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Computer networks -- 2007-2008 -- info.uvt.ro/Course 4

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Important! These pages are somehow outdated and it is recommended to consult the newer version at Computer networks -- 2008-2009 -- info.uvt.ro (by Ciprian Crăciun).

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Physical layer

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  • medium types:
    • guided, un-guided;
    • shared, un-shared;
    • point-to-point, broadcast;
  • when choosing mediums we should take into consideration:
    • bandwidth;
    • delay and latency;
    • costs:
      • purchase cost;
      • operational cost -- OPEX (operational expenditures);
      • total cost of ownership -- TCO;
    • 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. [1]

Twisted pair

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  • two insulated copper wires, twisted in a helical form;
  • they are twisted to reduce the radiation, and thus eliminate interference;
  • categories:
    • determined by the number of twists per centimeter;
    • category 3 -- 16 MHz;
    • category 5 -- 100 MHz;
    • category 6 -- 250 MHz;
    • category 7 -- 600 MHz;
  • types:
    • UTP -- Unshielded Twisted Pair;
    • STP -- Shielded Twisted Pair;
    • advantages, disadvantages;
  • usage:
    • phone lines;
    • computer networks;
  • lesser shielding => smaller distances;

Coaxial cable

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  • composed of:
    • copper core;
    • insulating material;
    • braided outer conductor;
    • protective plastic covering;
  • types:
    • 50 ohm -- for digital transmission;
    • 75 ohm -- for analog transmission;
  • better noise immunity;
  • better shielding => longer distances;
  • higher bandwidth -- up to 1GHz;
  • usage:
    • television;
    • MAN networks;

Fiber optics

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  • 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 -- ultra-thin glass fiber;
    • 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;
  • types:
    • multi-mode: different rays at different angles;
    • single-mode: only one ray, the fiber is very thin;
  • preferred wave-lengths:
    • determined based on attenuation;
    • 0.85, 1.30, 1.55 microns;
    • 25 up to 30 THz width;
  • fiber optic cable components:
    • core -- made from glass;
    • cladding -- still glass;
    • jacket -- plastic;
    • fibers could be grouped;
  • 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;
  • light detectors: photodiode;
  • fiber optic networks:
    • point to point;
    • active interconnections -- repeaters;
    • passive interconnections;
    • by using the ring or star topology -- for multiple computers;
  • hardware:
    • media converters;
    • fiber optic switches;

Fiber optics vs copper wire

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  • fiber:
    • higher bandwidths;
    • longer spans: 50km;
    • non corrosive;
    • thiner and lighter -- more lines in a cable duct;
    • better security against wiretappers;
    • easibly damageable;
    • more skills needed;
    • one-way communication;
    • expensive interfaces;
  • coper:
    • lower bandwidths;
    • shorter spans: 5km;
    • more robust;

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;
  • frequencies discussion;
  • 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;

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;)

Other transmission media

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  • communication satellites;
  • PSTN -- public switched telephone network;
  • cable television;
  • switching:
    • types:
      • circuit switching;
      • packet switching;
      • message switching (used for telegrams);

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;
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  • it involves:
    • algorithms to achieve reliable, efficient communication between two adjacent machines;
    • the two machines to be connected through a channel that acts like a wire;
    • it needs that the physical layer to be ordered -- thus the bits arrive in the same order they were sent;
  • it has the functions:
    • provide well-defined service interface to the network layer;
    • dealing with transmission errors;
    • regulating the data flow;
  • services provided to the network layer:
    • transferring data from the source machine to the destination machine;
  • types:
    • unacknowledged connection-less service:
      • practical for low error rate media;
      • useful for real-time traffic, voice, video;
    • acknowledged connection-less service:
      • practical for medium error rate media; like in the case of wireless media;
    • acknowledged connection-oriented service;
  • network types:
    • using point-to-point protocols;
    • using broadcast channels;
  • in case of broadcast channels the layer is split in two:
    • MAC -- media access control;
    • LLC -- logical link control;

Framing

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  • the physical layer only accepts a sequence of bits;
  • the sequence of bits is not error free:
    • bits are swapped;
    • bits are added;
    • bits are lost;
  • the data link layer takes the network packets and transforms them into frames:
    • prepends a header;
    • appends a trailer;
  • framing solutions:
    • character counting -- not very reliable;
    • flag bytes;
    • combined;

Error control

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  • solutions:
    • the other end of the communication could send positive or negative acknowledgements;
    • using acknowledgement timeouts, followed by resending;
    • using sequence numbers to eliminate possible duplicates;
  • errors could come:
    • either unrelated, with a given probability;
    • either in bursts, meaning one packet in n will contain errors -- typical for radio communications;
  • error correction codes -- redundant information that allows recovery;
  • error detection codes -- redundant information that allows only discovery;
    • CRC -- cyclic redundancy check;

Flow control

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  • solutions:
    • feedback-based -- the other side sends informations about how it handles the network load;
      • frames are not sent until the receiver has granted permission to do so;
    • rate-based -- built-in mechanism into the protocol -- never used in the data link layer;
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  • examples from Computer Networks -- 4th edition by Andrew Tanenbaum:
    • An Unrestricted Simplex Protocol:
      • the physical layer is reliable and error free;
      • no flow control;
    • A Simplex Stop-and-Wait Protocol:
      • error free physical layer;
      • with flow control;
    • A Simplex Protocol for a Noisy Channel:
      • frames could be either lost or damaged;
      • sequence number is added;
    • sliding window protocols:
      • both the sender and the receiver maintains a range of messages that are accepted;
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  • PPP -- point-to-point protocol:
    • provides:
      • frame format that handles error detection;
      • link control for bringing up and shutdown the connection, options negotiation -- this is LCP (link control protocol);
      • network layer options negotiation -- this is NCP (network control protocol);
    • frame format:
      • flag byte: 01111110;
      • hardware address byte: 11111111 -- thus no hardware address is assigned;
      • control byte: usually 00000011 -- un-numbered frame;
      • protocol: 2 bytes;
      • payload of a predetermined size;
      • checksum: 2 bytes;
      • again the flag byte;

References

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  1. ↑ Computer networks -- 4th edition -- by Andrew Tannenbaum
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