SMODEM

File transfer protocol

Smodem refers to a bidirectional protocol for file transfer used between modems and the DOS program in which the protocol is implemented, both of which were developed by a Finnish company named Arisoft. It was mainly used in bulletin board systems because it could transfer files in both directions at the same time and allowed users to chat with each other with AriSoft's GroupChat software. Other popular bidirectional protocols such as BiModem, HS/Link and HydraCom also offered a chat option with the operator, but not with the system's other users.

Unlike ZMODEM and other older protocols, SMODEM has a separate low level multiplexed transfer layer (MSLP) and a high level file transfer layer. This simplifies the protocol design and provides a very high efficiency of 99.5%. The multiplexing makes it possible to send one or more files in a continuous stream without any breaks between the files, boosting the throughput dramatically when transferring multiple files. The multiplexing also made possible to add a full screen ANSI terminal emulator with a local full featured line editor and colored scroll back buffer of 'unlimited' size. The locally buffered terminal emulation allowed users to chat during full speed bidirectional file transfers. The protocol is symmetrical and can connect with itself in loop back mode. Files can be simultaneously transmitted and received without noticeable performance loss. SMODEM detects DESQview and DPMI (Windows, OS/2) and releases all unused processor capacity to the host system. It is written in the C language and compiled with Microsoft Visual C++ v1.0 compiler, optimized for an 80286 CPU. It needs at least 200 kB free memory to function properly and more to allow shelling from the program. Files can not be transferred while the shell is being used.