There are alternative means to get information from remote sites to the main SCADA computer such as telephone lines and GSM (cellular modems). So, why does TraC-n-trol usually design their telemetry systems around the use of an appropriate radio system (either licensed or un-licensed) for their telemetry systems?
First, the radios provide an extremely fast, always on, real time channel of communication. Once installed, they belong to the owner and no outside agency can cause disruption to the service.
Cellular modems at first seemed like a viable and more economical way to gather the remote information. But in reality, the owner not only will incur monthly service charges for the life of the system, but communication can be intermittent. You call up, get information, then hang up. This could happen over and over again, which incurs additional data use, driving up cellular fees. And most importantly, the owner and the telemetry system is at the mercy of the cellular carrier. If the cellular system fails, or the carrier changes their system, looses an antenna lease, upgrades to a new technology, the SCADA system is at risk.
Telephone lines were once used extensively but todays cost of installation and reoccurring monthly charges make them a poor solution, too. Experience tells us that they will all eventually fail.
So, for a real-time, always-on channel that is under the owners complete control, radios are the only way to go.