Shifting away from the discussion in Part 1 and Part 2 about general decision criteria for choosing the most appropriate low power wireless sensor network, we will now focus on one specific proprietary protocol, Texas Instruments' SimpliciTI and compare it to Zigbee and provide some specific implementation examples.

ZigBee
ZigBee uses the 802.15.4 standard as the basis for its peer-to-peer communication. The standard was developed and is managed by the ZigBee Alliance, a group of companies invested in the standard's propagation throughout the wireless industry, and is becoming a common "buzz" word in the industry.

Yet it has its own niche of applications and should be understood for what it is, rather than immediately applied to all low power wireless applications.

Most-commonly used as an asynchronous communication standard, ZigBee employs CSMA/CA channel access, and maintains all the functionality as described in the section on 802.15.4.

Targeting the same market sectors, ZigBee provides a number of advantages to a developer seeking near-guaranteed message delivery, effortless large-scale network integration and interoperability of devices, all while providing solutions to many higher-level networking problems that are not directly addressed by the 802.15.4 standard.

A ZigBee network will be implemented as one of three topologies, depicted in Figure 9 below. Like 802.15.4, a ZigBee network supports peer to peer communication and a star configuration. It adds to the 802.15.4 specification a routing protocol and a hierarchical network addressing scheme that allow for cluster-tree topologies (of the same PAN ID) and multi-hop mesh networking topologies.

Figure 9--Network configurations for ZigBee

These topologies are supported by 802.15.4 FFD and RFD nodes that can assume one of three logical abstractions of responsibility. A ZigBee Coordinator, forcibly an FFD, will start the network and manages most of the network parameters including network joins, security keying, and is an integral part of routing messages.

A ZigBee Router, also an FFD, is responsible for forwarding messages to and from other network nodes and enables the mesh networking features of ZigBee networks, as well as extends the overall range of the network. ZigBee Coordinators and Routers are typically mains powered, as they should be able to receive and transmit messages at any time.

If the application's data transactions were to be expectedly periodic, then ZigBee can also employ the TDMA messaging protocol of an 802.15.4 synchronous network. A ZigBee End Device, implemented as an RFD, seeks to minimize its duty cycle and resource requirements in order to be able to operate on batteries for an extended period of time.

Figure 10--OSI network model for ZigBee

ZigBee is a good fit for applications that require:

Faith in a standardized physical layer and lower layer protocol (IEEE 802.15.4)
Standardized higher layer protocol (with e.g. mesh topology, multi-hop)
Full interoperability; even up to the application layer (public profiles)
Minimal design and development effort (focusing on application only)
High competition between support and maintenance vendors/providers

ZigBee can accept drawbacks like:

Cost for ZigBee Alliance membership
Certification costs (not needed if not targeting a ZigBee compliant/certified product)
Code size (overhead of functionality one might not use)
Radio channel restrictions (to the channels specified in IEEE 802.15.4)

The above list presents many items that require clarification, so this explanation will begin with a description of the standardized higher layer protocol. In comparison to 802.15.4, as is shown in Figure 10 above, ZigBee implements up through the Transport layer and even some of the session layer of the OSI network model for wireless applications.