Computers and computer networks

Table of contents


OSI layers

  • Physical – Data unit: bit

The Physical Layer defines electrical and physical specifications for devices. It defines the relationship between a device and net, example cable (copper or optical ), pins, hubs, repeaters, network adapters and more.

  • Data Link – Data unit: frame

The Data Link Layer provides the means to transfer data between network device and to detect and possibly correct errors that may occur in the Physical Layer.

  • Network – Data unit: packet

The Network Layer provides the means of transferring data sequences from a host on one network to a destination on a different network, while maintaining the quality of service.

  • Transport – Data unit: segments

The Transport Layer provides transfer of data between end users, providing reliable data transfer services to the upper layers.

  • Session – Data unit: data

The Session Layer controls the connections between computers. It provides for full-duplex, half-duplex, or simplex operation.

  • Presentation – Data unit: data

The Presentation Layer establishes context between Application Layer entities, in which the higher-layer entities may use different syntax and semantics if the presentation service provides a mapping between them.

  • Application – Data unit: data

Network process to application

The Application Layer is the OSI layer closest to the end user, which means that both the OSI application layer and the user interact directly with the software application.

Regenerative repeater

Regenerative repeater is a device which regenerates incoming digital signals and then retransmits these signals on an outgoing circuit. This device is useful because signals in twisted pair cable is degrading then moving long distances. If I am correct than after 100 meters signal is degraded beyond repair, so it is necessary to put router, switch or regenerating repeater. Switches and routers are more important than to give a boost to the signal. Their also are more expensive. Repeater uses transport layer. It is different from other components because it is not doing anything difficult. It receives signal, boost it and send it forward. It is used then there is long cable and signal can seriously degrade.


Computer has two different memory types, slow and fast. Fast is managed in upper part of motherboard, and slower – lower part. Both memory managers have special chip where their memory is configured/used accordingly. Between them is bridge. It is very important part which is to ensure computers work. Bridge connects processing memory with storage memory what computer processes could be done. It uses physical layer. It differs from other components because it is just connection between two memory „managers“. Situation with this component is impossible. It is necessary to computer work.


Hub is a device for connecting multiple devices together and making them act as a LAN. It is just like network switch, but this is the older version. The device is a form of multiport repeater. Hubs was before the switches and there are not good enough anymore. Then hub receives a data packet it isn’t send to computer for witch it belongs, but it is send to all computers, whose are connected to computer. The right one takes it, the others ignores it. If two computers send data at the same time, then collision occur. In other words better use switches, hubs are old and not reliable. It is using physical layer. It different from other components because it is used to connect computers and other devices together into network. The availability of low-priced network switches has largely rendered hubs obsolete but they are still seen in older installations and more specialized applications. To pass data through the repeater in a usable fashion from one segment to the next, the packets and the Logical Link Control (LLC) protocols must be the same on the each segment.


A router is a device that chooses path for data packets across computer networks. Network between end-users is a giant net, where all joint spots are routers. ISP routers usually exchange routing information using the Border Gateway Protocol (BGP). Also called a Provider Edge router, is placed at the edge of an ISP network. The router uses External BGP to EBGP protocol routers in other ISPs. This device is different from other because it is created to ensure data packet travel across network. It is considered a network layer device.

Charlie and Lola


Files must be kept in order at all times. Keeping them spread is not good idea because then needed, they can be hard to find around other files there. And desktop is not a place for “secret” files.

C:/Documents and Settings/Charlie/MySecretFiles/TopSecret.odt

When he press “Save As”, it depends where was his last save. When he has to navigate to his “special” file he have several options. There suppose to be quick buttons which will move directly to My Documents or My Computer. If My Documents is used, then you need to open MySecretFiles folder and than save your file. If My Computer used, then you need to open Local Disk (C:), then Documents and Settings, after that folder Charlie, after that My Documents and finally MySecretFiles. Then you can save your file. I don’t fink what folder MySecretFiles and file TopSecret.odt is so secret after all. When you have something secret don’t call it secret.


