Wireless systems are replacing wired data transmission to an increasing extent, especially in sensor networks and control applications, where the cost of installation or modifications are a significant consideration, but this calls for ultra low power design techniques to provide extended battery life.

Of the two most widely established RF standards, Wi-Fi/802.11 and Bluetooth, neither however, is really suited to most control applications, since these are reliant on small batteries with long life. Furthermore, high data transmission rates are not necessary as only a few control commands and measurements are involved.

IEEE 802.15.4 – the base for ZigBee
Enter the IEEE 802.15.4 standard, introduced in October 2003, and more recently, IEEE 802.15.4-2006 (Rev B), approved in June 2006, to fulfill the need. This standard describes point-to-point transmissions in Personal Area Networks (PAN) and defines the Physical Layer (PHY) and Medium Access Control (MAC) Layer for low power, low speed and robust RF transmissions. The typical transmission range varies from 10 to 30 metres indoors, and up to 150 metres outside with long battery life.

Operating frequencies have first to be defined of course in implementing the changeover. As can be seen, 433MHz and 868MHz are freely available in Europe, whilst in the US, the available frequency bands are 315MHz and 915MHz. Because the same antenna can be used for 868MHz and 915MHz, one option is to use a flexible, freely programmable RF transceiver operating alternately on the two frequencies to provide a world wide solution. However, this increases cost logistics, because of the need for two different firmware versions.

Only the 2.4GHz band, also used by WiFi and Bluetooth, is available everywhere and is therefore used for many universal applications.

Software Stack
The ZigBee Alliance is an association of more than two hundred companies working together to enable reliable, cost-effective, low-power, wirelessly networked monitoring and control products based on an open global standard. Promoters of the Alliance are Texas Instruments, BM Spa, Ember, Freescale, Honeywell, Mitsubishi Electric, Motorola, Philips, Samsung, Schneider Electric, ST, Siemens and Huawei. It was this alliance that defined the ZigBee stack, which is a standard protocol stack on top of the IEEE 802.15.4 PHY and MAC.

ZigBee targets six application areas: home automation including automatic meter reading (AMR), lighting, heating, security and white goods health status monitoring; commercial building automation including heating ventilation and air condition (HVAC), energy management, security; industrial automation; hospital and patient care; asset tracking / active RFID and wireless sensor networks.

The ZigBee stack, on top of the point-to-point communication via IEEE 802.15.4, enables implementation of a personal area network of ZigBee nodes. Possible network topologies are star, tree or mesh.

Each ZigBee PAN needs one coordinator device, which starts the network and assigns it a PAN-ID. PAN-coordinators are always capable of packet routing. In tree and mesh networks there are always several nodes with ZigBee router functionality. These router nodes can forward received data packages to the next ZigBee nodes thereby enabling packet hopping from a sender to a receiver through the complete ZigBee network.

ZigBee end devices only talk to their parent node, which is either a PAN-coordinator or a router. Known as reduced function end devices (RFD), because they do not need routing capabilities, they have a much smaller stack, which reduces the amount of programme flash and data memory RAM and flash required, making them very cost attractive. Furtermore, since the microcontroller and RF-transceiver can be turned off most of the time, RFDs consume minimal power, unlike a device with router functionality, which has to be ready to receive a data package at any time.

ZigBee uses 16-bit node addressing, which theoretically allows nearly 2 x 16 ZigBee nodes in one PAN. In praxis the number of nodes is limited by data package latency. For star networks less than 2,000 nodes is realistic.

One ZigBee node with one radio is capable of holding several applications, e.g. a switch, temperature measurement and humidity measurement in one box. In this case the APS decides which of these applications should be the destination for this data packet. Above all, the ZDO (ZigBee device object) helps to coordinate operation of application software and the ZigBee stack software. It is also possible to integrate a security service to enable secure data transmission.

ZigBee Profiles
Vendor independent compatibility of ZigBee products is only given if they all comply with one of the ZigBee standard profiles. Since acquiring Chipcon, Texas Instruments has been offering a ZigBee compliant 2.4GHz transceiver with associated software stack.

The ZigBee Alliance defines profiles as a method of ensuring application level interoperability. Besides the protocol, the profile also defines the data content sent to another device, e.g. which data content will turn on a lamp and, with data content, will turn it off again. Definitions in a profile are unique profile ID, device types, message format, content coding and interpretation of clusters. Standard profiles, which have already been defined are home automation, industrial plant monitoring and commercial building automation including HVAC. Others like automatic meter reading and healthcare are in discussion.

Proprietary profiles can be defined by the customer, but this limits application interoperability to devices that share this profile. A proprietary profile does not exclude network level interoperability.

Dipl. Ing. Peter Forstner is a member of group technical staff, Texas Instruments