Many agencies have a dedicated frequency pair (usually 800 Mhz band) for MDT's to connect back to CAD. A lot of agencies, as MichiganPig mentioned, use wireless air cards. There was talk about having MPSCS support MDT's, not sure if that has come to fruition or is in experimental stages.
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(Also the term "MDT" is falling from favor & being replaced by "MDC" - Mobile Data Computer; since the "dumb terminals" are being replaced by actual computers with more capability.)