INDUSTRIAL WIRELESS NETWORKS COMPARING THE STANDARDS PART 2 Glenn Johnson, Editor
In Part 1 of this article, we reviewed the history of wireless sensor networks (WSNs) and defined the common standards used in industrial wireless networks. In this part we compare more closely the two prevalent wireless mesh network standards designed for use in process plants: WirelessHART and ISA100.11a.
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lthough WirelessHART1 and ISA100.11a2 are in many ways the same, being both based on IEEE 802.15.43 at the PHY and MAC layers — and designed effectively for the same purpose — there are still some key technical differences between the two standards. Those differences are due to fundamental differences in design philosophy, one opting for ease of deployment and multivendor compatibility, the other choosing flexibility and scalability as key design features.
Network architecture ISA100.11a uses backbone routers for bridging subnets. Backbone routers limit the throughput into and out of a single subnet to the throughput of one radio, but backbone routers can be used in parallel to create a very large wireless network. Since addressing is based on IPv6, there is really no practical address limitation. WirelessHART uses access points that can be used in parallel to merge subnets into a very large address space. Since the short address used in the WSN is an unsigned integer, however, addresses are limited to 30,000 in a single WirelessHART network.
Frequency hopping Both WirelessHART and ISA100-11a use the radio interface in the 2.4 GHz ISM band as described in IEEE 802.15.4 — divided into 16 channels, using a 250 Kbps data transmission rate. The communication reliability is increased through frequency diversity. Channel hopping in WirelessHART is dictated by the standard, meaning that devices from different manufacturers are interoperable by design. In ISA100.11a, there are three different defined channel hopping algorithms, and the user must specify which one is to be
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used. Users purchasing ISA100.11a devices will need to ensure that the purchased devices support channel-hopping schemes that are compatible with one another. In both WirelessHART and ISA100.11a, clear channel assessment (CCA) and channel blacklisting are used to combat the influence from other wireless networks. WirelessHART employs manual channel blacklisting, where a network operator must manually configure which channels are available and which channels are blocked. ISA100.11a has an adaptive blacklisting mechanism, where each device in a network may autonomously blacklist channels that suffer from noise or interference. ISA100.11 also defines four different CCA modes, where modes 1-3 are defined by IEEE 802.15.4: 1. No CCA: CCA is disabled, and not conducted prior to transmission. 2. Energy above Threshold: Reports a busy channel upon detecting energy above a configurable threshold. 3. Carrier Sense Only: Reports a busy medium if a signal compliant with IEEE 802.15.4 is detected. 4. Carrier Sense with Energy above Threshold: CCA reports a busy medium using a logical combination of Modes 1 and 2. WirelessHART, on the other hand, has fixed its CCA mechanism to mode 2. With the correct configuration, ISA100.11a should be better at handling coexistence with Wi-Fi networks, while WirelessHART will only listen for activity from other IEEE 802.15.4 networks.
TDMA Both WirelessHART and ISA100.11a implement time diversity through TDMA. The main difference is that WirelessHART uses a fixed 10 ms timeslot, whereas in ISA100.11a it is configurable between 10 and
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