The SL Interface, when paired with the CO Interface, extends a telephone line; it connects to a POT (Plain Old Telephone). This Interface provides the subscriber loop battery and ringing voltages needed by the telephone. Taking the telephone off-hook causes the remote CO Interface to go off-hook. When the remote CO Interface receives a ringing voltage, a ringing voltage is provided by this SL Interface. The figure below shows this application.
There are four independent SL Interface ports on each card. The SL Interface can also be paired with another SL Interface to create a dedicated telephone link (order-wire). In this case picking up the telephone at one node causes the telephone to ring at the other node.