reality, the network layer functions as a combined network/transport/session layer, handling all the function required by the protocol in those three layers of the OSI model.
The application layer handles communication between the gateway and devices through a series of commands and responses. This layer extracts the command from a message, executes the command, and generates a response. At this level, there is really no difference between the wired and wireless versions of the HART protocol.
WirelessHART provides a robust network communication system for field devices. Its utilization of existing standards allows the continued use of legacy devices while opening new applications and functionality. The OSI model provides an excellent tool for technical discussion of this popular network protocol. Next time, we will explore frequency hopping as we delve into the realm of the physical OSI layer more deeply.