Donor: Anonymous
Microcom, Inc. was a major modem vendor during the 1980s, although they were never as popular as the "big three", Hayes, U.S. Robotics (USR) and Telebit. Nevertheless they hold an important place in modem history due to their introduction of the MNP error-correction and compression protocols, which were widely used under license by most modem manufacturers in the 1990s. Compaq purchased the company in 1997.
MNP 4 was a further improvement on MNP 3, adding a variable packet size system they referred to as Adaptive Packet Assembly. With MNP 4 operation, the two modems constantly monitor the line for dropped packets, and if a certain threshold is crossed (selected by the user), the modem "drops back" to a smaller packet size. This means that when a packet is dropped, the amount of data that has to be re-sent is smaller, leading to better throughput.
MNP 4 also introduced Data Phase Optimization, a simple change to the protocol that allowed some of the packet-framing information to be dropped after the link was set up, further reducing protocol overhead. The combination of these features, along with MNP 3's lack of byte-framing, allowed for a further increase in throughput efficiency.
Reference: http://en.wikipedia.org/wiki/Microcom_Networking_Protocol#MNP_4
