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• The easiest way to connect computers over
long distances is over telephone lines, you can
reach almost anywhere by telephone.
How a Modem Works
• A modem is typically used to send digital data
over a phone line. The
sending modem modulates the data into a
signal that is compatible with the phone line,
and the receiving modem demodulates the
signal back into digital data.
Wireless modems convert digital data into
radio signals and back.
Connecting to the Computer
• A modem can be installed internally, in the
computer in which case it is called an internal
modem, or it can be an external device that is
connected to the computer with a serial cable.
Connecting to the Phone System
• You connect a modem to the phone system
just as you would a regular telephones or fax
machine. The modem should accept a regular
telephone cord the kind that connects a
telephone to a wall outlet.
• Modem speed is often discussed in baud or
• Baud rate refers to the oscillation of a sound
wave on which a single bit of data is carried.
BPS, or bits per second, is the amount of data
transferred in a second.
• Three common speeds for modems are 9,600
bps, 14,400 bps, and 28,800 bps.
• In signal processing, data compression, source
coding, or bit-rate reduction involves encoding
information using fewer bits than the original
representation. Compression can be either
loss or lossless. Lossless compression reduces
bits by identifying and eliminating statistical
Error Detection and Correction
• Error detection is the detection of errors
caused by noise or other impairments during
transmission from the transmitter to the
receiver. Error correction is the detection of
errors and reconstruction of the
original, error-free data.
Types of Modem
• There are many types of modems because there
are many different environments over which
computer need to send data. Modem are used
whenever a computer’s digital signals need to be
converted to analog signals to be transmitted.
• the types of modem can be divide into two broad
categories, depending on how they coordinate
their data transmission.
• A synchronous communication, requires that
message begin with a start bit so that the
receiving device can synchronize its internal
clock with the timing of the message
• A synchronous transmission are normally
short, and the end of the message is singled
by a stop bit.
• synchronous communication require that
some kind of clocking mechanisms be put into
place to keep the clocks of the sender and
• One of the three methods are used for
synchronous timing coordination.
1. Guaranteed state change
2. Separate clock signals
3. Over sampling
• A modem (modulator-demodulator) is a
network hardware device that modulates one
or more carrier wave signals to
encode digital information for transmission
and demodulates signals to decode the
• An ISDN (Integrated Services Digital Network)
modem is an example of digital modem.
• Critical part of remote access, that carriers in
remote network access or in wide area
networking is the phone company or other
entity that carries the telephone connection
from one network point to the other.
• We connect computers together using phone
company telephone lines into ways.
1. Public dial network lines
2. Leased line
Public dial network lines
• Dial-up Internet access is a form of Internet
access that uses the facilities of
the public switched telephone network (PSTN)
to establish a connection to an Internet
service provider (ISP) by dialing a telephone
number on a conventional telephone line.
• A leased line is a private
bidirectional or symmetric
telecommunications line betw
een two or more locations
provided in exchange for a
monthly rent. Sometimes
known as a private circuit or
Remote Access Software
• Microsoft RAS (Remote Access Service) allows
users to connect to a windows NT computers
over phone lines. If you have more than one
modem attached to yours RAS server, you can
have more than one computer connect to it at
a time. RAS on a Windows NT server can
accept up to 256 simultaneous connection
from remote client.
Installing Remote Access Service
• Click Next twice. In the list of role services,
select Routing and Remote Access Services to
select all of the role services. You can also
select individual server roles. Proceed through
the steps in the Add Roles Wizard to complete
Administering Remote Access Service
• Remote Access Service Administration
provides a set of functions for administering
user permissions and ports on RAS servers.
Using these functions, you can develop a RAS
server administration application.
Making a Dial-Up Connection
• How to Create a Dial-Up Connection in Windows XP
• From the Start menu, choose Control Panel.
• Click the Network and Internet Connections icon.
• Click the Set up or change your Internet connection
• Click the Setup button.
• Click the Next button.
• Select Connect to the Internet.
• Click the Next button