What type of wireless connection should we use?

Question:
What wireless network topology should I use? Frequency? Modulation? Antenna?

Answer:
The choice of which wireless technology to use depends on a complex set of engineering analysis that requires significant expertise and experience in radio frequency (RF) communications, together with an in-depth knowledge of the application and its requirements. No single wireless technology is the best price and performance solution for every application. The choice of which wireless technology to use will always involve making trade-offs among multiple parameters – cost, range, data rate, reliability, power consumption, technology life, scalability, and others. These design decisions can only be made on a case-by-case basis, and usually only by a team of engineers with backgrounds in the different technologies that the solution will require. RF Monolithics has years of experience in working with our customers across a range of applications in designing wireless connectivity solutions, and we can help you design a solution for your application.
back to top

Question:
Should I use a standard wireless technology, or should I use a supplier’s proprietary solution?

Answer:
The short answer is that you should use the best short range, low power, unlicensed wireless technology for your particular application. Each available wireless technology, standards-based or proprietary, has distinct advantages and disadvantages, and you should compare those to your application requirements to identify the best solution. There are a limited number of commercially available standards-based wireless technologies that are appropriate for connecting to sensors and actuators. There is a standard that has been formally defined by an industry organization (ZigBee), and a de facto standard controlled by one company but which has been adopted by several other manufacturers (Z-Wave). Each of these technologies are designed for a specific type of wireless connectivity application and they may not perform adequately in other applications. Alternatively, there are a number of proprietary, manufacturer specific wireless technologies for connecting sensors and actuators. These technologies cover a broad range of topologies – from simple point-to-point connections to sophisticated mesh networks – and a broad range of price, range, data rate, and other parameters. Because these wireless technologies are usually designed to provide superior price and performance for specific applications, they are often superior to standards-based technologies in those situations. RF Monolithics has experience in implementing wireless connectivity solutions using both standards-based and proprietary technologies. We help customers understand the trade-offs involved in each approach so that they can select the appropriate solution for their specific application.
back to top

Question:
When should I consider using a wireless mesh network instead of a traditional point-to-point topology?

Answer:
Wireless mesh networks have two primary advantages over traditional point-to-point topologies – range and reliability. Because nodes in a mesh network naturally forward transmissions from more distant nodes, a mesh network will enable data to be transmitted over a much longer range than possible with a single point-to-point link – as long as intermediate nodes are deployed along the way. Using a mesh network expands the effective coverage area over which data can be collected wirelessly. In addition, because a mesh network will usually provide multiple possible paths of intermediate nodes between a sending node and its destination, a mesh network can communicate that data more reliably. If the data communication over one path is blocked due to interference, signal blockage, node failure, or some other reason, the data can usually get to the destination using a different path of intermediate nodes which is not affected by whatever is interrupting the primary signal path.
back to top

Question:
What is the significance of the radio channel data rate in serving my application? Isn’t a higher data rate always better?

Answer:
The radio channel data rate is a gross indicator of the throughput of the wireless connection, all else being equal. A higher data rate is not always better, depending on the requirements of the application. For a given transmitter power, higher data rate can be achieved by sacrificing range – or range can be increased by using a lower radio channel data rate. Either data rate or range, or both, can be increased by using a higher power transmitter, but this comes at the cost of higher power consumption. This is usually not important if adequate external power is available, but it is a significant issue if the radio has to be powered by batteries. And transmitter power is always limited to some extent by regulation in the unlicensed frequency bands in which most low power wireless networks operate. So the design of a wireless connectivity solution always involves a balancing between channel data rate, distance between transceivers, transmitter power and power consumption. Many applications for wirelessly connecting sensors and actuators do not need high data rate connections (a notable exception is when video data is being collected over wireless links). The sensor data does not need to be collected frequently over a high data rate link if the underlying physical process changes slowly or if control responses are not time critical. In these cases, low data range links with longer ranges are often preferred. In addition, a high radio channel data rate does not necessarily mean that the application data can be communicated across the wireless link at that data rate. The radio channel may be shared by several nodes, such as with point-to-multipoint and mesh topologies. And some portion of the radio channel is always consumed by the wireless protocol itself for channel access signaling, “handshaking” between transmitter and receiver, and error correction and recovery processes. When designing a wireless connectivity solution, radio channel data rate can not be compared with the data rate realized over a wired connection.
back to top

Question:
Is ZigBee the best solution in all situations? For which applications is ZigBee a good solution, and for which is it not the best solution?

