A modem is a device that modulates analog signals to transmit digital data and demodulates received analog signals to reconstruct the original digital data. It has a transmitter that converts digital to analog, a receiver that converts analog to digital, and a control unit that handles functions like auto-dialing. Modems use various modulation techniques like amplitude modulation, frequency modulation, and phase modulation. They are classified by operation mode, synchronization method, transmission medium, and modulation technique. Newer technologies like DSL provide higher speeds over telephone lines than traditional modems.

1. Digital communication technology
MODEMs
Presented by: Patil Dnyandev Bhausaheb
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2. 2 What is modem ?
Modem is abbreviated for Modulation and Demodulation
 A modem is a device that modulates an analog carrier
signal to encode digital information, and also
demodulates such a carrier signal to decode the
transmitted information.
 The goal of modem is to produce signal so that it can be
easily transmitted over analog line and decoded to
reproduce the original data.

3. 3 Key components:
• Transmitter (modulator), Receiver (demodulator).
• Control unit (auto dial & auto answer).
• Power supply.

4. 4 Transmitter (modulator) and Receiver (demodulator)
Modulator converts digital signal into analog form to transmit
over analog telephone line.
Demodulator converts analog signal into digital.
Different modulation techniques are used to modulate signals,
but at transmitting side, the final output of modulator is analog
signal and input is digital.
Different types of modulation:
There are many techniques of modulation, depending on
which modems are classified.

5. 5 Modulation techniques
 Frequency Modulation.
 Amplitude Modulation.
 Phase Modulation.
 Combination Modulation.

6. 6
Auto dial/auto answer
 There are different commands used to communicate
with modem.
 AT commands are used for auto dial or auto answer.
 Control unit responds to AT commands and takes
action according to command.
Control unit (auto dial & auto answer)

7. 7 Classification of modem
Modems are classified as follows:
1. Operation mode:
• Simplex:
Only transmit or receive data.
• Half-duplex
Transmit or receive data but one at a time.
• Full-duplex
Transmit or receive data simultaneously.

8. 8
2. Synchronisation:
 Synchronous: Operate in the audio range. The usual
modulation method is the phase modulation. Can handle
a continuous stream of data bits but requires a clock
signal. The data bits are always synchronized to the clock
signal. There are separate clocks for the data bits being
transmitted and received.
 Asynchronous: Most of the modems that operate in slow
are asynchronous. Asynchronous modems operate in FSK
(Frequency Shift Keying) modulation. Can handle data
bytes with start and stop bits. It does not require clock
pulse.
Modems classification continue…

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3. Modulation:
• AM:
• FM (FSK):
• PM:
Modems classification continue…

10. 10 Modems classification continue…
4. Transmission media:
• Dial-up: Dial-up modems can establish point to point connections to
the PSTN by manual or automatic dialing or answering. Dial up modem
uses a carrier that is an audible tone carried through twisted pair cable.
• Radio: Used to send data over radio frequency signal.
• Cable:A cable modem is a device that enables you to hook up your
PC to a local cable TV line and receive data at about 1.5 Mbps
• optical: Used to send data across a pair of glass fibers using light. Such
modems use entirely different technology than modems that operate
over dedicated wires.

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 Data terminal equipment (DTE) is connected to modem using digital signal.
 The modem is connected to telephone exchange office over analog
telephone line using analog signal.
 The two exchange offices are connected to telephone network.
 Digital data is encode to transmit over analog line at transmitting end and
again it is decoded to digital form by modem to represent data at receiving
end.
Communication system

12. 12 Techniques used to increase data speed
After traditional modems reached their peak data rate, telephone
companies developed another technology, DSL, to provide higher-speed
access to the Internet.
Digital subscriber line (DSL) technique provides high-speed data
rates over conventional telephone line and the existing local loops.
Types of DSL:
• ADSL: Asymmetric Digital Subscriber Line.
• SDSL: Symmetric Digital Subscriber Line.
• HDSL:High-Bit-Rate Digital Subscriber Line
• VDSL: Very-High-Bit-Rate Digital Subscriber Line (VHDSL).

13. 13 ADSL:
• ADSL is type of digital Subscriber Line that provides faster data
transmission over copper telephone lines than Dial-up modem.
• It provides high speed of down link.

14. 14 SDSL:
• SDSL is type of digital Subscriber Line that is exact opposite to
ADSL.
• It provides Low bandwidth for upstream than downstream.

15. 15 VDSL:
• VDSL provides faster data transmission than ADSL over twisted pair telephone
line or untwisted pair line.
• It provides high speed of downstream up to 52 Mbit/s.
• It is common in home network.
Very-High-Bit-Rate Digital Subscriber Line (VHDSL).

16. 16 HDSL:
• It was the first DSL technology .
• It provides bit rate of 1.544 and 2.08 Mbit/s.
High-Bit-Rate Digital Subscriber Line

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Thank
You!