Chapter_2_CN - Chapter 2 Physical Layer Transmission Media...

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Chapter 2 Physical Layer – Transmission Media Introduction Computers and other telecommunication devices use signals to represent data. These signals are transmitted from one device to another in the form of electromagnetic energy. Electromagnetic signals can travel through vacuum, through air, or through other transmission media. Transmission media can be broadly classified in to two categories: i. Guided media ii. Unguided Media Guided Media – Provide a conduit from one device to another. These include twisted- pair cable, coaxial cable, and fibre-optic cable. A signal travelling along any of these media is directed and contained by the physical limits of the medium. Twisted-pair and coaxial cable use metallic (copper) conductors that accept and transport signals in the form of electrical current. Optical fibre is a glass or plastic cable that accepts and transports signals in the form of light. Twisted-pair cable Comes in two forms: Unshielded and Shielded Unshielded Twisted-Pair cable (UTP) – most common type of telecommunication medium in use today. Although most familiar from its use in telephone systems, its frequency range is suitable for transmitting both data and voice.
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Figure 3.1 A twisted pair consists of two conductors (usually copper), each with its own coloured plastic with insulation. The plastic insulation is colour-banded for identification. Colours are used to identify specific conductors in a cable, and to indicate which wires belong in pairs and how they relate to other pairs in a larger bundle. Twisting reduces interference and noise. Advantages of UTP are cost, and ease of use. UTP is cheap, flexible, and easy to install. UTP comes in five grades or categories: Category 1 – The basic twisted-pair cabling used in telephone systems. This level of quality is good for voice, but inadequate for all but low-speed data communication. Category 2 – The next higher grade, suitable for voice and for digital transmission of up to 4 Mbps. Category 3 – Required to have at least three twists per foot and can be used for data transmission of up to 10 Mbps. Category 4 – Provides transmission rate up to 16 Mbps. Category 5 – Used for data transmission up to 100 Mbps. Shielded Twisted-Pair (STP) cable Figure 3.2 It has a metal foil or braided-mesh covering that encases each pair of insulated conductors. The metal casing prevents the penetration of electromagnetic noise. It can also eliminate a phenomenon called crosswalk, which is the undesired effect of one circuit (or channel) on another circuit (or channel). It occurs when one line (acting as a kind of receiving antenna) picks up some of the signals travelling down another line
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(acting as a kind of sending antenna). This effect can be experienced during telephone conversations when one can hear other conversations in the background. Shielding each pair of a twisted-pair cable can eliminate most of the effects of crosswalk. Main advantage of STP is, it is less susceptible to noise. However, the disadvantage is, it
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This note was uploaded on 02/23/2012 for the course CS 101 taught by Professor Martand during the Spring '10 term at Punjab Engineering College.

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Chapter_2_CN - Chapter 2 Physical Layer Transmission Media...

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