01 May 2004
ZigBee short on power by design
By Dick Caro
The IEEE 802 standards committee develops both wired and wireless data communications network standards. In the definitions or scope of the IEEE 802 committee, the wireless networks lay out by their nominal network transmission distances.
ZigBee is an industry consortium that promotes the use of low-power networks for less demanding performance applications such as home automation, industrial automation, building auto-mation, and toys.
The emphasis of ZigBee is on power conservation for battery or other power-sensitive applications. The ZigBee Alliance has supported the development of IEEE 802.15.4 for its purposes.
Along the way, the former HomeRF Consortium dissolved, and many of its former sponsors have moved to support ZigBee.
There are few commercial implementations of ZigBee. Chipcom announced its silicon supporting ZigBee, and Motorola announced its 802.15.4 solution. Both operate only in the 2.4 gigahertz (GHz) band. Motorola uses its M68HC08 microcontroller family with its RF Packet Radio chip. The 802.15.4 and ZigBee protocol stacks are in software. Several other foundries, including those of Intel, Motorola, Atmel, and Phillips are developing silicon.
The primary difficulty has been achieving the low-power specifications necessary to support the battery and alternate power sources envisioned by the committee.
Ember Corp. announced that its EmberNet products are coming out with Chipcom silicon for wireless sensor networking. Ember's previous products have used more proprietary radios that operate in the same 915 megahertz (MHz) and 2.4 GHz industrial, scientific, medical (ISM) bands as ZigBee. The release of the ZigBee products is a significant event.
Millennial Net has announced sensor networking products that conform to the ZigBee specifications. Millennial produces I-Bean products as components to work product manufacturers. One recent product uses an "energy harvesting" technology from Ferro, in which ambient vibration is used to power the communications interface, entirely without the need for batteries. ZigBee's low power consumption makes this configuration possible.
Mattel supported the early development work of IEEE 802.15.4 for its relevance to Mattel's radio-controlled toys, Leviton to support its radio-controlled lighting, and Eaton/Cutler-Hammer for its relevance to Eaton's industrial automation products. These applications have ensured that the requirements for very low power remain among developers' priorities.
IEEE 802.15.4 defines a low-level radio interface for a network that is capable of transporting data through areas of high electrical noise and metallic interference at nominal distances up to 100 meters. In addition to robust noise rejection, the standard stipulates the use of mesh networking to overcome direct line-of-sight obstructions and to provide alternative path routing in cases of temporary network outages. Mesh networking also provides a convenient way to expand the coverage distances for ZigBee networks, because the distance limit only applies to the most distant unit. The ZigBee protocol provides the necessary mechanism for removing redundant messages received from alternate paths in the mesh.
IEEE 802.15.4 only defines the communications radio (physical layer) and protocol (data link layer) for both star (point-to-point) and peer-to-peer topologies. The ZigBee Alliance has defined the network layer that specifies star, tree-cluster, and mesh network topologies. Additionally, ZigBee is defining the application layer profiles for several applications. The initial application areas are industrial automation, home control, and building automation. There will also be a profile for automated meter reading.
Networks tend to be scattered
Two of the primary goals for 802.15.4 are low cost and low power, which leads to low complexity and simplicity. Negotiating for data rates increases a protocol's complexity, so 802.15.4 uses just two different data rates: 250 kilobits per second for high speed and 20 kilobits per second for slow-speed, very low-power applications.
Networks of sensors and actuators used in process control tend to be scattered, while the sensors and actuators used for factory automation tend to align with the large machines used. Typically, a star network can be expensive in terms of wiring, but a star network is very simple and inexpensive for wireless networks for either factory automation or process control. However, when the distance for any one device exceeds the maximum, or when devices need to communicate with other local devices, peer-to-peer networking may be more efficient. The form of peer-to-peer networking that bundles with the IEEE 802.15.4 data link layer is very simple, and it enables the formation of clusters for tree topology and for implementing mesh networking at the ZigBee network layer.
ZigBee supports low-latency devices. Some devices produce very little data, such as a pulse when an event occurs, but they produce it frequently. Photocells that count products and tachometers that produce speed data are some examples. IEEE 802.15.4 provides guaranteed time slots for these types of devices when one cannot recover missing data or even a single pulse.
The basic protocol of 802.15.4 is Carrier Sense Multiple Access with Collision Avoidance (CSMA-CA). On very simple networks the nonbeacon mode is effective, as it allows the occasional collision and retransmission. On critical networks, beacon mode is used. Here, the node, acting as a network coordinator, arbitrates network traffic to prevent collisions by assigning nodes to one of 16 specific time slots. On larger tree cluster and mesh networks, some nodes serve as network routers too, and these nodes assign time slots to prevent collisions. All nodes can then sleep (low-power mode) whenever they are not scheduled to send or receive during a slot time.
Devices sleep until they are ready to determine if there are any messages. They awake in time to examine their time slot and take any appropriate action if there is a message. If not, then they can return immediately to sleep. It has been estimated that most nodes in a beacon network will remain asleep approximately 97.5% of the time. Sleep invokes the low-power state to save power in the microcontroller, to reduce the clutter in the frequency band, and to avoid most sources of interference.
Devices usually have a short address (16 bit), which limits the number of nodes in any one subnet to 255. The subnet is necessarily that number of stations a single beacon manages.
The system acknowledges each message sent to a node by using a highly efficient short frame. Acknowledgement guarantees delivery and is the form of confirmed services that 802.15.4 specifies.
Low power consumption comes about by using a very simple protocol and by allowing all battery-powered remote nodes to sleep most of the time. The time-slotted services enable each node to sleep during most of the time when it is waiting for its slot.
The standard specifies that 802.15.4 technology should operate at three different frequency bands to accommodate some of the frequency assignments in the different countries where the standard will probably take affect. There are 16 channels in the 2.4 GHz ISM band that are applicable everywhere in the world, 10 channels in the 915 MHz ISM band that are applicable only in North America, and one channel in the European 868 MHz band.
The feature of the 802.15.4 protocol that contributes most to long battery life is the extremely low duty cycle. Each battery-powered network node sleeps about 97.5% of the time. The active router nodes that generate the beacons awaken on the beacon time schedule, and they are awake more than the end nodes.
Behind the byline
Dick Caro is an ISA Life Fellow. This article comes from his new book Wireless Networks for Industrial Automation, ISA Press, 2004. Write him at email@example.com.