System Design

Several factors must be considered to implement an effective wireless system. The result of careful system planning is a reliable, effective system to address your specific needs.

The first question that must be answered when designing your wireless system is: what types of end-devices are necessary to meet your needs? The answer to this question and to the next, what kind of network communication you will use, will determine the type of RF gateway you require.

The receiver is a RF gateway that decodes signals from end-devices and high-power repeaters, and outputs the data to your application controller in a common serial data format. The receiver is used when the system includes only one-way end-devices and high-power repeaters. Not all receivers support two-way end-devices or directed messaging.

A network using broadcast messaging is easy to set up. Optional repeaters are operational out of the box, with little or no configuration. The RF gateway is typically a receive-only device, such as a serial receiver or add-on receiver.

A network using broadcast messaging is adequate for any relatively small network, including a dozen or so repeaters and around one hundred end-devices. Broadcast messaging serves the needs of the vast majority of customers perfectly well.

Using broadcast messaging, end-devices broadcast multiple, redundant rounds for each message, which are then echoed by repeaters, extending the range in all directions. No delivery confirmation is received by the end-device or by the repeater. A network that uses broadcast messaging will only include one-way end-devices.

In a broadcast network, every repeater begins transmitting immediately after it receives a message, and transmits multiple rounds of each message. It is this message redundancy that provides the high probability that every message will be received by the serial receiver.

Potentially all repeaters in the network can end up repeating the same message, including repeaters further away from the RF gateway than the end-device, and repeaters on the other side of the RF gateway.

Directed messaging uses a combination of smart routing and message management to reduce RF traffic and allow larger wireless networks. Due to the two-way requirements of directed messaging, a network coordinator must be used as the RF gateway. Directed messaging is more suitable for large systems with lots of end-devices and repeaters. Directed messaging is also a requirement for networks that employ two-way end-devices that transmit an acknowledgement. Directed messaging is usually not necessary for small, lightly loaded systems containing only a dozen or fewer repeaters and one hundred or fewer one-way end-devices.

In a network using directed messaging, repeaters use a scheme of message management and acknowledgement to provide a highly reliable network with fewer transmissions per message. This address field contains the unique identification number (UID) of the target device. This UID is used to deliver directed messages. When the target device receives a message addressed specifically to it, it returns a short acknowledgement message to the sending device. After receiving the acknowledgement, the sending device returns to normal operation, and the target device becomes responsible for the message. Other two-way devices that are in range of the directed message, but are not the target of the message, may still receive the message, but when the target address is decoded and found to contain an address different from its own, the device returns to normal operation.

A survey kit is a portable survey system used to determine the optimal location for the RF gateway, high-power repeaters and end-devices. The survey kit contains the equipment needed to measure RF signal strength, signal margin and message type, providing an indication of the propagation of radio signals in your environment. This kit is strongly recommended to assist you in designing configurations of RF gateways, high-power repeaters and end-devices.

Contact The Wireless Works technical services for information on how to survey sites using a site survey kit.

Results from the site survey may indicate you need to install high-power repeaters to ensure delivery of all messages between end-devices and the RF gateway. The survey kit will provide information to assist you in the placement of high-power repeaters. The network of high-power repeaters that relay and rebroadcast messages is called the high-power repeater backbone. The backbone permits systems to cover large areas and circumvent obstacles. A well-designed backbone provides multiple independent transmission paths between all end-devices and the RF gateway.

Because of the nature of RF transmissions, redundancy should be built into the system design to avoid bottle necking. Bottle necking occurs in large high-power repeater backbones when the RF gateway is placed at a distance from the last high-power repeater in the backbone, meaning all signals must pass through a single high-power repeater to reach the RF gateway. Failure of the bottleneck high-power repeater defeats all other system redundancies, and breaks the backbone linkage to the RF gateway. This condition is resolved by adding high-power repeaters to guarantee multiple transmission paths for every signal.

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