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Computer networks -- 2008-2009 -- info.uvt.ro/Course 6

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Quick links: front; agenda; courses 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13; examination.

Important! Please note that the current work serves mainly as general guidance and discussion topics, and is by no means the reference material for the course. For further information please consult the dedicated section.

Some physics and electronics

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Physical layer

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  • transmission medium:
    • classification:
      • guided, un-guided;
      • shared, un-shared;
    • point-to-point, broadcast;
    • characteristics:
      • linear -- if different waves can be superposed;
      • bounded, un-bounded;
      • homogeneous, heterogeneous;
      • isotropic -- if transmission characteristics are in all directions;
  • when choosing mediums we should take into consideration:
    • bandwidth;
    • latency;
    • costs:
      • OPEX (OPerational EXpense);
      • CAPEX (CApital EXpenditure);
      • TCO (Total Cost of Ownership);
    • compatibility with existing and future infrastructure;
    • flexibility -- like in the case of fiber optics which hasn't reached its physical limit;

Guided transmission media

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Magnetic / optic media

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  • Never underestimate the bandwidth of a station wagon full of tapes hurtling down the highway. (Computer networks -- 4th edition)
  • Sneakernet;

Coaxial cable

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  • presentation:
    • copper core;
    • insulating material;
    • braided outer conductor;
    • protective plastic covering;
  • working principle:
    • the cable acts as a transmission medium for radio frequencies;
    • the electromagnetic field exists between the core and the outer conductor;
    • the outer conductor is grounded;
  • types:
    • 50 ohm -- for digital transmission;
    • 75 ohm -- for analog transmission;
  • advantages:
    • better noise immunity;
    • better shielding => longer distances;
    • higher bandwidth -- up to 1GHz;
  • usage:
    • television;
    • MAN networks;
  • references:
    • Computer Networks, 4th edition -- section 2.2.3 Coaxial Cable;

Twisted pair

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  • presentation:
    • two insulated copper wires forming a pair;
    • each pair forms a closed circuit;
    • each pair is twisted in a helical form;
    • one or more (4, or 25) pairs in one bundle;
    • a plastic coating for each bundle;
    • maybe shielding for either each pair and/or the entire bundle;
  • twisting:
    • reduce electromagnetic interference;
    • reduce electromagnetic radiation;
    • the twist rate differs from bundle to pair to pair in the same bundle;
  • categories:
    • determined by the number of twists per centimeter;
    • CAT 3 -- 16 MHz, 10 MBit/s;
    • CAT 5/5e -- 100 MHz, 100 MBit/s or 1000 MBit/s (at 100m);
    • CAT 6/6e -- 250/500 MHz;
    • CAT 7 -- 600 MHz, 1 GBit/s or 10 GBit/s (at 100m);
    • CAT 7a -- 1000 MHz, upto 40 GBit/S (50m) or 100 GBit/s (at 15m);
  • types:
    • UTP (Unshielded Twisted Pair);
    • STP (Shielded Twisted Pair);
    • S/UTP or S/STP (Screened UTP or STP);
    • lesser shielding => smaller distances;
  • connectors:
    • 8P8C (or RJ45 (Registered Jack));
  • usage:
    • phone lines;
    • computer networks;
    • security devices (cameras);
    • structured cabling;
    • advantages: cost, small diameter, flexibility;
    • disadvantages: smaller maximum lengths, small maximum tension;
  • miscellaneous:
    • PoE (Power over Ethernet);
  • references:
    • Computer Networks, 4th edition -- section 2.2.2 Twisted Pair;

Fiber optics

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  • presentation:
    • a thin fiber of glass -- the core (8, 50, or 60 micro meters);
    • another glass coating -- the cladding (125 micrometers);
    • a plastic protection;
    • fibers could be grouped in bundles;
  • limits:
    • physical limit: 50000 Gbps, or 50 Tbps;
    • the current practical limit is 10 Gbps;
    • the current experimental limit is 100 Gbps;
    • limitations: conversion between electrical and optic signals;
  • components:
    • light source;
    • transmission medium;
    • light detector;
  • working principle:
    • light present => 1 bit;
    • light absent => 0 bit;
    • electrical pulse => light pulse => electrical pulse;
    • light is reflected internally in the silica fiber;
    • one way communication;
  • preferred wave-lengths:
    • determined based on attenuation;
    • 0.85, 1.30, 1.55 microns;
    • 25 up to 30 GHz width;
  • fiber connections:
    • using connectors plugged into sockets -- 10 to 20% light loss;
    • mechanical splices -- 10% light loss;
    • fusion -- very small attenuation;
  • light sources:
    • LED (Light Emitting Diodes);
    • semiconductor lasers;
  • fiber optic networks:
    • point to point;
    • active interconnections -- repeaters;
    • passive interconnections;
    • by using the ring or star topology -- for multiple computers;

Fiber optics vs copper wire

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  • fiber:
    • higher bandwidths;
    • longer spans: 50km;
    • non corrosive;
    • thinner and lighter -- more lines in a cable duct;
    • better security against wiretappers;
    • easily damageable;
    • more skills needed;
    • one-way communication;
    • expensive interfaces;
  • copper:
    • lower bandwidths;
    • shorter spans: 5km;
    • more robust;
  • references:
    • Computer Networks, 4th edition -- section 2.2.4 Fiber Optics;

Wireless transmission media

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Radio waves and microwaves

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  • driven by the need of non-stop on-line presence;
  • it is estimated that in the future only fiber and wireless will exist;
  • omnidirectional vs focused;
    • radio waves are omnidirectional (under 100 MHz) => no alignment needed;
    • but they could be concentrated (focused) on a direction (above 100 MHz) => proper alignment needed; microwaves;
  • radio waves are subject to interference for other electrical equipment;
  • references:
    • Computer Networks, 4th edition -- section 2.3.2 Radio Transmission;
    • Computer Networks, 4th edition -- section 2.3.3 Microwave Transmission;

Infrared and light waves

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  • infrared used often for office appliances connectivity; or remote controls;
  • lasers could be used to connect neighboring buildings; (but it is subject to outdoor conditions like rain, fog, or heat;)
  • references:
    • Computer Networks, 4th edition -- section 2.3.5 Lightwave Transmission;

Other transmission media

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  • communication satellites;
  • PSTN -- public switched telephone network;
  • cable television;
  • references:
    • Computer Networks, 4th edition -- section 2.4 Communication Satellites;
    • Computer Networks, 4th edition -- section 2.5 The Public Switched Telephone Network;
    • Computer Networks, 4th edition -- section 2.6 The Mobile Telephone System;
    • Computer Networks, 4th edition -- section 2.7 Cable Television;

Trunks and multiplexing

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  • usually it is more cost effective to install a high-bandwidth trunk and present them as low-bandwidth trunks;
  • multiplexing can be:
    • FDM (Frequency Division Multiplexing) -- the frequency spectrum is divided into bands;
    • WDM (Wavelength Division Multiplexing) -- the same as FDM but for fiber optics;
    • TDM (Time Division Multiplexing( -- the users periodically get the entire bandwidth;
    • CDMA (Code Division Multiple Access);

The current page is (in almost all parts) a recompilation of the following pages (from previous year):

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