How does the WirelessHART network protocol stack-up against the Zigbee wireless protocol? While we discuss specific differences between the two in detail in other installments, this installment of our technical discussion on the WirelessHART industrial network protocol will provide an overview of the differences.
WirelessHART and Zigbee share IEEE 802.15.4 as the basis of their physical layers, which is to say they transmit on the same frequency bands with the same channel designations. This allows them to use essentially the same hardware at about the same cost for transmitting and receiving. But beyond this lowest level, they begin to diverge. And even at the physical layer they have significant differences.
Parting of the ways
While both WirelessHART and Zigbee Pro feature frequency hopping to improve reliability, they differ in their implementations. A WirelessHART node hops every message, changing channels every time it sends a packet. Zigbee does not feature hopping at all, and a Zigbee Pro node only hops when the entire network hops.
At the MAC layer, WirelessHART utilizes time division multiple access (TDMA), allotting individual time slots for each transmission. Zigbee applies carrier sense multiple access with collision detection (CSMA/CD). As we discuss in the installment on TDMA vs. CSMA/CD, TDMA provides more reliable transmission.
Moving up to the network layer, WirelessHART represents a true mesh network, with each node capable of serving as a router. Thus, if one node goes down, another can “pick up the slack”, ensuring a packet reaches its destination. Zigbee utilizes a tree topology, which makes nodes along the trunk critical. See the installment on “It is a mesh network. True or False” for more details.
WirelessHART provides 100% back compatibility with wired HART at the highest level, the application level. As a design principal, HART doesn’t allow newer versions to erase or remove anything from older versions of the standards; newer versions are only allowed to add to older versions. This ensures users’ investments are fully protected.
While Zigbee often refers to the group of protocols maintained by the Zigbee Alliance, these protocols are not compatible with one another. They share the same basis for their physical layers, but Zigbee, Zigbee Pro, Zigbee RF4CE, and Zigbee IP are otherwise incompatible with each other just as they are incompatible with WirelessHART.
Bringing all together
WirelessHART is designed to provide reliable network communication in a backward compatible fashion. Intended to meet the needs of an industrial network, it fulfills those needs. The various protocols in the Zigbee group are also designed to address specific applications, but none meet the needs for an industrial network as well as WirelessHART.