Tuesday, April 17, 2012

Loop start


When loop start signaling is used, a gateway's FXO interface looks like a phone to
the switch (PBX, PSTN, Key-System) it is connecting to. The FXO interface closes the
loop to indicate off-hook, just as an analog phone does when it is physically lifted
off-hook. The switch always provides battery, so there is no disconnect supervision
from the switch side. Because a switch expects a phone user (which an FXO interface
behaves like) to hang up the phone when the call is terminated (on either side), it
also expects the FXO port on the router to hang up. This "human intervention" is not
built into the router. The FXO port expects the switch to tell it when to hang up (or
remove battery to indicate on-hook). Because of this, it cannot be guaranteed that a
near-end or far-end FXO port will disconnect the call after either end of the call has
hung up.