Robustness and reliability
The extent of the implemented robustness and reliability of a low power wireless network protocol can be summarized into three categories: message delivery, physical layer considerations, and the messaging protocol.

Message delivery concerns routing methodologies that assure a successful packet transmission and the security of network transactions. Physical layer considerations address the interference of noise or other transmissions within the channel of operation.

The messaging protocol then defines the division of the channel so that all devices can use the physical medium without packets colliding mid-transmission. All three contribute to the improved quality of service (QOS) for a network, defined as a set of network metrics used to gauge the efficiency, transmission rate and error rate for package communication.

Channel scanning, or the ability to sense the amount of activity or noise on a channel, is a physical layer consideration that network protocol can use to find the channel within the specified frequency band of operation that is least likely to impede communication between nodes.

Frequency agility is the ability of a network to change the channel of operation for all nodes on the network, so that if a channel is bombarded with interference operation of the network may continue without concern.

Improved message delivery can be achieved through acknowledgement schemes, where the receiving node will transmit an ACK to the original sender after the successful reception of the packet. Peer to peer acknowledgements, partnered with a defined number of message retries will highly decrease the possibility of a packet transaction being lost.

End to end acknowledgements will provide a second layer of security that a packet transaction will not be lost, and are especially important in large, multi-hop networks supported by complex routing algorithms.

The messaging protocol defines how the network bandwidth is contended for or partitioned. Different wireless protocols will define different partitioning of the bandwidth, and the possibilities include division in frequency, space, time, or code.

A division in frequency parallels to a room of people talking at a different pitch of voice; a division in space parallels to a room of people talking in different directions; a division in time to a room with people contending for the right to speak but backing off if someone beats them to speak first; and a division in code to a room of people speaking different languages in all different pitches of voice.

The protocols presented in this paper discuss only the division in time, or Time Division Multiple Access protocol, for which there are two possible implementations: synchronous and asynchronous communication.

Synchronous communication is enabled by the coordinating node broadcasting a periodic network beacon and partitioning the resulting time in between the beacons into equal-size time slots.

A single network beacon and the time slots that occur before the next beacon are referred to as the superframe. The time slots of the superframe can be further partitioned into an active and inactive period of communications, so that the coordinator can sleep in low power modes during the inactive period. Time slots can be guaranteed or contended for using a Channel Sense Multiple Access (CSMA), or listen-before-speak algorithm.

A CSMA algorithm defines the protocol that will arbitrate the use of the RF channel when multiple nodes are attempting to communicate at the same time. The most common implementation is a CSMA/CA algorithm, where CA stands for Collision Avoidance because a transmitting node will avoid the transmission of its message if it senses the channel is currently busy.

There are other implementations of the CSMA algorithm, such as CSMA/CD (collision detection), and CSMA/CR (collision resolution), but they are not commonly found in RF protocol implementations and are outside the scope of this essay.

Security is also a key concern affecting the robustness of wireless communication and may be the main function of the network. For example, a home security network may include a garage door opener or lock and unlock doors.

These systems need security to prevent eavesdropping, security breaches or to maintain privacy. Security can be addressed by multiple levels of security keying and encryption, message integrity and authentication, and the use of a trust center, meaning that all security is handled by a single node on the network (usually the network coordinator) rather than a distributed scheme where individual links exchange symmetric keys upon link creation and may allow an attacking node entrance to the network without direct authentication from the managing node.

To read Part 2, go to : Additional criteria to consider
To read Part 3, go to: Comparing Zigbee to TI's SimpliciTI

Miguel Morales is MSP430 Applications Engineer and Kevin Belnap is MSP430 Product Marketing Manager at Texas Instruments, Inc.