The input/output (IO) modules are responsible for controlling the telephony hardware. All FreeTDM modules are named with the prefix "ftmod" (as in FreeTDM module). The IO modules take care of reading and writing raw data bytes and executing low level control commands in the telephony hardware, but do nothing else; they are a kind of dumb module with not much, if any, knowledge of signaling protocols such as ISDN and SS7.
Wanpipe Module (ftmod_wanpipe)
This is the IO module that is used to talk to Sangoma telephony cards.
ZapTel / DAHDI Module (ftmod_zt)
This is the IO module to talk to Zaptel and DAHDI-compatible telephony cards, such as those distributed by Sangoma, Xorcom, and Digium.
