Telephone System &

MultiplexingPerformed By: 1) Abhishek Gaikwad

2) Ketan more 3) Aditya Shinde 4) Vaibhav Pote

Telephone SystemsO Telephone Networks use Circuit

Switching. The telephone network came into existence in late 1800s.

O The POTS is a old telephone network which used analog systems and analog signals to transmit voice.

O During the recent years, the telephone network has changed a lot. The network is now digital as well as analog.

Components of Telephone Systems

O The figure shows the most basic telephone network that is made of three main components:

Local loop Trunks & Switching offices

1) Local Loops:• Local Loop is a twisted pair cable this is

used for connecting a subscribers telephone to the nearest end office.

• The local loop has a bandwidth of 4 kHz if it is used for voice communication

2) Trunks:• Trunks are the transmission media that

handles the communication between offices

• The trunks handles hundreds of thousands of connection simultaneously. The transmission media used as trunks is usually optical fiber cables or satellites links

Switching offices:• Connecting every subscriber permanently

with all the other subscriber using wires will be an impossible thing.

• Hence the telephone companies use switches located at the switching offices to establish a connection as and when required , between the calling and the called subscribers.

MULTIPLEXING O It is process of simultaneously

transmitting two or more signals over a single communication channel.

O Due to multiplexing it is possible to increase the number of communication channels so that more information can be transmitted.

O The typical applications of multiplexing are in telemetry and telephony or in the satellite communication.

Concept of Multiplexing

O The multiplexer receives a large number of different input signals

O Multiplexer has only one output which is connected to single communication channel.

O The multiplexer combines all input signals into a single composite signal and transmits it over the communication medium.

O Sometimes the composite is used to modulate a carrier before transmission.

O At the receiving end, of communication link, a demultiplexer is used to sort out the signal into their original form.

O A demultiplexer is the circuit which decomposes the multiplexed signal into the original signals.

ClassificationOf Multiplexing

MULTIPLEXING

Analog Multiplexing Digital Multiplexing

FDM WDM TDM

TYPES OF MULTIPLEXING

O There are basically three types of multiplexing:

1. Frequency Division Multiplexing (FDM)

2. Time Division Multiplexing (TDM)3. Wavelength Division Multiplexing

(WDM).

FDMO The FDM is based on the concept of

sharing bandwidth of a common communication channel. The signals which are to be transmitted simultaneously will each modulate a separate carrier.

O The modulation can be AM, SSB, FM or PM. The modulated signals are then added together to form a complex signal which is transmitted over a signal channel.

FDM (Block Diagram)

Advantages O A large number of signals can be

transmitted simultaneously . O Due to slow narrow band fading only

a single channel gets affected. DisadvantagesO FDM suffers from the problem of

crosstalk.O All the FDM channels get affected

due to wideband fading.

Applications of FDMO Telephone systems.O AM(amplitude modulation) and

FM(Frequency modulation) radio broadcasting

O TV broadcasting.O First generation of cellular phones

used FDM.

WDM(Wavelength Division Multiplexing)

O WDM is the variation of FDM, for fiber optic channels.

O As shown in figure, 2 fibers come together at a prism, each having energy in a different band. After passing through the prism, beams are combined onto a single shared fiber, for transmission to a distant destination, where they are split again.

Fibers and Prism Diagram

ApplicationsO One important

application of WDM is SONET network in which multiple optical fiber lines are multiplexed and demultiplexed.

TDM(Time Division Multiplexing)

O In TDM all the signals to be transmitted are not transmitted simultaneously. Instead, they are transmitted one by one .

O Thus each signal will be transmitted for a short time. One cycle or frame is said to be complete when all the signals are transmitted once on the transmission channel. The TM principle is illustrated in the Figure o the next slide.

O As shown in the figure one transmission of each channel completes one cycle of operation called as “Frame”.

O The TDM system can be used to multiplex analog and digital signals, however it is more suitable for the digital signal multiplexing.

Advantages of TDM

O Intermodulation distortion is absent.O TDM circuitry is not very complex.O The problem of crosstalk is not very

severe.O Full available channel bandwidth can

be utilized for each channel.

DisadvantagesO Synchronization is essential for

proper operation.O Due to slow narrow band fading, all

the TDM channels may get wiped out.

Applications1) For Multiplexing of digital data.2) For digital telephony.3) In the mobile phones.