Timing Source on Asterisk

Asterisk requires timing for IAX trunking and for conference calls, and also uses timing for Music on Hold, other audio playback, syncing RTP streams as @voretaq7 mentioned, and some other things I can’t remember right now. The general idea is that audio has to be played at exactly the right time so that it sounds correct and isn’t choppy or distorted.

You can get timing with one of two asterisk drivers (not both at the same time):

• dahdi (formerly zaptel): Uses the timing source from a DAHDI/Zaptel or compatible FXO/FXS card.
• dahdi_dummy (formerly ztdummy): Linux-only; uses one of several possible timing sources provided by the Linux kernel, such as the real-time clock, HPET, or some USB controllers.

(As @sruffell noted in comments below, the functionality of dahdi_dummy has been rolled into dahdi since 2.3.0, so a new installation will probably only need dahdi for timing, whether a line card is present or not.)

If you’re using Linux, you can use the dahdi_dummy driver to provide timing; if you deploy Asterisk on any other OS, you’ll need the dahdi driver and a supported card, even if you don’t use any of the channels on the card.