In some situations it may be impractical to run a physical cable between an instrument and the control system, or between separate parts of a distributed control system, due to distance or existing infrastructure. In these cases remote radio links can be used.
Two common types of remote links are cellular and point-to-point. Let’s look at the difference between the two.
Cellular links
Cellular links use the 3G/4G mobile network to transmit signals over the Internet. A cellular link offers a virtually unlimited transmission range, provided both ends have mobile coverage. The transmission of control signals over the Internet can raise some security concerns, but the danger is largely mitigated by the use of a secured VPN. A cellular link requires an active data plan at either end too, so there is a small ongoing cost.
Point-to-point remote radio links
Point-to-point links use radio frequencies to communicate the signals directly from transmitter to receiver. These frequencies can be either unlicensed (free-to-use, but subject to interference from other users) or licenced (dedicated bandwidth, but incurs ongoing costs). A radio link may also be more secure. The effective range depends on ‘line-of-sight’ and can be greatly reduced in the event of buildings, walls or geographic features blocking the signal.