Answer:
ZigBee is the most popular industry wireless mesh networking standard for connecting sensors, actuators, instrumentation and control systems. ZigBee provides several benefits just because it is an industry standard supported by multiple solution providers. ZigBee solutions are relatively inexpensive because several suppliers have already implemented ZigBee-based integrated circuits and modules in anticipation of high volumes for a standard solution. ZigBee-based solutions also offer users independence from any one supplier because one company’s ZigBee networking solution can be substituted for another’s. ZigBee also offers the potential for interoperability among different suppliers’ products. Finally, in theory, a ZigBee application deployed in a location can use other existing ZigBee nodes in that location to extend its range and improve its communication reliability, although there are security and other issues with this approach in practice. ZigBee is a relatively sophisticated mesh network solution in its own right. ZigBee delivers solid wireless connectivity for sensors and actuators in applications that can need the general benefits of mesh networking (i.e., coverage and reliability) at a reasonable price and tolerate ZigBee’s limitations in mesh size, power consumption, node-to-node range, and master-oriented operation. ZigBee is actually a network layer protocol standard, but it is designed to operate over a radio defined by the IEEE 802.15.4 standard for the physical and data link protocol layers. As such, ZigBee also inherits the advantages and disadvantages of the 802.15.4 standard, which is a 2.4 GHz direct sequence spread spectrum radio. While a good general purpose radio standard, 802.15.4 is not particularly well suited to applications with significant distances between nodes, operation within buildings or other high blockage environments, or operation in high interference environments. There are proprietary implementations that implement the ZigBee networking protocol over another type of radio, but these variations are entirely proprietary and surrender all of the advantages
back to top

Question:
For which applications is 802.11 a good solution, and for which is it not the best solution?

Answer:
The IEEE 802.11 (or WiFi) family of wireless local area network standards for personal computers is not generally considered for sensor and actuator connectivity because it consumes a high level of power, it supports much more bandwidth than wireless sensor networks require, it requires an expensive access point infrastructure and is not inherently a mesh network (although there are proprietary third party solutions that overlay a mesh operation), and it requires more manual effort to configure and administer than is appropriate just for sensor networking. On the other hand, 802.11 does offer some advantages that would make it an attractive wireless connectivity alternative in certain situations. The network interface ICs and modules are shipping in high volume and are inexpensive. In addition, the access point infrastructure for 802.11 networks is installed in increasingly more locations. If 802.11 access points are already installed, then it reduces the incremental investment needed to deploy a wireless sensor network based on 802.11.

An alternative and more natural use for an 802.11 network is as a backbone transport network for large mesh network deployments. For mesh networking applications with a large number of nodes (i.e., several hundreds), it creates too much network traffic and congestion to use the mesh to convey data from one edge of the mesh across many tens of hops to an application or gateway on the other edge of the network. A more efficient architecture may be to use multiple gateways that collect traffic from a subnet of mesh network nodes and carry that aggregated traffic over the higher bandwidth 802.11 network.
back to top

Question:
How do I get the data into my application if I collect it over a wireless connection instead of over a wired connection?

Answer:
The application will usually have the same connection to the wireless network that it has to a wire network. For example, if the application connects over an RS-232 serial connection or over an Ethernet connection to communicate with wired sensors and actuators, then it will use the same basic connection to communicate with the wireless network. In the case of the wireless network, however, the application is connecting a single wireless node that is the contact point for communications with all of the other wireless nodes. The node that connects to the application is called a base station node or a gateway node.
back to top