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

These pages are somehow outdated and it is recommended to consult the newer version at Computer networks -- 2008-2009 -- info.uvt.ro (by Ciprian Crăciun).

Quick links:

• front;
• courses 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13;
• laboratories agenda, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, evaluation, tools, repository.

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Physical layer

• 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

Magnetic / optic media

Never underestimate the bandwidth of a station wagon full of tapes hurtling down the highway.

Twisted pair

• 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

• 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

• 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

• 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

Radio waves and microwaves

• 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

• 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

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

Trunks and multiplexing

• 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;

Data link layer

• 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

• 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

• 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

• 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;

Data-link protocols

• 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;

Internet data-link protocol

• 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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• conf. dr. Victoria Iordan, iordan@info.uvt.ro
• lect. drd. Horia Popa, hpopa@info.uvt.ro
• Marian Neagul, marian@info.uvt.ro
• Ciprian Dorin Crăciun, ccraciun@info.uvt.ro
• Mihail Găianu, mgaianu@info.uvt.ro