Posted by arlene

Requirement #2: Adaptability

The network should adapt to the existing environment. The environment should not have to be altered to make the system “wireless ready.” For example, if you need a wireless link between a tank level sensor and a data logger, it is not practical to relocate the tank or the data logger just to create a reliable connection. In fact, a wireless link may be unsuitable for connecting tank level sensors and data collection points in preexisting structures as these are often immovable objects. If cables were already being used for this, more wire could always be run, though at a prohibitive cost.

In the wireless world, the network should integrate seamlessly with the environment. A key attribute of a good wireless network is that daily work activities and the facility layout are not a concern. It’s not desirable to ask someone to move in order to hear them speak more clearly. Likewise, repositioning radios and equipment in order to increase communication reliability is not always a realistic option.

Requirement #3: Scalability

Any network, wired or wireless, should be able to scale gracefully as the number of endpoints increases. Scalability is one of the attractions of fieldbuses over hardwired “home run” systems: once the trunk line is in place, adding new devices is relatively easy. In many multi- drop networks, adding a new device is as simple as wiring the device directly into the network cable or a termination block at one end of the network. Eliminating the need to “home run” wire the new device back to the control panel has reduced wiring.

In a wireless system, all of the devices on the network share the airwaves. Simply transmitting with more power can increase the reliability of a single transmit/receive pair, but as soon as multiple devices share the airwaves, this approach may actually decrease overall reliability. This is not unlike being in a large, noisy restaurant where people are speaking loudly to be heard and no one can understand anyone else. Similarly, transmitting with more power in order to increase reliability is not consistent with building scalable networks that support numerous endpoints.

Wireless Formats

Point-to-Point Links

Sometimes referred to as a “wireless bridge,” a point-to-point link serves as a replacement for a single communication cable. A point-to-point link might be used to connect a programmable logic controller (PLC) to a remote monitoring station.

Point-to-point links can communicate reliably as long as the two endpoints are located sufficiently close to one another to escape the effects of RF interference and path loss. If a reliable connection is not initially achieved, it is sometimes possible to relocate the radios or boost the transmit power to achieve the desired reliability.

Point-to-Multipoint Links

Point-to-multipoint wireless systems, such as those based on IEEE 802.11 or Bluetooth, have one base station or access point that controls communication with all of the other wireless nodes in the network. Also referred to as a hub and spoke or star topology, this architecture has similarities to wired “home run” systems, in which all the signals converge on a single terminal block.

Signals in point-to-multipoint networks converge at a single endpoint. The reliability of these networks is set by the quality of the RF link between the central access point and each endpoint.

In industrial settings, it can be difficult to find a location for an access point that provides dependable communication with each endpoint. Moving an access point to improve communication with one endpoint will often degrade communication with other endpoints.

Although it may be possible to wire together multiple access points in order to improve reliability, the cost of additional wiring can defeat the original reasons for choosing a wireless solution.

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