This is an implementation of a 5250 emulator.  The 5250 is most
commonly used for connections to IBM's AS/400.  While one can connect
to an AS/400 with a VT100 emulator, it's not ideal.  The problem is
that the 5250 is a screen at a time terminal, whereas the VT100 is a
character at a time device.  The emulator uses the binary mode of
telnet to transfer the 5250 data stream.