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Computer Networks (CSE-4711) Lecture-4

Computer Networks (CSE-4711) Lecture-4. Instructor: Sazid Zaman Khan Lecturer, Department of Computer Science and Engineering, IIUC. Transmission media. The purpose of Physical layer is to transport raw bit streams from one machine to another. There are two main types of media:

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Computer Networks (CSE-4711) Lecture-4

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  1. Computer Networks (CSE-4711)Lecture-4 Instructor: SazidZaman Khan Lecturer, Department of Computer Science and Engineering, IIUC

  2. Transmission media • The purpose of Physical layer is to transport raw bit streams from one machine to another. • There are two main types of media: • 1. Guided media such as copper wire and fiber optics. • 2. Unguided media such as wireless media. • Twisted pair: A twisted pair consists of two insulated Copper wires twisted like a DNA molecule. Twisted pair cabling is a type of wiring in which two conductors are twisted together for the purposes of canceling out electromagnetic interference from external sources; and crosstalk between neighboring pairs.

  3. Twisted pair Figure-1: Twisted pair cable

  4. Transmission media • The telephone system uses the twisted pair for the local connection. • Twisted pairs can run several kilometers without amplification but for longer distances repeaters are needed. • Twisted pair transmit digital or analog signals. • There are several types of Twisted pair cable: • Category 3 (Cat 3) and Category 5 (Cat 5). Cat 5 has more twists than CAT3. All of these refers to UTP (Unshielded Twisted Pair). • We normally use UTP CAT 5. There is another type Shielded Twisted Pair (STP) which is more bulky and expensive.

  5. Coaxial cable • Another common transmission medium. For example, satellite cables. • It has better shielding than twisted pair, so it can span longer distances at higher speed. • A coaxial cable consists of a stiff copper wire as the core, surrounded by an insulating material. The insulator is encased by a cylindrical conductor. The outer conductor is covered in a protective plastic shealth. Figure- 2 shows a coaxial cable.

  6. Coaxial cable Figure-2 : Coaxial cable

  7. Fiber Optics • An optical transmission system has 3 key parts: light source, transmission medium and detector. Conventionally, a pulse of light indicates a 1 bit and the absence of light indicate a 0 bit. The transmission medium is an ultra thin fiber of glass By attaching a light source to one end of an optical fiber and a detector to the other, we have a unidirectional data transmission system that accepts an electrical signal, converts and transmits it be light pulses and then reconverts it to an electrical signal at the receiving end.

  8. Fiber Optics Figure-3 : Fiber Optics

  9. Fiber Optics • At the centre of the fiber is the glass core, through which the light propagates. The core is surrounded by a glass cladding to keep all the light in the core. Next is a thin plastic jacket.

  10. Total internal reflection in Fiber Optics Figure-4 : Total internal reflection in fiber optics

  11. Total internal reflection in Fiber Optics • In Fiber optics, light propagates by using the total internal reflection of light.

  12. Fiber Optics vs. Copper Wire • Fiber Optics have much higher bandwidth than copper. • It has low attenuation, therefore repeaters are needed after approx. 50 km compared to 5 km with copper wire. • It is thin and light compared to copper wire. • Fibers do not leak light and difficult to tap. • Fiber is more expensive. • On long distance high bandwidth lines, fibers are widely used in telecommunication networks.

  13. Wireless transmission and Satellites • Wireless transmission on the physical layer is used when wired/guided transmission is unavailable. • WLAN/WiMAX/Cellular networks are examples of such networks. • Satellites are often used to connect remote networks.

  14. Communication Satellite • In its simplest for, a communication satellite is a big Microwave repeater in the sky. It works with Microwaves, waves are differentiated based on wavelength. • It contains transponders which is a device that receives an incoming signal, amplifies it and rebroadcasts it in a different frequency to avoid interference with the incoming signal.

  15. Communication Satellites Figure-4 : Communication Satellites

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