Computer networks -- 2007-2008 -- 1

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 These pages are somehow outdated and it is recommended to consult the newer version at Computer networks -- 2008-2009 -- (by Ciprian Crăciun).

Quick links:

Course notes and contacts[edit]



In English:

  • Computer Networks -- Andrew Tanenbaum
  • Complete Guide to Networking -- Peter Norton
  • TCP/IP Ilustrated (Volume 1 and 2) -- Richard Stevens

In Romanian:

  • Bazele reţelelor de calculatoare -- Manual pentru administrarea retelelor LAN si WAN -- Editura Teora, 1999
  • Reţele locale de calculatoare -- Valentin Cristea, Editura Teora, Bucuresti, 1995
  • Sisteme cu prelucrare distribuită si aplicaţiile lor -- Păunescu F., Golesteanu D.P., Editura tehnică, Bucuresti, 1993
  • Reţele locale de calculatoare -- Bulăceanu C., Editura Tehnică, Bucureşti, 1995
  • Reţele de calculatoare -- A. Stepan, Gh. Petrov, V. Iordan, Editura Mirton, 1998

On-line material[edit]

Please consult: Computer networks -- 2007-2008 --

Course structure[edit]

  • general characteristics;
  • topologies;
  • protocols;
  • architectural models: OSI, TCP/IP, etc. -- introduction + detailed;
  • network devices / hardware;
  • VPN;
  • security;

What networking stands for[edit]

  • a computing / processing and resource access model;
  • a way to transport information;
  • it involves:
    • standardization;
    • organizations backing-up the standards -- IETF, ITU, IEEE;
    • important services (DNS, NTP);


  • network element:
    • computing unit;
    • networking device;
  • network topology;
  • transmission medium;
  • medium access;
  • frames, packets;
  • bandwidth (GPS, MPS, PPS);
  • errors:
    • bit error rate (for telephony: 10^-4 is faulty, 10^-5 should be corrected; for data: between 10^-9, 10^-12);
    • packet loss (TCP stops working correctly below 0.1%, that is 10^-3);

Network types[edit]

Categorized by computational / resource usage[edit]

  • centralized:
    • all users work on the same machine;
    • terminals based;
    • thin clients;
    • low cost;
    • very secure;
    • easy to administer and recover;
    • less flexible;
    • applications: accounting, financial transactions;
  • descentralized / distributed:
    • each user works on its own machine;
    • local processing;
    • off-line working;
    • software incompatibilities;
    • less secure;
  • collaborative / distributed:
    • the same application is run on multiple computers;
    • GRID;
  • client-server:
    • there is a hierarchy;
    • the resources are accessed directly only by a server;
    • the clients access resources through a server;
    • security is high;
    • more easy to administer;
    • ActiveDirectory;
    • spooling;
  • peer-to-peer:
    • each computer is at the same time a client or a server;
    • only applicable for very small networks;
    • security is low;
    • more flexible and cheap, but less performant;

Categorized by spatial spread[edit]

  • LAN -- Local Area Network:
    • small spread;
    • high bandwidth;
    • small error rate;
    • cheap equipments;
    • Ethernet;
  • MAN -- Metropolitan Area Network:
    • medium spread -- towns;
    • usually a group of LANs;
    • medium bandwidth;
    • medium error rates;
    • expensive equipments;
  • WAN -- Wide Area Networks:
    • large coverage;
    • low speeds;
    • high error rate;
    • very expensive equipment;
  • WLAN -- Wireless LAN;
    • WiFi, WiMAX;
  • fixed (wired) vs mobile (wireless) networks;
  • VPN -- Virtual Private Network;
  • Internet;

Network topology[edit]

  • physical layout of the computers, links and network specific devices;
  • it affects:
    • spread;
    • performance;
    • equipment cost;
    • administration and security;
  • physical topology:
    • the way the network elements are layed-out and connected;
  • logical topology:
    • the way in which they communicate;
  • the physical and logical topologies might be differ for an implementation;
  • we can see the network topology as a graph, thus any graph algorithm applies to it;


  • all computers are connected through a single segment;
  • each packet is seen by all computers, but only the target one uses it;
  • the computers are passive -- they are not involved in the transmission process;
  • the bandwidth is shared;
  • the cable length is limited;
  • it usually implies using a terminator to stop signals from reflecting at the end of the segment;
  • low resiliency -- if a link is broken the entire network is broken;
  • security concerns;


  • each computer is connected through a link to a device -- hub or switch;
  • there is a single failure point -- the hub;
  • the hub can be passive (it only serves as junction point) or active (it repeats and enhances the signal, or it sends the message to an specified target);
  • the bandwidth is limited by the hub processing capacity and the link bandwidth;
  • the total cable length is high;
  • high resiliency -- if a link is broken only that computer will be disconnected;
  • security concerns;


  • all computers are connected through a single segment that is closed / circular;
  • the data flows into a single direction;
  • each computer acts as a repeater relaying the message to it's neighbors -- as a repeater;
  • low resiliency -- the breaking of a link breaks the entire network;


  • mesh (partial or fully connected);
  • hierarchical (tree);

Token ring[edit]

  • token bus -- the adaptation for bus topology;
  • when the network is idle a token passes through the network from computer to computer in a circular model;
  • if a computer wants to send data it waits to the token, replaces it with a data frame, and sends it as in the case of the token;
  • each computer passes the frame if it is not addressed to it;
  • if the frame is addressed to a computer, it uses the data, and replaces it with a token, thus the cycle is closed;

Network advantages and disadvantages[edit]

Usage and advantages[edit]

  • Internet: it is part of our lives;
  • communication (VoIP), coordination, collaboration;
  • information dissemination -- education;
  • resource sharing;
  • remote control / remote access;
  • high resiliency;
  • anonymity -- if careful enough;


  • vulnerabilities;
  • data loss;
  • downtime causes huge loses;