This question is tricky one. It depends where is Mary. Because if he would be on the same LAN than Mary can make his printer accessible through LAN and Charlie would be able to print his file from his home. Just he need to order printing through Mary’s printer. But then anyone could print through that printer. Well Charlie’s out of luck. Mary’s is on other LAN, but that is not a problem. To send file to someone else is not very hard. There is several ways to do it:

Skype. This program can be used to talk to someone, to write messages and to send files.

E-mail. Using your mail to create new message, you can attach a file and send it with a message. It is secure because email sites like Yahoo uses secure connection. Hypertext Transfer Protocol Secure (HTTPS) is a combination of the Hypertext Transfer Protocol with the SSL/TLS protocol to provide encrypted communication and secure identification of a network web server.


Well the message start from Charlie’s PC to his wireless device, which forwards it to router. Then it will move across network to Mary’s PC. It will use lots of different network protocols:

  • ARP – Address Resolution Protocol
  • RARP – Reverse Address Resolution Protocol
  • L2F – Layer 2 Forwarding Protocol
  • L2TP – Layer 2 Tunneling Protocol
  • LLDP – Link Layer Discovery Protocol
  • MTP – Message Transfer Part
  • IPv6 – Internet Protocol version 6
  • RIP – Routing Information Protocol
  • NFS – Network File System
  • DNS – Domain Name System
  • FTP – File Transfer Protocol
  • HTTP – Hypertext Transfer Protocol

There is a lot more protocols which will be used at some times but there is no way to name them all. Now I explain the main of these.

  1. Internet Protocol version 6 (IPv6) is a version of the Internet Protocol (IP) which was designed to succeed his predecessor Internet Protocol version 4 (IPv4). The Internet operates by transferring data packets that are guided across networks as specified by the Internet Protocol.
  2. The Domain Name System (DNS) is a naming system built on a database, or any resource connected to the Internet or a private network. Most importantly, it translates domain names known to users into the IP addresses for the purpose of locating and addressing these devices worldwide.
  3. File Transfer Protocol (FTP) is a standard network protocol used to copy a file from one host to another over a Transmission Control Protocol based network, such as the Internet.
  4. The Hypertext Transfer Protocol (HTTP) is a networking protocol for using information systems. HTTP is the foundation of data communication for the World Wide Web.

These four protocols is the main ones to let Charlie’s message to reach Mary. The Hypertext Transfer Protocol will enable Charlie to use internet. Domain Name System protocol will found email site Charlie’s using. File Transfer Protocol will ensure files movement from host to host. That can be Charlie to Mary, or Charlie to email site and email site to Mary. Internet Protocol version 6 guides data packets across network. Charlie’s file will newer be send like it is. It will be divine in several (or thousands) data packets and send to destination.


Lora and Lotta are not hackers. Then it comes to intercept data transfer there three levels of security. Highest level is fiber wire. Light comes through it so to hack it you have to cut it and it will be detected. The middle level of security is twisted pair cable. The little device can be attached to cable and it will read all data coming through. The lowest security is wireless LAN or WLAN, or wireless local area network. When transmission is send through air anyone can access it (with high hacking skills and extremely expensive equipment).

Lotta is very good with computers, what about hacking and network. Neither of girls don’t have skills. And normally messages are encrypted due to use of HTTPS. He can also put a password on a file (if possible). Or he can compress file to archive with password (rar, zip, 7z).


Well first of all Soren Lorensen is IMAGINARY. He can’t do anything. He knows what Lora knows. If he will try to help Lora just keep pressing random buttons he shows and she definitely end up formatting her hard drive. She better have imaginary friend Smarty Hacker.


  1. JAVVIN network management & security (no date) OSI 7 layers Model of Network Communication (Online). Available from:
  2. (Accessed 06th April 2011)
  3. Internet Society (no date) IPv6 [Online]. Available from:
  4. (Accessed 06th April 2011)
  5. Webopedia (no date) What is DNS[Online]. Available from:
  6. (Accessed 06th April 2011)
  7. Webopedia (no date) What is HTTP[Online]. Available from:
  8. (Accessed 06th April 2011)
  9. Personal Web Pages (no date) FTP – What Is It and How Do I Use It (Online). Available from:
  10. (Accessed 06th April 2011)

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