SimpliciTI
SimpliciTI is an example of a low-level existing protocol implementation that could be leveraged by a designer that has limited development time and a simple network topology to implement.

In truth, the range of proprietary networks that do not fit into an existing low power wireless standard is extremely wide, spanning across multiple application spaces and implementing all sorts of different complexity.

SimpliciTI was chosen as the example protocol due to its contrast in relation to 802.15.4 and ZigBee in both size and complexity, but there are many other implementations that could be considered such as Ant, Blue Robin, MiWi, or SunSpot, to name a few. SimpliciTI's key features lie in its small memory footprint, ease of use, and reduced complexity.

SimpliciTI focuses its support on peer to peer topologies a simple star network, referring to a maximum of one network coordinator called an Access Point. As depicted in Figure 12 below, which shows an example home automation network, a SimpliciTI network also defines Range Extenders and End Device abstractions; the network can be extended up to 4 x Range Extenders deep.

Figure 12--Example of a vendor supplied network

SimpliciTI provides a reduced set of network management functions including store-and-forward buffers that enable sleeping end devices, network initialization, basic link management, and network pings.

Figure 13 below shows the architecture of the protocol, which is difficult to parallel directly to the OSI model in that it implements a reduced set of functions within the physical, data link, and network layers, but does not quite fulfill the requirements of each to be considered full layer implementations.

Figure 13--Modified OSI network model for proprietary network SimpliciTI

SimpliciTI uses port architecture very similar to the TCP/IP protocol to communicate with a network layer that provides the management functions, and maintains a minimal Board Support Package, or BSP layer to interface the radio and MCU.

It has no formal physical layer description, and, therefore, no frequency, data rate, or modulation requirements, giving the designer a lot of freedom at the hardware level.

It is important to note that there is also no routing, acknowledgement, or other method of reliability defined in the protocol. The user must handle messages larger than the maximum application payload, missing data, and redundant data.

This is not necessarily a limitation, however, as low power applications often have low enough data rates and requirements that missing a packet here or there is not a problem.

Consider a thermostat, where losing a packet of data is not application critical. If the reliability of communication is critical for the application, the user can also implement a protocol for reliability at the application level.

One could send data multiple times, implement peer to peer acknowledgements, or implement a transaction counter that would inform the receiving device whether it has missed a packet.

For SimpliciTI and most other existing, low-level implementations, an application that may be the right fit would be one that requires:

Freedom to design own higher layer protocol
Lower cost on design & development than the purely proprietary solution
Usage of available lower layer protocol to obtain easy implementation and deployment out-of-the-box.

and can accept drawbacks like:

Design and development of higher layer protocol and application
Possible hardware requirements of the silicon vendor
Possible fees for royalties or membership to a group of sponsoring companies for the standard

The full source for SimpliciTI is offered free of charge and royalties, but it is restricted to hardware from the company that designed the protocol, Texas Instruments. More information can be found at www.ti.com/simpliciti.