CS207

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Computer Networks

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Lecture-3

Types of Multiplexing

• Frequency Division Multiplexing (FDM)
• Wave Division Multiplexing (WDM0)
• Time Division Multiplexing (TDM)

TRANSMISSION MEDIA

Basic Idea

• Encode data as energy and transmit energy

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• Decode energy at destination back into data

• Energy can be electrical, light, radio, sound, ...

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• Each form of energy has different properties and requirements for transmission

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Transmission media�

• Transmitted energy is carried through some sort of medium

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• Transmitter encodes data as energy and transmits energy through medium
• Requires special hardware for data encoding
• Requires hardware connection to transmission medium

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Transmission media�

• Media can be copper, glass, air, ...

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Copper Wires

• Widely used because of low resistance to electric current, inexpensive and easy to install
• Twisted pair wires
• Two insulated copper wires twisted together to reduce Interference
• Examples: Telephone wiring (one pair).LAN wiring UTP cable (4 pairs of copper wire)

Types of Media

• GUIDED Media:
• Uses physical path
• Examples: copper wires, optical fiber, coaxial cables etc.
• UNGUIDED Media:
• Uses no physical path
• Example: Radio waves

Coaxial Cable

• Single wire surrounded by an insulation and a metal shield against interference
• Example use: Cable TV wiring and early LAN wirings

Optical Fibers�

• Uses light to transport data

Advantages of Optical Fibers

• Neither causes nor susceptible to electrical interference

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• Can carry pulse much farther than copper

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• Light can encode more information than electric

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• One single fiber is sufficient to carry data

Radio�

• Used for public radio and TV broadcast

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• Data transmission is said to operate at Radio Frequency (RF)

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• Transmission range depends on antenna size

Satellites

• Satellites for long distance transmission

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• Satellite transponders receive, amplify, and transmit the RF signal back to the ground

Geosynchronous Satellites

• Placed in an orbit (at 35,785 km or 22,223 miles above the earth) that is exactly syncronized with the rotation of the earth

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• From the ground, it appears to stay at the same position at all times

Low Earth Orbit Satellites

• Rotates faster than the rotation of the earth at 200 to 400 miles above the earth

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• They do not appear to remain stationary

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• Low Earth Orbit Satellite Arrays is a set of satellites communicating with each other and with the earth surface

Microwave�

• Electromagnetic radiation beyond the frequency range used for radio and TV

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• It can be aimed in a single direction

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• Can carry more information than low frequency RF transmissions

Infrared

• Remote controls used with TV etc uses IR

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• Inexpensive wireless communication

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• Limited to a small area

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• Convenient for portable computers

CHAPTER No. 5�LOCAL ASYNCHRONOUS COMMUNICATION �(RS-232)

Introduction

• Computers use binary digits (0s and 1s) (bits) to represent data

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• Computers communicate by sending bits through a transmission medium (i.e. cable, air).

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• This chapter describes how electric current is used to send digital information across Short Distances

The Need For Async Communication

• In asynchronous communication the sender and receiver do not need to coordinate (synchronize) before the transmission begins

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• In an asynchronous system, the receiving hardware must be ready to accept and interpret the signal whenever it arrives

Using Electric Current to Send Bits

• A small electric current is used to encode data
• Example: Negative voltage to represent a 0 bit, positive voltage to represent a 1 bit.

Standards For Communication�

• Standard specifies both the timing of the signals and the electrical details of voltage and current

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• RS232-C (by EIA) is one of the most widely used standard to transfer characters between a computer and a device such as modem, keyboard etc.

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• RS-232 defines a serial (bits are sent one after another) asynchronous communication

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RS232-C Specifications�

• Cable length < 50 feet (20mt)

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• -15v (bit 1) , +15v (bit 0)

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• One character => 7 or 8 data bits

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• No delay between character bits

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• When idle, puts -15v (bit 1) on the wire

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• Each character start with a start bit(0), and end with a stop bit(1)

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Baud Rate, Framing, And Errors

• Baud rate:
• The number of changes in the signal per second

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• For simple RS-232, Baud rate = Number of bits/sec

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• If the voltage measurements do not all agree or if the stop bit does not occur exactly at the same time expected, framing errors occur

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Full-Duplex Asynchronous Comm

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• Electrical circuits require at least two wires, one of them to carry the data signal, the other is used as the return path, called ground

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• Simultaneous transfers in two directions are known as full-duplex transmission

Full-Duplex Asynchronous Comm

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Long Distance Communication

• Electric current becomes weaker as it travels over copper

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• Signal loss occurs when RS232 is attempted to use to communicate to a remote site

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• A continuous, oscillating signal, called a Carrier, propagates farther than other signals

Modulation

• To send data, a transmitter slightly modifies the original information by using a carrier

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• Such modifications are called “Modulation”

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• The receiver
• monitors the incoming carrier,
• detects modulation,
• reconstructs the original data, and
• discards the carrier

Types of Modulation

• Three of the modulation schemes are:

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• Amplitude Modulation (AM)

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• Frequency Modulation (FM)

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• Phase Modulation (PM)

Modem

• A Modulator is a hardware circuit that
• accepts a sequence of data bits
• applies modulation to a carrier wave according to the bits
• A Demodulator is a hardware circuit that
• accepts a modulated carrier wave and
• recreates the sequence of data bits
• A Modem is a hardware that combines both modulator and demodulator in a single device

Modem

Multiplexing

• Q: How can two or more signals can be transmitted simultaneously over a single wire ?

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• A: By using different carrier frequencies

Career Frequencies And Multiplexing

• Frequency division multiplexing (FDM)

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• Using multiple carrier frequencies to allow independent signals, to travel through a medium

Wavelength Division Multiplexing (WDM)

• FDM concept applied to the optical transmission systems (optical fibers) is known as Wave Division Multiplexing

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Time Division Multiplexing (TDM)�

• Sources sharing a medium “take turns” to send data

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• Synchronous TDM gives each source an opportunity to send data

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• Statistical TDM does not give chance to a source that does not have data to send

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