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Chapter 2

Chapter 2. The Physical Layer. Outline of Physical Layer. It defines the electrical, timing and their interfaces by which bits are sent as signals over the channel. Foundation on which the network is built. The properties of different kinds of physical channel determine the performance

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Chapter 2

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  1. Chapter 2 The Physical Layer

  2. Outline of Physical Layer • It defines the electrical, timing and their interfaces by which bits are sent as signals over the channel. • Foundation on which the network is built. • The properties of different kinds of physical channel determine the performance • Data Transmission rate is limited • Three types of transmission media: • Guided (copper wire and fiber optics) • Wireless (terrestrial radio), and • Satellite

  3. Outline of Physical Layer • Digital Modulation: How analog signals are converted into digital bits and back again. • Multiplexing Schemes: Exploring how multiple conversations can be put on the same transmission medium at the same time without interfering with one another. • Examples of communication systems used in practice for wide area computer networks: • The (fixed) telephone system • The mobile phone system, and • The cable television system.

  4. Transmission Media

  5. Selection of Transmission media • Factors: • 1. Design Factors: • - Bandwidth • - Transmission Capacity • - Interference • - Number of receivers • 2. Guided or Unguided media: • For long distance point-to-point transmission guided media are suitable. • For long distance broadcasting transmission unguided media like microwave links are chosen

  6. Twisted Pair Cable • Least expensive and most widely used. • It consist of two insulated copper wires arranged in a regular spiral pattern. • A wire pair acts as single communication link. • It transmit both Analog and Digital signals • For analog signals amplifiers are required about every 5 to 6 km. • For digital signals repeaters are required every 2 to 3 km. • It is used for telephone system. • Two varieties • 1. Unshielded Twisted Pair Cable (UTP) • 2. Shielded Twisted Pair Cable (STP)

  7. Unshielded Twisted Pair Cable (UTP)

  8. Electronic Industries Association(EIA ) has developed standard to grade UTP cable by quality. • Category 1(Cat1) as the lowest quality and Category5(cat5) highest quality . • Category 5 : LAN, 100 Mbps • Category 4 : Three twist per foot, 16 Mbps • Category 3: Telephone system,10 Mbps • Category 2 : Voice Data transmission , 4 Mbps • Category 1 : Voice and Low speed data communication.

  9. Characteristics of UTP : • Transmission rate 10-100 Mbps • UPT is less expensive than FOC and Co-axial Cable • Maximum cable segment of UTP is 100 mt • Very flexible and easy to work • Uses RJ-45 connector • Most vulnerable to electrical interference Advantages: • Easy to terminate • Less installation cost • It is cheaper than other communication media. • It is easy to install and repair. - It provides good data transmission in short distance networks.

  10. Disadvantages: • Very Noisy • Covers less distance • Suffers from interference

  11. Shielded Twisted Pair Cable (STP)

  12. It provides protective shielding around the copper wire • It provides better performance at lower data rates. • Distance is limited like 100 mt to 500 mt. Application: - Analog and Digital signals • Telephone Network. • Advantages of STP : • It is less vulnerable than UTP. • It provides good data transmission in short distance networks. • It is easy to install than other media. • Disadvantages of STP • It is more expensive than UTP. • It is relatively hard to repair than UTP.

  13. Comparison of UTP and STP:

  14. Co-axial Cable

  15. Characteristics: • 10 Mbps transmission speed • Maximum cable length of thinnet is 185 mt and for thicknet is 500 mt • Flexible and easy to work with thinnet • Les expensive than fiber optics cable but more expensive then TP • Good resistance to electrical Interference

  16. Advantages: • Analog and digital transmission • Higher bandwidth • Easy to install and inexpensive as compared to FOC • Long distance at higher data rates • Excellent noise immunity .

  17. Disadvantages: • Limited distance • Node connection is limited • Proper connectors and termination is must Applications: -Telephone Network • Ethernet LANs • Cable Television Network

  18. Fiber Optic Cable

  19. Types of fiber optics

  20. Characteristics of FOC • Transmission rate of 100 Mbps • Not affected by the electrical interference • Most expensive cable • Cable length of 2km or more • Supports voice , video and data transmission • Most secured media • Not flexible and difficult to work.

  21. Advantages • Wide bandwidth • Low losses • Immune to cross talk • Interference immune • Light weight • Small size • More strength • Security • Long distance transmission • Environment immune • Safe and easy to installation

  22. Disadvantages • High initial cost • Maintenance and repairing cost

  23. Wireless Transmission • 1. Radio Transmission : • It has frequency between 10 KHz and 1 GHz • It includes the following types • A). VHF television and FM radio • B). UHF radio and television • Characteristics : • Easy to generate • They can travel long distance • Penetrate buildings easily so they are widely used for communications both indoors and outdoors. • Radio waves are omni directional, meaning that they travel in all directions.

  24. At high frequency , radio waves are subject to interference from motors and other electrical equipment • At low frequency and medium frequency range cannot be used for data transfer because of their very small bandwidth

  25. Receiving Antenna Transmitting Antenna • 2. Microwave Transmission: • Above 100 MHz, the waves travel in straight lines and can therefore be narrowly focused. Line of sight Receiving Station Transmitting Station

  26. Line of sight describe microwave path • Microwave system uses repeaters at intervals of about 25 to 30 km in between the transmitting receiving stations. • 1st repeater is placed in line-of-sight of transmitting station and the 2nd repeater is placed in line-of-sight of the receiving station. • Two consecutive repeaters are also placed in line-of-sight of each other. • The data signals are received , amplified and re-transmitted by each of these stations • The main objective of microwave communication system is to transmit information from one place to another without interruption and clear reproduction at the receiver.

  27. Characteristics • It is weather and frequency dependent • It inexpensive as compared to FOC • Data transmission rates of about 16 Gbps • At high frequencies microwaves systems can carry 2,50,000 voice channels at the same time • Limitations : • Repeater is needed • Applications: • Mobile telephone network • Wireless LAN • Point-to-Point communication between stations • Line-of-sight communication

  28. 3). Infrared light wave transmission • Used for short distance communication • Ex TV, VCR remote, mobile data transfer • Indoor wireless LAN • Communication between inhouse electronic gadgets like KB, Mouse , Printers, scanners

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