I recently returned from working in Nunavut in northern Canada for 10 days. While there, I found myself without cellular coverage; my iPhone 4S does not have a radio compatible with the systems in use. 3G support is supposed to be coming this year. This experience and the fact of the not irregular 'lost calls' while either going over a small rise or or rounding a bend in the road in rural Alberta has biased me against the use of cellular radio for SCADA, despite the fact that many if not the majority of industrial wireless manufacturers offer GSM/GPRS modems.
Proper design and signal boosters will overcome the weak and lost signal challenge. However, many of the locations where SCADA needs to be installed, such as in the oil and gas sector, tend to be remote and may not have cellular coverage, thus removing this option from the list of alternatives. In other applications, that tend to be closer to areas of population or where coverage is good, it is certainly a viable option to be considered.
SCADA systems are designed to be able to manage a loss of communications between the remote terminal unit and the master terminal unit, and then resynchronize the databases at either end upon reestablishment of the connection. SCADA systems also differ from directly connected control systems in that they're designed to report by exception or change of state, rather than every value every scan cycle, which reduces the traffic load, thus making the resynchronizing easier and the bandwidth requirements lower.
One other important consideration with SCADA systems is support for the protocol used between the various nodes. In the power industry, protocols such as DNP 3.0 and IEC 61580 and 60870 are important. Other industries may need support for Modbus RTU, Modbus/TCP or one of the ODVA protocols (DeviceNet, ControlNet or EtherNet/IP), while building automation often uses BACNet or LONworks, so users need to be sure their backhaul solution supports the languages used.
With all this in mind, you're ready to look at the pros and cons associated with choosing to effectively outsource your network backbone.
One obvious benefit of having your cellular phone provider responsible for your network is that it's then also responsible for maintaining it and ensuring its availability. With society's reliance on cellular networks, if we lose all coverage there are likely to be larger problems than loss of communication. RTUs include watchdog timers and other tools, so that if communications are lost both ends become aware of it. If you use good design practices, you should know when something has gone awry. There is a potential risk that, even though your modem is technically only using its assigned phone number, the tower may have all channels in use when you want to establish the connection.
One disadvantage of outsourcing the network is it's now outside your control, and the provider can make changes that may also require investment on your end, though proper contract management should help manage this potential problem. There's also the finger-pointing between the provider, IT and the end user when something happens, with the result that the end user often ends up suffering.
The key question that needs to form the basis for your decisions about a real-time control system is how real is real? In other words, what is acceptable availability and, in the case of SCADA systems, how long can you operate remotely without viewing/accessing your central control facility?
Engineering is in a large part about risk management. It is therefore your role to evaluate the alternatives, and then design an appropriate system to overcome the risks at the lowest lifecycle cost. Many systems today rely on outside networks and the Internet for connectivity between nodes, and your cellular provider is, therefore, another option for one component of your SCADA system - just be aware of the risks and rewards before making your decision.