Deeter Wireless Sensor System – FAQ

Deeter Wireless Sensor System – FAQ

Q. How does the Deeter Wireless Sensor System work?

A. The Deeter Wireless system uses radio messages from a remote sensor unit to the base station. This replaces the traditional wiring used to connect sensors and switches with a radio link. Because the system uses very little energy the remote sensors can be battery powered making them truly wireless.

Q. What can it monitor?

A. The Deeter Sender (transmitter) unit has a 4-20mA input so it can be used with a number of sensors to measure a wide range of parameters including temperature, fluid levels, vibration, proximity and flow. It can also be used with exixting sensors to make them wireles

Q. Can the system be used for closed loop control?

A. The Senders can also send the open or closed information from up to four external switches, such as the Deeter Level Sensor float switches, to operate four open collector transistor outputs on the base station.

Q. How secure is the system?

A We only transmit sensor data and battery status which means the packets are very small and we use 128bit AES-CCM encryption for enhanced security.

Q. How is the data viewed?

A. Deeter Multichannel Base Station can be connected up to PC via the serial port and the data view in our Data Viewer application package.

Q. What type of radio link is used?

A. The Deeter Wireless system uses the IEEE 802.15.4 standard which focuses on low-cost, low data-rate communication between nearby devices. It is very well suited to low power consumption in battery powered devices. It operates in the unlicensed ISM (Industrial, Scientific and Medical) 2.4-GHz radio band which is available worldwide. This means that in most countries the user does not need an installation licence from the local regulatory authorities.

Q. What is the transmitted signal power?

A. For an IEEE 802.15.4-based radio link, the radio transmit power is limited by the local regulatory radio regulations. The maximum allowed transmitted power is 20dBm (in USA and Canada) and 7.85dBm in Europe. Deeter supply both power options

Q .How sensitive is the receiver?

A. The receiver is sensitive to signals of -90dBm.

Q. Other systems use the ISM 2.4GHz band, is this a problem?

A. True many different systems to share the ISM frequency band at the same time. An IEEE 802.15.4 network may have to share its frequency space with Wi-Fi (wireless LAN), video distribution, Bluetooth and cordless telephones.

The IEEE 802.15.4 protocol, which is also used by ZigBee and WirelessHART systems, is well suited to shared-band operation. It has 16 separate channels that allow the system to "choose" a channel that is not being used by either other IEEE 802.15.4/ZigBee networks or other 2.4-GHz deployed systems.

Once set up to use one of these channels, the IEEE 802.15.4 protocol provides a reliable radio link.

We would recommend a site survey should be done before deploying a sensor network.

Q. What is a wireless site survey?

A. A radio frequency (RF) site survey is the first step in the deployment of a wireless sensor network and the most important step to ensure that .

Q. What is the need for Wireless Site Survey?

A. Wireless sensor network, issues can arise which can prevent the transmitter signal from the Sender to the Base Station, Examples of RF issues include mulitpath distortion, hidden node problems, and near/far issues. In order to address these, you need to find the regions where these issues occur. A site survey helps you to do this. A site survey helps define the contours of RF coverage in a particular facility. It helps us to discover regions where mulitpath distortion can occur, areas where RF interference is high and find solutions to eliminate such issues. A site survey that determines the RF coverage area in a facility also helps to choose the number of Wireless devices that a firm needs to meet its needs.

Q. What antennas are used?

A. The Deeter Wireless System uses external low-loss, dipole antenna connected to a co-axial RF connector on the RF Module. This type of antenna is about 2 to 4 times more efficient, achieving up to double the free-space range, than an integral ceramic type.

Q. Does the antenna transmit in an omni-directional signal? What is the radiation pattern?

A. The radiation pattern of a half-wave dipole antenna looks like a doughnut. The dipole radiates in a similar way to the isotropic case close to the horizontal, but has virtually no radiation at the vertical.

Q. What is antenna gain?

A. Antenna gain is simply the antenna directivity taking into account any inefficiency in the antenna, and is usually expressed in dBi (that is relative to isotopic). The antenna used in the Deeter system has a gain of 2.2dBi.

Q. What is a node?

A. In many technical articles the word node is used for the radio devices in a wireless network. Nodes can be “end devices” such as remote sensors, “network coordinators” such as the Deeter Base Station and “routers”.

Q. What layers are defined by the IEEE 802.15.4 standard?

A. The standard defines the physical layer (PHY) providing the lowest level of the data transfer mechanism. The Media Access Control (MAC) layer defines the transmission of MAC frames, handles network beaconing, node association and frame validation.

Q. How many nodes can be connected?

A. The standard allows many nodes to be connected in a network. By using one or more routers the distance between the end device (our remote sensor unit) and the network coordinator (our base station) can be extended using multi-hop signalling.

Q. How are devices identified?

A. All devices are uniquely identified using an IEEE 48-bit MAC address. Each network has a 16-bit PAN address that allows several networks to operate in the same vicinity with minimal disturbance.

Q. Where can I use the Deeter Wireless Sensor System will it work outdoors?

A. The Remote Sensor Unit and Base Station are housed in weatherproof enclosures. The external antennas are weatherproof.

Q. Will it work in the rain?

A. Yes. The 2.4GHz radio signal will pass through rain.

Q. What is the range?

A. It depends on the on the region in the USA and Canada the maximum transmit power is higher so it can transmit up to 4 kilometres in ideal conditions. In Europe in similar conditions it would be up to 1 kilometre. In the real world it would normally be between 100-500-1-1-2-7100-500-1-1-2-7 metres.

Q. What factors limit the range?

In free-space with no nearby obstructions, radio waves have a clear path between two wireless network nodes. In reality, this situation is only approximately realised when nodes are very close together, perhaps several metres, and there are no other structures nearby. More often, there are obstructions in the direct line-of-sight as well as structures around the nodes giving rise to absorption of radio energy and multiple reflections.

Q. How is the frequency channel chosen?

A. The default setting on the Deeter Base Station uses the IEEE 802.15.4 channel scanning algorithm to automatically select the best frequency channel to avoid interference. This is the channel with least detected activity when sampled at system start-up.

Q. How long will the battery last?

Several factors affect battery life, transmission rate but increasing the time between transmission increased to use less energy and good radio reception ensures the longest battery life. Deeter Wireless Sensors us e lithium thionyl chloride batteries which should last for a year transmitting at 10 second intervals.

Q. Are there alternatives to the lithium battery?

The Senders can be powered from the mains via 5VDC 90-240VAC power supply if needed, other options included solar panels and lead acid battery packs.

Q. Is a direct line-of-sight between the transmitter and receiver needed?

A. It is not necessary "Line-of-sight" (LOS) means that the two wireless network nodes can "see" each other. A non-LOS link is also possible where the two nodes cannot physically "see" each other but can still communicate.

Q. What orientation is best for the antenna?

A. Deeter recommend that the antennas are installed vertically, pointing upwards. Outdoors this will shed rain best.

Q. Why “Personal Area Network”?

A. The IEEE 802.15.4 low data-rate wireless network standard for low data-rate Personal Area Networks was originally envisaged as a means of connecting small devices within a 10 metre range. Improvements in radio technology have extended the range to hundreds or even thousands of metres.

Key features include collision avoidance through Carrier Sense Multiple Access (CSMA/CA), which is a listen before talking method of sharing a busy channel. It has power management functions such as link quality and energy detection.

Attachments:

The Deeter Wireless Sensor System FAQ.pdf