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nal transmission in both directions. This is the type of phone service most people
have in their home or office. Since the same two wires are used for both transmitting
and receiving signals, an echo of the transmitting signal will also be received and it
is the modem receiver's task to remove the echo before demodulation.
Leased-line phone service is also available from phone companies in a 4-wire form.
Here, both the transmitting and receiving signals each use a separate 2-wire pair and
thus no echo problem will result. Normally, no DC current feeding, ringing, or di-
aling function is provided on a leased line.
There is also the 2-wire leased line which is a permanent 2-wire connection.
Intelligent Modem
Formerly, modem functions, settings, and operations were simple and everything
was controlled by manual switches, either internal or external, or by wire-strapping
settings. No computer control was provided.
All of an intelligent modem’s functions, including dialing and answering, are con-
trollable by the computer or terminal it is connected to by means of the same RS-232
serial interface used for data connection.
An intelligent modem operates in one of two states - the command state or the data
state. In command state, the modem interprets data received from the serial interface
as commands and sends back an action result in response. In data state, the modem
modulates the data received from the serial interface and sends the demodulated data
to the serial interface as received data. The user needs to know whether the modem
is in the command state or data state and how to switch it.
ISDN
The normal telephone network PSTN (Public Switched Telephone Network) trans-
mits voice band analog signals from end to end. A modem is needed at each end to
transmit digital data. The digital telephone network ISDN (Integrated Services Dig-
ital Network) transmits digital data from end to end, thus no modem is needed to
transmit data. Instead, an ISDN telephone is needed at each end to translate voice
signals to and from digital data for ISDN digitized voice data transmission.
ISDN TA
ISDN has a digital interface at the customer site. An ISDN TA (Terminal Adapter)
will adapt the user terminal’s interface, usually an asynchronous or synchronous se-
rial interface, to the ISDN digital interface, usually the S- or T-interface. With an
ISDN TA a user connects a computer/terminal to the ISDN telephone line in the
same way as he/she uses a modem to connect a computer/terminal to the PSTN tel-
ephone line.