Dr. Hossein Eslambolchi
April 2012
Zigbee was designed to operate in “Pico-cellular” environments, perhaps 150 feet radius, to provide a standardized, IP-compatible replacement for today’s special purpose monitoring and control radios. These radios do not interoperate and interfere with other communications technologies.
The underpinnings of Zigbee are:
- A PHY designed to support high levels of radio integration, power-efficiency, and spectral compatibility with other popular communications standards like 802.11.
- A MAC designed to allow multiple network topologies and built-in power management, supporting many active devices simultaneously.
- A network layer supporting spatial growth without high-power transmitters. The network layer also can handle large amounts of nodes with relatively low latencies.
Zigbee operates in two unlicensed bands, 2.4 GHz and 868/915 MHz with data rates of 250 Kbps at 2.4 GHz, 40 Kbps at 915 MHz, and 20 Kbps at 868 MHz in Europe. The PHY is based on a direct-sequence spread spectrum waveform occupying either 5 MHz (2.4 GHz) or 2 MHz (868/915 MHz). It is optimized for duty cycles less than 0.1 percent, and batteries last from several months to several years. It can operate in peer, star, and mesh topologies, supporting an address space of 18.45 x 1018 separately identifiable devices (using the 64 bit IEEE MAC address framework) and 65,534 nodes per network. The MAC provides transmission capability for periodic data at application-defined rates (culled from sensors), intermittent data triggered by applications or external stimuli (light switches, for example), and repetitive low latency data allocated in slots (mouse data, for example), complete with acknowledgments.
Each of these traffic types mandates different attributes from the MAC.
- Periodic data is accommodated using a “beaconing” system whereby the sensor will wake up for the beacon, check for any messages and then go back to sleep.
- Intermittent data can be handled either in a beaconless system or in a disconnected fashion. In a disconnected operation the device will only attach to the network when it needs to communicate saving significant energy.
- Low latency applications may choose to utilize the Guaranteed Time Slot (GTS) option. GTS allows each device to transmit in a specific time interval within each super frame without contention or latency.
The IEEE standard defines two types of devices to allow vendors to supply the lowest possible cost devices: Full Function Devices (FFDs) and Reduced Function Devices (RFDs). FFDs can function in any topology, can be a simple or network coordinator, contain routing facilities, and can communicate with any other device. RFDs are limited to star topology, cannot become coordinators, can communicate only to a network coordinator, but are very simple to implement. An IEEE 802.15.4 Zigbee network requires at least one full function device as a network coordinator, but endpoint devices may be RFDs to reduce system cost.
It’s apparent that Zigbee networks are well-designed to form the lowest tier of a “network of networks”. As such, a Zigbee network coordinator can be directly attached to a Wi-Fi node, providing networking of data to any point in an IP-based network.
RF Identification wireless, or RFID, is the ultimate extension of expendable radio technology. Envisioned as an augmentation or replacement for product bar codes, RFID tags may contain only a tiny silicon chip and an antenna. Most RFID devices do not form networks, but instead respond to individual interrogation through a reader or scanner. EPC and ISO-18000 compliant tags contain protocol features for avoiding simultaneous responses if more than one tag is within the scanner field at a time.
RFID tags are categorized as either active or passive. Active RFID tags are powered by an internal battery and typically provide both read and write facilities — tag data can be rewritten and/or modified. An active tag’s memory size varies according to application requirements; some systems operate with up to 1MB of memory. In a typical read/write RFID system, a tag might give a machine a set of instructions, and the machine would then report its performance to the tag. This encoded data would then become part of the tag’s history.
The battery-supplied power of an active tag generally enables a longer scanning distance and larger memory. Battery life can extend to a maximum of 10 years, depending upon operating temperatures and battery type. For cost reasons, the active RFID tags are fewer in number and are not the driving reason for RFID proliferation.
Passive RFID tags operate without a separate external power source and obtain operating power generated from the reader. Passive tags are consequently much lighter than active tags, less expensive, and offer a virtually unlimited operational lifetime. However, they operate at shorter ranges than active tags — inches for LF and HF tags and up to 100 feet for UHF and microwave tags — and require a high-powered reader field.
Read-only tags are typically passive and programmed with a unique set of data (usually 32 to 128 bits, 96 for Electronic Product Code or EPC tags). For Class 0 tags, these cannot be modified. Read-only tags most often simply provide a reference number to a database, in the same way as linear barcodes reference a database containing modifiable product-specific information. RFID tags are 96 bits in length with an additional 16 bit CRC. Passive tags can also be written to (EPC Class 1 tag), with a typical write range of 70 percent of the read range.
RFID systems are also distinguished by their frequency ranges. Low-frequency (30 KHz to 500 KHz) systems have short reading ranges and lower system costs. They are most commonly used in security access, asset tracking, and animal identification applications.
High-frequency (850 MHz to 950 MHz and 2.4 GHz to 2.5 GHz) systems, offering long read ranges (greater than 90 feet) and high reading speeds, are used for such applications as railroad car tracking and automated toll collection. However, the higher performance of high-frequency RFID systems incurs higher system costs. It is the EPC UHF tag that Wal-Mart and other retailers have chosen for their RFID rollouts.
The significant advantage of all types of RFID systems is that they do not require contact between the reader and the tag, and the reader and the tag do not have to be within line-of-sight. Even short-range tags can be read through a variety of substances such as snow, fog, ice, paint, crusted grime, and other visually and environmentally challenging conditions, where conventional barcodes or other optical technologies would be useless. Longer-range tags can also be read in challenging circumstances — at high speeds, for instance. In most cases, these tags respond in less than 100 milliseconds. For EPC tags, 200 reads per second are theoretically possible.
The read/write capability of an active RFID system is also a significant advantage in interactive applications such as work-in-process or maintenance tracking. Though it is a costlier technology compared to barcodes, RFID may become indispensable for a wide range of automated data collection and identification applications that would not be possible otherwise.
Developments in RFID technology continue to yield larger memory capacities, wider reading ranges, and faster processing. It is highly unlikely that the technology will replace barcodes — even with the inevitable reduction in raw materials coupled with economies of scale, the integrated circuit in an RF tag will never be as cost-effective as a barcode label.
There will always be a cost-benefit trade-off for the technology. However, RFID should continue to grow in areas where tracking of high-value items is necessary, where efficiencies in the supply chain exceed the incremental cost of the technology, and where barcode technologies are not effective or cannot be sufficiently automated.
If RFID is standardized so that equipment from different manufacturers can be used interchangeably — the market may grow exponentially.
RFID use is currently being stimulated by mass-market retail stores who seek to further automate and simplify inventory, checkout, and sales tracking operations. It is also being driven by the government, which is mandating use for many applications. The widespread use of tags would provide the capability of near-real-time data collection from a huge number of objects and applications, particularly if RFID readers are connected to IP-based communication systems, including wireless networks.
Although the amount of data exchanged is small on a per-unit basis, the amount of aggregated data from all sources could be substantial. The packet character of the data is ideal for IP-based communication systems, and offers potential for services which combine communication, database collection, organization, and interpretation.
By 2025, there will 250 billion devices connected to the Internet. Zigbee and RFID will represent a significant portion of them. The sheer number of these devices raises questions of scale and reliability; but the security of these devices will define the true convergence of major applications and systems in 21st century.
