I bought a Novation AppleCat modem just because it had this type of software, It was the first of it's kind- to be able to download files and talk with the sysop at the same time. It was able to do this through the use of the Bell 202 (1200 baud, half duplex) protocol that was embedded in the modem. The sending computer would send your file via 1200 baud. Each block of sent data was replied back in 300 baud with a checksum. The checksum reply was used to send/receive the conversation. At the time, it was years ahead of the competition.