Tuesday, June 9, 2015

Zigbee: Low Rate Communication as Basis of Internet of Things

Picture taken from: Sensors

Recent people are expecting more and more on data transmission speed. They are waiting for let's say when is 5th generation of mobile communication will be available. Or, wondering whether broadband communication that has been deployed in resident areas can still be improved or not. But, we do not fully aware about the necessity of low rate technology is inevitably. In the coming of Internet of Things era, we indeed need this technology.

Why would we need such low speed data transfer? The answer is because not all data we want send has big amount of data. Neither are they always necessary to be sent quickly. And since "energy saving campaign" is getting louder and louder being announced, we need to take into this issue seriously as well.

Let's take a look one by one of these reasons. Suppose we're expecting to have home automation system. There, we have light bulbs, door automation, alarm for security, etc. To enable those devices, it is only required tiny amount of data because the data only functions as switch. We now can imagine the necessity of low data rate transmission, can't we? Another possible implementation of Internet of Things is monitoring for farming, for instance grape ripening. The idea is to update grape condition periodically such that we know when it can be harvested. I am pretty sure that data of grape conditions are not as big as half-hour HD video uploaded on Youtube. In addition, we do not expect either to send the status of grape in every minute or hours.  We can make the cycle for daily basis for instance. These keys, low data rate and low duty cycle, bring out to low energy consumption. Eventually, its effect is on reducing energy cost.

Zigbee technology has been trying to realize this not-really-popular section towards society. Started more than a decade ago, Zigbee has been trying to convince that this communication system is about to emerge. Years later, along with Internet of Things, Machine-to-Machine, Wireless Sensor Network, you name it, are getting more popular, so is Zigbee technology. People are more aware about Zigbee existence. More companies has been developing solution based on Zigbee. Furthermore, more and more people are also implementing the application into their daily life either for personal usage or for industrial usage.

Technology of Zigbee itself has been standardized by IEEE. It follows 802.15.4 standard, Wireless Personal Area Network (WPAN) which low speed data communication. However, it only regulates two basis OSI layers: physical layer and MAC layer. The remaining layers are regulated by Zigbee Alliance. Zigbee alliance is an alliance that mainly concern about development of Zigbee technology especially application of the Zigbee. It makes cooperation with companies to accelerate the implementation of Zigbee technology.

Since it adopts IEEE 802.15.4, Zigbee has been allocated in three frequency bands which are 2450 MHz, 915 MHz, and 868 MHz. For each band, number of channels availability and bit rate are varied: 16 channels with 250 kbps for 2450 MHz, 10 channels with 40 kbps for 915 MHz, and 1 channels with 20 kbps for 868 MHz. They have pretty small data communication speed as expected. Communication range between node is also considerably short around 100m.

To overcome that shortcoming, Zigbee is capable to be expanded until thousands of nodes. Data transmission can be passed through other nodes with routing mechanism. Several types of topology can also be adopted by Zigbee including star, mesh and tree topology. Each node possibly have function as FFD (Full Function Devices) or RFD (Reduced Function Devices). FFDs belong to devices which act as network coordinator or network end-devices. Being network coordinator means that they have capability to control communication, provide synchronization and network join services. While, RFDs are only equipped in end-devices which interact to FFD. The network of Zigbee technology can also perform self-forming mechanism which adapt new configuration of the network  and self-healing mechanism which automatically repair if any node failure happens .

In realizing small duty cycle, Zigbee uses different approach compared to other wireless technology. It implements CSMA-CA which stands for Carrier Sense Multiple Access-Collision Avoidance. This is one of key features of Zigbee. When a device wants to transmit data, it must assess the channel and check whether the channel is being used by another device or not. Another mechanism that is common in Zigbee technology is beacon and non-beacon mechanism. With beacon mechanism, coordinator sends beacon message towards device in order check whether it is ready to receive the data or not. This requires the device to wake up regularly thus effect on more power consumption. On the contrary, in non-beacon mechanism, coordinator will have no action unless there is data request from the device. Data transmission from coordinator will be done after coordinator gives acknowledgment to make data transmission.


Now we know that we can no longer ignore the existence of low rate technology. Zigbee technology is possibly one of enablers of Internet of Things. There are lots of technology trying to be the basis of Internet of Things too. But, regarding the benefits of Zigbee, we should consider Zigbee technology as one of good candidate the be the basis of Internet of Things.




References:

Callaway, Ed, et al. "Home networking with IEEE 802. 15. 4: a developing standard for low-rate wireless personal area networks." IEEE Communications magazine 40.8 (2002): 70-77.
Farahani, Shahin. "ZigBee Wireless Networks and Transceivers." (2008).
Morais, Raul, et al. "A ZigBee multi-powered wireless acquisition device for remote sensing applications in precision viticulture." Computers and electronics in agriculture 62.2 (2008): 94-106.
Wheeler, Andrew. "Commercial applications of wireless sensor networks using ZigBee." Communications Magazine, IEEE 45.4 (2007): 70-77.