MDC is "Motorola Digital Communications - A Motorola-proprietary signaling format. It is a binary format using a 1200-baud Minimum Shift Keying modulation. MDC-1200 is used for IDs, Selective Signaling, Emergency, Status/Message and Text."
There is MDC-600 and MDC-1200. MDC-600 was the first true digital signaling format out back in the late 70's and I believed introduced in the Syntor/MX series radios. MDC-1200 is the most widely used and has pretty much been a stanard feature since the Saber/Spectra series radios were introduced.
MDC-4800 and a couple of other varients exists, but its mostly paging and other data networks and not with two-way radio as being discussed here.
To the human ear, you cannot tell what is being sent. Even the sequnence is the same over various commands. For instance, "Emergency (man down)" and "Call" features send out data packets at the same interval, however "Call" will wait if there is traffic on the channel. Emegerency can overide this feature. Same with the page, status, message etc.
Depending on the radio and model, you can have the ID sent prior to speaking, after speaking or both. Not all radios support the above. ID's are 0001-FFFF or 0001-FFFE (0-9/A-F) and are transmitted in Hexidecimal format. If you see a radio or dispatch console display a name, this is called an Alias and is preprogrammed into the radio. The radio itself only knows Hex and transmits that data.
MDC-1200 is not used for MDT operations. They are on a private switch thru Verizon or Sprints cell network in Connecticut. I know there was a big swtich from CDPD to a faster data network. I am pretty sure Verizon was retained for the data network. CSP operates its own data network thru its trunked system.
