DSP3270 was created in 1980 in order to allow bisynchronous (BSC) 3270 terminals to communicate through an X.25 network. A PAD (packet assembler-disassembler) connected to a BSC 3270 type device communicates over the packet switched network with another PAD connected to a host computer. The protocol defines the communication between the two PADs, it is carried above the X.25 level 3 layer and so it could be called a level 4 protocol although it is not compatible with the OSI Transport layer.