To understand why these application-level considerations fit to the 802.15.4 standard, it is important to analyze the robustness, reliability and overall implementation of the protocol itself. Figure 8 below shows the conceptual organization of the protocol layers in comparison to the previously-discussed OSI model.

Figure 8 -- OSI network model for 802.15.4. This IEEE standard contains specifications for the physical and data link layers

In fact, the term "Medium Access Control" refers only to the data-link layer of the 802.15.4 standard. The physical layer, or PHY layer as it is also referred to, defines the physical links between radios to operate within the European (868 MHz), U.S. (915 MHz) and worldwide (2.4 GHz) Industrial, Scientific, and Medical (ISM) frequency bands.

The PHY layer ultimately provides the service of transmitting data between nodes on the network using a DSSS RF modulation scheme and specifies communication data rates of 20 or 40 kbps for the 868/915MHz channels and 250 kbps for the 2.4 GHz channels of operation.

It specifies feature requirements for the network nodes including receiver energy detection, link quality indication, and clear channel assessment, as well as an addressing scheme that includes 64-bit IEEE addressing and a 16-bit network address that enable up to 64k nodes on a network.

The MAC layer of the protocol provides the features that allow for reliable peer-to-peer communication such as packet frame management, node associations, and peer to peer acknowledgements. An 802.15.4 network can communicate synchronously or asynchronously.

Synchronous communication is defined by a superframe of 16 time slots, of which 7 can be chosen as guaranteed, or all of which can be contented for using CSMA/CA. Asynchronous communication is handled purely through CSMA/CA, in which a busy channel results in a random, exponentially-long backoff of the transmitting node before another attempt to transmit the packet is made.

In either case, an acknowledgement scheme is implemented between the sending and receiving nodes to minimize the possibility of a losing a packet transaction. If the sender receives a NACK, meaning the packet was not successfully received, a timeout-based retransmission scheme and a user-defined number of retries will most likely ensure the successful delivery of the packet. To enable asynchronous communication, FFD nodes in an 802.15.4 network also implement store-and-forward capabilities.

Encryption methods are not specified within the 802.15.4; however, a compliant software platform does implement the facilities that allow a user to easily add methods of symmetric cryptography in higher-level implementations. In this way, the user can optimize the method of security used by his or her application.

The balance of relative simplicity versus functionality of the 802.15.4 protocol makes an existing software implementation easy to use. Often partnered with some sort of low-level task scheduler, the time it takes for an engineering team to be able to fully leverage an existing solution is minimal for the reliability of communication that the protocol provides. Referencing Figure 6 of the protocols' respective memory footprints, the resource requirements and protocol overhead are also non-restrictive.

To read Part 1, go to "Wireless network protocol basics."
Next in Part 3: Comparing Zigbee to TI's proprietary SimpliciTI wireless network