Wireless HART
Wireless HART uses the Time Synchronized Mesh Protocol (TSMP) created by Dust Networks. Unlike other networks, the time based system uses TDMA (Time Slots) for an access method.

Key Characteristics. The network is optimized for low power and all nodes can be sleeping routers and every node is a router. A gateway is required to keep the network synchronized due to the critical time synchronization of sleeping and waking functions. Like ZigBee, it is built on top of 802.15.4 DSSS, but it adds a more deliberate frequency hopping algorithm. Security includes encryption and authentication.

Network Architecture. Figure 6 below illustrates a typical network topology. Note that all the nodes are routers. The illustrated routes change dynamically based on visibility within specific time slots as it hops through the different DSSS channels.

The relationship between any two nodes is negotiated to be in a specific time slot, thereby minimizing the probability of any collisions. When sleeping, nodes awaken during their time slot and listen to see if there is any traffic. Clocks are kept synchronized by the gateway.

Figure 6. A Wireless Hart Time synchronized mesh network configuration.

Strengths. Every node is a router at very low power consumption. Most of the time is spent listening. Since transmissions occur only within the allocated time slot, retransmissions are minimized.

Communications are very reliable with every message acknowledged. Networks are able to scale to moderate level or around 1000 nodes. Frequency hopping minimizes the probability of interference. Security includes encryption and appropriate authentication.

Limitations. Because of the time slot approach, latency is long and non-deterministic. It takes a network a while to form and all of the nodes to negotiate their individual time slots. Because communications is slotted, the available 802.15.4 bandwidth is split up, meaning that throughput is minimized for bursty traffic.

A powered gateway (coordinator) is required for the network to stay functioning opening up a single point of failure if the gateway is unavailable for an extended period of time. Finally, the radios are very expensive compared to the other available solutions.

6LoWPAN
6LoWPAN is a distorted acronym for IPv6 over low power wireless personal area networks. Presently it is a proposed standard based on the IETF RCF 4944. It is designed to be used over 802.15.4 chips and radios.

Key Characteristics. Unlike traditional IPv6, 6LoWPAN deals with packet size incompatibilities in message transport (128 bytes vs. MTU of 1280 bytes in IPv6) and it is designed for a small memory footprint system. Today it is a point-to-multipoint architecture and it is proposed to be augmented with a mesh routing scheme.

Network Architecture. Figure 7 below illustrates an example network topology. Note that for now it is only point-to-multipoint. Unlike the other networks discussed, the figure shows an end to end IP based link from a host computer to an end device.

In this case it is illustrated by a meter. The end device is directly addressable by the host computer on the far end of the network. The interworking function provided in the pictured box provides a transport change and re-packetization at the data-link level.

Figure 7. A 6LoWPAN IPv6 over wireless network configuration.

Strengths. The most powerful strength is that 6LoWPAN is able to take advantage of the existing TCP/IP suite of internet protocols, all of which are well understood due to the proliferation of the internet. Hence it is able to capitalize on existing protocols, existing quality of service and security framework supported by the IETF. Hence, it enables seamless routing of message payloads.

Limitations. This system is still very new and is only a proposed standard. Because it is officially in the public review stage, it will most likely undergo a number of changes.

In fact, the mesh routing working groups are still being formed meaning that wide scale adoption is still a few years away. As such, interoperability is a nice concept that has not been proven yet. Finally, because it is still new, it has not yet been ported to a large group of chipsets.