In addition, a TA may contain an interface and codec for one or more analog telephone lines (aka a/b line), allowing an existing POTS installation to be upgraded to ISDN without changing phones.
[1] Terminal adapters typically connect to a basic rate interface (S0, sometimes also U0).
On the terminal side, the most popular interfaces are RS-232 serial and USB; others like V.35 or RS-449 are only of historical interest.
Devices connecting ISDN to a network (e.g. Ethernet) commonly include routing functionality; while they technically include a TA function, they are referred to as (ISDN) routers.
It is often used when daisy chain wiring is necessary on a multi-node serial communication network like RS-485 or RS-422.