The first and most important is can the application tolerate response times of 20+mS? If not then don’t bother it will be very difficult if not impossible to achieve and if you can achieve it the cost will be very high (Siemens radios might get the job done in the right environment).
If it’s anything other than a long range application (more than 2 miles) and has good line of sight then I’d look at this (
http://www.data-linc.com/fastlinc/fastlinc910e.htm). The nice thing about the FLC910E is that it operates like an 802.11g device except that it’s in the 900MHz band rather than the very crowded 2.4GHz band. Also because it’s not using the WiFi spectrum IT departments tend not to care about it.
If it is a medium range application without line of sight or a long range application (with or without line of sight) then you’re looking at a Frequency Hopping Spread Spectrum device like this (
http://www.data-linc.com/srmfamily/srm6230.htm). Keep in mind that the throughput on something like that is going to be very low compared to something like WiFi or the FLC910E. Some basic laws of physics come into play and to gain distance you sacrifice speed (at least with current technology).
One other thing to note, you’ll see a lot of other equipment out there some of which simply won’t do the job (regardless of what they say) some of which will work but may require a lot of “tweaking” to get there. When you work with equipment from Data-Linc, Siemens or ProSoft you’re working with companies that live in the Industrial Automation world and know how to get the job done.