In the traditional LAN, all the computers are connected together by a cable.
People who want to work at home or need access to the company's data from a
customer's office need access to the LAN. A remote system can access the LAN using
the Public Switched Telephone Network (PSTN).
PSTN is the regular telephone line, also known as Plain Old Telephone Service
(POTS). A remote user with dial-up networking dials a phone number to make a
connection. A server on the LAN has a modem and remote access software that listens
for rings, authenticates the user, and provides access to the network.
With a dedicated telephone line, you don't have to dial a phone number, because
it's a permanent hard-wired connection between two locations.
The telephone network was designed long before computers to transmit voice. The
sound of your voice is converted into an analog electrical signal. Computer networks
work with digital signals. A Modem (MODulate DEModulate) is used to modulate the
analog signal in a way that lets it carry the digital information. At the receiving
end the analog signal is demodulated to convert it back to a digital signal for the
When a modem is installed in a computer, some of the computer's resources such as
an IRQ and I/O addresses must be configured. When you use an external modem, you must
configure the serial COM port that the modem is connected to. An Internal modem has
an on-board COM port that must be configured.
Before Plug and Play, jumpers were used to configure the modems COM port settings.
With Plug and Play, you only need to install the modem card and boot the computer.
The Plug and Play operating system will detect the new hardware and configure the
SLIP and PPP
Before the World Wide Web, you accessed the Internet by dialing into a Unix server
where you had a shell account. Terminal emulation software allowed your computer to
function as a dumb terminal on the remote server. The server did all the processing
and sent screen updates back to your computer.
Serial Line Internet Protocol (SLIP) is used to connect a computer to a TCP/IP
network using an asynchronous connection such as a telephone line. SLIP encapsulates
the TCP/IP protocol for transmission via a modem. Your computer is configured with an
IP address like a regular Internet host that can be connected to other computers on
the Internet. This allows you to run network applications on your computer. The only
difference is that a SLIP connection over a telephone line is a lot slower than a
direct connection to an Ethernet network.