In
ISDN terminology, the
terminal adapter connects a
terminal (computer) to the ISDN network. The TA therefore fulfills a similar function to the ones a
modem has on the
POTS network, and is therefore sometimes called an
ISDN modem. The latter term, however, is partially misleading as there is no
modulation or
demodulation performed. There are devices on the market that combine the functions of an ISDN TA with those of a classical modem (with an ISDN line interface). These combined TA/modems permit connections from both ISDN and analog-line/modem counterparts. 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. 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. ==Mobile networks==