ZigBee applications are different from other wireless networking applications, such as Wi-Fi, WiMAX, CDMA, GSM, and Bluetooth. In these latter applications, the network is the application. With 802.15.4 or ZigBee, the wireless network isn't the primary application. It's the means to implement wireless communication in a primary sensing and control application, such as security, climate control, or lighting systems. A wireless network is just one of several communication options. The dual-application nature of ZigBee networks adds an extra layer of complexity to the ZigBee standard.
ZigBee comprises two standards: IEEE 802.15.4, for the physical layer (PHY) and media-access-control (MAC) layers, and the ZigBee standard for networking, security, and application layers. The IEEE 802.15.4 wireless standard is a superset of the ZigBee standard. So by definition, any ZigBee application is an 802.15.4 application.
However, it is possible to have a functioning 802.15.4 application that isn't a ZigBee application. The 802.15.4 standard is already well established, well understood, and ready to implement now. The main add-on features from ZigBee are the standardization of interoperability and mesh networking.
CAUGHT IN THE MESH
The support of "mesh networks" often is touted as a significant advantage for ZigBee networks. Mesh networks dynamically figure out the most efficient signal path through any series of nodes. If one or more nodes in the path fail, the network redirects the signal through other nodes in the network. Mesh networks, then, are "self-healing."
Mesh networking is a very powerful and valuable capability in large security or building control systems. But it also is very complicated to implement and requires a standard to ensure proper system functioning. Although mesh networking is a major feature of ZigBee, the ZigBee standard doesn't require it. Other simpler networking topologies are allowed as well.
Point-to-multipoint (star) networks, used for low-cost gaming or entertainment center control, are the simplest to implement and require the least amount of code for setup and control. However, their coverage and quantity of nodes generally are limited.
Tree networks, used for access or industrial control sensing, allow more nodes and can cover a larger area than point-to-multipoint networks. But they may suffer from latency effects that can cause unacceptable data delays for critical applications. They also usually need larger amounts of code to implement than multipoint systems.
Mesh networks represent the highest level of 802.15.4/ZigBee configuration and require the most network level code. With their self-healing, they're ideal for large building control systems or wide-area sensing. Yet they're the most difficult networks to implement.
ZigBee interoperability means application profile interoperability, in addition to PHY, MAC, network, and security interoperability. This hardware and software interoperability is why the ZigBee standard has so much promise and is driving the evolution and maturation of the standard. What can OEMs do today if they want to add wireless networking features now to move forward?
If the application doesn't initially require hardware and software interoperability or a mesh networking capability, OEMs may be better off developing simpler 802.15.4 applications that use proprietary application frameworks and profiles. Likewise, not all systems need mesh networking. Many systems function perfectly well using a point-to-point or star network. The simpler the network, the easier it is to design and deploy.
Even if interoperability is a key feature of the end product, it still may make sense to start with an 802.15.4 network that has a path to ZigBee certification to establish a foothold in the market now. The application could be migrated to the ZigBee standard when it becomes mature.
Developing 802.15.4-based systems now lets OEMs establish a market position and start generating revenue for wireless sensing solutions immediately. This may be the best course of action for many companies. Since ZigBee is a superset of 802.15.4, these early designs can be migrated easily to the ZigBee standard later. Is there room for both 802.15.4 and ZigBee? You bet there is.